EASA cautions 737 operators of radio altimeter errors

Washington DC
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EASA has published a safety information bulletin alerting Boeing 737 operators of an "erroneous low range radio altimeter (LRRA) indication" that has been linked to the fatal crash of a Turkish Airlines Boeing 737-800 at Amsterdam Schiphol on 25 February. The accident killed nine of the 134 on board, including three pilots in the cockpit.

Central to the investigation is a radio-altimeter fault that caused the aircraft's autothrottle to enter retard mode at too high an altitude, reducing thrust to idle speed before the aircraft was in position for its final flare above the runway.

Pilots lost control of the aircraft after speeds decreased to 110kt (204km/h) at approximately 500ft (152m) above the ground due in part to the fault. The system is designed to automatically reduce thrust to idle when the aircraft enters its landing flare approximately 27ft (8m) above the ground.

Dutch investigators revealed on 29 April that several radio altimeter failures had occurred on the accident aircraft in its previous eight flights. EASA notes that "there are reports of further incidents attributed to the same cause".

In its 30 April alert, which references a flight operations technical bulletin published by Boeing, EASA says if one of an aircraft's two LRRAs provides erroneous altitude readings, the associated "flight deck effects" may typically include "inappropriate flight mode annunciation indication of autothrottle retard mode during approach phase with the airplane above 27ft above-ground-level."

The agency is recommending that flight crews, whether operating in automated or manual flight modes, "carefully monitor" primary flight instruments including airspeed and attitude, for aircraft performance and the flight mode annunciation for autoflight modes.

"When the autothrottle mode is selected during critical phases of flight, the pilot flying may consider to keep a hands-on position on the engine throttles to guard against and correct any abnormal behaviour," EASA continues, adding that, "Early intervention prevents unsatisfactory airplane performance or a degraded flight path."