Straight and level

[goldox]

Hopefully not a silly question...

If an aircraft is flying straight and level, in unchanging conditions (ie wind, temp, pressure etc. remain constant).

As fuel is burnt, and the aircraft becomes lighter and lighter, would it tend to climb? I'm not 100% certain why I think it might but if throttle setting was the same, as it powers less mass, I think the tendency would be to climb? And assuming autopilot is on, maybe that deals with this anyway?

Just wondering...

[Discorde]

As the weight reduces the pilot (or autopilot) selects a progressively lower pitch attitude, thus reducing lift generated to match current weight. The new attitude will result in an increase in airspeed unless engine thrust is correspondingly reduced.

Pitch attitude (nose high or level with the horizon or low) is used to control either vertical flight path or airspeed, depending on phase of flight.

Further info available from 'How Do We Fly The Plane' (Kindle edition) or 'How Airliners Fly' (paperback or Kindle edition). The aerodynamics are identical for both small and large aircraft.

[EW73]

Also as the airplane progressively gets lighter with fuel usage, the economical cruise speed gets slower and slower, so as the weight decreases (as does the speed), the performance capabilities will allow a climb to a higher altitude, where the cycle starts over. The initial speed at the new altitude is again slightly faster, slowing as the weight continues to decrease, so the airplanes slowest econ cruise speed for the flight is just before top-of-descent.

Cheers

[goldox]

Thanx you both for the informative replies. It's very interesting and shows how even 'straightforward' flying needs careful consideration and tweaking!