When doing acceptance flights for purchased aircraft one of the issues is
always how much is the true air speed. Since you are working with an airspeed
indicator that you don't know, an outside air temperature probe that you do not
know temperature rise factor, etc, it is best to use you GPS ground speed.

Technique usually requires a pilot-flying and pilot-non-flying crew. During the
test use the same power (rpm/manifold pressure), fuel flow, and holding
altitude.

Fly a track of 000 for about 30 seconds so that the acceleration/deceleration
forces can equalize, note down the ground speed. Call it groundspeed A

Fly a track of 120 for about 30 seconds so that the acceleration/deceleration
forces can equalize, note down the ground speed. Call it groundspeed B

Fly a track of 240 for about 30 seconds so that the acceleration/deceleration
forces can equalize, note down the ground speed. Call it groundspeed C

A^2 + B^2 + C^2 = sum of (ground speeds^2)

sum of (ground speeds^2) / number of tracks (3) = average of (ground speeds^2)

square root of average of (ground speeds^2) = true air speed at that altitude
with that outside air temperature.

Just a $1.99 calculator from Office Depot can do the trick.

Now of course the question comes in about the validity of the formula.

Lets try 100 knots true air speed, wind 045 at 30 knots (realistic winds at
less than 12k feet). Using our handy dandy Sporty's E6B-F lets input the values
in the HDG/GS function;

WDIR 045
WSPD 30
CRS 000
TAS 100

GS A 76.5
HDG 12.2 degrees

WDIR 045
WSPD 30
CRS 120
TAS 100

GS B 87.9
HDG 103.2 degrees

WDIR 045
WSPD 30
CRS 000
TAS 100

GS C 128.7
HDG 244.5 degrees

Ground speed A^2 (5852.25)
Ground speed B^2 (7726.41)
Ground speed C^2 (16563.69)

sum of A^2, B^2, C^2 = 30142.35

sum of the squares / 3 = 10047.45

square root of average = 100.23696922792508295043396893834 knots

Please note that even with a wind that is 30% of your true air speed, the
percent error using this formula 0.236%. This would cumulate to about
8.5308922053029862156228817788 seconds of leg time error for every 1 hour of
flight.

Close enough I'd say.




paul k. sanchez, cfii-mei
on eagles’ wings
2011 south perimeter road, suite g
fort lauderdale, florida 33309-7135
305-389-1742 wireless
954-776-0527 fax
954-345-4276 home/fax

www.iflyqpa.com/cas