# What is the reason for changing the speed reference (IAS or Mach number) with altitude?

Inspired by that question: How is the airspeed-Mach number transition handled in modern airliners?

When pressure and density decrease IAS also decreases. When temperature and pressure/density decrease speed of sound also decreases. As air pressure, density and temperature all decrease with altitude, if one fly a constant IAS or a constant Mach number while constantly climbing or descending, the aircraft actually constantly accelerates or slows relative to the air mass.

But at lower cruise altitude an aircraft will be piloted to maintain a constant IAS, while at upper cruise altitude a constant Mach will be flown.

Machmeter combined with airspeed indicator, source

What is the reason for changing the speed reference (IAS or Mach number)?

Aircraft are limited by both air speed (VMO, affects loads on the structures) and Mach (MMO, formation of shock waves resulting in buffet).

At low altitudes, the speed of sound is high so an aircraft is most limited by indicated airspeed (IAS). At higher altitudes, the speed of sound is lower so the aircraft will be limited by Mach number. Aircraft typically fly towards the upper limit of their speed, so at some point they will have to switch from remaining under the IAS limit to remaining under the Mach limit.

The chart below shows the changes in different speeds as altitude increases, assuming standard atmosphere.

Trend data source, speeds for 737-800

The aircraft climbs at 250 KIAS from sea level. An acceleration from 250 to 300 KIAS is included at 10,000 feet. You can see that as IAS is held constant, TAS (True Airspeed) and Mach both increase.

The switch from 300 KIAS to Mach 0.76 is done at FL280. You can see the Mach speed is approaching its MMO. From this point, both TAS and IAS decrease. After the tropopause, around 35,000 feet, temperature stops decreasing and TAS remains nearly constant.

At FL400 the aircraft levels off and cruises at Mach 0.765, and a step climb to FL410 is included at this speed to show the trends there.

More detailed charts and an explanation of IAS vs. Mach as it relates to climb and aircraft performance can be found here.

• @mins There are two limits: IAS (Vne) and critical Mach number (Mcr). Too much IAS can exceed structural limits. Over critical mach you get undesirable compressability effects (more drag, mach tuck) etc. In warm dense air the critical mach num is higher than the Vne, so you don't need to worry about exceeding it. In cold, thin air the Vne is higher than the critical mach, so you don't have to worry about exceeding that Commented Feb 9, 2017 at 20:53
• @mins look up "coffin corner." The "coffin corner" is the angle on the graph where the mach limitation is lower then the IAS limitation Commented Feb 9, 2017 at 20:55
• To late to edit, but I think I'm using critical mach wrong. I'm talking about Mmo - the maximum operating mach vs Vmo - maximum operating IAS. Commented Feb 9, 2017 at 21:05
• Thank you very much for the good diagram and the clear explanations. The Leeham article is very interesting too.
– mins
Commented Feb 9, 2017 at 22:57
• @Sean: Speed of sound decreases with altitude, due to temperature decreasing. It's true it increases when density decreases, but this is cancelled by the fact it decreases by the same quantity when pressure decreases.
– mins
Commented Sep 18, 2019 at 21:34

In a climb:

• The air density decreases. That means for a given IAS, the TAS becomes faster.
• 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).

(Note: The changeover altitude is not fixed. It depends on what IAS / Mach number is most economical for a plane.)

(Getting to Grips with Aircraft Performance, Airbus, via SKYbrary.aero)

• Is it decreasing density or temperature that decreases the local speed of sound? I thought it was the former. Commented Feb 10, 2017 at 5:04
• @ToddWilcox, it is decreasing temperature that is decreasing local speed of sound. Speed of sound is ONLY a function of temperature, see here Commented Feb 10, 2017 at 6:22
• Ah I get it. As long as the medium is a gas, it's the temperature that matters. When comparing gasses to liquids, though, the huge difference in density is relevant. I never knew that before. Thanks! Commented Feb 10, 2017 at 12:47