Faulty Altimeter factor in Turkish Airlines crash

Chairman of the investigation Pieter van Vollenhoven today announced that the Boeing 737-800 was flying with a faulty altimeter reading. This in turn caused the automatic pilot, on which the aircraft was operating, to reduce thrust and caused the aircraft to stall. The crew was unable to correct the effect of this stall in time to prevent the collision. The accident ended in 9 fatalities (5 passangers and 4 crew) with 80 passengers wounded.

The aircraft was on the way to Schiphol from Istanbul and was expected to land at 10.40 local time. The aircraft had experienced a normal flight before the final approach. In the cockpit was the Captain, the 1st Officer under training supervision and a second 1st Officer who was in the jump seat. The crew reported to Schiphol approach at 10.04 and were transferred to the tower frequency at 10.14. The tower then gave instruction for the approach to RWY 18R (The Polderbaan).

Part of the approach to RWY 18R included the descent to 2000ft. This descent was undertaken with help of the automatic pilot.

At an altitude of 1950ft the left altimeter was giving a reading of -8ft. This faulty reading caused the automatic pilot to decrease thrust. The crew were warned of the problem with a "engage landing gear" voice warning from the aircraft. From the investigation it was found that no reaction was given to this warning. At this point the autopilot disengaged thrust and the aircraft began to lose speed and then eventually stall. At the stalling point a warning (vibration of the joystick) was given and the crew reacted by increasing thrust.

Unfortunately the increase in power was too late and the aircraft's tail hit the ground at a distance of 1km from the runway threshold. From the Blackbox data, which can hold up to 25 hours of data, 8 previous flights were recorded. In 2 previous flights the exact same problem occurred during the landing phase. The speed of the aircraft before collision was 95kts. The landing speed of such an aircraft in this configuration should have been 140kts.

The investigation will now concentrate on the functioning of the altimeters in conjunction with the automatic landing system. The weather conditions at the time, a low cloud base and mist meant that during the beginning of the descent the runway was not visible from the cockpit. The Dutch Safety Board was assisted in this investigation by amongst others: The NTSB, BEA (Bureau d'Enquetes et d'Analyse), AAIB (UK), DGCA (Turkey), Boeing, Turkish Airlines, FAA, IVW (Inspectie Verkeer en Waterstaat-NL).

Reference /A/ provides Boeing's previous fleet communication on the subject event. The US NTSB, FAA, Boeing, the Turkish DGCA, the operator, the UK AAIB, and the French BEA continue to actively support the Dutch Safety Board's (DSB) investigation of this accident. The DSB has released a statement on the progress of the investigation and has approved the release of the following information.

While the complex investigation is just beginning, certain facts have emerged from work completed thus far:
- To date, no evidence has been found of bird strike, engine or airframe icing, wake turbulence or windshear.
- There was adequate fuel on board the airplane during the entire flight. - Both engines responded normally to throttle inputs during the entire flight.
- The airplane responded normally to flight control inputs throughout the flight.

The Digital Flight Data Recorder (DFDR) data indicates that the crew was using autopilot B and the autothrottle for an ILS (Instrument Landing System) approach to runway 18R at Amsterdam Schiphol airport. During the approach, the right Low Range Radio Altimeter (LRRA) was providing accurate data and the left LRRA was providing an erroneous reading of -7 to -8 feet. When descending through approximately 2000 feet the autothrottle, which uses the left radio altimeter data, transitioned to landing flare mode and retarded the throttles to the idle stop. The throttles remained at the idle stop for approximately 100 seconds during which time the airspeed decreased to approximately 40 knots below the selected approach speed.

The two LRRA systems provide height above ground readings to several aircraft systems including the instrument displays, autothrottle, autopilots and configuration/ground proximity warning. If one LRRA provides erroneous altitude readings, typical flight deck effects, which require flight crew intervention whether or not accompanied by an LRRA fault flag, include: - Large differences between displayed radio altitudes, including radio altitude readings of -8 feet in flight. - Inability to engage both autopilots in dual channel APP (Approach) mode - Unexpected removal of the Flight Director Command Bars during approach - Unexpected Configuration Warnings during approach, go-around and initial climb after takeoff - Premature FMA (Flight Mode Annunciation) indicating autothrottle RETARD mode during approach phase with the airplane above 27 feet AGL.

There will also be corresponding throttle movement towards the idle stop. Additionally, the FMA will continue to indicate RETARD after the throttles have reached the idle stop Boeing Recommended Action - Boeing recommends operators inform flight crews of the above investigation details and the DSB interim report when it is released. In addition, crews should be reminded to carefully monitor primary flight instruments (airspeed, attitude etc.) and the FMA for autoflight modes.

More information can be found in the Boeing 737 Flight Crew Training Manual and Flight Crew Operations Manual. Operators who experience any of the flight deck effects described above should consult the troubleshooting instructions contained in the 737 Airplane Maintenance Manual. Further, 737-NG operators may wish to review 737NG-FTD-34-09001 which provides information specific for the 737-NG installation. Initial investigations suggest that a similar sequence of events and flight deck indications are theoretically possible on the 737-100/-200/-300/-400/-500. Consequently the above recommendations also apply to earlier 737 models.