Accident: Turkish Airlines B738 at Amsterdam on Feb 25th 2009, landed on a field
By Simon Hradecky, created Wednesday, Mar 4th 2009 13:59Z, last updated Wednesday, Mar 4th 2009 15:15Z
The Dutch Accident Investigators have revealed first results in a press conference in Den Haag today. With the exception of the left radio altimeter the Dutch Safety Board has not found any irregularities with the aircraft. The airplane was carrying 127 passengers and 7 crew. 9 occupants were killed in the accident, thereof 4 crew and 5 passengers. 28 of the injured are still in hospital care. The captain was occupying the left hand seat, a fully qualified first officer occupied the right hand seat. For this first officer this was a training flight. Another first officer occupied the observer's seat.
The airplane was performing an ILS approach to runway 18R on autopilot in fairly bad visibility according to standard operating procedures by Turkish Airlines. Mist and low cloud prevailed suggesting, that the crew most likely did not see Polderbaan (runway 18R) while commencing the final descent. While the airplane was at 1950 feet MSL with the autoflight systems coupled to the ILS already, the left hand radar altimeter produced a wrong height reading of -8 feet. This resulted in the autothrottles to pull the thrust to idle. The landing gear warning horn sounded as a result of the erroneous reading, but was not considered to be a problem according to the cockpit voice recorder. However, this warning should have alerted the crew of a radar altimeter problem. The stick shaker activated at a height of about 500 feet AGL, at which point full power was applied. This however was already too late to recover the flight and the airplane subsequently impacted ground. The airplane hit ground with the tail first at a speed of 92 knots (normal landing speed would have been 140 knots) and slid for about 150 meters until coming to a stop. The airplane's tail broke off and the fuselage ruptured at the business class, the landing gear separated according to its design. The engines separated and continued for an additional about 250 meters due to the thrust they developed and the sudden deceleration of the airframe.
The fatalities were mainly along the line of rupture through the business class. The cockpit crew was fatally injured mainly because of the enormeous deceleration forces and the nose wheel, that got partially embedded into the cockpit.
The flight data recorder stores 25 hours of flight data, which covered the last 8 flights of the airplane. During those flights another two failures of the left hand radar altimeter had been recorded. A Boeing Alert sent to all operators of 737s (all types) said, that no evidence of bird strike, engine or airframe icing, wake turbulence or windshear has been found so far. There was adequate fuel on board throughout the entire flights. Both engines responded normally to throttle inputs, the airplane responded normally to flight control inputs all the time. The digital flight data recorder indicated, that the autopilot B and autothrottle was being used for an ILS approach to runway 18R. The right hand radar altimeter was providing proper data, while the left hand radar altimeter provided faulty readings of -7 and -8 feet. The autothrottles, using the left hand radar altimeter, transitioned to the flare mode and retarded the throttles to idle thrust. The throttles remained at the idle stop for about 100 seconds, while the airspeed dropped to 40 knots below selected approach speed.
According to Boeing, if one of the radar altimeters provides erroneous height readings (regardless of whether the radar altimeter indicates itsself failed or not - failure flag or not), typical flight deck effects requiring crew intervention would be erratic radar altimeter readings on the instrument displays with differences between the two readings, inability to engage both autopilots in dual mode for the approach mode, unexpected removal of flight director bars, unexpected configuration warnings and a premature autothrottle RETARD announciation on the pilots' primary flight displays.
