Pilot suicide or mechanical failure? The Egyptians and the Americans will never agree on EgyptAir 990

No proven cause has been found for the crash of an EgyptAir Boeing 767-300ER off the US East Coast south of Nantucket on 31 October, 1999. When the accident report on EgyptAir flight 990 was adopted on 13 March this year, two causes were advanced, one by the US National Transportation Safety Board (NTSB), the other by the Egyptian investigation team (Flight International, 26 March - 1 April).

The two conclusions were completely at odds. The only point at which interpretations of evidence converge is on the flight trajectory. In its conclusion, the NTSB provides a 118-point argument leading to a declaration that the "probable cause" was "the aircraft's departure from normal cruise flight and subsequent impact with the Atlantic Ocean as a result of the relief first officer's [RFO] flight control inputs. The reason for the RFO's actions was not determined."

The Egyptian Civil Aviation Authority (ECAA) is more circumspect, providing a 22-point argument before concluding that "a dual [right elevator] power control actuator [PCA] failure is consistent with the known and predicted behaviour of the aeroplane and of all the recorded data concerning the accident. Although a dual PCA failure cannot be confirmed, however, it must be considered as a plausible cause of the accident."

After taking off from New York Kennedy Airport at 01:20, the aircraft climbed uneventfully to 33,000ft (10,000m) with the autopilot engaged. At about 01:42, 2min before top of climb, the RFO replaced the command first officer (CFO) in the right-hand pilot seat, having persuaded him to take his rest early. At 01:48 the captain said that he was going to go to the toilet "before it gets busy". The RFO acknowledged this, and the captain left the flightdeck.

Both NTSB and Egyptian reports concur to this point, but then diverge. According to the Egyptian report, 12s after the captain had left the cockpit, an unidentified voice, possibly another person on the flightdeck, said in Arabic: "Control it." A few seconds later there was "a click and a thump", and the RFO said quietly: "I rely on God."

During the next minute, according to the ECAA, there were "six recorded events that were described as thumps or faint thumps", and then "a rapid, 0.7° movement of the left elevator. Seven seconds later the autopilot was disengaged." The RFO said again: "I rely on God." At autopilot disconnect, the Egyptians maintain, both elevators moved slightly in a trailing-edge down direction, but were not synchronous. At just after 01:50 both elevators moved to 5° trailing edge down in about 4s, with a 1s time delay between the movement of the left and right elevators, according to the ECAA.

This was the start of the accident sequence, with the aircraft nosing over at near zero g to a 40° nosedown attitude, reaching Mach 0.99 at 21,000ft with the ailerons showing abnormal movement. By that time the captain, asking "what's happening?", had regained his seat and both left and right elevators had begun moving up, reaching neutral deflection 20s after 01:50.

Then the flight data recorder shows a left and right elevator split, the left continuing to move up, the right down. A possible explanation for this, says the ECAA, is that the right elevator might have broken away from the tail at that point because the aircraft had almost reached Mach 1. The ECAA observes that, during the rapid descent, "there was an effort confirmed by the flight data recorder [FDR] to maintain the aircraft in a stable, wings level attitude".

At the same time, says the ECAA report, "the engine start lever was changed from engine run to cut-off" for both turbofans and the speed brake was deployed. Four seconds later, the ECAA says: "The captain ordered 'shut the engines', to which the RFO replied 'it's shut', and the captain said 'pull' or 'pull with me' four times."

The NTSB's version of events differs. It maintains the captain says: "What is this? What is this? Did you shut the engine?" and that several seconds later he ordered "shut the engines". On the other hand, the NTSB makes little of the noises recorded in the cockpit just before the dive, the voice saying "control it", and reads nothing into the small elevator movements.

The ECAA points out that there are no words of disagreement, anger or physical violence while the FDR and cockpit voice recorder (CVR) are running.  The NTSB interprets the captain's repeated calls of "pull with me" as a suggestion that the RFO was not doing so, and observes that it would be difficult to tell visually in which pitch direction the RFO was exerting force.

When the recorders stopped they did so because they were deprived of electrical power by the engine shut-down. Radar traces have been analysed to indicate that the aircraft bottomed out of its initial dive at about 16,000ft, climbed to 25,000ft, stalled, turned right through about 50° and entered its final fall.

The 767 fell into international waters, making Egypt responsible for the investigation because the aircraft was registered there. Within 24h of the accident, however, Egypt had voluntarily handed responsibility for the investigation to the NTSB. Nevertheless, an Egyptian team was present throughout the inquiry and reports that, although it was kept informed of progress and attended tests, the US safety board frequently "ignored" its input.

The NTSB reported four possible mechanical failure modes for the elevator operating system as being worthy of investigation in the light of the aircraft's manoeuvres, whereas the ECAA advanced 18 possible failures. Only the four chosen NTSB scenarios were run in a 767 engineering simulator. The ECAA argued that many of the tests were beyond the design capability of the simulator.

In the final analysis, all that seems beyond dispute is that the investigation report has failed to lay this accident to rest.

Source: Flight International