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I've had this discussion with my FI: Are speeds such as Vx and Vy referring to IAS at all times or TAS that needs to be compensated for?

Here's an example. Let's assume the following:

  • Vy of my single piston engine, fixed prop aircraft is specified at 70 kts
  • I'm taking off from 0 ft MSL, climbing to 5000 feet pressure altitude
  • it's +15° celsius outside temperature with a QNH of 29.92 Hg / 1013 hPa (all ICAO standard)

Which of these two scenarios would actually get me to my target altitude faster?

  1. I'm constantly pitching for 70 kts IAS under full power
  2. I'm constantly pitching for 70 kts TAS under full power, i.e.:
  • 70.0 kts IAS @ 0 ft
  • 68.6 kts IAS @ 1000 ft (70 / 1.02)
  • 67.3 kts IAS @ 2000 ft (70 / 1.04)
  • 66.0 kts IAS @ 3000 ft (70 / 1.06)
  • 64.8 kts IAS @ 4000 ft (70 / 1.08)
  • 63.6 kts IAS @ 5000 ft (70 / 1.10)

I was assuming an appoximate increase of 2% per 1000 feet in TAS compared to IAS. My FI says that scenario two would actually yield in an optimal climb, but I'm uncertain as to how the reduced air density would affect lift and therefore the optimal speed to maintain.

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IAS -- simply because the POH tries to present information in a form that is immediately useful to the pilot. You need Vx and Vy quickly -- they show up as a mark on the airspeed indicator so you can quickly glance and know where you stand.

You need TAS when you're navigating -- figuring out how far you fly in a certain amount of time, don't forget winds, etc. You likely don't fly at best climb speed or minimum sink speed for the extent of the mission.

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  • $\begingroup$ Thank you! So IAS is to be used for the reasons you said, which makes a lot of sense, however is this also physically the optimum as you climb or would the gradient in scenario two actually result in a faster climb? $\endgroup$
    – Double M
    Commented Feb 20 at 19:41
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    $\begingroup$ That gets down to subtle details of how the engine and propeller performance changes with altitude. What we know for sure -- at the absolute ceiling, Vx is equal to Vy -- no matter whether they are measured in IAS or TAS. Consequently, one or the other (or both) must vary with altitude. Fortunately, this variation is relatively slow and we mostly care about the value of Vx and Vy near the ground. $\endgroup$
    – Rob McDonald
    Commented Feb 20 at 20:38

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