Re: Aircraft speed and weight at cruise.

Date:         24 Oct 97 04:33:14 
From:         David Guyer <>
Organization: Boeing Commercial Airplanes Group
References:   1
Next article
View raw article
  or MIME structure

jf mezei wrote:
> -A wing generates a lift value which is proportional to the air speed.
> -The force of the lift should always equal the weight of the aircraft
>  otherwise the plane would either climb or drop.
> -The weight of an aircraft changes as fuel is exhausted during cruise.
> Assuming a LAX-SYD flight with no winds. Towards the end of the flights
> when the plane is much lighter, doesn't the lift provided by the wings
> exceed by quite a bit the lighter weight of the aircraft ?

If no adjustments during the flight were made, this would be true, but
minor adjustments are constantly being made.

> Is this change so trivial that it is not an issue, or do pilots take
> this into consideration (reducing speed to reduce lift or what ?) ????
> If speed is adjusted to match the lift with the weight of the aircraft,
> how does this affect airline schedules  where cargo loads may influence
> the time it takes for the airctaft to get to destination ?
> Would a fully loaded plane not travel faster then a same plane but
> lightly loaded ?

Typically, a plane has both an autopilot and an autothrottle, but even
without these pilots would be making the same adjustments.  During the
flight, as fuel is exhausted, and the aircraft becomes lighter, the
throttles are reduced to maintain a constant airspeed (Mach number).
Since the same airspeed is maintained, if the wing's angle of attack is
not changed, there would be excess lift, but the standard practice of
maintaining altitude, naturally causes a reduction in angle of attack,
which maintains the lift=weight balance.