I'm aware of the difference between TAS and IAS, and how IAS gets lower in reference to TAS as you climb and the air gets thinner. What I'm trying to figure out is whether or not an aircraft's IAS stays somewhat constant as it flies higher, translating to an increase in TAS.

Do you get a benefit in TAS at FL250, or is your TAS similar to what your TAS was at 3000ft, and now your IAS is much lower?

Does this answer change if it's jet or turboprop or turbocharged piston or piston?


Airplanes are flown with respect to indicated airspeed; in terms of true airspeed, jets (and powerful turboprops) go faster and faster up to a point, and then they slow down a bit – that point is a crossover point: switching to an essentially constant Mach number.

In a climb:

  1. As you wrote: the air density decreases, that means for a given IAS, the TAS becomes faster.
  2. The local speed of sound decreases due to the decreasing temperature, that means it takes a slower TAS to get to any given Mach number the higher the plane climbs.

So, as a plane climbs at a constant IAS, the plane will be fast approaching its limiting Mach number (MMO).

Here's a graphic representation:

enter image description here
Source: Getting to Grips with Aircraft Performance, Airbus, via SKYbrary.aero

The reason for maintaining 250 KIAS below 10,000 (shown above) is here:

Also, see related:

  • 1
    $\begingroup$ Great answer and very helpful to the OP. $\endgroup$
    – 757toga
    Aug 18 '21 at 1:07
  • $\begingroup$ @757toga: Much appreciated, I like your explanation too! $\endgroup$
    – ymb1
    Aug 18 '21 at 1:09
  • $\begingroup$ Thanks. That's super helpful. So should I assume that the reason piston and turbo-piston aircraft lose IAS as they climb is because of the physics of a piston engine in thinner air? $\endgroup$ Aug 18 '21 at 20:50
  • $\begingroup$ @IsaiahTaylor: There are powerful turbo-pistons out there. See this Q&A and this for the inverse thrust of pistons (props really), and props vs fans. $\endgroup$
    – ymb1
    Aug 18 '21 at 21:12

High performance aircraft (e.g. Airbus, Boeing, Gulfstream, etc.) will generally climb out at a specific IAS limited and based on regulatory requirements, altitude, and max efficiency (fuel/performance). Once the aircraft reaches a specific mach number (ratio to speed of sound, such as .78 or .82, etc.) as the climb continues the crew will continue operating at that Mach number instead of IAS. The indicated Mach speed will provide for the best fuel and performance based on the type of aircraft (based on company requirements or manufacturers' information). For example an older B737 may cruise at .76 Mach and a B757 may cruise at .80 Mach.

The higher the aircraft goes maintaining the same Mach speed, the lower the IAS will be for that Mach speed.

You do get a bigger magnitude benefit TAS vs IAS at 25000 ft vs 3000 ft.

These aerodynamic principles don't fundamentally change with aircraft type (turbocharged, piston, etc). Although much of the information above would not apply to turbocharged or piston aircraft just based on performance limitations.


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