Re: Calculating best alt

Date:         29 Dec 98 03:12:35 
From:         "Phil" <dredbyte@concentric.net (remove)>
Organization: Concentric Internet Services
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Zirkball wrote in message ...
>Is there a formula airline dispatchers use for assigning best altitudes
>given distance between cities?

Dispatchers would consider both wind and aircraft
performance at the predicted weight. Higher is
better. Tailwinds are better than headwinds and
crosswinds enroute.

What one may see on a long flight is a planned
climb to a fairly low cruise altitude (FL290 for
example) followed by a climb to a higher level.
Initially the aircraft will be so heavy that it
can't climb way up there AND maintain enough
margin above stall speed.
Without a safe buffet margin, turbulence and
turning could result in controllability
problems...After burning off fuel, the lighter
airplane can then climb to a higher and more
efficient cruise altitude. The upper limit is both
a matter of efficiency (reaching the tropopause)
and safety (convergence of high speed and low
speed buffet limits of the wings).

Regarding winds, the flight may be routed to avoid
a jetstream if it is in the opposite direction, or
planned to use the jetstream if it is going the
same way as the flight. I've seen flights go way
off the great circle route in order to utilize a
strong jetstream and save fuel. I've also seen
flights from Chicago to Sarasota stay at FL290 to
remain below a strong jetstream headwind.

Hope this helps,
Phil Collier