Dutch Safety Board presents results investigation into crash Turkish Airlines
On 6 May 2010, the Dutch Safety Board, chaired by Professor Pieter van Vollenhoven, presented their final findings of the investigation into the accident of Turkish Airlines’ Boeing 737-800 which crashed shortly before the intended landing on runway 18R (the so-called ‘Polderbaan’) at Amsterdam Schiphol Airport on 25 February 2009. This accident claimed the lives of four crew man and five passengers and three crew man and 117 passengers were injured.
During the investigation was ascertained that on approach, the left radio altimeter system displayed an incorrect height of -8 feet. As a result, the autothrottle system reduced thrust as if the aircraft was ready for the final phase of the approach and caused a reduction of the speed too large. The speed remaining was below the stalling speed. This resulted in a situation in which the wings could not provide enough lift and the plane crashed.
During approach the flight crew had received instructions from air traffic control to approach runway 18R with a short turn-in. A short turn-in is – frequently – used to deal with traffic as efficiently as possible. The instructions for a short turn-in were not accompanied by an instruction to descend. Because of this, the glide path had to be intercepted from above. Interception of the glide path from above masked the malfunctioning of the autothrottle for the pilots. In this phase of the flight it is normal for the thrust to be reduced so there is no need for extra thrust.
Next, the approach was not stabilised at 1000 feet so the crew should have executed a go-around. According to Turkish Airlines procedures, an approach should be stabilised at 1000 feet when flying under instrument conditions. Stabilised means that the aircraft – at 1000 feet – must be completely prepared for landing. If this is not the case, the approach ought to be abandoned and a go-around must be executed.
Because of the reducing speed, the nose pitch increased even more. Both speed decrease and nose pitch were not noticed by the crew until a signal, the stick shaker, warned that the aircraft threatened to stall. The recovery procedure for this situation was not performed correctly. The Board is of the opinion that the international training rules are inadequate. In some cases dealing with an approach to stall situation is not trained for many years. The fact that the approach to stall warning is a last safety means entails that, if the situation arises, there is an immediate and acute emergency situation. An adequate response of the crew is crucial and the necessary actions will have to be trained more often.
In spite of the fact that Boeing and many airlines had been aware of the problems with the radio altimeter system, this was not considered to be a safety problem. With exception of this accident, crews confronted with similar radio altimeter problems had always been able to solve these problems. In most cases problems with the radio altimeter system were not reported. If Boeing had received more reports, the manufacturer might have reached the conclusion sooner that a new analysis was necessary.