A mystery from the past may have relevance for the present.
David Learmount/LONDON
When Capt. Harvey "Hoot" Gibson's aircraft, a Trans World Airlines Boeing 727-100, suddenly rolled out of control and dived 32,000ft (10,000m), Gibson had to pull more than 5g before recovering control at 6,000ft.
The near-disaster happened at night over Saginaw, Michigan, USA, on 4 April 1979, years before the unexplained March 1991 United Airlines 737-200 fatal crash at Colorado Springs, or the mysterious USAir 737-400 disaster at Pittsburgh on 8 September, 1994. Before those incidents, Gibson had already embarked on a fight to force the US National Transportation Safety Board (NTSB) to re-examine the evidence of his own experience at Saginaw. The NTSB had blamed the incident on pilot procedures - an assessment based on what the US Air Line Pilots Association (ALPA) calls "assumptions", rather than evidence. Despite investigation by TWA and ALPA, which demonstrated that many of the NTSB technical assumptions could not have been right, the Board did not respond.
The Colorado Springs and Pittsburgh accidents, however, have added weight to Gibson's plea: in all three, loss of directional and roll control resulted in a dive, but, while Gibson managed to save TWA flight 841, the other two flights were not so lucky. In those two accidents, failure of the aircrafts' roll or yaw control systems, has been stated by the investigators, to have been possible but, in the Saginaw investigation it was not seriously considered.
Gibson is now to take his plea again to the district court, hoping that concern over Pittsburgh will improve his chances of being heard.
Keith Hagy of ALPA's accident department admits that Gibson's chances of forcing the NTSB's hand are receding with time. There are many NTSB reports on record, he points out, which are now acknowledged as being wrong in the light of increased technical and operational knowledge, but, because of lack of resources, they have not been re-examined.
The NTSB's Saginaw report alleges that the event started at 39,000ft, when Gibson's crew attempted to disable the leading-edge slats by tripping the appropriate circuit-breakers, so that he could deploy 2¡ trailing-edge flap without them. The motive ascribed to him was "...to improve aircraft performance". The crew denies it.
The next NTSB "assumption", as ALPA calls it, was that, because leading edge slats 2, 3, 6 and 7 began to extend anyway, the crew retracted all high-lift devices. Slat 7 remained deployed, however, leading to a starboard roll and lateral control problems.
ALPA produces evidence to show that the ascribed cause was highly unlikely. For example, the NTSB report assumes that control was restored by the separation of Slat 7 near the bottom of the aircraft's dive, whereas the manufacturer says that the most likely slat-separation point - if it had been extended at the time - would have been in the early dive through 32,000ft.
A crucial issue in the NTSB's investigation was that the Saginaw 727's cockpit-voice-recorder (CVR) tape had been bulk-erased and, without checking the CVR's erase mechanism and its behaviour in the light of the aircraft's seriously damaged state at the time of its emergency landing, the Board accused Gibson of erasing the tape. ALPA says: "The combination of disparaging presumptions was the origin of a bias planted in the minds of the investigators; this bias fouled each phase of the investigation."
The ALPA submission, based on the aircraft's behaviour as revealed by the flight-data recorder, says: "The probable cause of the TWA 841 accident was a complex interaction that involved the tightly coupled response of lateral and directional flight controls on the 727-100 aircraft. The failure of a component in the outboard right aileron [it was established that a hinge-bolt had failed, inducing aileron up-float and flutter at the beginning of the incident] induced roll and yaw. A malfunction of the automatic rudder-control system produced further yaw, sideslip and roll. This interaction led to a loss of control from which recovery was effected only after hydraulic power to the lower rudder failed." ALPA also refers the NTSB to numerous similar control-related incidents and to the fact that the TWA 841 aircraft, N840TW, "...suffered a prior near loss of control...less than two years prior to the accident". The NTSB, however, failed to research or to consider the relevance of these, alleges ALPA.
If ALPA is right in its investigation, however, and Hagy is right in his fears about the effects of time on Gibson's chance of a hearing, the pilots will remain under blame and NTSB database statistics could continue to hide real trends.
Source: Flight International
