15
$\begingroup$

For an aircraft to remain in level flight, it must generate lift force that is exactly equivalent to its weight. However, modern airliners lose a lot of weight when they are in cruise flight in the form of fuel burning off. Therefore, it stands to reason that to remain in level cruise while the aircraft's weight reduces, the lift the wing generates must also reduce. How do airliners accomplish this? They generally don't adjust their cruising speed, and the wing geometry itself doesn't change (purposefully), so I assume that airliners fly at reduced angle of attack to remain level?

Edit: It's been made clear in the comments that there are multiple ways this could be accomplished. I'm curious how exactly it's done by airliners today.

$\endgroup$
5
  • 9
    $\begingroup$ AoA must be reduced, which will reduce drag. The real question is whether they reduce throttle or speed up. $\endgroup$
    – StephenS
    Jul 25 at 23:40
  • 1
    $\begingroup$ Why not hold a-o-a constant at the most efficient value, and throttle back as needed to maintain altitude, accepting the resulting reduction in cruise speed? $\endgroup$ Jul 25 at 23:40
  • 3
    $\begingroup$ Not to confuse this with orbital mechanics, but they can go higher and slow down their IAS. This would keep their TAS more "on schedule" and maintain optimal AoA. $\endgroup$ Jul 26 at 0:09
  • 1
    $\begingroup$ Does this answer your question? $\endgroup$
    – Koyovis
    Jul 26 at 5:25
  • $\begingroup$ RE "and throttle back as needed to maintain altitude, accepting the resulting reduction in cruise speed?" -- or, if permission is obtained, leave the throttle setting fixed and use the excess thrust to climb-- as several answers have suggested-- since lift is nearly equal to weight regardless of whether the aircraft is climbing or flying level, and is in fact slightly LESS than weight in a climb, this method too must involve a DECREASE in airspeed, if a-o-a is left constant and a/c is not meant to follow a looping trajectory! $\endgroup$ Jul 26 at 22:40

2 Answers 2

27
$\begingroup$

Yes. As fuel is burned the airplane will want to climb, so the autopilot (which is flying the plane 99% of the time and will normally be in altitude hold mode once you're at your clearance Flight Level) will reduce pitch attitude and AOA ever so slowly as weight and lift required is reduced.

Without autothrottle, if you leave the thrust alone and are paying close attention, you will also notice the Mach# creep up slightly and may tweak the thrust back eventually to reduce it, to keep speed constant if that is necessary, taking the bonus reduction in fuel burn.

If the plane has an autothrottle system, the autopilot will be maintaining both the flight level and the airspeed. So if you are just sitting there watching it carefully over time, you should observe the pitch attitude slowly decrease just about imperceptibly, and the thrust setting also decreasing almost imperceptibly to maintain speed/mach, with the fuel burn going down.

$\endgroup$
20
$\begingroup$

It's almost always more economical to fly higher if you can. So, as an airliner burns off its fuel load, it will naturally want to climb higher, and the pilot will request clearance to do so in stages as the gross weight of the plane goes down.

If you can't get clearance for more altitude then you trim off the AoA to maintain constant altitude, and you fly a little faster.

$\endgroup$
4
  • 2
    $\begingroup$ The change is (of course) continuous, so there is constant minute necessity for trimming. Pilots rarely do this, as autopilots are controlling airliners most of the time. $\endgroup$
    – Jpe61
    Jul 26 at 22:08
  • 1
    $\begingroup$ There is an interesting nuance here that I'm sure has not been fully addressed by this answer. If a-o-a is held constant, and weight decreases, airspeed must decrease, or else the excess airspeed will cause the lift force to be greater than weight, in which case the trajectory will resemble the entry to a loop. If a-o-a is held constant, and thrust is not reduced as weight is reduced, and the aircraft is allowed to climb as a result, there will be some decrease in airspeed. To a first approximation, the decrease will be proportional to the square root of the change in wing loading. $\endgroup$ Jul 26 at 22:48
  • 1
    $\begingroup$ Only "to a first approximation", because this is ignoring the fact that lift actually is slightly less in a steady-state climb than in level flight. So the actual decrease in airspeed must be slightly greater than described above. If airspeed is held constant and airplane is allowed to climb as weight decreases and thrust is held constant, you can be sure that the angle-of-attack has decreased. $\endgroup$ Jul 26 at 22:50
  • 6
    $\begingroup$ There was an 'exception' which demonstrated the effect more naturally: Concorde. Its design cruise mode was Mach hold with altitude, and it was gradually climbing as it got lighter (or as air got colder, etc.) It was normally cleared for all levels above. $\endgroup$
    – Zeus
    Jul 27 at 1:00

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.