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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). $\endgroup$ – UnrecognizedFallingObject May 13 '15 at 22:53
  • $\begingroup$ @JanHudec -- make that an answer and I'll upvote it :) $\endgroup$ – UnrecognizedFallingObject 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 at 22:13
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  1. The system never ‘votes’ on anything. Which input to use is decided by manual configuration. Even for the triply redundant systems (airspeed, baro-altitude, attitude) the system will only degrade and show appropriate ECAM warning and the pilots must verify which system is fauly and disconnect it.

    So this is no different for radio altimeter. Only because there is just two of them, it is even more important to cross-reference the other systems to identify which is faulty.

  2. The radio altimeter does not affect auto-thrust. AFAIK, it is only used for:

    • altitude callouts,
    • scaling autopilot response to ILS deviation and
    • flare during auto-land.

    For thrust, the only feature is that if thrust levers are not in idle by 20 ft AGL, it will call out the famous “retard, retard” instead of altitude to remind the pilots of that condition.

    And since Halifax runway 5 has only localiser, but no glideslope, ACA 624 was not doing ILS approach, so radio altimeter only affected the callouts.

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