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Can someone please explain how exactly cruise (I think it’s called straight, level and un-accelerated flight) is maintained for like an airliner please?

I’ve read a few answers on this website for questions in the same ballpark, but none seem to answer my question and are too complicated.

For example: Do airliners reduce their angle of attack to maintain lift = weight as fuel burns off during cruise?

That Question says: reduce pitch attitude and AOA ever so slowly as weight and lift required is reduced

But how is aoa and pitch changed?

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    $\begingroup$ Your confusion isn't clear, are you asking about how trim, or an autopilot works, or how throttles might be reduced to maintain a given airspeed as the plane gets lighter? $\endgroup$ Oct 19, 2023 at 20:13
  • $\begingroup$ Hi, so my question is referring to all of those bits (autopilot, trim,…,)- I dont know what they exactly do to maintain cruise (so the cruise altitude, speed so the plane maintains cruise). To be fair, I don’t even know what trim is and whether it maintains cruise $\endgroup$
    – James
    Oct 19, 2023 at 20:48
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    $\begingroup$ I’m voting to close this question because it sounds like you're looking for a short tutorial on how flight works. That's a bit broad, though some valiant efforts have been made to address this. $\endgroup$
    – FreeMan
    Oct 20, 2023 at 18:03

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AS the plane burns fuel, it gets lighter, lift exceeds weight, and the plane climbs a little. The autopilot notices the climb, and trims the plane a little nose-down to decrease the AoA and hence the lift. The autopilot thus forces the plane to maintain a constant altitude even though its weight is continuously decreasing during cruise.

The pilot does this manually if there's no autopilot, trimming away lift as the plane gets lighter and lighter.

IN practice, that plane will always get better range the higher it flies, so instead of setting the altitude hold feature in the autopilot, the pilot instead asks ATC for a clearance to higher and higher altitudes as more and more fuel gets burned off during its cruise phase of flight.

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  • $\begingroup$ Hi, how exactly does trim work please? And how would the pilot do this manually? That’s sort of where my confusion is please? $\endgroup$
    – James
    Oct 20, 2023 at 12:00
  • $\begingroup$ @James Trim is a way to set where the elevator (or other control surface) will sit if you aren't pulling on the stick (or pushing on the pedals). So rather than needing to pull hard on the stick for the whole flight you can set the trim and then make only small adjustments. $\endgroup$
    – Chris
    Oct 20, 2023 at 16:13
  • $\begingroup$ Hi James, thanks for that. I’m gonna read more into trim- it seems like a complicated topic $\endgroup$
    – James
    Oct 20, 2023 at 16:29
  • $\begingroup$ Oops, sorry I meant hi Chris not James $\endgroup$
    – James
    Oct 20, 2023 at 17:55
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Well, ”exactly” is a bit broad concept, but the basic idea is rather simple. Four main forces affect the aircraft in flight. Gravity has to be compensated by lift and drag by engine thrust. When the forces are in balance, straight and level flight is maintained.

The heavier the plane (more fuel), stronger the gravity, so more lift is needed. But increasing the lift also increases drag, thus engine thrust must be increased.

The additional lift required is achieved by pitching up slightly, but in practice the difference is only a fraction of a degree.

So, when the aircraft burns fuel it gets lighter, pitches down, drag decreases and engine thrust is reduced. But the changes in are so minor that any atmospheric disturbance causes much larger corrections so it is very difficult to notice the difference in short term.

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An airplane in level flight can begin to climb by raising its elevator. The air hitting the elevator causes the nose of the plane to rise, presenting more of its wing to oncoming air. This increases its lift and establishes a climb, and also causes its speed to slow.

Similarly, an airplane in level flight can descend by lowering its elevator. This also increases the speed.

As an airplane begins to burn its fuel, it begins to climb. The pilot (or autopilot) can counter this tendency by lowering the elevator a little bit. This will cause the speed to increase slightly, which can be countered by lowering the throttles a bit.

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  • $\begingroup$ Hi, so I looked online, and it says flaps aren’t really used in cruise? Is that correct? If it’s not, so does that mean that the pilot just adjusts the flap to maintain cruise? $\endgroup$
    – James
    Oct 20, 2023 at 12:02
  • $\begingroup$ @James The flaps are not usually used in cruise, no. Flaps are parts of the wing that can be deployed to increase lift and drag, but not at cruise speeds. The elevator is a part of the tail which moves up and down to cause the nose to go up or down. $\endgroup$
    – Chris
    Oct 20, 2023 at 14:42
  • $\begingroup$ Hello Chris, thank you for clarifying that. So the elevator is used only in cruise? Is it used at all in take offf and land please and, how is it that it can be used safely whilst going so fast in cruise please? $\endgroup$
    – James
    Oct 20, 2023 at 14:49
  • $\begingroup$ @James The elevator is used in all phases of flight. As the airspeed increases, the elevator is changed by a smaller amount for the same effect. Beyond a certain speed, too large of a deflection of the elevator can have catastrophic effects, so only small deflections are made at cruise speed. $\endgroup$
    – Chris
    Oct 20, 2023 at 16:11
  • $\begingroup$ Hi Chris, sorry I’m still a little confused- so why are there the flaps please then on the back end of the wings please? Are they only used in take off and landing phase please? And elevator is universal? $\endgroup$
    – James
    Oct 20, 2023 at 16:27

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