# Mach is slower than G/S or TAS?

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I've noticed that when flying at high altitudes in mach mode (like a good boi), the conversion between my groundspeed and true airspeed convert to be a slower mach number than what's showing as my mach speed.

For example, right now in the FF 757, I'm holding a .83 knot cruise. My GS is showing 465 knots.

But 465 knots = mach .68.

Why?

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Where did you get Mach .68 = 465 knots? The relationship between airspeed and Mach number is not a fixed relationship - Mach number depends on the outside air temperature. For a given airspeed, the Mach number increases the colder the air becomes.

In any case, you cannot use GROUND speed to calculate aircraft performance, because ground speed is affected by the wind. If you are flying at a True Airspeed of 460 knots into a 100 knot headwind, your ground speed will be only 360 knots. With a 100 knot tailwind, your ground speed will be 560 knots.

The only speed that "matters" is True Airspeed (TAS) which depends on your altitude and the outside air temperature.

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Actually, for aircraft performance (by this we mean 'aerodynamic performance') the only speed that matters is IAS (indicated air speed). TAS only starts to matter when you want to know how soon am I going to arrive to my destination. Therefore in initial pilot's training - you don't even have to know what TAS is.

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Actually, for aircraft performance (by this we mean 'aerodynamic performance') the only speed that matters is IAS (indicated air speed). TAS only starts to matter when you want to know how soon am I going to arrive to my destination. Therefore in initial pilot's training - you don't even have to know what TAS is.

The OP's question related to Mach number, which is directly related to TAS corrected for temperature. My concern was that it appeared he was trying to relate Mach to ground speed, (and/or thought that airspeed vs. Mach is a fixed relationship).

Of course in a simulator, unlike r/w, one has the option to turn all surface and upper winds off, in which case TAS and ground speed will be the same.

It's absolutely true that aircraft performance from the pilot's perspective is based on indicated airspeed, and performance tables for high altitude flights show Mach numbers in terms of indicated airspeed for specific altitudes.

In the typical "low & slow" piston-powered aircraft used for initial training, there is not much difference between IAS and TAS in any case in most situations.

If the OP was using FSX/P3D it's possible he might have inadvertently set the sim to display TAS instead of IAS on the airspeed indicator, but since he mentioned the FlightFactor 757, I assume he is using X-Plane.

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...Of course in a simulator, unlike r/w, one has the option to turn all surface and upper winds off, in which case .......

FSX won't rip your aeroplane apart :wink: . There may be add-on weather engines deal with them satisfactorily. Not sure how X-plane fares.

465 = M0.68 ? No.Maybe. Sometimes. At 75000 feet in an XB-70, Mach 3.0 is showing less that 450kt on the ASI. Admittedly GS is up around 1700 knots or so

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1. Don't confuse ground speed with any other speeds. I am sure you have seen seagulls in the air with TAS 20knots and Ground speed zero.

2. IAS is not exactly a speed. Its is more a pressure than a speed.

3. Mach is not exactly a speed. It is more a ratio than a speed.

4. TAS. At last. Yes it is a speed relative to the air not to the ground.

5. Mach , IAS when and why?

You have to understand what crossover altitude is.

When the aicraft climbs to the crossover altidute at a constant IAS , Mach increases.

When the aicraft descents to the crossover altidute at a constant MACH the IAS increases.

Some more details here:

https://www.quora.com/What-is-a-crossover-altitude

Emmanuel Argiropoulos

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