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How does the A320 (or other Airbus FBW aircraft, for that matter) decide which radio altimeter reading to use (say, for FMGC functions like autothrust), or whether to reject the radio altimeter data altogether and throw an ECAM to the pilots? It's certainly not a full voting system, considering that the A320 only has two radio altimeters, not three...

(For the curious: I ask because the flight profile from the ACA 624 mishap bears some resemblance to what would happen if the power rolled back on very short final without the crew noticing.)

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    $\begingroup$ A320 auto-thrust does not roll back depending on altitude. It only cares about airspeed. AFAICT radio altitude is only used for setting ILS sensitivity and for flare during auto-land and since ACA624 was a non-precision approach (Halifax runway 5 does not have glide-slope), it shouldn't have had any effect. $\endgroup$
    – Jan Hudec
    May 13 '15 at 8:54
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    $\begingroup$ @JanHudec -- got it, interesting that the Airbus A/T lacks a Flare mode (i.e. you have to pull the power back manually, albeit with an automatic callout for it) while the Boeing version has such a thing, even outside of autolands (you'd normally expect it to be the other way around). $\endgroup$ May 13 '15 at 22:53
  • $\begingroup$ @JanHudec -- make that an answer and I'll upvote it :) $\endgroup$ Aug 3 '17 at 2:21
  • $\begingroup$ @UnrecognizedFallingObject, please note Airbus has the flare mode in autoland(AP+Autothrust) thrust is automatically reduced irrespective of levers position. However autothrust alone in speed mode will require manual thrust reduction, otherwise after landing thrust is slightly increased automatically to maintain the approach speed. $\endgroup$
    – user40476
    Jun 7 '19 at 22:13
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First of all, RA1 is showed on the captain PFD and RA2 is showed on the F/O PFD. If one is detected as failed, the functional one is showed in both PFDs. This behavior is fixed, there is no way for the pilots to change it (instead, you can change the IR and ADR data with the relative knobs on the pedestal).

Now, two questions bring up:

  1. How the system detects a failure in one RA if you have only 2 RAs and it cannot perform voting?

In this case, the only way to detect the failure is the self-detection, i.e. the control hardware and software specific to each RA performs the sanity checks. Since you cannot vote, there is no way to externally detect which one failed if they disagree (unless you cross-check with other data like database terrain or similar, but I doubt it does that). The disagreement triggers the ECAM message "NAV RA DEGRADED".

  1. What the systems (GPWS, GLS, windshear, FWC, etc.) use if the two RAs disagree?

According to this Airbus document:

  • GPWS uses RA1
  • AP1/FD1 uses RA1
  • AP2/FD2 uses RA2

It doesn't state about other systems using RAs, probably they use only one side like GPWS and AP. At least these systems use RA information: reactive windshear, GLS autoland, TCAS, ROW, because in case of RA1+2 failure they are on the INOP SYS list. The FWC (Flight Warning Computer) is also using the RA signals to compute the flight phase (for example, to inhibit certain ECAM messages on takeoff and final). I can't find any information on FWC, it probably just disables message inhibition.

Differently it is in the A380, where you have 3 RAs, so they can vote, and all the systems use to the same value.

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