Cues should alert 737 pilots to altimeter fault: Boeing

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Boeing has highlighted to 737 operators the symptoms of a malfunctioning radio altimeter, after investigations into the Turkish Airlines approach crash at Amsterdam found evidence that a faulty altimeter prompted the autothrottle to reduce thrust.

The airframer points out that such symptoms might warrant crew intervention and it is reiterating the importance of monitoring flight instruments.

Boeing says the autothrottle uses data from the left-hand altimeter which, in the Turkish jet, suddenly switched to an incorrect reading while still at nearly 2,000ft.

This reading apparently prompted the autothrottle to transition to landing-flare mode, retarding the thrust levers to the idle stop, where they remained for about 1min 40s, bleeding off the airspeed. The 737 lost altitude and the crew failed to recover the aircraft before it struck the ground.

Boeing has pointed out that an erroneous radio altimeter reading - even without a direct fault flag - will typically generate a number of possible effects on the flight deck, requiring crew action.

Apart from differences in the displayed radio altitudes, these effects include unexpected configuration warnings, premature annunciation of authrottle retard, removal of flight-director command bars, and movement of the throttle levers towards the idle position.

"Crews should be reminded to carefully monitor primary flight instruments and the flight-mode annunciation for autoflight modes," says Boeing.

It states that the Turkish aircraft and its engines responded properly to flight-control and throttle inputs, and adds that the investigation into the 25 February crash has produced no evidence of wake turbulence, windshear, icing, bird strike or fuel exhaustion.