I always calculated my TAS at any cruise altitude by

  1. Using my planned cruise (pressure) altitude, temperature
  2. Plug desired CAS, PA, temperature into e6b and get TAS

But I've also seen people get TAS only from the POH for cruise. And many books say that e6b is only used for TOC and that for cruise you should use the POH.

The discrepancy I see is that the POH assumes a constant power setting. But I'm assuming a constant CAS. So, in that case it seems my way should be more accurate.

What's the proper procedure that a student pilot should follow?

p.s. I fly a pa-28-181.

  • 1
    $\begingroup$ You’re asking about flight planning, not in-flight use, correct? $\endgroup$
    – StephenS
    Commented Jun 8, 2022 at 20:14
  • $\begingroup$ there are many replies and comments on the identical Reddit thread $\endgroup$ Commented Jun 8, 2022 at 21:13
  • $\begingroup$ @StephenS yes, flight planning. Not in flight use $\endgroup$
    – Jonathan
    Commented Jun 9, 2022 at 3:46
  • $\begingroup$ It doesn't matter, you'll get the same TAS either way as it's just a calculation. $\endgroup$
    – GdD
    Commented Jun 9, 2022 at 8:04

1 Answer 1


Using the POH with a fixed power setting is more correct, and as a student, you should absolutely do it that way.

The POH is where you get your fuel burn per hour. That, combined with GS and ETE per leg (which you’ll get from TAS and winds aloft) will give you the fuel burn per leg, which is a critical part of your flight planning.

  • 1
    $\begingroup$ This doesn’t make sense. TAS is simply the actual speed the AC is moving through the air. It is calculated from indicated airspeed by accounting for the density of air at altitude. It has nothing to do with power settings. The POH can only display expected TAS vs power settings for a nominal engine- which is often not the case in real life. $\endgroup$
    – Jim
    Commented Jun 8, 2022 at 18:33
  • $\begingroup$ But OP says: Noting my cruise (pressure) altitude, temp. This leads me to believe he’s in the airplane not planning. Maybe I interpreted wrong? $\endgroup$
    – Jim
    Commented Jun 8, 2022 at 18:49
  • $\begingroup$ Me either. My ASI had a temperature knob and did the E6B calculation implicitly. But in planning where would you even get CAS from?? $\endgroup$
    – Jim
    Commented Jun 8, 2022 at 20:31
  • $\begingroup$ @Jim I suspect OP is planning to fly a fixed IAS/CAS and adjust power as needed to achieve it. My CFI disabused me of that idea when I started XC flight planning. $\endgroup$
    – StephenS
    Commented Jun 8, 2022 at 21:05
  • $\begingroup$ Ok. May be. I agree- dumb idea. It’s coming at it bassackwards. $\endgroup$
    – Jim
    Commented Jun 8, 2022 at 21:50

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