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Old 27th Apr 2021, 14:10
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Capn Bug Smasher
 
Join Date: Oct 2014
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You can control altitude with an auto-throttle but not, necessarily, a pitch controller.

If you use the throttles only, the aeroplane will - with a bit of lag - maintain the trimmed speed, and climb or descend according to the excess or deficiency of thrust. (Steady state climb performance is dictated by how much more - or less - thrust than gravity you have, and not the pitch angle of the aircraft or power of the elevator. Otherwise you could steadily climb quite nicely in any glider just by pulling back on the stick.)

Pitch alone, on the other hand, is inadequate because the thrust setting can still endanger the aircraft. For instance, the thrust may be to low to safely climb the aircraft without losing speed and stalling, but the aircraft pitches up anyway. Or you may have too much power and you exceed VNE as you pitch down for the descent. You still need to control throttle setting by hand, but you don't have to control pitch by hand, so long as the aircraft is trimmed to fly at the speed you want it to.

In reality you would use both pitch and power for a VNAV controller (pitch to prevent "long-period" oscillation round the climb angle and power to establish the climb rate) but if you could only use one it would have to be power.
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