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.

Posted by B. N. Sullivan at 14:20
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3 comments:

  1. The wingspan of the Boeing 737/800 is 117.5 feet. Even if one wing points to the ground and the other points to the sky the difference between the altitude recordings from the two radio altimeters cannot exceed 117.5 feet, let alone 1942 feet. It is shocking that the automatic throttle system ("which provides more or less engine power") bases its decision on data that openly violates the physical dimensions of the plane. Apparently there is not a single line of code in the Boeing control system that checks for consistency between the two radio altimeters before feeding the left altimeter reading to the throttle system.

    Wake turbulence was being considered as one of the possible causes for the sudden decrease in altitude (http://www.reuters.com/article/latestCrisis/idUSLR365468): "A Dutch Safety Board spokesman said wake turbulence was one of the options its investigators were looking at. "We look at all options and we certainly are not excluding this one," spokesman Fred Sanders said."”

    Dutch Safety Board did not communicate in their preliminary report their conclusions regarding this potential cause.
    "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).""

    In other words, the Dutch Safety Board have kept to themselves their findings, if any, from the investigation of wake turbulence as a cause of the crash. It is not clear why the Board would not publish the landing schedule data for the flight immediately preceding the Turkish flight and how much time was allowed between the two landings on the same runway. Regardless of whether one of the altimeters malfunctioned, wake turbulence may have aggravated the situation. Hopefully the full report on this issue will contain this information.

    The question is, does Schiphol space the landing times too closely? If so, other flights may also be in danger.

    The Board states in its report: “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.”

    The Board seems to suggest that the crew, who had access to both the left and the right altimeter readings, should have realized that the autopilot was driven by the altitude from the left altimeter readings only, when the system issued the warning signal "landing gear must go down". And then, they should also have realized that the automatic throttle system was driven by the left altimeter reading only.

    The tone of the report is not neutral toward the pilots, who lost their lives within the Board’s jurisdiction, while it does not address any issue that may implicate responsibility for Ducth authorities, such as whether the Turkish flight was authorized to land too soon after a bigh airliner.
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  2. N. Nielsen F/E B747 Ret.Mar 14, 2009 01:52 AM
    It is of course nice to know that the wingspan of the B 737/800 is 117.5 feet, but in this case totally irrelevant. The Radio Altimeter antennas are located along the aircraft centerline in the bottom of the fuselage. Whatever the bank angle, they would indicate the same altitude if working correctly. Furthermore, they would not indicate anything if the aircraft was in a 90 degrees bank as the R/A beams would be directed along the earths surface into thin air. That is, unless the aircraft is flying along a mountain wall. But if this situation should ever occur the crew would have failed to peform their duties miserably.

    The anonymous writer wonders why there is no consistency check between the two R/A's before the signal is fed to the Auto Throttle system. All such a check could be used for would be to disconnect the Auto Throttle in case of a discrepancy as the system would not be able to tell which R/A was erroneous. Also, that a R/A fails without a "Flag" warning is extremely rare. But it is exactly in such a case that a well trained crew becomes of utmost importance.

    When anything unexpected happens, like in this case: The throttles retard to idle, the engines spool down (shurely confirmed by the engine instruments and noise level), The Ground Proximity Warning System calls for "Gear Down", the Airspeed starts dropping while the Auto Pilot keeps increasing the pitch attitude in an attempt to maintain the ILS Glidepath. The crew should (must) disconnect Auto Pilot and Auto throttle and fly the airplane manually. If in doubt about the status of the airplane, carry out a missed approach and sort things out at a safe altitude. In this case the crew, in a time span of 1 minute and 40 seconds let, the airspeed drop to about 95 Kts and waited until the stick shaker came on before applying power, unfortunately too late. Why none of the two pilots at the controls took over manually a lot sooner and flew the airplane as they shurely have been tought to, we will never know, since, until now, none of the "Black Boxes" can record the thoughts of the pilots.

    Just to put things straight. According to the Boeing "Warning", which is issued on the basis of the DSB report, there was no indication of Bird Strike, Engine or Airframe Icing, Wake Turbulence or Wind Shear.

    The writer wonders if Schiphol space landing flights too closely. The spacing between landing and starting airplanes is regulated by ICAO according to airplane weight classifications. The controllers at Schiphol, as at any other proper airport, of course adhere to these rules.

    The writer feels that the DSB report is not neutral towards the crew. But all it does is to present the facts discovered so far. Whether it is neutral or not is in the eyes of the beholder.
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  3. This crash took place in Feb '09 so my comments are quite late (this being Sept "10). However I would like to note the following:-
    ATC vectored the 'plane unto the LOC too close the the airport.
    ATC did not clear the crew for the approach until the aircraft was ABOVE the GS. This is unacceptable
    NO aircraft is capable of capturing the GS from ABOVE it.
    The warning to the crew that the Radio Altimeter is inop does NOT tell the crew that the Auto Thrust is no longer working.
    The airline industry is full of nonsense. The manufactures, airlines and government agencies are not providing the necessary support to the pilots to do their jobs effectively. I know as I recently retired after 32 years in that ungrateful industry.
    The crew should have responded sooner to the reduced speed.
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