How exactly is cruise maintained as fuel is burnt?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.