FiLE NO. 1-0042

AIRCRAFT ACCIDENT REPORT TRANS WORLD AIRLINES, INC. BOEING 707-331B, N8705T LOS ANGELES, CALIFORNIA AUGUST 28, 1973 Adopted: July 10, 1974

NATIONA~.TRANSPORTATION S AFETY B O ARD Washington, D.C. 20591 REPORT NUMBER: NTSB-AAR-74-8

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TECHNICAL REPORT STANDARD TITLE PAGE No. 2.Government Accession No. 3.Recipientls Catalog No. NTSB-AAR-74-8 5.Report Date I . T i t l e and Subtitle A i r c r a f t Accident Report " 9 1974 ' r a n s World Airlines, Inc., Boeing 707-331B, N8705T ,, ,OS Angeles, California, August 28, 1973 6.Performing Organization Code 1 . Authorb) 8.Performing Organization Report No.

. Report

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).

IO.Work U n i t No. 1327 i1.Contract or Grant No.

Performing Organization Name and Address National Transportation Safety Board B u r e a u of Aviation Safety Washington, D. C. 20591

13.Type of Report and Period Covered A i r c r a f t Accident Report August 28, 1973

12.Sponsoring Agency Name and Address

NATIONAL TRANSPORTATION SAFETY BOARD Washington, 0. C. 20591

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14.Sponsoring Agency Code

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15.Supplementary Notes T h e r e a r e no new safety recommendations contained in this report. 16.Abstract A T r a n s World Airlines, Inc., Boeing 707-331B porpoised while descending approximately 35 m i l e s west of Los Angeles, California, at 2150 p. d. t. on August 28, 1973. T h e flight was bound f o r t h e Los Angeles International Airport. T h e longitudinal instability p e r s i s t e d f o r about 2 minutes during which m o r e than 50 pitch oscillations occurred. Peak acceleration f o r c e s of t 2 . 4 g to -0.3g were m e a s u r e d a t t h e a i r c r a f t ' s c e n t e r of gravity. T h e r e w e r e 141 p a s s e n g e r s and 11 c r e w m e m b e r s aboard. A s a result of t h e accident, one p a s s e n g e r was injured fatally; one flight attendant and two other p a s s e n g e r s w e r e injured seriously. T h e National Transportation Safety Board determines that the probable c a u s e of this accident was a combination of design tolerances in the a i r c r a f t ' s longitudinal control s y s t e m which, under c e r t a i n conditions, produced a c r i t i c a l relationship between control f o r c e s and a i r c r a f t response. T h e atypical control f o r c e c h a r a c t e r i s t i c s which w e r e present in this p a r t i c u l a r a i r c r a f t ' s control s y s t e m w e r e conducive t o overcontrol of t h e a i r c r a f t by the pilot. The pilot's n o r m a l reaction to a n unexpected longitudinal disturbance led t o a pitching oscillation which w a s tempor a r i l y sustained by his subsequent application of control column f o r c e s t o regain stable flight. 17. Key Words 18.Distribution Statement Longitudinal stability T h i s report is available t o the public through the Pilot-induced Oscillation National Technical Information Service, Springfielc Virginia 22151 19.Security Classification 20.Security Classification 21.No. of Pages 22.Price (of t h i s report) (of t h i s page) 43 UNCLASSIFIED UNCLASSIFIED ,NTSB Form 1765.2 (11/70) ii

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TABLE OF CONTENTS Page

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1 1.1

1.2 1.3 1.4 1.5

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1 7 1.8

1.9 1.10 1.11 1.12 1 13 1 14 1. 15 1 16 1 16 1 1 . 16.2 1. 1 6 . 3 1 17 2 2 1 ,2.2

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Synopsis Investigation History of the Flight InjuriestoPersons Damage t o Aircraft Other Damage Crew Information Aircraft Information Meteorological Information Aids t o Navigation Communications Aerodrome and Ground Facilities FlightRecorders Aircraft Wreckage Medical and Pathological Information Fire Survival Aspects T e s t s and R e s e a r c h B-707 C o n t r o l S y s t e m Inspections and T e s t Flights T e s t Results Other Information Analysis and Conclusions Analysis Conclusions ( a ) Findings (b) P r o b a b l e Cause Recommendations

1 2 2 3 3 4 4 4 4 5 5 5 5

................................ ................................ 6 ................ 6 ............................................. 6 .................................. 6 ............................... 7 .............................. 7 ....................... 8 .................................... 14 ............................... 16 ......................... 16 ....................................... 16 .................................... 20 .................................... 20 ............................. 21 ............................... 22

Appendices Appendix Appendix Appendix Appendix

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A .Investigation and Hearing B . Crew Information C .Flight R e c o r d e r Readout D .Boeing 707 Stabilizer . Elevator Assembly Appendix E .Safety Recommendations A-73-76 through78 Appendix F .Safety Recommendation A-74-41

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Glossary

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25 26 27 29 30 38 42

File No. 1-0042

NATIONAL TRANSPORTATION SAFETY BOARD WASHINGTON, D. C. 20591 AIRCRAFT ACCIDENT REPORT Adopted: July 10, 1974 TRANS WORLD AIRLINES, INC. BOEING 707-331B, N8705T LOS ANGELES, CALIFORNIA .-AUGUST 28, 1973

SYNOPSIS At 2150 on August 28, 1973, T r a n s World Airlines, Inc., Flight 742, a Boeing 707-331B (N8705T), experienced longitudinal oscillations (porpoised) while descending t o the Los Angeles International Airport, Los Angeles, California. One hundred forty-one p a s s e n g e r s and 11 crewmembers were aboard. As a result of the accident, one passenger was injured critically and died 2 days l a t e r ; one flight attendant and two other passeng e r s were injured seriously. T h e flight was a scheduled p a s s e n g e r flight f r o m Honolulu t o Los Angeles. T h e flight was routine until the a i r c r a f t was about 35 miles west of Los Angeles. While it was descending through 22, 000 feet p r e s s u r e altitude a t 350 knots indicated airspeed, the a i r c r a f t began t o porpoise. Over 50 oscillations were experienced which produced peak acceleration forces of f2. 4g to - 0. 3g at the a i r c r a f t ' s center of gravity. The oscillations subsided a s the indicated a i r s p e e d was reduced t o about 300 knots. T h e flight continued to Los Angeles without f u r t h e r difficulty. T h e National Transportation Safety Board determines that the probable cause of t h i s accident was a combination of design tolerances in the aircraft's longitudinal control s y s t e m which, under c e r t a i n conditions, p r o duced a c r i t i c a l relationship between control f o r c e s and a i r c r a f t response. The atypical control force c h a r a c t e r i s t i c s which were present in this particular a i r c r a f t ' s control s y s t e m were,conducive t o overcontrol of the aircraft by t h e pilot. T h e pilot's n o r m a l reaction t o a n unexpected longitudinal disturbance led t o a pitching oscillation which was temporarily sustained by his subsequent application of control column f o r c e s t o regain stable flight. T h e c a u s e of the death and injuries was the impact of unrestrained persons with unyielding objects in t h e cabin environment.

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As a result of t h e investigation, the Safety Board has submitted five recommendations t o the F e d e r a l Aviation Administration. 1.

1. 1

and t at a

INVESTIGATION

History of the Flight

T r a n s World Airlines, Inc. (TWA), Flight 742, a Boeing 707-331B (N8705T), w a s a n international, scheduled passenger flight operating between Bangkok and San F r a n c i s c o with en route stops at Hong Kong, Taipei, Okinawa, Guam, Honolulu, and Los Angeles. One hundred fortyone passengers and 11 c r e w m e m b e r s were aboard the flight. Flight 742 departed Honolulu a t 1709 11 and operated routinely at c r u i s e flight level ( F L ) 330 (33, 000 feet p r e s s u r e altitude). About 2110, the flight established radio contact with the Los Angeles Air Route T r a f f i c Control Center (LAX Center). About 2129, the LAX Center controller cleared Flight 742 t o descend t o F L 110. The crew acknowledged the clearance, disengaged the autopilot, reduced power, and s t a r t e d t o descend. Since cabin entertainment had just ended, flight attendants were cleaning up the galley and preparing f o r landing. The "fasten seatbelt" sign was off and five o r s i x passengers were standing n e a r the aft galley and lavatories. As the flight descended through F L 220 a t 350 knots indicated airspeed (KIAS), the a i r c r a f t pitched up abruptly, then pitched down, and began a n oscillatory, o r porpoising, motion. More than 50 oscillations were experienced within about 2 minutes. The a i r c r a f t ' s nose attitude 0 pitched f r o m about 5' t o 7O noseup t o about 5O t o 7 nosedown during that period. At the onset of the oscillations, the c r e w turned on the "fasten seatbelt" sign and verified that the autopilot was disengaged. The engine power was reduced t o idle and the rudder power, mach t r i m , and yaw damper were turned off. Appropriate circuit b r e a k e r s were pulled. T h e captain, a s s i s t e d by the f i r s t officer, attempted t o counteract the porpoising motion by inputs through the control column. The a i r c r a f t continued t o descend while decelerating. T h e pitching oscillations abated,

trim subs aPPr aft c c e ili turn the 1 in t h

abili t o fo FL 1 diffit disc, that nat i c

crev for i 742

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and the a i r c r a f t regained stable flight as the KIAS reduced to about 300 at a p r e s s u r e altitude of 19, 500 feet. The crew did not notice the t r i m position o r motion of the s t a b i l i z e r t r i m wheels during the porpoising. However, t h e stabilizer t r i m s y s t e m subsequently operated normally. The c r e w observed that t h e r e was no appreciable turbulence before o r a f t e r the incident. Those flight attendants and p a s s e n g e r s who were standing in the aft coach section of the a i r c r a f t were thrown repeatedly f r o m floor t o ceiling while the a i r c r a f t porpoised. Some p a s s e n g e r s managed to r e t u r n to s e a t s and escaped s e r i o u s injury. According t o t h e p a s s e n g e r s , the v e r t i c a l acceleration f o r c e s were progressively l e s s f a r t h e r forward i n the cabin. After regaining stable flight, the c r e w determined that the controlability of t h e a i r c r a f t w a s normal except f o r a slightly high r e s i s t a n c e to forward control column movement. The descent w a s continued to F L 110 (11,000 feet), and t h e c r e w notified LAX Center of the control difficulty. The TWA dispatcher was a l s o notified, and the situation w a s discussed with company maintenance personnel. The c r e w requested that emergency medical a s s i s t a n c e standby at the Los Angeles International Airport. After f u r t h e r evaluating the a i r c r a f t flight c h a r a c t e r i s t i c s , the c r e w declared a n emergency,and the flight was c l e a r e d by LAX Center f o r a n instrument landing s y s t e m (ILS) approach t o runway 7L. Flight 742 landed at 2243 without f u r t h e r incidents. The accident o c c u r r e d during hours of d a r k n e s s . 1.2

Injuries to P e r s o n s Injuries

Crew

Fatal Nonfatal None

0 2

: :

1. 3

9

,

Passengers

Others

1

0 0

2: 138

Includes only those p a s s e n g e r s who were immediately hospitalized.

Damage t o Aircraft

An inspection of t h e a i r c r a f t did not r e v e a l any s t r u c t u r a l damage which could be attributed to the accident.

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1.4

Other Damage clea The] west

None 1. 5

Crew Information 1. a

T h e c r e w m e m b e r s w e r e certificated f o r the flight.

(See Appendix

B.) At t h e t i m e of the accident, c r e w m e m b e r s had been on duty f o r 6 to 7 hours. Before reporting f o r duty, the c r e w m e m b e r s had a r e s t period of about 2 1 hours.

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Aircraft Information

N8705T, a Boeing 707-331B, s e r i a l No. 18916, w a s owned and operated by TWA. Its date of manufacture is December 9, 1965. T h e a i r c r a f t had accumulated 31, 136 flight-hours at the time of t h e accident. A block overhaul had been performed on January 4, 1970, a t the TWA Maintenance Facility, Kansas City International Airport, Kansas City, Missouri. T h e a i k r a f t had flown 14,305 hours since this overhaul and 48.7 hours since i t s l a s t C-check maintenance. N8705T w a s certificated, equipped, and maintained according to F e d e r a l Aviation Administration (FAA) regulations. T h e a i r c r a f t ' s estimated g r o s s weight and center of gravity (c. g. ) were 204,000 pounds and 30 percent MAC, respectively. Both a r e within specified limits. N8705T had a l s o experienced pitching oscillations on July 18, 1972. T h e r e was no damage reported as a result of that accident. 1. 7

Meteorological Information

T h e Los Angeles International Airport s u r f a c e weather observations we;e a s follows:

2100 -

1200 feet scattered, visibility-7 miles; t e m p e r a t u r e 63O F., dew point-58' F . , Wind-260' at 5 knots altimeter setting-29.89 inches.

2200 -

1200 feet scattered, visibility-8 miles, t e m p e r a t u r e 0 0 62' F . , dew point-50 F., wind-250 a t 5 knots, a l t i m e t e r setting-29.90 inches.

1.11

Rec Flig

the whi(

The mat airs ing at 3 The bet7 300 alti; the pit c

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- 5 The c r e w of Flight 742 reported that the weather w a s good with c l e a r skies. They reported no turbulence during the e n t i r e flight. T h e r e were no other pilot r e p o r t s of turbulence o r a d v e r s e weather west of LOSAngeles. 1.8

Aids to Navigation Not applicable.

1.9

Communications Not applicable.

1.10 Aerodrome and Ground Facilities Not applicable. 1.11 Flight R e c o r d e r s N8705T w a s equipped with a Fairchild A-100 Cockpit Voice R e c o r d e r (CVR) and Lockheed Aircraft Service (LAS) model l09C Flight Data R e c o r d e r (FDR). The CVR tape will r e t a i n only 30 minutes of recorded audio. Since the r e c o r d e r continued to operate a f t e r the oscillations, the conversations which took place during the accident were e r a s e d by subsequent recordings.

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The FDR m e t a l foil was removed and examined. (See Appendix C. ) The porpoising was evident on the vertidal acceleration t r a c e as approximately 55 cycles o c c u r r i n g during a 2-minute period. The indicated a i r s p e e d and p r e s s u r e altitude t r a c e s showed that the a i r c r a f t was descending about 2,100 feet p e r minute and was passing through 22,400 feet altitude a t 352 kn. when the f i r s t significant v e r t i c a l acceleration peak occurred. The a i r s p e e d reduced t o 340 kn. during the next 20 seconds and remained between 340 t o 345 kn. f o r about 1 minute. The a i r s p e e d then decayed to 300 kn. where the v e r t i c a l acceleration excursions converged. The altitude remained constant at 22,400 feet f o r about 2 5 seconds, a f t e r which the a i r c r a f t descended. The a i r c r a f t w a s leveled at 19, 500 feet as the pitching oscillations ceased. The v e r t i c a l acceleration reached maximum values of t 2 . 4 g and -0.3g. Peak acceleration f o r s u c c e s s i v e cycles varied slightly as did the t i m e interval between the peaks.

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1. I t

Not applicable. 1. 13 Medical and Pathological Information The fatally injured person and the t h r e e seriously injured persons sustained f r a c t u r e s , internal injuries, and cuts and bruises. P o s t - m o r t e m examination revealed that the cause of death was "subarachnoid and retroperitoneal h e m o r r a g e " which was caused by "blunt f o r c e t r a u m a t o the e n t i r e body. " 1. 14 F i r e

fligl cial. stab 1.1(

stab and of tl cont

Not applicable. 1. 15 Survival Aspects

The person who died and those who were seriously injured had been standing in the aft galley and lavatory a r e a when the oscillations began. The "fasten seatbelt" sign was off but most p a s s e n g e r s were seated and t h e i r seatbelts were fastened. The persons standing in the aft part of the aircraft w e r e thrown f r o m the floor t o the ceiling s e v e r a l times. T h e r e w e r e many sharp, hard, and protruding objects in the aft galley and lavatory a r e a which could have inflicted the injuries t o the unrestrained persons. One p a s s e n g e r was reportedly in the aft lavatory while the aircraft w a s porpoising; however, that passenger was not injured. S e v e r a l persons were cut and bruised a s they were thrown against the a i r c r a f t ' s interior, a r m r e s t s , and other portions of s e a t s . The contents of overhead r a c k s spilled onto the floor and s e a t s . One passeng e r was s t r u c k on the head by a c a m e r a that had been stowed in a n overhead rack. The contents of the auxillary b a r waste container spilled onto the floor in the aft galley a r e a . A f t e r the porpoising ceased, two of the injured passengers, who w e r e unconscious, remained on the floor in the aft a i s l e . The other injured persons were strapped into s e a t s . The uninjured flight attendants, a Navy flight surgeon, and two p a s s e n g e r s who were trained in nursing, administered f i r s t aid. Two p a s s e n g e r s moved t o other s e a t s because of seatbelt failure. The cause and type of seatbelt failures was not d e t e r mined since the seatbelts had been repaired o r replaced before they could be inspected by Safety Board personnel. However, maintenance r e c o r d s indicate that a p a i r of "mismatched" belts were replaced with a "matched" set.

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- 7 1.16 T e s t s and R e s e a r c h The investigation centered around an evaluation of the a i r c r a f t ' s flight c h a r a c t e r i s t i c s and an examination of i t s control system, especially those p r i m a r y control s u r f a c e s which could affect longitudinal stability.

1. 16. 1 B-707 Control System The B-707 a i r c r a f t is controlled longitudinally by movable horizontal stabilizer and elevator control surfaces. The stabilizer is used f o r t r i m and the elevators a r e normally used for t r a n s i e n t maneuvers. The position of the s u r f a c e s may be commanded by the pilot o r by the automatic flight control s y s t e m (autopilot). The angle of incidence of the horizontal stabilizer is varied by repositioning a linear ball nut jackscrew-type actuator. During normal manual flight operation, the actuator is driven by a unidirectional, t h r e e - p h a s e induction motor which i s energized by movement of trim switches on either pilot's control wheel, The motor operates through two electro- magnetic clutches which control the direction of the jacks c r e w actuator motion. During automatic flight, the stabilizer t r i m jackscrew actuator i s driven by a s e p a r a t e servomotor in response to autopilot signals. If the n o r m a l electrical. s y s t e m malfunctions, the jackscrew actuator can be operated manually by rotating t r i m control wheels which a r e mounted on each side of the c e n t e r control pedestal. This action will disconnect the autopilot s e r v o and t r i m motor and d r i v e a mechanical s y s t e m which repositions the ball nut and jackscrew mechanism. The elevators a r e floating aerodynamic surfaces which extend the f u l l span of t h e stabilizer. T h e left and right elevators a r e structurally independent and a r e interconnected only through the control system. The position of the elevators is controlled by motion of the control column which is connected by a cable and linkage s y s t e m to a trailing edge control tab on each elevator. The elevators rotate about t h e i r hinge line by reaction to aerodynamic f o r c e s imposed by displacement of t h e s e control tabs.

S i x aerodynamic balance panels a r e attached t o each elevator s u r face forward of the hinge line. The balance panels a r e supported by a p a r a l l e l o g r a m linkage attached to stabilizer s t r u c t u r e and move within a cavity in the stabilizer a s the elevators a r e deflected. During flight,

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a p r e s s u r e differential i s created between the upper and lower surfaces of the panels by the airflow a c r o s s the stabilizer- elevator assembly. This p r e s s u r e differential produces a moment about the elevator hinge line which opposes the moment caused by the aerodynamic load on the elevator s u r f a c e and thus a s s i s t s the control tabs in moving the elevators.

r1i

i !

t h e Boe maint ex Compax tions, i

In inspect the em1

In addition to the main control tab, a stabilizer actuated elevator control tab (SAE tab) is hinged to the trailing edge of each elevator. The position of the SAE tab i s controlled by the elevator position relative to the stabilizer. The tab functions to balance elevator loads, which a r e imposed by stabilizer t r i m changes, and to optimize elevator net hinge moment c h a r a c t e r i s t i c s . The position of each elevator at any instant is determined by the aerodynamic loads on the elevator surface, the balance panels, the elevator control tab, and the SAE tab. F o r a given control column position, the elevators will move to a position where the s u m of the moments about the hinge line is zero. Since the left and right elevator control tabs a r e operated f r o m a common control system, their movement will b e substantially identical, and if tab efficiency, aerodynamic balance, and SAE tab rigging a r e equal, the two elevators will a s s u m e identical positions. The f o r c e within the control s y s t e m that the pilot must counteract t o control the a i r c r a f t by elevator movement is that f o r c e induced through the mechanical s y s t e m by the control tab hinge moment and a f o r c e generated by deflection of a control s y s t e m centering spring. The relationship between pilot control f o r c e and aircraft response i s thus dependent upon the relationship between a i r c r a f t response to elevator deflection and those variables affecting the elevator hinge moment balance.

1

includi

: During automatic flight, the cable and linkage s y s t e m which connects the control column with the elevator control tabs i s driven by the autopilot elevator servomotor. The servomotor produces a load sufficient to counteract the control tab hinge moment and the centering spring force. The autopilot stabilizer t r i m s e r v o may be considered an elevator s e r v o "helper. " A computer within the s y s t e m monitors the load on the elevator servo. When the elevator control f o r c e reaches a given threshold, the stabilizer s e r v o will run and reposition the stabilizer until the elevator control f o r c e i s relieved. 1. 16. 2 Inspections and T e s t Flights

Flight t e s t s and inspections were conducted at TWA's maintenance facilities in Los Angeles, California, and Kansas City, Missouri, and

the air a mort I

noted 1 altitud 1

-1 contrc

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the Boeing Company's facility in Seattle, Washington. Engineering, maintenance, and flight t e s t personnel f r o m both TWA and t h e Boeing Company participated in a i r c r a f t inspections, maintenance, modifications, and t e s t flights.

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The initial Initial Inspection of Aircraft in Los Angeles inspection of che longitudinal concrol s y s t e m of N8705T, including the empennage structure, disclosed thk following discrepancies:

1.

T r i m m i n g the horizontal stabilizer by t h e n o r m a l "beep" t r i m s y s t e m caused a ground power circuit b r e a k e r t o open. T h e circuit b r e a k e r opened because of a burned contact on the t r i m control relay. T h e discrepancy would result in an open phase of the threep h r a s e winding on the stabilizer t r i m motor. T h e inflight effect would be a n approximate 25-percent r e duction in t r i m speed under maximum t r i m load conditions.

2.

T h e elevator hinge line friction exceeded values specified i n the applicable maintenance manual. F o r c e s of 13. 5 lbs. and 15 lbs. were required at the trailing edge to move t h e left and right elevators, respectively. T h e maximum allowable f o r c e is 8. 5 Ibs. Friction was reduced t o a n acceptable level by lubricating the elevator hinges and balance panel mechanisms.

All other a s p e c t s of t h e s t r u c t u r e and the longitudinal control system, including the autopilot, conformed t o specifications. Since t h e noted discrepancies w e r e not sufficient t o cause the accident, the a i r c r a f t was f e r r i e d t o Kansas City, where facilities were available f o r a m o r e detailed investigation. During the f e r r y flight f r o m Los Angeles t o Kansas City, the pilot noted that on one occasion the a i r c r a f t pitched up decidedly when the altitude hold autopilot function was disengaged. T e s t s Conducted in Kansas City - T h e a i r c r a f t ' s longitudinal control system, including the stabilizer and elevator surfaces, was reinspected after a r r i v a l at Kansas City. Although no significant defects were observed, minor rigging adjustments were made; the right-hand SAE tab, the right-hand elevator control tab, and both the left and right gust d a m p e r assemblies were replaced to improve system performance.

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N8705T was flown four t i m e s t o evaluate the flight c h a r a c t e r i s t i c s in the "as received" condition and changes in those c h a r a c t e r i s t i c s subsequent to specific maintenance and modification. Another B- 707 aircraft was flown to establish a baseline by which to compare the p e r formance of N8705T. A l l flights w e r e conducted with an a i r c r a f t g r o s s weight and c. g. position a s n e a r a s possible to those believed to have existed during the accident. Boeing Company personnel with B-707 flight test experience piloted the a i r c r a f t . The objective of the f i r s t flight of N8705T was t o a s s e s s the c h a r a c t e r i s t i c s of the longitudinal control s y s t e m and the a i r c r a f t r e sponse t o static and dynamic control surface displacement. Since instrumentation was not installed, the t e s t s and the observed results w e r e of a qualitative, r a t h e r than quantitative nature. The t e s t s , which w e r e conducted at 15, 000 feet and 2 9 , 0 0 0 feet, included wind-up t u r n s , elevator pulses, s t a b i l i z e r - e l e v a t o r t r a d e s and autopilot operation. The flight c h a r a c t e r i s t i c s of the a i r c r a f t for the wind-up turns, elevator p u l s e s , and autopilot operation w e r e satisfactory. However, it was determined 'during the stabilizer- elevator t r a d e t e s t s that the magnitude and variation of the f o r c e required to displace the control column for increasing elevator deflections differed f r o m those generally expected by a pilot. T h e control column push f o r c e s n e c e s s a r y to counter a i r c r a f t noseup out- of- trim conditions did not satisfy acceptable stability c r i t e r i a .

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The control column f o r c e s required to produce elevator trailing edge down deflections throughout the range of elevator t r a v e l were weaker than desirable. F o r steadily increasing elevator deflections, the push f o r c e required on the controls increased, then became constant, and eventually, f o r l a r g e deflections, decreased. However, the f o r c e remained a "push" f o r c e and did not reduce to z e r o a t the maximum deflection. Upon completion of the t e s t s at 29, 000 feet, the a i r c r a f t was taken to 33, 000 feet. F r o m this point a descent was initiated s i m i l a r to the descent initiated before the accident. Nothing unusual was observed during the descent to 1 5 , 0 0 0 feet. Following the f i r s t flight of N8705T, another TWA B-707-300 a i r craft was flown and stabilizer- elevator t r a d e t e s t s were conducted at 15,000 feet. The pilot observed that the control column f o r c e s required

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during these t e s t s were significantly different f r o m those required on N8705T. The f o r c e s w e r e of an acceptable magnitude and the gradient was positive throughout the range of elevator travel. After establishing that the longitudinal control f o r c e c h a r a c t e r i s t i c s of N8705T differed f r o m those of other B-707 aircraft, the longitudinal control s y s t e m was examined in m o r e detail. P a r t i c u l a r emphasis was placed on those components, fits, and tolerances known to affect control forces. SAE tab rigging and spring preloads were changed to specified nominal values. Balance panel seals and c l e a r a n c e s were checked and readjusted. The second flight of N8705T was to evaluate the effects of these changes. Stabilizer- elevator t r a d e s w e r e conducted at 15,000 feet. The test results were s i m i l a r to those experienced on the f i r s t flight. Before the third flight, the internal s t r u c t u r e s of the stabilizer and elevator a s s e m b l i e s were inspected. T h e r e were no significant findings. However, the left elevator was replaced a s a precautionary measure. Other minor dimensional changes were accomplished. The third flight was terminated before completion of t e s t s because of an unrelated problem. Before the fourth flight, the balance panel sliding seals were r e adjusted to achieve the maximum allowable clearance with stabilizer * structure. This change theoretically would produce heavier control forces. The fourth flight of N8705T consisted of f u r t h e r evaluation of control f o r c e s during stabilizer- elevator trades. Although the control s y s t e m c h a r a c t e r i s t i c s w e r e improved, the control f o r c e s n e c e s s a r y to produce high elevator deflections were still weaker than desirable. The ability to proceed with f u r t h e r evaluation of the problem at Kansas City was limited by the lack of quantitative data. N8705T was therefore f e r r i e d to the Boeing Company facility in Seattle f o r m o r e comprehensive examination using flight instrumentation.

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T e s t s Conducted in Seattle Instrumentation was installed aboard the aircraft to provide inflight measurements of control s y s t e m forces, differential p r e s s u r e s a c r o s s selected balance panels and elevator surface and control tab positions. Another s e r i e s of test flights was conducted. Before the f i r s t flight at Seattle, the aircraft was r e s t o r e d to its August 28 configuration except for the replacement of the right-hand

r t

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elevator control tab and balance panel s e a l c l e a r a n c e s which were r e adjusted to nominal values. The control f o r c e c h a r a c t e r i s t i c s observed on the flight w e r e essentially the s a m e a s those observed at Kansas City. Examination of data following the flight disclosed that the left elevator deflected f u r t h e r than the right elevator for a given control column displacement. The left control tab hinge moment was significantly lower than predicted values while the right control tab hinge moment was normal. The f o r c e which the pilot must exert to displace the control column relates directly to the algebraic sum of the left and right control tab hinge moments. A s a result of this finding the Boeing Company's engineering p e r -

sonnel theorized that the aerodynamic performance of the left elevator was degraded by a disturbance of the a i r flow a c r o s s the stabilizer and elevator s u r f a c e s . Although previous inspections of the empennage had disclosed no structural deficiencies, a ground test was conducted to examine the effect of a i r loads on the stabilizer elevator assembly. Loads proportional to a 280 kn. 12' elevator deflected condition were applied at t h r e e points on the stabilizer. Spanwise and torsional deflections corresponded to p r e dicted values. However, during the t e s t , the upper skin of both stabilizers exhibited a spanwise waviness, the double amplitude of which appeared excessive. The double amplitude measurements w e r e 0.42 inches and 0. 32 inches f o r the left and right stabilizers, respectively. An identical loading test was conducted on a s i m i l a r a i r c r a f t which had acceptable control f o r c e c h a r a c t e r i s t i c s . The compression waviness m e a s u r e d on that aircraft was 0. 1 2 inches and 0.28 inches for the left and right sides, respectively. A subsequent examination of the upper skin of the stabilizer surfaces of N8705T under no-load conditions disclosed that the residual waviness of the left stabilizer s u r f a c e exceeded the s u r f a c e smoothness described on fabrication drawings for new a s s e m b l i e s . The objective of the second and third flights of N8705T at Seattle was to investigate the c h a r a c t e r i s t i c s of the boundary l a y e r on the stabilizer and elevator s u r f a c e s . Tufts w e r e installed on the upper surface of each stabilizer f o r the second flight. Although conclusions could not be drawn f r o m observations of the tufts in flight, the control f o r c e s observed were weaker than those encountered previously which indicated f u r t h e r degradation of elevator performance a s a result of the tuft installation. Vortex generators w e r e temporarily installed on the upper s u r f a c e of each stabilizer f o r the third flight. The control column f o r c e c h a r a c t e r i s t i c s

w e r e improve1 strumentation control tab hi1 Investig upper surface skin panels w waviness was tolerances go with those of skin panels h, The foc The t e s t res1 effect

.

After t facility in Ka F i n a l '1 lizer-elevatc f r o m anothe: replacement N8705' and stabilizt longitudinal control disp i s t i c s obser The s N8705T we1 w e r e no din installed to stabilizer-( N786' ducted at 11 a s those ob The ! between h tm account f o ~ upper surf left elevate

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were improved by the vortex g e n e r a t o r s . However, analysis of the instrumentation data disclosed that the left elevator performance and control tab hinge moments w e r e still unacceptable. Investigators w e r e concerned about the influence of the stabilizer upper surface skin waviness on boundary l a y e r c h a r a c t e r i s t i c s . New skin panels were fabricated and installed on both stabilizers. Skin waviness w a s reduced only slightly. The waviness was attributed to the tolerances governing the match of the rivet hole patterns of the skin panels with those of the stabilizer s t r u c t u r e s . The rivet hole pattern of the new skin panels had been established f r o m the removed panels. The fourth test flight consisted of m o r e stabilizer- elevator t r a d e s . The t e s t r e s u l t s indicated that the new skin panels had no significant effect. After the fourth test flight, N8705T was f e r r i e d back to the TWA facility in Kansas City.

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3

F i n a l Testing in Kansas City Both the left and right stabil i z e r - e l e v a t o r a s s e m b l i e s were removed and replaced with a s s e m b l i e s f r o m another B-707-300 a i r c r a f t . Instrumentation was installed in the replacement stabilizers to m e a s u r e elevator control loads. N8705T was flown and subjected to the wind-up turn, elevator pulse, and s t a b i l i z e r - e l e v a t o r t r a d e t e s t s . The operating c h a r a c t e r i s t i c s of the longitudinal control s y s t e m and a i r c r a f t response to static and dynamic control displacements w e r e acceptable and comparable to those c h a r a c t e r istics observed on other B-707 a i r c r a f t . T h e stabilizer- elevator a s s e m b l i e s which had been removed f r o m N8705T were installed on a s i m i l a r B-707-331 a i r c r a f t , N786TW. T h e r e were no dimensional o r rigging changes introduced. Instrumentation was installed t o achieve a configuration identical to that of N8705T b e f o r e stabilizer- elevator a s s e m b l y removal. N786TW was flown and stabilizer- elevator t r a d e tests were conducted at 16, 000 feet. The cohtrol column f o r c e s w e r e about the s a m e a s those observed on N8705T before the stabilizer change.

:tics

The investigation r e c o r d s were examined f o r significant differences bet,ween the right and left stabilizer- elevator a s s e m b l i e s which could account for the lighter control loads evident on the left side. Besides the upper s u r f a c e waviness, one measured physical difference existed: The left elevator upper nose s u r f a c e contour at the hinge line was predominantly

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below the f a i r e d contour extension of the upper stabilizer surface, while the right-hand elevator nose was predominantly faired o r very slightly above the f a i r e d contour extension of the upper stabilizer surface. (See Appendix D. ) A l l alinements were, however, within p r e s c r i b e d limits. Boeing Company engineers theorized that the boundary l a y e r effects caused by the upper s u r f a c e skin waviness would be aggravated by the step down that was predominant at the hinge line on the left elevator. The stabilizer to elevator contour fair was changed on both a s s e m blies to produce a condition wherein the elevator nose contour was p r e dominantly above the stabilizer contour extensions. The alteration was accomplished by removing shims between the stabilizer trailing edge beam and the elevator hinge support structure. N786TW was then flown to evaluate the effect of this change on the longitudinal control f o r c e c h a r a c t e r i s t i c s . The data obtained during the stabilizer- elevator t r a d e t e s t s showed that both the left and right elevator control loads were s i m i l a r to those observed on other B-707 aircraft. The push f o r c e required to displace the control column increased steadily with increasing elevator deflections; that is, the gradient remained positive with acceptable f o r c e levels. T h e pilot a s s e s s e d the maneuvering c h a r a c t e r i s t i c s of N786TW (with N8705T's stabilizers and elevators) a s acceptable at maximum and intermediate out- of-trim conditions. One final flight consisted of stabilizer elevator t r a d e s , elevator pulses, and wind-up t u r n s conducted at 150 kn. and 390 kn. at 15,000 feet and . 8 6 mach at 29,000 feet. Out-of- trim conditions were f u r t h e r evaluated in a dive f r o m 35, 000 feet at 90 mach. The a i r c r a f t ' s handling qualities w e r e satisfactory for all conditions tested.

.

1. 16.3 T e s t s Results

The test results w e r e a s follows: ( a ) The push f o r c e required to displace the control column was weak f o r intermediate to maximum trailing edge down deflections. The change in the f o r c e required f o r increasing elevator deflections deviated f r o m acceptable c r i t e r i a . These control f o r c e c h a r a c t e r i s t i c s were caused by low hinge moments on the left elevator control tab.

loadel on an1 specir exces strucl on thc

predo surfa, ing st The d distri the m contr

place prodr t h e S, mom1 of mc great eleva

move lowel pilot

the SI Boeil deser ward

follo1

varia was I the h desig

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( b ) The upper s u r f a c e skin of the left stabilizer was wavy under loaded conditions, the double amplitude of which exceeded that m e a s u r e d on another aircraft. The residual no-load waviness exceeded limits specified f o r new a s s e m b l y fabrication. The waviness was attributed t o excessive t o l e r a n c e s of the rivet hole p a t t e r n s in the skin panel and mating structure. Waviness d a r u p t e d the flow and thickened the boundary l a y e r on the stabilizer upper surface. ( c ) T h e upper nose contour of the left elevator at i t s hinge line was predominantly below the f a i r e d contour extension of the upper stabilizer surface. Although the alinement was within p r e s c r i b e d limits, the resulting step down caused f u r t h e r disturbance to the thickened boundary layer. The disturbed flow of a i r within the boundary l a y e r affected the p r e s s u r e distribution and thus the resultant lift vector acted on the elevator so that the moment about the elevator hinge caused by air loads on a deflected control s u r f a c e w a s lower than normal. ( d ) The position of the elevator for any given control column d i s placement depends upon the balance of moments about the elevator hinge produced by air loads on the control surface, the elevator control tab, the SAE tab, and the balance panels. The lower- than- normal hinge moment caused by a i r loads on the control surface affected the total balance of moments about t h e elevator hinge s o that the balance was achieved at a greater- than- normal control s u r f a c e deflection with a lower- than- normal elevator control tab hinge moment. The f o r c e within the control s y s t e m that the pilot counteracts to move the elevator depends directly on control tab hinge moment. The lower- than-normal control tab hinge moment produced lower- than-normal pilot f o r c e r e q u i r e m e n t s . Acceptable control f o r c e c h a r a c t e r i s t i c s were obtained by eliminating the s t e p down at the s t a b i l i z e r - t o - e l e v a t o r contour fair by removing shims. Boeing Company engineers theorized that elimination of this step down desensitized the elevator-to-boundary l a y e r thickening which occurred f o r ward of the hinge line. The Boeing Company p r e p a r e d a report of the test findings. following i s .excerpted f r o m that report:

The

"The recent testing has provided new information on the effects of variations of the dimensional tolerances a t the elevator hinge line that was not discovered on previous developmental t e s t s where airflow over the horizontal tail s u r f a c e s was apparently normal. However, the b a s i c design of the control s y s t e m is such a s to provide protection against even

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g r e a t e r degradation of elevator hinge moment c h a r a c t e r i s t i c s than those observed on this airplane in that any e r r a t i c elevator motion that causes a rapid pitching oscillation can be controlled and damped by preventing rapid control column movements. Control displacement should be smoothly limited t o that n e c e s s a r y t o provide a slow variation in attitude a s required to obtain a smooth and gradual approach to turbulenc6 penetration speed. With the control column movement so limited, the control tab antibalance actions tend to r e s i s t elevator motion. E r r a t i c pitching motion can be aggravated by an improperly phased attempt to manually damp the airplane. An e r r a t i c pitching motion due to the c a u s e s discussed above has constituted a v e r y infrequent phenomena. Such e r r a t i c pitching, if it a p p e a r s , would be associated with an out- of- trim stabilizer not readily apparent to the c r e w because of light control f o r c e s in the out- of- trim condition. The procedures to be followed by the flightcrew if unusual rapid longitudinal pitching of any nature o c c u r s a r e essentially the s a m e a s those outlined for turbulence penetration in the F A A Approved Flight Manual and a s d i s cussed in the Boeing Flight Training Manual and the November-December, 1963, Boeing Airliner. The airlines should review t h e s e procedures with their pilots to emphasize the prevention of rapid control column movements. 1. 17 Other Information

N8705T experienced a pitching oscillation on July 18, 1972, during which one flight attendant was injured. That accident o c c u r r e d 10 m i l e s east of Bradley Field International Airport, Windsor Locks, Connecticut. The porpoising s t a r t e d a f t e r the a i r c r a f t climbed and leveled at 12, 000 feet with the autopilot engaged. Flight data obtained f r o m the FDR showed vertical accelerations s i m i l a r to, but f o r s h o r t e r duration than, those encountered on August 28, 1973. The c r e w stated that they held the control column fixed until the oscillations stopped. Inspection of the a i r c r a f t a f t e r the accident disclosed no evidence of control s y s t e m o r s t r u c t u r a l irregularities. Consequently, it was concluded that the oscillation was a result of turbulence encounter. Maintenance r e c o r d s reflect no f u r t h e r difficulties of this kind f r o m July 18, 1972, to August 28, 1973. 2. 2. 1

ANALYSIS AND CONCLUSIONS

Analysis

A pitching oscillation which imposes l a r g e vertical acceleration loads on an a i r c r a f t can be caused by: (1) Turbulence, ( 2 ) e r r a t i c pilot

actio o r (4 aircr consj

1973, turbu that 1 Saf et accic

and q WOUll

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the e Iongi invef the a inver lencc caus

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than in th of os indic acci syst

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actions, ( 3 ) a stability problem inherent to the a i r c r a f t ' s b a s i c design, o r (4) a malfunction o r out-of-tolerance condition within the p a r t i c u l a r aircraft's longitudinal control system. Each of these possibilities is considered. The weather f o r e c a s t f o r Los Angeles on the night of August 28, 1973, did not indicate turbulence, n o r w e r e t h e r e any pilot r e p o r t s of turbulence o r unusual weather in the a r e a . The c r e w of Flight 742 stated that the skies w e r e c l e a r with no apparent turbulence. Therefore, the Safety Board concluded that inflight turbulence was not a factor in this accident. T h e c r e w m e m b e r s of Flight 742 w e r e properly certificated, trained, and qualified for the flight. T h e r e was no indication of any problem which would have affected the performance of t h e i r duties. The Boeing 707 a i r c r a f t has been used for nearly 15 y e a r s . During the e a r l y s e r v i c e life of the aircraft t h e r e w e r e occasional incidents of longitudinal upsets caused by an encounter with s e v e r e turbulence. During investigation of those incidents the longitudinal control c h a r a c t e r i s t i c s of the a i r c r a f t were thoroughly analyzed and tested. T h e findings of t h e s e investigations led to minor changes t o the a i r c r a f t and to published turbulence penetration procedures. T h e r e was no evidence that the upsets were caused by longitudinal instability. T h e r e have been two other instances of e r r a t i c pitching motions which were not attributable to turbulence. Both of the aircraft involved were found t o have out- of-tolerance discrepancies within t h e i r longitudinal control systems. When corrected, both a i r c r a f t exhibited acceptable flight c h a r a c t e r i s t i c s . Therefore, the Safety Board concluded that the stability problem encountered by Flight 742 was not one inherent to the basic 707 design. N8705T was certificated, equipped, and maintained according to requirements and regulations. The g r o s s weight and C . g. w e r e within prescribed limits throughout the flight f r o m Honolulu t o Los Angeles. Although the c i r c u m s t a n c e s of the July 18, 1972, accident were somewhat different and the pitching motions were damped in l e s s t i m e than those encountered by Flight 742, t h e r e were definite similarities in the entry a i r s p e e d , t h e peak loads reached, and the average frequency of oscillation. The similarities a r e too great to be coincidental. Evidence indicates a strong possibility that the longitudinal instability in both accidents was caused by some fault unique to the longitudinal control system of N8705T.

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During the postaccident flight t e s t s , it w a s determined that the magnitude and variation of the f o r c e required to displace the control column to produce elevator trailing-edge-down deflections differed f r o m that generally expected by a pilot. The unusual c h a r a c t e r i s t i c s w e r e most apparent when undesirably weak push f o r c e s were n e c e s s a r y to counter a i r c r a f t noseup out- of- trim conditions. The low control f o r c e s w e r e caused by the effect of a thickened boundary l a y e r on the left stabilizer which was produced by waviness on the stabilizer upper s u r f a c e combined with the step down at the stabil i z e r - t o - e l e v a t o r contour fair. The control column f o r c e c h a r a c t e r i s t i c s play a m a j o r role in determining the pilots feel of the a i r c r a f t and h i s consequent ability t o maintain stable flight. When the control column f o r c e gradient i s low, the slightest change in f o r c e applied by the pilot will result in overresponse by the a i r c r a f t . Such response will tend to c a u s e a pilot to o v e r c o r r e c t any initially sensed pitch disturbance. This, in turn. will lead to o v e r c o r r e c t i o n in the opposite direction and thus induce an oscillation. T h e tendency to o v e r c o r r e c t will be magnified as the control column f o r c e gradient becomes negative. Continued efforts by the pilot to regain stable flight can result in a critical phasing between the pilot's control column movement and the a i r c r a f t ' s pitching motion, which will sustain r a t h e r than damp the oscillation.

A second effect of low control f o r c e s relates to autopilot operation. Automatic pitch control of the B-707 i s effected through the b a s i c elevator control and stabilizer t r i m s y s t e m by signal inputs to the respective s e r v o motors. T r a n s i e n t pitch e r r o r signals a r e nulled by elevator deflections. A computer monitors the elevator control load and when the load r e a c h e s a given threshold, the stabilizer s e r v o will run, retrimming the stabilizer until elevator control f o r c e s a r e relieved. If the elevator control f o r c e gradient is lower than normal, the autopilot will d r i v e the elevators to a g r e a t e r - t h a n - n o r m a l deflection before the stabilizer t r i m threshold is reached. The condition can manifest itself by an out- of- trim condition when autopilot is disengaged.

'

If then, the autopilot is disengaged with no f o r c e applied to the control column, a pitch excursion will occur. Such a pitchup accompanied autopilot disengagement on the flight f r o m LOSAngeles to Kansas City. The Safety Board believes that t h e s e a r e the c i r c u m s t a n c e s which caused the pitching oscillation experienced by Flight 742.

cruis nosec flight ment t h e tl. craft

autop pat ed appli requi been the ti allow allow furth t h e 1t negli by US push pres whicl

stabi oper: disc] out c the h little

COlUI

colul offic forct the t aggr allov resp ceas

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The autopilot probably had been engaged for some t i m e during the cruise at 33, 000 feet. F u e l burn-off would have required an a i r c r a f t nosedown t r i m change. The change in t a i l loading required for level flight was probably effected by the autopilot through elevator displacement and, because of the lower-than-normal elevator control loads, the threshold required f o r stabilizer t r i m was not reached and the a i r craft was out- of- trim in the noseup direction. When the captain received the descent clearance, he disengaged the autopilot, reduced power, and s t a r t e d the descent. He probably anticipated a mild pitch change when he disengaged the autopilot and immediately applied f o r c e to the control column to c o r r e c t the change. The push f o r c e required would have felt n o r m a l and the out- of-trim condition would have been noticed only i f the instrument panel t r i m indicators w e r e checked at the t i m e of autopilot disengagement. The captain apparently intended to allow a i r s p e e d to i n c r e a s e and maintain a speed just below the maximum allowable during the descent. This flight profile would have required further t a i l load changes in the a i r c r a f t nosedown direction. Because of the low gradient, the i n c r e a s e in control column f o r c e could have been negligible. Although the captain could have relieved the control p r e s s u r e s by u s e of the stabilizer t r i m , it is likely that he p r e f e r r e d to hold some push f o r c e on the column. In this c a s e the forward control column p r e s s u r e may have been relatively low and not indicative of the degree to which the a i r c r a f t was out-of-trim. After the accident, it was determined that a burned contact in the stabilizer trim control r e l a y would c a u s e a 25-percent reduction in the operating speed of the trim motor. The Safety Board believes that the discrepancy would have had little effect on the captain's ability to t r i m out control loads and thus was not a f a c t o r in the accident. In addition, the higher-than-allowable elevator hinge line friction would have had little effect except to f u r t h e r mask the abnormal control f o r c e s .

In any event, with the aircraft out- of- trim and the low control column f o r c e gradient, even a slight relaxation of p r e s s u r e on the column would cause an abrupt pitchup. The captain and the f i r s t officer reacted naturally t o such a pitchup by applying m o r e push f o r c e to the control column. Again, because of the low f o r c e required, the tendency was to o v e r c o r r e c t . Subsequent control column motions aggravated the oscillation. The c r e w properly reduced power and allowed the a i r c r a f t to slow. As the a i r s p e e d d e c r e a s e d the a i r c r a f t response became l e s s sensitive to control inputs and the oscillations ceased. !

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Although the stick f r e e stability of the a i r c r a f t was probably not impaired, the out- of- trim condition would preclude the relaxation of control column force. The best procedure would have been t o hold the control column in a fixed position and allow the a i r c r a f t to stabilize. In view of the acceleration loads being experienced, this procedure may have been difficult to apply. Since the a i r c r a f t experienced g f o r c e s alternating f r o m positive to negative, the a i r c r a f t occupants w e r e thrown to the ceiling and then back to the floor. If the occupants fell to the floor when the aircraft floor was rising, the f o r c e of impact would have been great. Probably this type of f o r c e and movement caused the serious injuries. Two passengers said that although they w e r e able to grab onto something with their hands, their feet and legs were t h r m up in the a i r and then slammed back to the floor. Other p a s s e n g e r s who w e r e not strapped to, but w e r e sitting in, t h e i r seats escaped s e r i o u s injury by holding onto a r m r e s t s o r other seats. Debris in the a i s l e f r o m the overhead racks caused difficulty for the persons administering f i r s t aid. The flight surgeon and others giving f i r s t aid noted that f i r s t aid supplies w e r e inadequate. They improvised by using pillowcases and other aircraft m a t e r i a l s . The Safety Board p r a i s e s the flight attendants' and p a s s e n g e r s ' actions during the emergency and the orderly evacuation of the injured after landing. 2. 2 Conclusions

( a ) Findings 1.

The pitching oscillation o c c u r r e d at night during the descent f r o m c r u i s e altitude and consisted of about 55 cycles with maximum peak to peak vertical acceleration loads at the a i r c r a f t ' s c. g. of t 2 . 4 g to -0.3g.

2.

T h e r e was no evidence of turbulence in the a r e a of the accident.

3.

The crewmembers w e r e properly certificated, trained, and qualified for the flight.

(1

pr ob a t in t h e ditions aircra

- 21 4.

The a i r c r a f t was certificated, equipped, and maintained according to regulations.

5.

A waviness of the upper skin on the left stabilizer disrupted the boundary l a y e r on the surface. The thickened boundary l a y e r was f u r t h e r disturbed by a step down in the stabilizer- to- elevator contour f a i r produced by the elevator hinge alinement. The resulting p r e s s u r e d i s t r i bution on the elevator affected the longitudinal control loads.

6.

Critically weak push f o r c e s w e r e required t o displace the control column and produce elevator trailing edge down deflections which w e r e n e c e s s a r y to counter a i r c r a f t noseup, out- of- trim conditions.

7.

The a i r c r a f t was out- of- trim when the autopilot was disengaged. The light control f o r c e s masked the out-oft r i m condition.

8.

The light control f o r c e s induced the flightcrew to overc o r r e c t in response to a i r c r a f t pitching motions and initiate and sustain the longitudinal oscillation.

9.

The c r e w reacted properly by reducing power and slowing the a i r c r a f t .

10.

The "fasten seatbelt" sign was off before the porpoising began.

11.

Unrestrained p e r s o n s in their seats and those standing in the aft cabin a r e a w e r e the most severly injured.

12.

The injured occupants w e r e repeatedly thrown f r o m floor t o ceiling in an environment which included hard edges, s h a r p c o r n e r s , and protruding surfaces.

(b) P r o b a b l e Cause 'The National Transportation Safety Board determines that the probable c a u s e of this accident w a s a combination of design tolerances in the a i r c r a f t ' s longitudinal control s y s t e m which, under c e r t a i n conditions, produced a c r i t i c a l relationship between control f o r c e s and aircraft response. The atypical control f o r c e c h a r a c t e r i s t i c s which w e r e

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present in this p a r t i c u l a r a i r c r a f t ' s control s y s t e m w e r e conducive t o overcontrol of the a i r c r a f t by the pilot. The pilot's n o r m a l reaction to an unexpected longitudinal disturbance led to a pitching oscillation which was t e m p o r a r i l y sustained by his subsequent application of cont r o l column f o r c e s to regain stable flight. The c a u s e of the death and injuries was. the impact of unrestrained p e r s o n s with unyielding objects in the cabin environment. 3.

RECOMMENDATIONS

As a result of this accident, on October 18, 1973, the Safety Board submitted Safety Recommendations A-73-76 through 78 to the Administrat o r , FAA. Copies of the recommendations and the Administrator's responses a r e included in Appendix E. The Safety Board on May 15, 1974, submitted a n additional r e c o m mendation to the Administrator which .will r e q u i r e that the c o r r e c t i v e m e a s u r e s described by the Boeing Company be accomplished. This recommendation i s included a s Appendix F. This accident reemphasizes the need for improvement in the design of a i r c r a f t interiors to reduce the potential for s e r i o u s injury a s a result of abrupt maneuvers o r an encounter with inflight turbulence. Recommendations previously submitted t o the FAA in the Board's Special Study, "In-flight Safety of P a s s e n g e r s and Flight Attendants Aboard A i r C a r r i e r A i r c r a f t " L1 pertaining to seatbelt discipline, improvements in padding of h a r d s u r f a c e s , elimination of s h a r p edges and c o r n e r s , and improvements for s t o r a g e of a r t i c l e s in overhead racks a r e relevant to the c i r cumstances of Flight 742's porpoising encounter.

2 1 NTSB Report Number AAS-73-1.

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23

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BY THE NATIONAL TRANSPORTATION SAFETY BOARD /s/ JOHNH. REED Chairman

/ s i FRANCIS H. McADAMS

Member

Is/

LOUIS M. THAYER Member

IS/ ISABEL A. BURGESS Member

/ S f

July 10, 1974

WILLIAM R. HALEY Member

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25

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APPENDIX A INVESTIGATION AND HEARING

I.

Investigation

The Safety Board was notified of the accident on August 28. 1973, before the flight landed at Los Angeles International Airport. Representatives f r o m the Safety Board, the F e d e r a l Aviation Administration, and T r a n s World Airlines w e r e present when the aircraft landed. Investigation groups w e r e established for operations, s y s t e m s , human factors, and flight data r e c o r d e r . The Airline Pilots Association and the Boeing A i r c r a f t Company a l s o participated in the investigation.

2.

Public Hearing T h e r e was no public hearing.

- 26 APPENDIX B

I

CREW INFORMATION

Captain John Wilber H a r p s t e r Captain John Wilber H a r p s t e r , 53, held Airline T r a n s p o r t Certificate No. 116002, with ratings in Boeing 707. He had accumulated about 26,171 total flight-hours and about 8,170 flight-hours in Boeing 707 a i r c r a f t . His l a s t proficiency check in the Boeing 707 was May 30, 1973. His l a s t line check was in a Boeing 707 on July 14, 1973. His f i r s t - c l a s s medical certificate was issued on June 6 , 1973. First Officer Robert Cooper Evans F i r s t Officer Robert Cooper Evans, 39, held Airline Transport Certificate No. 1410739, with ratings Reciprocating Engine Powered (F/D Turbo J e t powered Boeing 707 (F/E). He had accumulated about 6, 128 total flight- hours with about 4,378 flight- hours in Boeing 707 a i r c r a f t . His last proficiency check in Boeing 707 was March 22, 1973. His last line check in Boeing 707 was January 30, 1973. His first-class medical certificate was issued June 18, 1973. Flight Engineer Don Wilbur Jackson Flight Engineer Don Wilbur Jackson, 53, held Flight Engineer's Certificate No. 725778, and Commercial Pilot Certificate No. 1586750 with type rating Reciprocdting Engine Powered ( F / E ) Turbo J e t Powered Boeing 707 ( F / E ) . He had accumulated About 19, 000 flight-hours. His l a s t proficiency check i n Boeing 707 a i r c r a f t was July 18, 1973. His l a s t line check was on t h e s a m e date. His last medical certificate was issued April 24, 1973.

i 1

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I! i

i

I I

F

29

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APPENDIX D

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W

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- 30 UNITED STATES OF AMERICA Honora

NATIONAL TRANSPORTATION SAFETY BOARD WASHINGTON, D.C.

persis time i t o bel tribut

APPENDIX E ISSUED:

October 18, 1973

T

Adopted by the NATIONAL TRANSPORTATION SAFETY BOARD a t i t s o f f i c e i n Washington, D. C . on the 3rd day o f October 1973

FORWARDED TO:

Honorable Alexander P. Butterfield Administrator Federal Aviation Administration Washington, D. C. 20591

thus f which a seri Engine Compan

1 11 1 1 1

SAFETY RECOMMENDAT IONS

T (

c

A-73-76 t h r u 78

The National Transportation Safety Board's i n i t i a l investigation indicates that one o r more f a u l t s i n a Boeing 707-331B longitudinal control system might have been contributory t o the cause of a recent accident involving P a n s World Airlines Flight 742, on A u g u s t 28, 193. Although t h e investigation has not yet been completed, the Safety Board believes t h a t t h e i n i t i a l findings a r e s u f f i c i e n t t o j u s t i f y certain interim actions designed t o preclude t h e serious consequences which may r e s u l t from similar occurrences. The subject accident occurred as t h e f l i g h t , en route from Honolulu t o Los Angeles, was descending from cruise a l t i t u d e approximately 35 miles west of t h e destination a i r p o r t . Upon passing through f l i g h t l e v e l 220 a t approximately 350 K LAS, t h e a i r c r a f t entered a porpoising o s c i l l a t i o n which persisted f o r approxhately 2 minutes. Over 50 pitching cycles were experienced, with peak acceleration forces h t t h e a i r c r a f t c.g. of +2.4g and -0.3g. Unrestrained passengers and stewardesses i n the a i r c r a f t were subjected t o violent displacements. Of the 9 crewmembers and 141 passengers, 1 passenger sustained f a t a l i n j u r i e s ; 2 stewardesses and 2 passengers sustained serious i n j u r i e s before t h e a i r c r a f t regained s t a b i l i z e d f l i g h t . A review of records after t h i s occurrence disclosed that the S&me -707 a i r c r a f t had been involved i n a similar accident on July 18, 1972, a t Windsor L O C ~ S , Connecticut. I n that accident, one stewardess was seriously injured when t h e a i r c r a f t experienced a s e r i e s of p i t c h oscillations which

1181

TI frictil and ba: was no1

TI by 8 Bc includt stabil: tudes 1 induce control The chE instabj

TI are:

Honorable Alexander P. Butterfield

31

-

(2)

persisted f o r approximately 1 5 seconds. Although our findings a t that time indicated an encounter with in- flight turbulence, we now have reason t o believe that a longitudinal control system fault might have been contributory t o that mishap as well.

The current investigation of the August 28th accident has consisted, thus f a r , of (1)an examination of those a i r c r a f t systems and components which could a f f e c t t h e longitudinal s t a b i l i t y of the a i r c r a f t , and (2) a series of f l i g h t tests t o evaluate t h e a i r c r a f t f l i g h t characteristics. Engineering, manufacturing, and f l i g h t t e s t personnel from t h e Boeing Company were key p a r t i c i p a n t s i n t h i s a c t i v i t y . The a i r c r a f t examination disclosed: (1) an open c i r c u i t i n one phase of the three-phase AC parer a t t h e s t a b i l i z e r trim control relay, which would l i k e l y cause a degradation i n t h e torque output of t h e s t a b i l i z e r trim jackscrew motor, and (2)

excessive force was required a t t h e elevator surface when it was subjected t o t h e breakaway force check specified i n t h e applicable Boeing 707 Maintenance Manual. More detailed inspection t o determine t h e f r i c t i o n source revealed that many of t h e inboard and outboard s e a l s between the elevator balance panels and s t a b i l i z e r s t r u c t u r e were compressed excessively.

The s t a b i l i z e r trim control relay was replaced and t h e elevator breakout f r i c t i o n was brought t o an acceptable l e v e l by lubrication of elevator hinge and balance panelmechanisms. Tbe compression fit of t h e balance panel s e a l s was not corrected. The a i r c r a f t was then subjected t o a f l i g h t c h a r a c t e r i s t i c evaluation by a Boeing p i l o t , accompanied by engineering personnel. The f l i g h t test included a s e r i e s of elevator/stabilizer trade t e s t s wherein variations of s t a b i l i z e r trim were compensated by elevator deflection a t different a l t i tudes throughout t h e a i r c r a f t ' s speed range. Although t h e p i l o t d i d not induce an uncontrollable porpoise, he d i d note a significant anomaly i n the control column force gradient during conditions of elevator darn deflections. The c h a r a c t e r i s t i c s noted were of a nature which tended toward longitudinal instability.

Two poqsible factors which a r e known t o contribute t o such a condition are:

i

I

1

graaent c h a r a c t e r i s t i c was noted. The subjective evalwtion of t h e P i l o t was that, although nearer t o normal, the aircraft s t i l l exhibited low-force gradient c h r a c t e r i s t i c s a t h i g h elevator angular deflections under Cona t i o n s of high dynamic pressure. The ongoing investigation w i l l be directed toward Complete inStruUIent a t i o n t o explore f u r t h e r t h e longitudinal f l i g h t characteristics of t h i s aircraft.

Although OUT findings a r e incomplete, we believe that the f a c t s developed thus f a r provide evidence that one o r more control system f a u l t s can produce a longitudinal i n s t a b i l i t y induced by e i t h e r external disturbance or p i l o t control input under i s o l a t e d conditions. We believe that this i s more l i k e l y t o occur if t h e a i r c r a f t i s out of trim i n a high dynamic pressure environment. The Board i s understandably concerned about t h e existence of other Boeing 707/720 a i r c r a f t which might exhibit similar undesirable characteri s t i c s i f exposed t o such conditions. We believe that a measurement of higher-than-normal elevator breakout forces might be indicative of such a problem. I n order t o minimize the p o s s i b i l i t y of future occurrences of this nature, t h e Safety Board recommends that t h e Federal Aviation Administration i n i t i a t e t h e following interim actions: 1. Issue an Air Carrier Operations Bulletin which describes t h e circumstances of t h i s accident, applicable cautions regarding such i n s t a b i l i t y , and recommended p i l o t procedure t o reduce t h e p o s s i b i l i t y of a sustained high "g" o s c i l l a t i o n , should an i n s t a b i l i t y manifest i t s e l f .

Honorable Alexander P. Butterfield 2.

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33

(4)

Issue an Airworthiness Directive which would require: (a)

that a l l Boeing 7O7/72O a i r c r a f t be subjected t o an elevator breakout force check i n accordance with t h e approved maintenance procedures a t the next scheduled maintenance v i s i t ; and

( b ) that those a i r c r a f t on which t h e breakout f r i c t i o n determined i n p a r t (a) exceeds t h e maximum allowable values be subjected t o f u r t h e r inspection t o ensure that t h e elevator balance panel s e a l compression i s not excessive.

3. Require changes i n the approved Maintenance Manual f o r a l l Boeing 707/720 a i r c r a f t t o : (a)

specify a more precise method of measuring t h e net elevator hinge f r i c t i o n throughout t h e e n t i r e range of control surface travel; and

(b)

specify a more definitive method f o r adjusting the balance panel seals within the desirable tolerance. A t present, t h e manual s p e c i f i e s a OdO.020-inch fit between the balance panel seal and t h e s t a b i l i z e r structure. A -0.020-inch measurement implies s e a l compression which i s extremely d i f f i c u l t t o measure accurately.

Our technical staff i s available for any f u r t h e r assistance they may be able t o provide. REED, Chairman, M c W ,

i n the above recommendations.

T H A Y E R , BURGESS,

and

HALEX, Members,

j & & -

By.

ohn H. Reed

concurred

- 34 PEPAR

FEDERA

P! O'J Honorable John H. Reed Chairman, National T r a n s p o r t a t i o n Safety Board Department of T r a n s p o r t a t i o n i&ashington, D.C. 20591

OFFICE OF THE ADMINISTRATOR

Notation '/ / &'/

Honor, Chair1 Depar Washil

Dear Mr. Chairman:

Dear 1

'This w l l l supplement o u r l e t t e r of November 2, 1973, r e g a r d i n g Safety Recommendations A-73-76 thru 18.

This

'The s t a b i l i z e r f r o m N8705 which had shown s e v e r a l anomalies was reworked in s e v e r a l suspect a r e a s , installed on N876TW and t e s t flown. T h e r e s u l t s were unsatisfactory. Modifications w e r e then made to the elevator by making the " ski jump" negative. (The " s k i jump" is the v e r t i c a l distance between a projection of the horizontal stabilizer and the s u r f a c e of the elevator a t the hinge l i n e . ) Results of the flight t e s t w e r e satisfactory.

A s the r e s u l t of the investigation, the following a c t i o n s will b e undertaken. Two r e v i s i o n s to the maintenance manual will b e made. One will reduce the l i m i t s and specify negative values for the s k i jump for both upper and lower s u r f a c e s . The other will r e d u c e the l i m i t s f o r the elevator balance bay gaps.

An operations bulletin will be issued which w i l l d i s c u s s the problem and specify the c o r r e c t i v e action to be taken in c a s e of pilot induced oscillation. We believe that this w i l l be a s a t i s f a c t o r y solution to the problem. Sincerely, L

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35

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DEPARTMENT OF TRANSPORTATION FEDERAL AVIATION ADMINISTRATION

Honorable J o h n H. Reed Chairman, N a t i o n a l T r a n s p o r t a t i o n S a f e t y Board Department of T r a n s p o r t a t i o n 20591 Washington, D. C.

Dear

OFFICE OF ~EAOMINI~RATOR

M.r

Chairman:

T h i s i s i n r e s p o n s e t o NTSB S a f e t y Recommendations A-73-76 t h r u 78. Recommendation No. 1. I s s u e a n Air C a r r i e r O p e r a t i o n s B u l l e t i n which describes t h e circumstances of t h i s accident, applicable cautions r e g a r d i n g s u c h i n s t a b i l i t y , and r e c o w e n d e d p i l o t procedure t o reduce t h e p o s s i b i l i t y of a s u s t a i n e d h i g h "g" o s c i l l a t i o n , should an i n s t a b i l i t y manifest i t s e l f . Comment. We s h a r e y o u r concern t h a t o t h e r Boeing 707-720 a i r p l a n e s might e x h i b i t characteristics similar to t h o s e o f t h e TWA a i r p l a n e , However, a f t e r a review of t h e B o a r d ' s f i n d i n g s , we do not f i n d p e r s u a s i v e argument o r f a c t u a l d a t a t o j u s t i f y t h e i s s u a n c e o f a n W e b e l i e v e t h a t issuance of a b u l l e t i n a t operations bulletin. t h i s time might t e n d t o c o n f u s e concerned f l i g h t crewnembers on t h e proper a c t i o n t o t a k e i n a s i t u a t i o n s i m i l a r t o t h a t of t h e TWA a i r p l a n e o f August 28.

We are w i t h h o l d i n g a c t i o n on t h i s recommendation pending completion We a r e p r e p a r e d t o meet w i t h y o u r t e c h n i c a l of t h e investigation. s t a f f t o d i s c u s s any a d d i t i o n a l i n f o r m a t i o n o r d a t a i n s u p p o r t of t h e recomnendation which may be a v a i l a b l e . R e c o m e n d a t i o n No. require:

2.

Issue a n A i r w o r t h i n e s s D i r e c t i v e which would

(a) t h a t a l l Boeing 707-720 a i r c r a f t be s u b j e c t e d t o a n e l e v a t o r b r e a k o u t f o r c e check i n accordance w i t h t h e approved maintenance procedures a t t h e n e x t scheduled maintenance v i s i t ; and @) t h a t t h o s e a i r c r a f t on which t h e breakout f r i c t i o n determined i n p a r t (a) exceeds t h e maximum a l l o w a b l e v a l u e s be s u b j e c t e d t o f u r t h e r i n s p e c t i o n t o e n s u r e t h a t t h e e l e v a t o r b a l a n c e p a n e l compression is not e x c e s s i v e . Comnent. We do not b e l i e v e t h a t a n A i r w o r t h i n e s s D i r e c t i v e i s a p p r o p r i a t e a t t h i s time.

-

36

3

2

Our e n g i n e e r i n g p e r s o n n e l i n t h e Northwest Region are wdrking w i t h t h e Boeing Companyto a s s e s s t h e c h a r a c t e r i s t i c s , both aerodynamically and s t r u c t u r a l l y , o f t h e TWA a i r p l a n e i n v o l v e d i n t h e August 28 a c c i d e n t which r e s u l t e d i n h i g h peak a c c e l e r a t i o n f o r c e s due t o a n i n f l i g h t l o n g i t u d i n a l o s c i l l a t i o n . A test program, b e i n g conducted by t h e Boeing Company, i s p r e s e n t l y under way t o determine t h e phenomenon r e l a t e d t o t h e o s c i l l a t o r y c h a r a c t e r i s t i c s experienced by The e l e v a t o r breakout f r i c t i o n does n o t appear t h e TWA a i r p l a n e . t o be a r e l a t e d c a u s e a s s o c i a t e d w i t h t h e l o n g i t u d i n a l o s c i l l a t i o n experienced by t h i s a i r p l a n e . The Boeing Company conducted a test i n which t h e e l e v a t o r breakout f o r c e exceeded the m a x h i u m a l l o w a b l e v a l u e and no o s c i l l a t i n g was experienced.

Ret f 01

(4 hit and (b) sea a 0

str is

F l i g h t t e s t s were conducted on September 25, 29, and 30 of t h e TWA a i r p l a n e i n which i n v e s t i g a t i o n s were made t o measure t h e e l e v a t o r b a l a n c e p r e s s u r e s i n t h e upper and lower c a v i t y . T u f f s were i n s t a l l e d on t h e e l e v a t o r and tabs. Also, s t r a i n gages were i n s t a l l e d on t h e structure. The r e s u l t s i n d i c a t e d a boundary layer t h i c k e n i n g o c c u r r e d on t h e e l e v a t o r . Vortex g e n e r a t o r s were i n s t a l l e d on the upper s u r f a c e of t h e s t a b i l i z e r and no o s c i l l a t i o n s were experienced.

C om ba 1

S t a t i c t e s t s on t h e s t a b i l i z e r were conducted on t h e a i r p l a n e which i n d i c a t e d a severe w r i n k l i n g of t h e s k i n o n t h e h o r i z o n t a l s t a b i l i z e r due t o t o r q u e loading. A h i g h t o r q u e l o a d i n g would b e experienced by t h e s t a b i l i z e r i n a n o u t - o f - t r i m c o n d i t i o n due t o t h e loads imposed on t h e s t a b i l i z e r by t h e e l e v a t o r . The s t a b i l i z e r w r i n k l i n g c h a r a c t e r i s t i c s a f f e c t the boundary l a y e r over t h e e l e v a t o r . It was noted i n t h e s t a t i c t e s t t h a t t h e w r i n k l i n g p a t t e r n d i f f e r e d between t h e l e f t and r i g h t s t a b i l i z e r . A check made on a n o t h e r a i r p l a n e with t h e same l o a d i n g c o n d i t i o n s r e s u l t e d i n a d i f f e r e n t s k i n w r i n k l i n g p a t t e r n on t h e s t a b i l i z e r which was less severe.

Sin

Boeing i s s t i l l a s s e s s i n g t h e s t r u c t u r a l c h a r a c t e r i s t i c s o f t h e s t a b i l i z e r of t h e TWA a i r p l a n e t o determine t h e s t r u c t u r a l f e a t u r e s They are i n h e r e n t in t h e s t r u c t u r e which r e s u l t e d i n t h i s problem. checking f a b r i c a t i o n c h a r a c t e r i s t i c s , s k i n gages, and m a t e r i a l c h a r a c t e r i s t i c s f o r conformity. Until the structural characteristic i s i d e n t i f i e d which c a u s e s t h i s problem on t h e TWA a i r p l a n e , we have no c r i t e r i a t o determine t h o s e a i r p l a n e s i n service t h a t may have t h e same d i f f i c u l t i e s .

When t h e t e s t program i s completed and t h e d a t a a s s e s s e d , we will t a k e a p p r o p r i a t e a c t i o n a t t h a t time.

wo r Rev pro

Adm

- 37 3 Recowendation No. 3. Require changes i n the approved Maintenance Manual f o r a l l Boeing 707-720 a i r c r a f t to:

(a) specify a more precise method of measuring the net elevator hinge f r i c t i o n throughout t h e e n t i r e range of control surface t r a v e l ; and (b) specify a more d e f i n i t i v e method for adjusting the balance panel s e a l s within the desirable tolerance. A t present, t h e manual s p e c i f i e s a Of0.020-inch f i t between the balance panel s e a l and the s t a b i l i z e r structure. A -0.020-inch measurement implies s e a l compression which i s extremely d i f f i c u l t t o measure accurately. Comment. The Boeing Company agrees t h a t accurate measurement of balance s e a l compression i s d i f f i c u l t and have advised t h a t they w i l l work toward an improved procedure. Revisions t o the Maintenance Manual are expected as soon a s improved procedures can be developed. Sincerely,

I

n

exander P. Butterfield Administrator

- 38 NATIONAL TRANSPORTATION SAFETY BOARD WASHINGTON, D.C. APPENDIX F

ISSUED: May 15, 197.4

Honorable Alexander P. B u t t e r f i e l d

Administrator Federal Aviation Administration Washington, D. C. 20591

SAFETY

RECOMMENDAT ION (S)

A-74-41

During i t s p r e l i m i n a r y i n v e s t i g a t i o n of t h e a c c i d e n t i n v o l v i n g Trans World A i r l i n e s , F l i g h t 742, on August 28, 1973, t h e N a t i o n a l T r a n s p o r t a t i o n S a f e t y Board s u b m i t t e d S a f e t y Recommendations A-73-76 through 78. I n your i n i t i a l r e s p o n s e t o t h e s e recommendations, you s t a t e d that a d d i t i o n a l a c t i o n would b e t a k e n a f t e r t h e i n v e s t i g a t i o n was completed.

As i n d i c a t e d i n your subsequent r e s p o n s e of March 4, 1974, t h e i n v e s t i g a t i o n has been completed. The conclusion was t h a t t h e abnormal , produced f l i g h t c o n t r o l c h a r a c t e r i s t i c s of Boeing 707-33E3, ~ 8 7 0 5 ~were when t h e boundary l a y e r on t h e h o r i z o n t a l s t a b i l i z e r - e l e v a t o r assembly thickened. The t h i c k e n i n g was caused by a combination of e x c e s s i v e s k i n waviness on t h e upper s u r f a c e of t h e s t a b i l i z e r and t h e e x i s t i n g v e r t i c a l dimension of t h e s t a b i l i z e r t o e l e v a t o r f a i r a t t h e e l e v a t o r h i n g e l i n e . S i n c e t h i s dimension can be modified by adding o r s u b t r a c t i n g shims between t h e s t a b i l i z e r t r a i l i n g edge beam and t h e e l e v a t o r h i n g e support s t r u c t u r e , c o r r e c t i v e a c t i o n i s p o s s i b l e . However, s i n c e t h e dimensions on t h e a c c i d e n t a i r c r a f t were a l l w i t h i n t o l e r a n c e s s p e c i f i e d i n a p p l i c a b l e drawings and maintenance documents, t h e s e t o l e r a n c e s should be changed and o t h e r f l e e t a i r c r a f t s h o u l d b e i n s p e c t e d t o ensure that t h e y a r e n o t s u s c e p t i b l e t o t h e c o n t r o l problems. The S a f e t y Board i s aware of Boeing Company's i n t e n t i o n s t o e s t a b l i s h new t o l e r a n c e s , modify maintenance manuals, and i s s u e a s e r v i c e b u l l e t i n t o r e q u i r e i n s p e c t i o n and accomplish m o d i f i c a t i o n , i f needed. The S a f e t y Board b e l i e v e s that F e d e r a l A v i a t i o n A d m i n i s t r a t i o n a c t i o n i s r e q u i r e d t o ensure thal c o r r e c t i v e measures a r e implemented.

sub the fri and

the

- 39 Honorable Alexander P. Butterfield

(2)

Therefore, the National Transportation Safety Board recommends that the Federal Aviation Administration: (Safety Recommendation A-7&-41) 1.

Issue an Airworthiness Directive which: (a) specifies new tolerances for the vertical dimensions of the Boeing 707/TO stabilizer to elevator fair at the elevator hinge line;

) describes procedures for measuring and establishing proper dimensions;

) requires that all Boeing 707/720 aircraft be

inspected, at the next scheduled maintenance visit, for the proper dimensional relationship of the stabilizer to elevator fair at the elevator hinge line in accordance with the procedures established, and

(a)

requires those aircraft found to have an out-of-tolerance condition to be modified according to prescribed procedures.

The findings of the investigation and tests made subsequent to submission of Safety Recommendations A-73-76 through 78 notwithstanding, the Safety Board continues to believe that excessive control surface friction can further aggravate undesirable control system characteristics and that these recommendations are still relevant.

THAYER, and HALEY, Members,

REED, Chairman, McADAMS, the above recommendation. BURGESS,

Chairman

concurred in

DEPARTMENT

OF

- 40 TRANSPORTATION

-

FEDERAL AVIATION ADMINISTRATION

2 WASHINGTON, D.C.

20590

MAY 2 2 1974 Horiorable John H. Reed Chairman, National T r a n s p o r t a t i o n Safety B o a r d Department of T r a n s p o r t a t i o n Washington, D. C. 20591

OFFICE OF THE ADMINISTRATOR

Notation 1181A

Dear Mr. Chairman:

'This is in r e s p o n s e t o NTSB Safety Recommendation A-74-41. i s s u e d May 15. Recommendation No. 1

1. I s s u e an a i r w o r t h i n e s s directive (AD) which: ( a ) Specifies new t o l e r a n c e s f o r t h e v e r t i c a l dimensions of t h e Boeing 7071720 s t a b i l i z e r t o elevator fair at t h e elevator hinge line. (b) D e s c r i b e s p r o c e d u r e s f o r m e a s u r i n g and establishing p r o p e r dimensions. ( c ) R e q u i r e s that all Boeing 707/720 a i r c r a f t b e inspected, at t h e next scheduled maintenance visit, f o r the p r o p e r dimensional relationship of the s t a b i l i z e r t o elevator fair at the elevator hinge line i n accordance with t h e p r o c e d u r e s established. (d) R e q u i r e s t h o s e a i r c r a f t found to have an out- of-tolerance condition to b e modified according to p r e s c r i b e d procedures. Comment

1. An A D covering t h e following technical a r e a s is c u r r e n t l y being p r e p a r e d f o r e a r l y adoption: ( a ) New t o l e r a n c e s f o r the v e r t i c a l dimensions of t h e Boeing 7071720 s t a b i l i z e r t o e l e v a t o r fair at the elevator hinge line.

(b) P r o c e d u r e s f o r m e a s u r i n g and establishing p r o p e r dimensions.

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(c) Requirement f o r all Boeing 707/720 aircraft t o b e inspected

for t h e p r o p e r dimensional relationship of the s t a b i l i z e r t o elevator fair at t h e elevator hinge line in a c c o r d a n c e with t h e established procedures. T h e time of inspection m a y not b e at t h e next scheduled maintenance visit, as you recommend, s i n c e this t i m e differs between o p e r a t o r s . However, a t i m e of inspection will be established to achieve timely coverage, and m a y b e e x p r e s s e d in terms of flight hours. (d) Requirement that t h o s e a i r c r a f t found t o have an out-oft o l e r a n c e condition t o be modified in accordance with p r e s c r i b e d procedures, and within a specified t i m e period. Recommendations f r o m A-73-76 through 78 T h e Safety Board continues to believe that e x c e s s i v e control s u r f a c e friction can f u r t h e r aggravate undesirable control s y s t e m c h a r a c t e r i s t i c s , and that t h e s e recommendations are still relevant. Comment Flight t e s t data indicates that reduced control s y s t e m friction may i m p r o v e control c h a r a c t e r i s t i c s but is not d i r e c t l y r e l a t e d t o the porpoising problem. T h e AD will include provisions t o inspect balance panel c l e a r a n c e s with a p p r o p r i a t e c r i t e r i a f o r adjustments to prevent excessive control s u r f a c e friction. Sincerely,

Administrator

- 42

GLOSSARY Boundary Layer - That thin region of r e t a r d e d a i r flow immediately adjacent to the s u r f a c e of an airfoil in flight. Disruption o r impeding the flow of the boundary l a y e r will cause p r e m a t u r e stagnation and a i r flow separation f r o m the surface. The point of separation will affect the p r e s s u r e distribution and thus the resultant lift produced by the airfoil.

lie an

of the enc se1

Thelongitudinal control d

f o r c e s of a n a i r c r a f t a r e discussed a s "stick f o r c e gradient. " In t e r m s of static stability c h a r a c t e r i s t i c s , the stick f o r c e gradient is described a s the change in the f o r c e required t o be exerted on the control column a s the a i r s p e e d i n c r e a s e s above (push f o r c e ) o r d e c r e a s e s below (pull f o r c e ) a specified t r i m speed. In t e r m s of maneuvering o r dynamic stability c h a r a c t e r i s t i c s , the stick f o r c e gradient is described a s the change in the f o r c e required to be exerted on the control column to produce a change in load factor. F o r positive maneuvering stability, the a i r c r a f t m u s t r e q u i r e a steady i n c r e a s e in control column f o r c e to produce an i n c r e a s e in load factor. F o r purposes of this report, the stick f o r c e gradient i s described a s the change in the control column push f o r c e required to produce an increasing elevator trailing edge down deflection for a constant set of operating conditions. Elevator P u l s e T e s t s - The a i r c r a f t is t r i m m e d f o r a stabilized a i r s p e e d and the control column i s pulsed to introduce a transient pitch disturbance. The tendency of the a i r c r a f t to r e t u r n to the condition f r o m which it was disturbed is a m e a s u r e of the longitudinal stability.

Stabilizer- Elevator T r a d e T e s t s - The a i r c r a f t is initially t r i m m e d in level flight at a stabilized airspeed. The stabilizer angle of incidence is then changed by u s e of the stabilizer t r i m system. Level flight is maintained by displacing the control column to produce that elevator deflection n e c e s s a r y to keep a constant t a i l load. The variation of the f o r c e required to displace the control column for increasing elevator deflections is a direct indication of the a i r c r a f t ' s handling qualities. Stick F r e e Stability - The tendency of the a i r c r a f t to r e t u r n to stable flight a f t e r an initial disturbance with the control column f r e e , i. e. hands off.

and to I

P

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43

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Tufts - P i e c e s of cloth or string tacked to the surface which will l i e streamlined in a n a r e a of unseparated flow but will l i e forward in an a r e a behind the separation point. Vortex Generator - A s m a l l airfoil placed vertically on the s u r f a c e of a l a r g e airfoil. The vortex generated by the s m a l l airfoil mixes with t h e air in t h e boundary l a y e r of the l a r g e airfoil t o i n c r e a s e the kinetic energy within the boundary l a y e r thereby delaying stagnation and airflow separation. t

-

Wind-Up T u r n The a i r c r a f t is t r i m m e d for a stabilized a i r s p e e d and then placed in a positive "g" t r i m . Elevator deflection i s required to maintain the d e s i r e d load factor.