Turkish Airlines Flight 1951 pilots not at fault in the accident.
The final report for the accident that TC-JGE registered Turkish Airlines aircraft suffered prior to its landing while performing the TK1951 Istanbul-Amsterdam flight on Wednesday, 25 February 2009, has been released by the Dutch Safety Board (DSB).
From the published report the pilots of a Turkish Airlines plane, which crashed shortly before landing at Schiphol airport last February killing nine people, were largely not to blame for the accident.
In the report there is heavy criticism of aircraft manufacturer Boeing, which built the 737 and was aware of problems with the radio altimeters.
In their preliminary report on the crash last March, accident investigators said a faulty altitude meter had led to the engines going idle power. The plane was being flown on automatic pilot until the troubles start at about 1,950 feet.
Boeing is being sued by crash survivors and the victim's families in the US. two of those killed worked for the Boeing aerospace company.
Turkish Airlines Flight 1951 was a passenger flight which crashed near Amsterdam's Schiphol airport on 25 February 2009, killing nine passengers and crew including all three pilots.
During approach for landing a faulty radio altimeter powered down both engines of the Turkish Airlines flight before it crashed near Amsterdam airport killing nine people. When flying at about 1,950 feet the plane's left radio altimeter indicated minus 8 feet and the auto pilot assumed Boeing 737-800 had already landed, prompting the automatic pilot to power down both engines to idle.
"The crew initially did not react to these events," because the aircraft was already too fast for the approach to land and when it become too slow an alarm went off that the plane's speed would drop below the minimum, the pilots reacted and increased power both engines but it was too late and too close to the ground.
The weather was misty with low clouds and the runway was not yet visible at the height at which the descent started and the pilots were busy looking for the runways besides other duties. The airplane stalled at 150 meters and crashed 1 kilometer before the runway.
The aircraft initially hit the ground in a field with its tail followed by its undercarriage, with a forward speed of 175 km per hour on impact. An aircraft should normally have a speed of 260 km per hour for landing.
After the accident, instead of correcting the faulty radio altimeter system Boeing issued a reminder to all 737 operators to carefully monitor primary flight instruments during critical phases of flight.
The plane's black box -- which can register 25 hours of flying time and in this case had covered 8 flights-- showed the problem had occurred twice previously during landings but for a too short duration for anyone to be concerned.
Five Turks and four Americans were killed when the plane plunged into a boggy field 1 kilometer from the runways of Amsterdam's Schiphol airport. Passengers said the plane suddenly dropped to the ground during landing. When the plane hit the ground the aircraft broke into two pieces, the tail and the engines broke off.
Most of the fatally wounded were near the rupture, in business class and in the cockpit in which the three crew members died. The section that remained most intact was situated around the plane's wings.
Report examines what happened during crash at Amsterdam’s Schipol Airport in February 2009 which left nine dead. Turkish Airlines disagrees with Dutch Safety Board report; says approach stabilisation was not a factor in the causation of the accident. Says that even though the crew promptly reacted to stall warning, the authothrottle kicked back unexpectedly. The second attempt by the crew, after disengaging the autothrottle, to advance thrust levers was successful, but too late.
The purpose in these types of reports is to identify how the event has occurred and to determine what can be done to prevent it from happening again. Turkish Airlines has reviewed the report and, in respect for the public interest in this accident, provides the following comments on the report:
“The aircraft had two radio altimeter systems to measure its vertical distance from the ground. During the approach of TC-JGE aircraft into Amsterdam Schiphol Airport’s runway 18R, the right-hand radio altimeter system used by the First Officer, who was flying the aircraft, was performing normally, while the left-hand system showed an incorrect altitude of -8ft.
The aircraft was designed so that the autothrottle system, which is the part of autoflight system that controls the speed of aircraft by altering engine power, receives the data on ground proximity only from the left-hand radio altimeter system without any comparison with the right-hand system's data, and the autothrottle system accepted the left-hand system's incorrect altitude information as an indication that the aircraft was at the touchdown stage of landing.
The autothrottle system reduced the engine power, causing the aircraft speed to go below the minimum speed needed to keep the aircraft controllable in the air (the stall speed), and the resulting loss of control of the aircraft occurred at an altitude too close to the ground for recovery.
This error-prone relationship between the autothrottle and left-hand radio altimeter system had not been previously explained in the aircraft manufacturer’s documentation for the flight crews; this was described in an update only after this accident.
Because Amsterdam Air Traffic Control brought the aircraft in with a shorter-than-normal approach route, the aircraft was flying higher and faster than usual. Due to this attitude and speed, it was to be expected that the thrust levers would go to the idle position to reduce engine power as the aircraft was slowing down and descending to reach normal approach speeds and altitudes. This appropriate expectation prevented the system error from being realised in time.
As the speed approached the required minimum controllable speed, aural and visual warning systems were activated, and the crew immediately initiated steps to recover control of the aircraft. But the prompt advancement of the thrust levers by the flight crew was initially counteracted by the autothrottle system, which retarded the thrust levers again.
The crew disengaged the autothrottle and advanced the thrust levers again, but the loss of time due to this system behaviour in close proximity to the ground resulted in the recovery efforts being unsuccessful.
The aircraft manufacturer’s documents had previously advised the pilots of the need to advance the thrust levers immediately, but the documents had not mentioned the need to disconnect the autothrottle system during such a recovery.
Listed below are some of the aspects included in the report:
1) THY Teknik AŞ (Turkish Technic) had identified incorrect radio altimeter outputs in the THY B737-800 fleet long before the accident, and it had conducted studies regarding these outputs. In numerous communications, it brought these incorrect outputs to the attention of the aircraft manufacturer and the radio altimeter antenna manufacturer. Neither took sufficient steps to address these outputs.
These incorrect outputs were also raised by Turkish Technic with the aircraft manufacturer via its official internet portal, which lead numerous other B737-800 operators to share online that they were having similar radio altimeter problems, but the topic was removed from the forum without the problems having been resolved.
2) The report states that the aircraft manufacturer had failed to see these incorrect altimeter outputs, which were observed in numerous B737-800 aircraft, as a serious flight safety problem. But it is a remarkable fact that the B737-800 aircraft manufactured after 2006 were equipped with a system that does compare the left-hand and right-hand radio altimeter data and informs the flight crew when there is a difference between the two.
The report indicates that the aircraft manufacturer, as well as the American Federal Aviation Administration, which provided the certification for the relevant aircraft, failed to attach proper significance to addressing the design problem that led to the incorrect altimeter outputs on earlier B737-800 aircraft such as the accident aircraft.
3) According to the Dutch Air Traffic regulations, an aircraft performing an instrument approach to Amsterdam Schiphol’s runway 18R should be aligned with the runway extension at least eight nautical miles before the runway threshold at a height of 2000ft. Despite this, to increase the flow of traffic, air traffic controllers often use their discretion to bring the aircraft into alignment as close as five nautical miles.
The report states that this practice requires an aircraft to fly higher and faster than would be required for a properly aligned approach and increases the workload of the flight crew in the critical final approach phase. The report therefore evaluates this practice of the air traffic controllers as unsafe.
4) This air traffic control practice caused the aircraft to end up with more energy than required for its location in relationship to the runway threshold, and the autothrottle system kept the thrust levers in the idle position to reduce the airspeed to the required final approach speed.
As the approach speed properly selected by the pilot on the automatic flight system was reached, the incorrect radio altimeter value prevented the autothrottle system from advancing the thrust levers to maintain that speed, and the speed began to decrease even more rapidly.
At an altitude of approximately 460ft, the aural and visual warning systems gave warning that the aircraft was entering an uncontrollable state (i.e., stall), and the flight crew promptly reacted and initiated the recovery steps. Thrust levers were advanced, and the control column was operated to recover control. But the aircraft was designed so that the autothrottle system would continue to use the incorrect radio altimeter data and would retard the thrust levers back to idle.
The flight crew, realising this, deactivated the autothrottle system and advanced the thrust levers again. The report notes that the aircraft manufacturer’s emergency procedures documentation did not include an item that calls for deactivation of the autothrottle during a stall recovery manoeuvre.
5) Simulator tests carried out after this event show that an altitude of at least 500ft is required for B737-800 aircraft to recover from stall. According to the report, stall warning was received at an altitude of about 460ft and, as a result of the above evaluation, stall was unrecoverable.
In light of the information revealed in the accident investigation, Turkish Airlines have informed all its B737-800 pilots how the aircraft's error-prone design can result in the autothrottle using incorrect radio altimeter data. Turkish Airlines have also trained these pilots in the simulator in stall-recovery techniques at low altitudes.
In the report released by the DSB there are also certain aspects Turkish Airlines disagree with, principal two of which are listed below:
a) Approach stabilisation is not a factor in the causation of this accident.
b) It is claimed by the report that the crew could have recovered the aircraft after the stall warning was received. However, even though the crew promptly reacted, authothrottle kicked back unexpectedly. The second attempt by the crew, after disengaging the autothrottle, to advance thrust levers was successful but too late.
“After the accident, four similar incidents were brought to the attention of the DSB,” the report said, noting that in each case the aircraft was landed without further incident after the crew disengaged the autothrottle.
Moreover, the report said, “Radio altimeter system problems within the Boeing 737-800 fleet had existed for many years.” For example, Turkish Airlines had complained to Boeing about fluctuating and negative height measurements that caused landing gear warnings, autopilot disconnects and ground-proximity warning system warnings. “Turkish Airlines and other operators dealt with the problems as a technical problem and not as a safety problem,” the report said. “As a result, the pilots were not informed of this issue.”
Suspecting that corrosion was causing the problems, Turkish Airlines installed gaskets between the radio altimeter antennas and the fuselage skin, and wrapped the connectors to block moisture. But this did not eliminate the problems. The greatest success was achieved by replacing the antennas, but tests of some of the removed antennas did not reveal why the problems had occurred. “It is almost impossible to take the correct measures if the cause of the fault cannot be identified,” the report said.
Boeing in 2004 added a warning in the 737-800 dispatch deviation guide that an autopilot or autothrottle must not be used during approach and landing if its associated radio altimeter is found to be inoperative before the flight begins.
However, the report noted that the aircraft’s quick reference handbook and flight crew operating manual do not contain similar guidance for a radio altimeter malfunction that occurs during flight.
Investigators also found that the ATC handling that resulted in the accident aircraft’s interception of the localizer course high and close to the runway, without prior consultation with and approval by the crew, was not an isolated event but was characteristic of more than 50 percent of the approaches to Runway 18R at Schiphol.
Based on the findings of the investigation, the DSB recommended improvement of the reliability of the 737-800 radio altimeter system, evaluation of the benefits of installing an aural low-speed warning in the aircraft, and monitoring to ensure that air traffic controllers in the Netherlands adhere to ICAO and national standards for lining up aircraft for approach.
The full report is available at http://www.onderzoeksraad.nl/docs/rapporten/Rapport_TA_ENG_web.pdf