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Some aircraft are equiped with auto landing to assist on zero visibility and ceiling land. Actually, how does this system work?

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    $\begingroup$ Possible duplicate of How does ILS (Instrument landing system) work? $\endgroup$ – fooot Nov 24 '15 at 17:12
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    $\begingroup$ @fooot Why is this a dupe? The question is how autoland works, not how the ILS works. The ILS is used by the autoland system, but they aren't the same thing. $\endgroup$ – Pondlife Nov 24 '15 at 17:20
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    $\begingroup$ The question isn't clear which part it's asking about, and ILS is the guidance system being used. $\endgroup$ – fooot Nov 24 '15 at 17:27
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    $\begingroup$ @fooot True, but it requires a radar altimeter too, as far as I know (which isn't much). Anyway, I think you're right that the question isn't clear but in that case I think we should ask for clarification and/or close as "unclear what you're asking" rather than closing as a dupe. That's my two cents anyway. $\endgroup$ – Pondlife Nov 24 '15 at 17:44
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    $\begingroup$ The ILS question doesn't provide details about the autoland requirements (e.g. the precision required, the redundancy). Also, the way the flare out is executed or the ground roll are interesting. I think these elements deserve a dedicated answer. $\endgroup$ – mins Nov 24 '15 at 21:13
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Autoland is an autopilot feature which allows the autopilot to control the aircraft all the way to touch-down and even a bit further.
Basic idea is that the autopilot is accurate enough to keep the aircraft on the correct approach path. This is achieved by (on top of really good software engineering) with at least 2 autopilot/flight director calculators. One of the calculators is the one in control, anfd the other one is monitoring the first one. In case of disagreement between the systems, a fault is announced and the approach must be aborted (fail-passive system).
There are also fail-active systems that require 3 autopilot channels. These are capable of continuing the approach even if one system gets faulted.
Some systems have a roll-out guidance to keep the aircraft aligned on the runway centerline after touch-down during braking.
Functionality is the same as with autopilot. The calculator gives commands to the control surfaces and thus steers the aircraft. In addition a radio altimeter gives accurate altitude information to enble flare and thrust reduction.

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