How accurate is the TAS calculator on a Garmin GPS?

[David Megginson]

1. Your plane's TAS will vary by your power setting, so you have to correct RPM and MP for density altitude.
2. Your plane's TAS will vary by gross weight (one very rough rule of thumb is that you'll gain about a knot for every 100 lb you get rid of).
3. Your plane's TAS will be lower with a forward CG (but also more stable in pitch in rough air).
4. If there's any vertical motion in the air, that can raise or lower your TAS by quite a few knots, because, aerodynamically, you're climbing or diving while flying level.

Taken all together, these can easily change your TAS by more than 10% from flight to flight, or even in the same flight as you burn off fuel and fly into different weather.

 

[Piperonca]

Nauga,
who does them quick and dirty as a first pass, but not using any of the methods discussed here

Ah, that means extraction from rectal defilade...

 

[nauga]

Ah, that means extraction from rectal defilade...

PIDOOMA - there are times when that's good enough, but I'm referring to flying a circle at constant IAS with a logging GPS. It's not acceptable for cert, but it's good enough for an initial look, especially if you'll be VFR. I'm usually much more interested in the altimeter error (same source, cause, testing and reduction) than the airspeed error but I get airspeed from the tests anyway.

Nauga,
and a little ∆P/qc

 

[k9medic]

Ah. That makes sense. Do you have the airspeed TAS calculator thing or was that just for the later model PA32s?

No, just the old ASI.

Here is the thing. I always fly the same power settings and I always fly the same route (at least one segment of about 100nm that is always from one waypoint to another with no ATC interference.)

The only thing that has changed is the slight rigging and the step removal.