The European Aviation Safety
Agency recently published an airworthiness directive (AD) for the flight manuals
of the Airbus A320, A330 and A340 series to address an undesirable nose-down
movement that could occur should an angle-of-attack (AOA) probe on the aircraft
become blocked. The AD was issued as the result of a benign occurrence reported
on an A321. The FAA followed up with emergency AD
2014-25-52. “When the alpha protection mode is activated due to blocked AOA
probes, the flight control laws order a continuous nose-down pitch that, in a
worst-case scenario, cannot be stopped with backward sidestick inputs, even in
the full aft position,” stated EASA. The Airbus-developed interim actions that
became effective December 11 begin with keeping one air data reference on, while
turning off the other two. However, there is still a risk of erroneous stall
warnings appearing on the speed display, EASA noted. For the A330/A340 models,
on which there may still be a false stall warning, Airbus suggests “using the
flight path vector” in the recovery process.
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