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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, 2013 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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