Wednesday, March 04, 2009

Dutch Safety Board's preliminary report on the Turkish Airlines crash at Amsterdam

Dutch Safety BoardThe Dutch Safety Board has issued a preliminary report regarding the Turkish Airlines Boeing 737-800 accident at Amsterdam last month.  Turkish Airlines Flight TK 1951, which was arriving from Istanbul,  was on approach to runway 18R (AKA 'the Polderbaan') at Amsterdam's Schiphol Airport on February 25, 2009 when it crashed into a field short of the runway threshold. Four crew members and five passengers were killed in the accident. Twenty-eight of the 80 people who were injured remain hospitalized, according to the Dutch report.

The Dutch Safty Board's preliminary report states that the flight experienced no problems "until just before the approach." According to information obtained by investigators from the aircraft's cockpit voice recorder and flight data recorder, the aircraft was descending, with the auto-pilot engaged, when "an irregularity occurred" at 1950 feet.

Quoting from the English version of the Dutch Safety Board report posted on the Board's website:
At a height of 1950 feet the left radio altimeter suddenly indicated a change in altitude -- from 1950 feet to -8 feet -- and passed this onto the automatic pilot. This change had a particular impact upon the automatic throttle system which provides more or less engine power.

The radio altimeter normally measures the altitude of the plane above the ground very accurately and can start registering this from 2500 feet. As already mentioned, this radio altimeter is very significant for providing the appropriate power for an automatic landing.

A Boeing is fitted with two radio altimeters, a left one and a right one. The black box has shown that this deviation only occurred in the left radio altimeter.

The voice recorder has shown that the crew were notified that the left radio altimeter was not working correctly (via the warning signal “landing gear must go down”).

Provisional data indicates that this signal was not regarded to be a problem.

In practice, the plane responded to this sudden change as though it was at an altitude of just a few meters above the Polderbaan and engine power was reduced.

It seems that the automatic system -- with its engines at reduced power -- assumed it was in the final stages of the flight.

As a result, the aircraft lost speed.

Initially the crew did not react to the issues at hand.

As a result of the deceleration, the aircraft's speed was reduced to minimum flying speed (stalling situation) and warning signals (the steering column buzzes at an altitude of 150 metres) were given.

The black box shows that full power was then applied immediately. However, this was too late to recover the flight, the aircraft was too low and, consequently, the
Boeing crashed 1 kilometre short of the runway.

The black box -- which can register 25 hours of flying time and which, in this case, covered 8 flights -- showed that this problem had occurred twice previously in a similar situation, before landing.

The aircraft initially hit the ground with its tail and then the undercarriage followed.

The forward speed was about 175 km per hour upon impact. An aircraft of this weight should normally have a speed of 260 km per hour for landing.

The aircraft came to a rapid halt (after about 150 m) as a result of the arable land being made up of boggy clay.

The braking caused by the ground meant that the aircraft broke into two pieces; the tail broke off and the aircraft’s hull ruptured at business class.

The landing gear broke off, in accordance with its design.

This also applied to the two engines.

The full power and the sudden braking resulted in both engines continuing forwards for a further 250 meters.
The report goes on to note that the Board's investigation "will now focus fully on the workings of the radio altimeters and the connection to the automatic throttle (automatic steering system)."

Separately, the Dutch Safety Board announced that it has issued a warning to Boeing as a result of the initial findings of the Turkish Airlines accident investigation. Boeing, in turn, has given notice that a warning will be issued to all users of this type of plane to make them aware of this possible risk.

RELATED: Click here to view all posts about THY Flt 1951 on Aircrew Buzz.