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My electronic E6B does calculations for both planned and actual true airspeed (yielding different results). I was told the difference has to do with assumptions about temperature, but the manual doesn't give any details. What is the difference between the two?

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    $\begingroup$ When you do the calculation for actual, are you using CAS or IAS to do the true calculation? That's a step that's often overlooked (or impossible)! $\endgroup$ – egid Dec 18 '13 at 3:54
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From the manual for an electronic E6B which seems to be similar to yours:

PLANNED TRUE AIRSPEED

This function is used to calculate true airspeed for preflight planning. It will compute the true airspeed in knots and Mach number and density altitude, given the pressure altitude, temperature, and calibrated airspeed in knots.

and

ACTUAL TRUE AIRSPEED

This function calculates true airspeed, Mach number and density altitude given pressure altitude, indicated temperature in Celsius and calibrated airspeed.

The manual doesn't give any more information on the difference between the two calculations so we can only guess...

Both methods use pressure altitude and calibrated airspeed. One clue is that the "planned" version uses "temperature", while the "actual" version uses "indicated temperature".

So my guess is:

  • you use the "actual" version in flight, and it expects you to input Total Air Temperature (TAT), read from the gauge in the airplane, which it then converts to Outside Air Temperature (OAT) to do the density altitude and true airspeed calculation.
  • You use the "planned" version for flight planning, where the temperature you have available is the OAT.

The TAT is the OAT plus the rise in temperature due to the air being brought to rest relative to the airplane. Since the TAT is a bit higher than the OAT, the "actual" version should give slightly lower density altitude and a slightly lower true airspeed.

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It's very possible that your flight computer's TAS calculation is expecting calibrated airspeed rather than indicated airspeed; the difference between these two can often be enough to affect your results by a meaningful amount.

Most aircraft do not have a formula for the conversion, or enough information to convert airspeeds other than those specified in the manual. This means that the "CAS" used for TAS calculations is often actually IAS.

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  • $\begingroup$ Good thought, but both functions take calibrated airspeed as an input. $\endgroup$ – jrdioko Dec 18 '13 at 16:19
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    $\begingroup$ Are you converting indicated airspeed to calibrated before entering it into the computer? That's the step that is usually harder to do as the conversion from IAS to CAS is often not available. $\endgroup$ – egid Dec 18 '13 at 16:23

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