Mach vs. IAS

[Track]

Does anybody have a simple answer as to why we change over from IAS to Mach nr. from a certain altitude for speed reference and traffic separation?

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Track

[Genghis the Engineer]

Most aircraft have a Vne and an Mne - obviously the relationship between the two is a function of height.

So at lower altitudes it's always CAS/IAS that's the big player - at altitude IAS becomes fairly small but Mach becomes much more significant. Since that becomes your operating limit, it makes sense to use it for other purposes as well.

Example (fictional aeroplane) Vne=300kn, Mne=0.7. At ISA S/L conditions 300kn = 0.45M, so you can ignore Mach and fly to IAS limits quite safely.

However at FL240, 300 kn=0.7M, and at FL 300, 300kn=0.85M - well above Mne. So above FL240 you may as well ignore IAS (well apart from for converting to TAS for Navigation anyway) and use Mach all the time.

G

[Check 6]

In high performance aircraft (jets), Mach number is an important limitation. Each jet has a limiting Mach number (Mmo = Mach-maximum operating), therefore Mach numbers are used at the higher speeds which occurs at mostly higher altitudes for most jets. The limiting Mach number occurs because the "local Mach number", i.e. the Mach speed of portions of the wing is much higher than the aircraft, therefore portions of the wing could exceed Mach 1.0. The Lear 35 for instance has an Mmo of .83 Mach. The wing is transonic near this speed, and will exceed Mach 1.0 (supersonic) at a Mach indicated somewhere above .83 Mach. IAS is used at lower altitudes where the IAS is limiting. Typically, jets transition to Mach numbers in the range of FL 250-300. For example at FL 370 a Lear 35 will be cruising at Mach .79 with an IAS of 240 KIAS and a TAS of approximately 455 Kts. The exact numbers depend on the OAT. The above is the simple explanation, I hope it helps.

Check 6

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Kick the tires, light the fires, first off is lead, brief on guard.

[This message has been edited by Check 6 (edited 08 October 2000).]

[Checkboard]

Just to clarify:

The Mach number/CAS relationship changes only with pressure altitude. (which is why some manuals specify a change over altitude)

The TAS changes with temperature.

[Eli Vator]

A note of caution must be applied to Genghis' above post. IAS should never be ignored. It's true that at high altitudes Mach No. is the reference to be used for the aircraft's maximum speed limits. However, IAS must still be referenced with regard to the lower speed limits at any altitude.

[Genghis the Engineer]

Fair cop, thanks for that Eli.

G