Investigators are refining simulations of a Sudanese Boeing 707-330 freighter crash in order to determine whether a serious fault affecting the outboard right-hand engine was responsible.
The aircraft came down shortly after departure from Sharjah 15 months ago, killing all six occupants, but the United Arab Emirates inquiry has been hampered by an absence of information on either flight recorder.
Radar and video data has been used in a bid to reconstruct the dynamics of the accident through a simulation which has focused on two scenarios affecting the outboard Pratt & Whitney JT3D powerplant: a typical loss-of-thrust failure, or a rapid activation of reverse thrust at high power.
Wreckage analysis discovered the engine's core thrust reverser was in a deployed position, as were the outer translating sleeve and the right-hand reverser clamshell. The left-hand clamshell was also "beyond the normally-stowed position", says the General Civil Aviation Authority.
The GCAA has also carried out an extensive examination of the engine components and cockpit instruments in order to ascertain whether the powerplant was operating.
Although flight recorder information might have yielded the crucial data, the GCAA says the only usable tape from the cockpit-voice recorder contained data unrelated to the accident flight, while the flight-data recorder was "very unlikely" to have been recording at the time of the crash. It has made appropriate recommendations to the Sudanese authorities.
The aircraft was being operated by Azza Air Transport, on behalf of Sudan Airways, at the time of the 21 October 2009 accident. As the aircraft took off, the suspect engine's cowl fell from the 707.
Its crew informed air traffic control of having "lost" the engine, says the GCAA, before the jet entered a right turn with 70° bank and dived to the ground some 20s after becoming airborne.
Simulations have drawn from only a limited data set, including six radar hits from the control tower, and investigators have had to make several assumptions about airspeed, flap setting and the behaviour of the aircraft taken from surveillance images and witnesses.
Initial results of the simulation suggest that, for a typical engine failure, rudder could be used to keep the aircraft on a straight and level course without sustained use of the control yoke.
But for a thrust-reverser activation scenario, maintaining the runway heading would - in the worst case - have required full rudder, 50° yoke and 9° opposite bank to maintain heading. "Any less input would have the aircraft heading turn right, which is the direction the accident aircraft took," says the GCAA.
Boeing is examining engine parts from the 707 and assisting with developing a more detailed simulation, which could provide a better match with known data, while a human factors analysis is attempting to determine the probable reaction of the crew.
