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?
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.
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.
It depends on the airframe, but in this case, I guess Airbus test pilots determined that 20 ft AGL was the best place to retard the throttles to expedite the roundout with enough energy left to do it safely.
As a general rule, commercial and business jet aircraft are aerodynamically pretty slippery and do not slow up quickly when the thrust levers are pulled to idle. This fact is compounded when the a/c is in ground effect, which is why jets are generally flown onto the runway as opposed to flow to a stall prior to touchdown. I would guess aircraft with newer generations of high bypass engines slow up faster that low-mid bypass turbofans and turbojet powered airplanes do. Some business aircraft, for example the Embraer Phenom 300, suggest the thrust levers should be pulled to idle as the aircraft passes over the threshold markers (piano keys) prior to touching down some 750-1500 feet thereafter (by ATPL standards).
A lot of guessing going on here. The reason the auto thrust calls for the reduction is simply a reminder to the pilot that it is engaged. It has nothing to do with any of the reasons sited above. These thrust levers are stationary and do not move during flight once the automatic function is engaged, hence the reminder.
