The Dutch Oenderzoeksraad (Dutch Safety Board, DSB) released their final report concluding (in length):
During the accident flight, while executing the approach by means of the instrument landing system with the right autopilot engaged, the left radio altimeter system showed an incorrect height of -8 feet on the left primary flight display. This incorrect value of -8 feet resulted in activation of the ‘retard flare’ mode of the autothrottle, whereby the thrust of both engines was reduced to a minimal value (approach idle) in preparation for the last phase of the landing. Due to the approach heading and altitude provided to the crew by air traffic control, the localizer signal was intercepted at 5.5 NM from the runway threshold with the result that the glide slope had to be intercepted from above. This obscured the fact that the autothrottle had entered the retard flare mode. In addition, it increased the crew’s workload. When the aircraft passed 1000 feet height, the approach was not stabilised so the crew should have initiated a go around. The right autopilot (using data from the right radio altimeter) followed the glide slope signal. As the airspeed continued to drop, the aircraft’s pitch attitude kept increasing. The crew failed to recognise the airspeed decay and the pitch increase until the moment the stick shaker was activated. Subsequently the approach to stall recovery procedure was not executed properly, causing the aircraft to stall and crash.
The Dutch Safety Board has reached the following sub conclusions:
Technical aspects
The problems with radio altimeter systems in the Boeing 737-800 fleet had been affecting several airlines, including Turkish Airlines, for many years and were known to Boeing and the Federal Aviation Administration of the United States of America.
Several airlines, including Turkish Airlines, regarded the problems with radio altimeter systems as a technical problem rather than a hazard to flight safety. As a result, the pilots were not informed of this issue.
It has become clear that the existing procedures, tests and routines applied by several airlines, including Turkish Airlines, were not sufficient in order to resolve the problems with erroneous radio altitude values.
The investigation failed to find a single cause for the origin of the erroneous radio altitude values.
Tests showed that the Rockwell Collins Enhanced Digital Flight Control System (EDFCS) uses radio altitude values that are characterised as ‘non computed’ (unusable,) whereas this characterisation should have prevented this. The operating software designed to compare the two radio altimeter systems cannot be applied in the entire Boeing 737 NG fleet. The introduction of operating software capable of making comparisons has failed to fully eliminate the undesired activation of the retard flare mode.
Not all certified Boeing 737 operating software versions for the autothrottle and flight control computers respond to an erroneous radio altitude signal in the same way. This situation is undesirable, especially in cases where an airline is using several versions that respond differently and without having informed its pilots.
Reports
Despite the fact that Boeing and the Federal Aviation Administration of the United States of America had been aware for many years that the radio altimeter system was causing many problems and was affecting the operation of other systems, this situation was not designated as a safety risk. Reports of problems with the radio altimeter system that could not be resolved by Boeing justified an effort to analyse the radio altimeter system and other related systems. Boeing and the Federal Aviation Administration of the United States of America could have recognised the fact that the problems caused by the radio altimeter system, especially the potential for activating the autothrottle retard flare mode, posed a safety risk.
Most of the problems regarding the radio altimeter system were not reported. If the manufacturer had received more reports, Boeing might have recognised the need for renewed analysis.
All radio altimeter components (antennas, cabling and radio altimeter computers) are certified in accordance with the applicable standard.
With regard to certification in situations related to a specific flight phase, such as the approach, it could be useful to calculate the risk based on the actual exposure time of the flight phase (in this case the approach) instead of the total flight time. This could yield a more accurate assessment of the actual risks.
Air traffic control
There are no indications that wake turbulence caused by the Boeing 757 scheduled to land before flight TK1951 had effect on the sequence of events of flight TK1951.
The line-up of flight TK1951 took place at a distance of between 5 and 8 NM before the runway threshold without prior ‘offer’ to the crew, and without instruction to descend to an altitude lower than 2000 feet. This is not in accordance with the Rules and Instructions air traffic control applied by Air Traffic Control the Netherlands, which are based on the International Civil Aviation Organization guidelines. This method of lining up the aircraft is used for over 50% of all approaches on this runway.
A turn-in, whereby interception takes place at between 6.2 and 5 NM, with no instruction to descend to an altitude below 2000 feet is in deviation of the International Civil Aviation Organization guideline specifying that the aircraft must be flying level on its final approach course before the glide slope is intercepted.
Operational
Due to the fact that the localizer signal was intercepted at 5.5 NM from the runway threshold at a altitude of 2000 feet, the glide slope had to be intercepted from above. As a result, the crew were forced to carry out a number of additional procedures, resulting in a greater workload. This also caused the landing checklist to be completed during a later moment in the approach than standard operational procedures prescribe.
The cockpit crew did not have information regarding the interrelationship between the (failure of the) left radio altimeter system and the operation of the autothrottle. Of all the available indications and warning signals, only a single indication referred to the incorrect autothrottle mode, namely the ‘RETARD’ annunciation on the primary flight displays. With the knowledge available to them at that time, the crew had no way of understanding the actual significance of these indications and warning signals and could not have been expected to determine the pending risk accurately.
Within the Turkish Airlines pilot corps was no clarity on calling out flight mode annunciations, while it has been demonstrated that calling out these annunciations raises the pilots’ awareness of the automatic flight system status.
As a result of intercepting the glide slope signal from above, the incorrect operation of the autothrottle was obscured for the crew.
In accordance with Turkish Airlines’ standard operating procedures, the approach should have been aborted at 1000 feet followed by a go-around, as the approach had not yet been stabilised at this time. However, this procedure was not carried out.
Despite the indications in the cockpit, the cockpit crew did not notice the too big decrease in airspeed until the approach to stall warning.
With the cockpit crew - including the safety pilot - working to complete the landing checklist, no one was focusing on the primary task: monitoring the flight path and the airspeed of the aircraft. It can thus be concluded that the system based around the presence of a safety pilot on board flight TK1951 did not function effectively.
The total time between the activation of the stick shaker and the moment the throttle was set to the maximum thrust position was nine seconds. Simulator tests have shown that the situation could have been recovered and the flight continued if the crew immediately after stick shaker activation had moved the thrust levers to maximum thrust as part of the approach to stall recovery procedure.
The aircraft entered a stall situation with the autopilot engaged. The autopilot was disengaged at an altitude of between 400 and 450 feet above ground level.
Test flight data by Boeing and subsequent analysis of this data have demonstrated that when the aircraft has stalled the altitude loss for recovering from the stall situation after selection of maximum thrust is approximately 500 to 800 feet. When the aircraft arrived in the stall situation the remaining altitude of 400 to 450 feet was not sufficient to recover the situation.
The fact that the thrust levers were not immediately moved to their maximum thrust positions in accordance with the approach to stall recovery procedure indicates that the crew were not sufficiently trained to deal with a situation of this type.
The information featured in the Quick Reference Handbook regarding the use of the autopilot, the autothrottle and the need for trimming in the approach to stall recovery procedure is unclear and insufficient.
Crew resource management and crew communications during the approach were not in accordance with the standard operating procedure of Turkish Airlines regarding cockpit communication.
Turkish Airlines safety programme
In accordance with Joint Aviation Requirements - Operations 1, Turkish Airlines has a programme for the prevention of accidents and to enhance flight safety.
As a part of its quality assurance programme, Turkish Airlines has drawn up an internal auditing schedule. None of the audits conducted up until the time of the accident yielded any findings as regards adherence to the standard operating procedures described in the Operations Manual or the application of crew resource management procedures.
In 2008, the Flight Safety department received 550 aviation safety reports from cockpit crews. None of these reports concerned problems with radio altimeter systems, unintended warnings relating to the landing gear, ground proximity warnings or autothrottle ‘RETARD’ mode indications during approach.
Each year, the Flight Safety department conducted approximately fifteen incident investigations. No investigation was ever conducted with regard to problems involving radio altimeter systems.
Turkish Directorate General of Civil Aviation
The requirements with regard to stall training set out in the Joint Aviation Requirements - Operations 1 (as applied by Turkish Airlines) and Joint Aviation Requirements - Flight Crew Licensing are too limited. This limited amount of training is insufficient, as automatic flight systems and procedures cannot always ensure that the crew does not encounter a stall situation. Recovery from a stall situation should also be included in recurrent training programmes.
Transport and Water Management Inspectorate
The Transport and Water Management Inspectorate tests whether procedures published in the Aeronautical Information Publication Netherlands are in line with national and international regulations. The audits conducted by the Transport and Water Management Inspectorate have not provided insight as to whether individual air traffic controllers act in accordance with the Rules and instructions air traffic control. The Transport and Water Management Inspectorate has not assessed whether the procedures set out in the Rules and instructions air traffic control are in line with the recommendations of the International Civil Aviation Organization.
The DSB added to their preliminary report, that when the stick shaker activated at about 460 feet AGL, the first officer reacted promptly by moving the thrust levers about half way forward, the still active autothrottle pulled the levers back to idle. The captain however reacted as well and took over control, the airplane was travelling at 107 KIAS and a nose up attitude of 11-12 degrees. 3 seconds after the stick shaker activation one of the pilots disengaged the autothrottle, the throttle levers still remained at their idle position.
The DSB believes, that the transfer of control to the captain interrupted the application of thrust, so that the levers were not moved and kept fully forward.
9 seconds after stick shaker activation the throttle levers were moved fully forward. The engines spooled up to their full thrust in less than 4 seconds but not before the airplane contacted ground 12 seconds after the stick shaker activated at a nose up attitude of 22 degrees . The black boxes stopped recording 15 seconds after the stick shaker activated.
The safety pilot (first officer in the observer seat) had called out the failed radar altimeter indication 2:21 minutes before the stick shaker activated. The safety pilot also called "Speed" three times after the stick shaker activation
???Also ein weiterer eindeutiger Pilotenfehler der TK Crew?
