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I watched this video of a delivery of an A350.

at 14:50 there is the sequence I'm interested in: the ground radar normally counts down the distance to the ground, but shortly after the 20 feet mark, it calls the typical "retard" to pull the levers back.

Why is it like this and not like normally at 0 feet?

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  • $\begingroup$ What do you mean by "normally"? And do you mean the callout or the reduction of the thrust? $\endgroup$ – Noah Krasser Aug 12 '18 at 16:42
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    $\begingroup$ I am offended that you refer to the pilot as a typical retard. $\endgroup$ – Michael Hall Aug 7 at 14:35
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Why is it like this and not like normally at 0 feet?

Normally is not 0 feet: all Airbus (maybe with the exception of the A380, that might have a longer flare) have a "Retard" call at 20ft, not at 0ft. From the A320 Flight Crew Training Manual:

As a reminder, the ”RETARD” aural alert will sound. In flare, this aural alert will occur at 20 ft, except in the case of autoland, where it occurs at 10 ft.

enter image description here

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    $\begingroup$ Where is the answer to the question - WHY? This statement only re-enforces what the original question stated - that the Airbus flight computer calls out to retard the throttles at 20ft. $\endgroup$ – jwzumwalt Aug 12 '18 at 15:21
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    $\begingroup$ @jwzumwalt NO. The question "why is this aircraft different?" is based on the false premise that this is the only Airbus doing it, while others have it at 0. the answer is: the premise is wrong, all do it at 20. $\endgroup$ – Federico Aug 12 '18 at 15:28
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    $\begingroup$ The last sentence that encapsulates the question, "Why is it like this and not like normally at 0 feet?" $\endgroup$ – jwzumwalt Aug 12 '18 at 15:33
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    $\begingroup$ @jwzumwalt exactly. I show that normally is not 0ft $\endgroup$ – Federico Aug 12 '18 at 15:38
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    $\begingroup$ @RonJohn because that's a different question. If a question will come that will ask why in general it is like that, I will answer that different question. Also the use of feet is a different question (already asked, see aviation.stackexchange.com/q/1387/1467 aviation.stackexchange.com/q/12600/1467 aviation.stackexchange.com/q/2572/1467 aviation.stackexchange.com/q/31758/1467 aviation.stackexchange.com/q/33531/1467) $\endgroup$ – Federico Aug 13 '18 at 10:30
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I'm no expert on this aircraft but I think it's because jet engines don't respond instantly to throttle changes because combustion is a continuous process, rather than a series of individual strokes in a piston engine.

If you just suddenly reduced the fuel input, you reduce the pressure in the combustion chamber and the flame might go out. Instead you have to do it gradually and wait for the turbine to reduce speed.

These days, the throttle just tells the engine controller what power setting the pilot would like to have, and the engine controller reduces the fuel flow at a rate that the engine can handle.

So (again, at a guess) 20 feet is how long it takes the engine to reduce power.

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