Tragic Accident with a Turkish Airlines B 737-800

What may have happened to flight TK 1951 on Wednesday 25 February 2009?

The flight with the Turkish B 737-800 arrived at Schiphol airport without any problems. An on-time arrival with a good aircraft and three pilots on the flight deck. Route training was in progress, a standard airline procedure. Weather conditions, although not very beautiful, were excellent for landing. Runway 18 Right was in use. Then suddenly, a few miles from touch down, something went wrong. It apparently started with a faulty annunciation of one of the radio altimeters, normally not a very serious problem. However, as the crew decided to make an automatic landing, the role of the altimeter is suddenly very important. And on this flight it has lead to a catastrophe!

By: ir. Hessel Benedictus, chief pilot Cessna Citation II, department Control & Simulation

ircraft accidents should not happen, but they unfortunately do. Being related to the world of aviation they come in hard, certainly when it happens almost in your backyard. Like flight TK 1951 at Schiphol Airport. Why did it happen, how could it happen, what went wrong? Flight 1951 approached Schiphol after an uneventful flight from Istanbul. The Boeing 737-800 was cleared by the Approach controllers to descend to 2000 feet and was instructed to turn left to a heading of 210 to intercept the ILS for runway 18R. Maybe because of the weather conditions or the fact that one of the co-pilots was receiving route instruction, but the crew apparently decided to make an automatic landing.

The mode control panel in the glare shield is used to send the commands of the crew to the auto flight system. For an auto land both autopilots are activated, as well as the auto throttle system. Intercept heading is dialed in the heading window, required speed in the speed window. Localizer and glide slope, signals from the ILS for lateral and vertical guidance to the runway, are armed. The autopilots will eventually couple the aircraft to localizer and glide slope. The auto throttle system will maintain the final approach speed, as selected by the crew. When, approaching the touch down area, the radio altimeters sense a couple of feet above the runway, the auto throttle will bring the thrust levers to the idle position (RETARD), while the autopilots lift the nose for a flare into a landing. The pilots have to monitor the performance of the auto flight system. The most important instrument is the Primary Flight Display (PFD), as can be seen in Figure 1, with the artificial horizon in the center, speed tape on the left, altitude and vertical speed tapes on the right, heading at the bottom.

The vertical and lateral inputs for the autopilot are also sent to the flight director, cross pointers over the aircraft symbol in the artificial horizon. When the aircraft is on the localizer and on the glide slope, both pointers are centered. Any deviation will appear as a movement of the pointers to left or right, up or down. An important part of the PFD is also the Mode Annunciator at the top of the artificial horizon, showing the mode in which the auto flight system is operating. For example, when the localizer is armed while the auto flight system is still receiving heading inputs, the annunciator will show HDG in green, LOC in white. When the auto flight system captures the localizer, HDG will disappear, and LOC will change from white into green. These mode changes have to be confirmed by the pilots by calling the changes.

Figure 1: Secondary (left) and Primary (right) Flight Displays of a Boeing 737-800 during and ILS approach. Note the radio altimeter showing 620 feet.

It looks like flight 1951 came in a bit high, in which case the glide path is intercepted from above, with a slightly higher rate of descent compared to the three degree glide slope. In combination with a deceleration to the selected final approach speed (with a margin of approximately forty percent over the stall speed), the throttles may well have been in the idle position. This would than be a normal and to be expected situation. At an altitude of approximately 1900 feet, a radio altimeter annunciates a faulty value of minus eight feet. Since it is this altimeter which is connected to the auto throttle computer, with auto land selected by the crew, it sends a command for idle thrust to the throttles. With the throttles already in idle, this command would not be noticeable, except from the annunciation « RETARD » on the annunciator panel.

The autopilot system follows the localizer, aligning the aircraft with the runway centerline. The system also tries to follow the three degree glide slope and the selected final approach speed. However, with the throttles held in the idle position by the auto throttle system, this is an impossible task. The aircraft descends slowly below the glide path, the speed drops below the selected reference speed. At an altitude of approximately 600 feet the aircraft is well below the glide path and reaches the stall speed. The artificial stall warning system, attached to the control column, rattles its alert. Then the throttles are jammed forward in an effort to climb the aircraft out of the stall, but the crew cannot avoid the aircraft from hitting the ground in a stalled, nose high, position.

This may well have been the history of the final part of flight 1951. But why? According to the conversation between the flight crew and the air traffic controllers everything was normal when they reached the initial approach altitude of 2000 feet. What happened in the cockpit during the one and a half minute between 1900 and 600 feet? Why did the crew not notice the abnormal changes in altitude, speed and attitude? The flight displays seemed to have provided the expected data and information. Has there been some sort of distraction? In any way, the three crewmembers may, in the last part of the flight, not have been fully aware of the situation they were in, specifically when still in clouds. Situational awareness! The term pops up again! It is a fact that, due to circumstances, with all the beautiful information presented in the cockpit, pilots may mentally still lag behind the actual course of events. Already many years attention has been given to improve situational and also energy awareness. Studies were focused on improved horizontal situation displays, implementation of terrain information, energy management, vertical situation displays and infrared based outside world displays.

A solution which appeals to me very much is the so called tunnel in the sky. The Faculty of Aerospace Engineering has been very actively involved in research into these revolutionary future flight displays for many years. The planned flight path of the aircraft is presented to the crew by a three dimensional tunnel on the PFD, see Figure 2. As long as the aircraft stays in the tunnel, either manually flown or guided by the auto flight system, the aircraft safely reaches the selected runway. In addition, there are ways to show the energy situation, planned versus actual energy state. Let us hope that these kinds of very significant improvements will not just stay a subject of research, but that they will be implemented in every future airline cockpit! Aircraft accidents just should not happen.

Figure 2: Future Primary Flight Display with tunnel in the sky and terrain visualization, under development at the Control & Simulation Division and the Aerospace Software and Technologies Insitute (ASTI)