Lauda Air B767 Accident Report
Prepared for the WWW by
Safety Promotion Comt.
All Nippon Airways
Following are excerpts from the Lauda Air B767-300ER Accident Report dated July 21, 1993 issued by AIRCRAFT ACCIDENT INVESTIGATION COMMITTEE, MINISTRY OF TRANSPORT AND COMMUNICATIONS, THAILAND.
See the FULL REPORT for details.
Lauda Air airplane, Boeing 767-300 ER of Austrian nationality and registry OE-LAV, flight number NG 004 was on a scheduled passenger flight Hong Kong-Bangkok-Vienna, Austria. NG 004 departed Hong Kong Airport on May 26, 1991 , and made an intermediate landing at Bangkok Airport for unloading and loading of passengers and cargo. The flight departed Bangkok Airport at 1602 hours. The airplane disappeared from air traffic radar at 1617 hours about 94 nautical miles northwest of Bangkok. Local police authorities near the accident site notified the Rescue Co-ordination Centre, Department of Aviation in Bangkok of the accident. The Department of Aviation notified aviation authorities in the Republic of Austria (state of the operator and state of registry) and the United States of America (state of manufacture). The Republic of Austria and the United States of America sent their Accredited Representatives to participate in the investigation.
All times in this report are UTC.
1. FACTUAL INFORMATION
1.1 History of Flight
Lauda Air Flight 004 (NG004) was a scheduled passenger flight from Hong Kong to Vienna, Austria with an en route stop in Bangkok, Thailand. The flight departed Bangkok at 1602 hours on May 26, 1991 for the final flight sector to Vienna Austria.
All pre-flight, ground, and flight operations appear routine until five minutes and forty five seconds after the cockpit voice recorder (CVR) recorded the sounds of engine power being advanced for takeoff. At this point a discussion ensued between the crew members regarding an event later identified as a crew alert associated with a thrust reverser isolation valve.
The crew discussed this alert for some four and one half minutes. The Quick Reference Handbook (QRH) was consulted to determine appropriate crew actions in response to the alert. No actions were required, and none were identified as being taken.
Ten minutes and twenty seconds into the flight the co-pilot advised the pilot-in-command of the need for rudder trim to the left. The pilot-in-command acknowledged the co-pilot's statement.
Fifteen minutes and one second into the flight, the co-pilot stated "ah reverser's deployed." Sounds similar to airframe shuddering were then heard on the CVR. Twenty nine seconds later the CVR recording ended with multiple sounds thought to be structural breakup.
Flight conditions were recovered from non-volatile memory in the left engine electronic engine control (EEC). At the suspected point of reverser deployment, the EEC readout indicated that the airplane was at an approximate altitude of 24,700 feet, a speed of Mach 0.78, and developing climb power.
The airplane crashed in mountainous jungle terrain at 14 degrees 44 minutes North latitude and 99 degrees 27 minutes East longitude at approximately 1617 hours. Night time visual meteorological conditions prevailed.
NOTE:1.2 through 1.18 are omitted.
The crew members were trained, qualified and certificated for their respective duties according to the laws and regulations of the Republic of Austria. There was no evidence that medical factors or fatigue affected the flight crew's performance.
The airplane was certificated, equipped and maintained according to regulations and approved procedures. Flight documents indicate that the gross weight and c.g. were within prescribed limits. With the exception of some recurring maintenance PIMU messages pertaining to the thrust reverse system which did not preclude dispatching the airplane's (sic).there was no evidence of pre-accident failure or malfunction of the airplane's structure, powerplants, and systems.
The weather in the area was fair at the time of the accident. Although there were no reported hazardous weather phenomena, isolated lightning was possible. There are few visible landmarks and population centers on the ground along the route of flight and it is possible that the horizon was not distinguishable. Recovery from any unusual flight attitude could have been affected by the lack of outside visual references.
The flight appeared normal until five minutes and forty-five seconds after takeoff (takeoff = the CVR recorded sound of engine power advanced). At this time the crew began to discuss an event in the cockpit that was later identified as illumination of a REV ISLN indication. The pilot-in-command stated "that keeps coming on." The REV ISLN indication could consist of either a REV ISLN amber (yellow) light illumination on the center pedestal or a L REV ISLN VAL advisory amber (yellow) EICAS message or both indications. This indication appears when a fault has been detected in the thrust reverser system. It indicates a disagreement between the respective hydraulic isolation valve (HIV) and the associated thrust reverse lever position or an anomaly in the air/ground system. No corrective actions were necessary and none were identified as taken by the crew.
The crew's discussion of the REV ISLN indication was of an informative nature and continued for about four and one-half minutes. The co-pilot read information from the Airplane Quick Reference Handbook as follows:
"Additional systems failures may cause in- flight deployment" and "Expect normal reverser operation after landing." The pilot-in-command remarked "....its not just on, its coming on and off," he said, "...its just an advisory thing...," and shortly thereafter stated, "could be some moisture in there or something." The critical nature of an in-flight thrust reverser deployment in this phase of flight was not known and therefore the flightcrew was not provided with operational guidance. Airplane design changes implemented after this accident eliminated the need for operational guidance for the flightcrew.
Review of the thrust reverser system design indicates that when the auto-restow system function is required, system pressure to close the reversers is applied during restow and for 5 seconds after restow is sensed. The REV ISLN light illuminates for this period except for the first 2 seconds. The associated EICAS message appears 2 seconds after the REV ISLN light illuminates. Interpretation of the crew's comments regarding the reverser ISLN indication, "Coming on and off' indicates that they may have been observing cycling of the auto-restow system (see Appendix C). The specific interval of illumination of the light, and the possibility that the light ceased to be observed, could not be determined from the cockpit voice recorder comments nor from any other evidence. Also it could not be determined if the REV ISLN light was accompanied by an EICAS message; nothing was verbalized by the crew. There was no recoverable data from the nonvolatile memory available in the recovered EICAS components.
At ten minutes twenty seven seconds into the flight, the co-pilot advised the pilot-in-command that there was need for, "a little bit of rudder trim to the left." The crew discussion of trim took place from an elapsed time of 10:27 and lasted nine seconds. About four and one-half minutes separated the REV ISLN indication event from the trim discussion. It ended with the pilot-in-command saying "O.K., O.K.". It is probable that the trim requirement was a normal event in the flight profile. The trim requirement does not appear to be related to the upcoming reverser event, and there was no apparent reason for the crew to interpret it as such.
Fifteen minutes and one second into the flight the co-pilot's voice was heard to exclaim, "ah reverser's deployed," accompanied by sound similar to airframe shuddering, sounds of metallic snaps and the pilot-in-command stating "here wait a minute." The cockpit voice recording ended twenty nine seconds later with multiple bangs thought to be structural breakup of the airplane.
An assessment of flightcrew attempts to control the airplane's flightpath was not possible due to loss of the FDR data as a result of ground fire damage to the recorder tape.
The physical evidence at the crash site conclusively showed that the left engine thrust reverser was deployed. Nonvolatile computer memory within the electronic engine control (EEC) indicated that an anomaly occurred between channel A and B reverser sleeve position signals. It was concluded that this anomaly was associated with the thrust reverser deployment of one or both sleeves. The EEC data indicated that the thrust reverser deployed in-flight with the engine at climb power; based on EEC design, it was also concluded that the engine thrust was commanded to idle commensurate with the reverser deployment, and that the recorded mach number increased from 0.78 to 0.99 (the actual maximum speed reached is unknown due to pressure measurement and recording uncertainties). The left EEC data indicates that the fuel cutoff switch was probably selected to cutoff within 10 seconds of thrust reverser deployment. Examination of the cutoff switch also indicates that it was in the cutoff position at impact.
NOTE: Para 2.2 through 2.7 are omitted.
3.2 Probable Cause
- The crew members were trained, qualified, and certificated for their respective duties according to the laws and regulations of the Republic of Austria.
- The airplane was certificated, equipped and maintained, and operated according to regulations and approved procedures of the Republic of Austria.
- The weather in the area was fair. There were no reported hazardous weather phenomena although lightning may have been present. It is possible that the horizon was not distinguishable.
- The physical evidence at the crash site showed that the left engine thrust reverser was m the deployed position.
- Examination of nonvolatile computer memory within the left EEC indicated that the engine was at climb power when the reverser deployed, engine thrust was reduced to idle with the reverser deployment, and the recorded Mach number increased from 0.78 to 0.99 after the deployment. The actual maximum speed reached is unknown due to pressure measurement and recording uncertainties.
- The scatter of wreckage indicated that the airplane experienced in-flight breakup at a steep descent angle and low altitude.
- There was no indication on the available wreckage of an in-flight fire prior to the breakup of the airplane.
- Examination of the available wreckage revealed no evidence of damage from a hostile act, either from within the airplane or from the exterior.
- Simulations of a 25 percent lift loss resulting from an in-flight deployment of the left engine thrust reverser indicated that recovery from the event was uncontrollable for an unexpecting flight crew.
- From an airplane flight performance standpoint, questions remain unanswered regarding thrust reverser plume behavior at high Mach numbers and in-flight reverse induced airframe buffeting at high Mach numbers, and effects of inlet spillage caused by a reversed engine at high Mach numbers.
- Thrust reverser system certification by the FAA required that the airplane be capable of continued safe flight and landing under any possible position of the thrust reverser (FAR 25.933(a)(2)). However, wind tunnel tests and data used in the simulation of this accident demonstrated that aerodynamic effects of the reverser plume in-flight during engine run down to idle resulted in a 25 percent lift loss across the wing. Simulation of the event disclosed that the airplane was not capable of controlled flight unless full wheel and full rudder were applied within 4 to 6 seconds after the thrust reverser deployed.
- Investigation of the accident disclosed that certain "hot-short" conditions involving the electrical system occurring during an auto-restow command, could potentially cause the DCV to momentarily move to the deploy position. However, no specific wire or component malfunction was physically identified that caused an uncommanded thrust reverser deployment on the accident airplane.
- Testing identified hypothetical hydraulic system failures that could cause the thrust reverser to deploy. However, no specific component malfunction was identified that caused an uncommanded thrust reverser deployment on the accident airplane.
- No specific Lauda Air maintenance action was identified that caused uncommanded thrust reverser deployment on the accident airplane.
- The design changes recommended by Boeing and thereafter mandated by U.S. Federal Aviation Administration Airworthiness Directive 91-22-09 for the B767/PW4000 thrust reverser system should effectively prevent in-flight deployment even after multiple failures.
The Accident Investigation Committee of the Government of Thailand determines the probable cause of this accident to be uncommanded in-flight deployment of the left engine thrust reverser, which resulted in loss of flight path control. The specific cause of the thrust reverser deployment has not been positively identified.
The Aircraft Accident Investigation Committee recommends that the United States Federal Aviation Administration examine the certification philosophy of all airplane certificated with ground only engine thrust reverser systems to provide appropriate design safeguards to prevent in-flight deployment.
The Aircraft Accident Investigation Committee also recommends that the United States Federal Aviation Administration revise the certification standards for current and future airplane flight recorders intended for use in accident investigation to protect and preserve the recorded information from the conditions of prolonged thermal exposure that can be expected in accidents which occur in locations that are inaccessible for fire fighting efforts.
Back to "TOP"
Back to `Computer-Related Incidents'.
Peter B. Ladkin, 1999-02-08
Last modification on 1999-06-15
by Michael Blume