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).]