Pat,

The out-and-back ground speed run has been the standard method of
calibrating the airspeed indicator for "position error" for a number of
years now. Your example is absurd, but a more realistic exaggeration will
make the point of just how good the method is: take your 80 mph plane out on
a measured course in a 40 mph tailwind, then return. You'll find that the
outbound leg took one-third the time of the return leg. Turn this process
around: you measure out a ten-mile leg, then fly it both directions and note
the times. Each time value will give you a groundspeed, and the difference
between the groundspeeds will be twice the wind speed. Subtract the
windspeed from the fast groundspeed value, or add it to the slower, and
you'll have the true groundspeed.

Run the true groundspeed through your E6B computer for altitude and
temperature and it becomes True Indicated Airspeed. Be careful, though,
because you need to make that calculation backwards from the TAS calculation
you're used to. Whereas you would normally take your indicated airspeed in
cruise and use your E6B to find your TAS, here you have the TAS (true
groundspeed) and you're trying to find out what your airspeed indicator
should read. Once you've done that, compare this against what your airspeed
indicator actually indicated during the flight (you did fly a steady
airspeed at constant altitude and RPM, didn't you?) and you'll have one data
point on your airspeed calibration card for your airplane flight manual. See
AC 90-89A Chapter 4, Section 3, paragraph 5.

With the advent of GPS, we no longer have to fly a measured course over the
ground; just take your GPS groundspeed readings while traveling in two
opposite directions, then average them. That's your true airspeed as you
would find it in no wind conditions. If you wish, plug that into your E6B
for temperature and altitude (backward calculating, again), and you'll know
what your airspeed indicator >should< have been reading at the time.


David J. Gall