The Boeing 737-800 can be flown either manually or automatically. This also applies to the management of the engines. The autothrottle regulates the thrust of the engines. The aircraft is fitted with two radio altimeter systems, one on the left and one on the right. In principle, the auto-throttle uses the altitude measurements provided by the left radio altimeter system. Only if there is an error in the left system that is recognised as such by the system, the autothrottle will use the right-hand radio altimeter system.
The aircraft involved in the accident was being flown by the first officer, who was sitting on the right-hand side. His primary flight display showed the readings measured by the right radio altimeter system. The right-hand autopilot was in use and, once air traffic control had provided a heading and altitude to be flown, it was in the ‘altitude hold’ mode in order to maintain that altitude. During the approach, the left radio altimeter system displayed an incorrect height of -8 feet. This could be seen on the captain’s (left-hand) primary flight display. The first officer’s (right-hand) primary flight display, by contrast, indicated the correct height, as provided by the right-hand system. The left-hand radio altimeter system, however, categorised the erroneous altitude reading as a correct one, and did not record any error. This is why there was no transfer to the right-hand radio altimeter system. In turn, this meant that it was the erroneous altitude reading that was used by various aircraft systems, including the autothrottle. The crew were unaware of this, and could not have known about it. The manuals for use during the flight did not contain any procedures for errors in the radio altimeter system. In addition, the training that the pilots had undergone did not include any detailed system information that would have allowed them to understand the significance of the problem.
When the aircraft started to follow the glidepath (the ideal path to the runway) because of the incorrect altitude reading, the autothrottle moved into the ‘retard flare’ mode. This mode is normally only activated in the final phase of the landing, below 27 feet. This was possible because the other preconditions had also been met, including flaps at (minimum) position 15. The thrust from both engines was accordingly reduced to a minimum value (approach idle). This mode was shown on the primary flight displays as ‘RETARD’. However, the right-hand autopilot, which was activated, was receiving the correct altitude from the right-hand radio altimeter system. Thus the autopilot attempted to keep the aircraft flying on the glide path for as long as possible. This meant that the aircraft’s nose continued to rise, creating an increasing angle of attack of the wings. This was necessary in order to maintain the same lift as the airspeed reduced.
In the first instance, the pilots’ only indication that the autothrottle would no longer maintain the pre-selected speed of 144 knots was the RETARD display. When the speed fell below this value at a height of 750 feet, they would have been able to see this on the airspeed indicator on the primary flight displays. When subsequently, the airspeed reached 126 knots, the frame of the airspeed indicator also changed colour and started to flash. The artificial horizon also showed that the nose attitude of the aircraft was becoming far too high. The cockpit crew did not respond to these indications and warnings. The reduction in speed and excessively high pitch attitude of the aircraft were not recognised until the approach to stall warning (stick shaker) went off at an altitude of 460 feet. This warning is activated shortly before the aircraft reaches a stall situation. In a stall situation the wings of the aircraft are not providing sufficient lift and the aircraft cannot fly anymore.
If the prescribed recovery procedure - i.e. selecting full engine power and reducing the pitch attitude of the aircraft - is implemented correctly and immediately when the stick shaker starts, then the aircraft will continue to fly normally. Boeing’s procedures also prescribe that the throttle levers should be pushed fully forward in such a case.
The first officer responded immediately to the stick shaker by pushing the control column forward and also pushing the throttle levers forward. The captain however, also responded to the stick shaker commencing by taking over control. Assumingly the result of this was that the first officer’s selection of thrust was interrupted. The result of this was that the autothrottle, which was not yet switched off, immediately pulled the throttle levers back again to the position where the engines were not providing any significant thrust. Once the captain had taken over control, the autothrottle was disconnected, but no thrust was selected at that point. Nine seconds after the commencement of the first approach to stall warning, the throttle levers were pushed fully forward, but at that
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point the aircraft had already stalled and the height remaining, of about 350 feet, was insufficient for a recovery.
The Board concludes that the improper functioning of the left-hand radio altimeter system led to the thrust from both engines being reduced by the autothrottle to a minimal value too soon, ultimately causing too big a reduction in speed. The airspeed reached stall speed due to a failure of monitoring the airspeed and pitch attitude of the aircraft and a failure to implement the approach to stall recovery procedure correctly. This resulted in a situation where the wings were no longer providing sufficient lift, and the aircraft crashed.