# Why is it that when autopilot is maintaining cruise, we don’t feel the changes when lift increases?

When we’re inside an aeroplane, when we’re in cruise, why is it that we don’t feel the changes in altitude, lift and the adjustments for example from the elevator on the plane as the weight is burned in cruise?

• Can you say what difference Question or exposition make between people and auto-pilots, other than that in theory, auto-pilots do things more smoothly? Commented Oct 25, 2023 at 19:48

When the airplane's weight is reduced, as fuel is burned, the pitch attitude (angle of attack) and required thrust necessary to maintain the desired speed are also reduced.

This happens very slowly and the adjustments in pitch and power are so small that the changes are essentially imperceptible.

• Hi, @RTO thanks for the answer. I read that reducing thrust was not good practise please as it means that you use the engine at ‘sub-optimal’ levels? Commented Oct 22, 2023 at 18:00
• @James What you read is incorrect, or you misunderstood what you read. Where did you read that? Commented Oct 22, 2023 at 18:28
• @James As the airplane gets lighter (fuel burn) and you are still maintaining level flight it takes less thrust/power to maintain the same airspeed.
– user22445
Commented Oct 22, 2023 at 18:30
• @James that somewhat can be true for certain engines, but it's not a general rule. In general you change several engine parameters together to keep the engine operating within its optimal range of rpm and temperature (and other factors). Commented Oct 23, 2023 at 8:57
• The only thing you feel when riding in any vehicle is the accelerations. The accelerations are caused by real (not fictitious, inertial) forces exerted on the vehicle. If the autopilot is programmed to maintain a constant altitude heading and velocity, there will be no acceleration and you will not feel anything, except for the 1G acceleration of the earth frame of reference (upwards), and momentary deviations that are corrected by the A/P to return to the desired target heading speed and altitude. Commented Oct 24, 2023 at 14:25

People can feel even small changes in acceleration if they happen quickly, but slow changes in acceleration would need to be very large to be perceived at all. While adjustments in flight parameters to deal with changing fuel load wouldn't affect acceleration, a plane in altitude-hold mode would nonetheless have a certain amount of undulating motion, and the perceptually smoothest autopilot algorithms would likely have larger variations in acceleration than an autopilot that was designed to minimize such variations.

Focusing on horizontal acceleration because it's a bit easier to reason about, imagine that one had a ball hanging on a 1m string from the ceiling of a plane that was accelerating forward with an acceleration of 0.01g. That ball would seek equilibrium point that was about 1cm away from vertical. If the plane's acceleration were to instantly drop to zero, the equilibrium point of the ball would instantaneously change by 1cm, and the ball would swing noticeably with a period of about two seconds and a peak speed of well over a centimeter per second.

If instead the plane started with an acceleration of 0.02g, but reduced its acceleration gradually over a period of a minute, the ball's initial displacement would be about twice as big as in the earlier example, but it would take a minute to reach vertical, and its maximum horizontal speed would be an order of magnitude below that of the earlier example.

The human vestibular system is a bit more complicated than a hanging ball on a string, but similar principles apply.

• Re "changes in acceleration": It goes by the name jerk... ("5. (physics, engineering) The rate of change in acceleration with respect to time.") Commented Oct 23, 2023 at 17:21
• @PeterMortensen jerk oit is, but it conveys the wrong impression to all but those who deal with it. The next derivative after that is jolt or jounce or, importantly, snap. That's important because it allows the next two derivatives after that to be crackle and pop. Really :-) . wiki2.org/en/Jerk_(physics) . || Sime of Larry Niven's ships had 'jolt meters'. Whether they meant what they should have is moot. Commented Oct 24, 2023 at 0:27
• @PeterMortensen Niven Pournelle use below. They use the term jolt correctly. In the linked passage below a ship increases acceleration from rest to 3g. Jolt alarms warn of the jolt conditions then stop as they settle at a steady 3 g. A nice account of a space battle using the well integrated assumption set their story is based on. baen.com/Chapters/1011250018/The_Mote_in_Gods_Eye.htm Commented Oct 24, 2023 at 1:18
• @RussellMcMahon: I'd not learned the term "jerk" in that usage until after I was out of college, and wanted to avoid any unfavorable impressions or associations the term might have to anyone. Commented Oct 24, 2023 at 5:17

Part of the answer is that even violent control surface action is not felt if the result is level flight. While you may have significant changes in the vertical airspeed, corresponding to significantly changing lift forces (for example in order to counteract up- or downdrafts), ideally the airplane experiences little change in vertical ground speed: It stays level in space and is not accelerated much. In fact, maintaining level flight was the very reason for the control surface action.

The same is true for the adjustments due to the weight loss from expelled fuel. Ideally, the reduced lift forces stay equal to the reduced gravitational forces so that the result is zero vertical acceleration and hence level flight (except perhaps minor, slow attitude changes). The cabin is not accelerated so that nothing is felt.