Say I want to find out EAS and I'm given a time that a pilot will reach a waypoint that is "X" Nautical Miles away, an altitude & I'm also given a ground speed that is increased by a tailwind. Is it possible to derive equivalent airspeed from these factors.

My current state of research is as follows: I know the traditional EAS equation is useless in this case as I dont know the air density and other equations I've looked at require the Mach number which I also can't obtain. Basically all the EAS equations I've found require a density or the Mach number and I'm not sure how to proceed.

  • 1
    $\begingroup$ Is this a homework question? Can you specify the inputs you have more precisely? $\endgroup$
    – Jan Hudec
    Commented Nov 25, 2019 at 20:54
  • $\begingroup$ How comes you don't know density and mach number? You have altitude, and then you either also have temperature, or assume standard temperature (both density and speed of sound depend on temperature, so you can't make any progress without it—different temperature, different answer). $\endgroup$
    – Jan Hudec
    Commented Nov 25, 2019 at 20:57
  • $\begingroup$ It's not a homework question, it's a practise exam question. I'm given FL400, a waypoint that is 300NM away, with the pilot expected to arrive at the waypoint in 32 minutes. The aircraft ground speed is increased by a 100 knot tailwind. $\endgroup$ Commented Nov 25, 2019 at 21:05
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    $\begingroup$ I see two options. You can assume FL400 is also density altitude, or you can assume the standard temperature in tropopause and calculate the Mach number, because in practice anything that flies in FL400 maintains a Mach number. The difference is probably negligible, and you have to assume temperature either way. $\endgroup$
    – Jan Hudec
    Commented Nov 25, 2019 at 21:38
  • $\begingroup$ Okay I see, I will attempt this and post my solution in the forthcoming days. Thank you for the help. $\endgroup$ Commented Nov 25, 2019 at 22:11

1 Answer 1


No, it is not possible to derive EAS from the data provided.

p=Actual density

p(o) = standard Sea Level Density

TAS = True Airspeed.

You can get to actual density from the pressure altitude and temperature (ISA+ or raw). Without temperature, it is impossible to derive air density from pressure altitude; only estimates are possible. For a more in depth derivation, this is a good source.


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