Re: Fuel Weight versus Initial Cruise Altitude

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         14 Oct 94 02:23:34 
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In article <airliners.1994.1618@ohare.Chicago.COM> (Larry Massaro) writes:
>On a couple of transcon flights, I have noticed that that pilot has
>sometimes spoken about leveling off at an intermediate FL to burn
>off fuel.  Is it really more efficient to burn fuel first at lower
>cruise altitudes then climb to a final?  Does anyone have any
>data/equations etc. which indicate this is true.  My first impression
>would be that winds aloft would be the more probable reason 
>for flying at the lower "less eficient" altutude initially.

There are two considerations.

The heavier you are, the higher the stall speed gets.  Therefore, the heavier
you get, the faster you have to go.  The faster you go, the closer you'll get
to the airspeed limitations of the airplane.  At a certain point, the 
weight vs. speed curves define the maximum altitude of the airplane.

The second primary consideration is efficiency.  Saturn V didn't haul all
its stages to the Moon, and neither does a 747 haul all its fuel to cruise
altitude.  The less fuel to haul, the more efficient the climb will be, 
and the more fuel you will save.  All this translates into operating econ-

Most airliners use a climb to some "initial" altitude.  Depending on ATC
considerations and the length of the flight (which defines the optimal 
altitude), the airplane could have several more "steps," which should 
be pretty much imperceptible to the passengers.  Crews attempt to fly to 
as optimal a flight altitude as possible.  It is a common practice.

What is bad is when ATC constraints keep an airplane to a low altitude for
an unusual period of time.  Jockeying for altitude has ramifications on 
range, ground speed, and overall operating economies and passenger 

To give an example, here are some values for the 737-200, with JT8D-9
engines.  FL = flight level, = altitude / 1000.  Schedule is the speed
schedule, in Mach.  This is a high-speed schedule for the 737, and less
efficient.  Optimal weight is the computed economical weight for a specific
configuration.  Limit weight is the cruise thrust limit weight at that al-
titude, for ISA+0.

FL    Schedule    Optimal weight    Limit weight
370    0.78        75,000 lbs        90,600 lbs
350    0.78        82,000 lbs        99,700 lbs
330    0.78        90,000 lbs       108,500 lbs
310    0.78        99,000 lbs       117,700 lbs
290    0.78       108,000 lbs       120,000 lbs

There's a pretty dramatic change there, amounting to two or three hours of
flying time, which is probably why one rarely sees these airplanes at 
"Winds aloft" enters into the equation to some degree, but the above are
the *limits*, regardless of how beneficial the winds turn out to be on
flight planning (i.e., fuel load and initial selected altitude for the
desired altitude).

Robert Dorsett                         Moderator, sci.aeronautics.simulation