Lack of flight data and cockpit voice recorders hampers effort to reach conclusion

The crew of a de Havilland Twin Otter that crashed on the island of St Barthélémy, Guadeloupe, in the French Caribbean, are suspected of having selected reverse thrust when the aircraft was about to enter the steep final approach to the runway, says the report from French accident investigation bureau BEA.

Both pilots and all 17 passengers were killed in the accident on 24 March 2001, when the aircraft, in fine weather, suddenly went out of control and crashed.

The 18-seat Air Caraibes aircraft (F-OGES), which entered service in 1970, was not fitted with flight data or cockpit voice recorders, so the investigators had to rely upon evidence from, among other things, a passenger's video camera.

The wreckage indicated an approximately 37° nose down impact in a left turn, the report says, but there was no evidence of technical malfunction or of a birdstrike, and meteorological data suggest that any turbulence approaching the ridge was insufficient to upset the aircraft.

The left propeller's pitch at impact was only just above that applicable for reverse thrust, and the similarity of the damage to the propeller tips suggested to the investigators that both propellers were operating approximately symmetrically at impact.

The approach to runway 10 at St Barthélémy involves flying over the top of a ridge, then closely following the steep slope to the runway in a valley at its base. A video camera had survived the impact and post-crash fire partially intact. The only relevant usable part of the film showed a view taken out of a cabin window looking through the left propeller disc at the extreme north of St Barthélémy island when there was about a minute to go to the accident.

The sound track of the film did not indicate any engine malfunction or unacceptable propeller asymmetry. Many witnesses, however, heard dramatic changes in the engine tone during the flight's last few seconds.

The BEA suggests that the captain probably intentionally selected the reverse Beta range to control the steep approach, and then while selecting from Beta to idle, a momentary large thrust asymmetry rendered the aircraft uncontrollable.

This type of operation has previously been the cause of a Twin Otter accident, says the BEA. But it admits that there are three other possibilities: loss of control during a go-around, stalling, or pilot incapacitation.

A fully-laden Tropical Airways Let 410 19-seat turboprop crashed within 5km (3nm) of its take-off from Cap-Haïtien airport, Haiti, on 24 August, bound for Port-de-Paix. It came down in a sugar cane field at around 16:00, killing all 21 people on board. Witnesses report smoke coming from the Walter M601E-powered Let 410 during its descent to impact.

Source: Flight International