What are the chances of autopilot failing to do a flare in an otherwise stable autopilot controlled approach and landing? Has it happened before and how was it dealt with?
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16$\begingroup$ Form aircraft technical log by captain: "Defect: autoland fails to flare." - Response by mechanic: "Action: this plane does not have autoland capability." $\endgroup$– Jpe61Feb 11, 2022 at 6:41
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$\begingroup$ This deserves the answer: "You find out if the pilot is paying attention or not." $\endgroup$– JoshuaFeb 13, 2022 at 20:18
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3 Answers
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In the B767 and B757 the autopilot (autoland) system (triple redundancy) monitors required system components throughout the approach. The flare mode is displayed at 1500 ft and at 45 ft it activates. At any point should the autoland system components (continuously being monitored by the autopilot systems) fail an autoland status annunciator (ASA) and the display directly in view of the crew will alert them of the failure. Based on the weather/visual references they will take the appropriate action (manually land, missed approach, etc.).
I think it is highly unlikely that failure of the flare mode would go unnoticed considering the triple redundancy autopilot system and continuously monitored individual system components (including flight crew active monitoring in accordance with procedural SOP's).
It's a bit more complex than my general description above, but it is pretty straightforward.
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2$\begingroup$ Question: by the time they realize it didn't activate at 45 ft, they will be already at 30 ft, going down at 3 degrees. Isn't that too late to do anything, let alone go through the go around procedure? (Maybe I should make this a separate question). $\endgroup$ Feb 11, 2022 at 12:58
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2$\begingroup$ This probably assumes the radar altimeter is reading correctly, whatever the local 5G band is doing. $\endgroup$ Feb 11, 2022 at 13:46
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1$\begingroup$ @LatestGlitch: If the flare mode did not activate even at 45 ft. the crew could initiate a go around or disconnect the A/P and land manually. It would require immediate action but the crew is monitoring the system very closely. But, again, it's unlikely that with 3 A/P's monitoring all system components (showing the system is fully functional) that a failure would occur at that precise moment. Anything is possible but the reliability calculations show a failure scenario like you are asking about is unlikely. $\endgroup$– user22445Feb 11, 2022 at 15:15
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5$\begingroup$ @user_1818839: I have no idea how the altimeter is implemented but I’d very much hope that simple electromagnetic interference (assuming that 5G even causes interference despite 220MHz of separation) is not enough to cause an incorrect reading. In any case, they are doing plenty of testing now just to make sure … $\endgroup$– MichaelFeb 11, 2022 at 18:31
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$\begingroup$ @Michael It's radar. Yes, interference could definitely cause it to go inop or an incorrect reading, as is the case with any radar. Radar reflections are typically extremely low magnitude. Enough interference will make any radio go inop, for that matter, though some are much more interference-tolerant than others. It's especially trivial to cause erroneous indications with ILS. For radio altimeters, the principle of operation is Frequency-Modulated Continuous-Wave Radar. $\endgroup$– reirabFeb 12, 2022 at 22:30
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In Airbuses, the autopilot always performs an autoland after an ILS approach, unless the autopilot is disconnected by the pilots before landing. At 400 ft the autopilot modes freeze (indicated by "LAND" on FMA) and autoland can only be prevented by disconnecting autopilot or performing a missed approach. The "FLARE" mode activates by 30 ft. Should the "FLARE" mode not activate for any reason, pilots would perform a go-around.
My technical knowledge is not deep enough to tell how it would be possible that "FLARE" mode would not activate after "LAND", except if there would be some critical system failures, like dual autopilot fail or failure of the radio altimeter. In that case, it would also be indicated by a bright red AUTOLAND failure light.
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Autopilot is not an autonomous flight control system. In any case in which the autopilot is flying the airplane, there is officially still at least one human pilot monitoring the flight. Unfortunately, that doesn't guarantee anything.
Without considering exactly how improbable it is to ever happen, should such an event ever occur, the likely result is an extremely hard landing, or even a crash and a very angry letter going to the factory.
I don't recall ever hearing of specifically this happening, but for instance the crash of Turkish Airlines Flight 1951 on the 25th of February 2009 in the Netherlands, involved malfunctioning of automated flight control systems during approach. In that case, the autopilot acted too early rather than too late, as a response to a defective ground proximity sensor. That case also proved, that having a full crew awake and present, doesn't stop crucial indications of going unnoticed or noticed too late.