Re: Flying High ?

Date:         19 Feb 98 01:34:27 
From:         g3av8tor@aol.com (G3AV8TOR)
Organization: AOL http://www.aol.com
References:   1
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A jet aircraft at altitude, 30 to 45,000 feet, is flying much faster and
is more fuel efficient than one at , let's say, 10,000 feet.

You may be confusing your airspeeds. There is indicated airspeed, which
is what the pilot in the cockpit reads; calibrated airspeed, which is
indicated airspeed corrected for instrument errors; and true airspeed,
which is calibrated airspeed corrected for altitude and temperature.
Most jet aircraft also have a display showing true arispeed.

Assuming no instrument errors, indicated and calibrated airspeeds are
the same, as  an aircraft climbs, the true airspeed increases for a
given indicated airspeed. This rate of increase is about 2% per 1000
feet up to the tropopause on a standard temp day.

As an example, an aircraft indicating 300 knots at 10,1000 feet would
have a true airspeed of about 360 knots. (300+20%). The same aircraft
flying at 300 knots indicated at FL310 (31,000 feet) would have a true
airspeed of about 486 knots.(300+62%).

Most jets I fly are not able to maintain the same indicated at altitude,
but they are still faster at altitude. For example, at an altitude of
35,000 feet, I can indicate 250 knots. This translates to about 425
knots true airspeed (250+70%), which is still faster than the 300 KIAS
example above.

A pilot wishing to be more exact with their calculations can use charts
or a computer to calculate true airspeed. Large deviations from standard
temperatures can wreak havoc with the 2% equation.

Most jets I fly use N1 or fan speed expressed in %RPM to indicate engine
power.  The engines will need less fuel for a given N!% as the aircraft
climbs do to the decreased air density. While thrust for a given N1% is
not constant as the aircraft climbs, the aircraft is still more fuel
efficient at altitude.

Hope this helps.

Gary