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Old 14th Jan 2018, 22:11
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Winnerhofer
 
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wfpalmer 15/01/2018 at 8:47 am
It’s not like Airbus has done nothing. Have they gone back to redesign and recertify the A320/330 flight control and airspeed indication systems? Well, no. But, they have designed and implemented a much more robust airspeed indication scheme for the A350 where data on the PFD automatically switches between sources and if all the pitot tubes are blocked will then switch to a system that displays airspeed based on data provided by the FADEC (engine cowl pressure sensors)and AOA data.

Additionally, the autopilot doesn’t just click off when the going gets tough but enters an extended protection envelope where it will attempt a recovery back to the normal flight envelope. This extended envelope includes extremes in speed, AOA, pitch (50° up!) and bank (120°). Indeed, the A350;s autopilot can remain engaged (or be reengaged) even in cases of dual engine failure, complete hydraulic failure and emergency electrical configuration!

>> For example in the Airbus design although AoA and Mach number are calculated by the ADR and transmitted to the PRIM fourteen times a second they are not directly available to aircrew.<>For example in the Airbus design the current procedure is to reach up above the Captain’s side of the overhead instrument panel, and deselect two ADRs…which ones and the criterion to choose which ones are not however detailed by the manufacturer.<<

This procedure applies when the AOA probe is stuck/frozen and the airplane inappropriately enters an alpha protection mode (i.e., it wants to pitch down to correct a high AOA). This is not an unusual airspeed recovery procedure. It doesn't matter which two ADRs are selected off, as the objective is to force the airplane out of Normal law where that protection operates. Perhaps the crew will have had some indication from prior failures which of the ADRs is most likely to blame, but for the immediate action it doesn't make any difference. The objective is to regain positive control of the airplane.
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