Excellent responses from
Keith Williams well supported by wise comment from
John_T with respect to why Mach Number becomes the limiting speed of the aircraft at higher altitudes. Also wise comment from other posters with respect to EAS/CAS/DAS/TAS/Mach relationships.
The original poster asked why we use Mach Number in lieu of IAS at high altitudes, and the responses mentioned above correctly allude to
ONE of the reasons, i.e. Aircraft Operating Limitations.
The other, and more common reason why we do so (because we rarely operate at the limiting speeds) is Aircraft Performance. After the aircraft speed has passed Mcrit, Wave Drag enters the picture, and severely modifies the Total Drag curve of the aircraft. Up until Mcrit, the conventional drag curves as we know them, are the sum total of Form Drag and Induced Drag. Thus, all performance is predicated upon EAS, which (unfortunately) is presented to the pilot as CAS.
Above Mcrit, Total Drag then becomes the sum total of Form Drag, Induced Drag, and Wave Drag. The "new" Drag Curve upon which we now predicate all aircraft performance is referenced to Mach Number. An Example - Maximum Range Cruise Speed is found by projecting a line from the 0/0 origin to a point tangential to the Drag Curve. For a given weight, this will always result in the same EAS for MRC
UNTIL Mcrit is reached at that speed. After Mcrit the Drag curve then (after a small initial negligible effect) then curves up much more steeply than the original Drag Curve. The point of tangency then occurs on the Drag Curve at ...... a given Mach Number, EAS, CAS are irrelevant.
At increasing Altitudes, the EAS at which Mcrit occurs begins to 'slide' backwards down the curve, with typical operating speeds somewhat above Mcrit. (Except on low level short sectors, almost all jet aircraft Climb, Cruise, and Descend at a Mach Number except at lower altitudes, where Mcrit begins to 'slide' up the curve again).
Thus, at the normal operating levels for jet aircraft (High), almost all NORMAL performance is predicated upon Mach Number. For very high flying aircraft, Mcrit may descend to Vmd, and begin to creep upwards on the rear side of the conventional drag curve, and Vmd is no longer related to EAS, it too, is related to Mach Number (Mmd).
So, in short, there are 2 major reasons why jets operate at Mach Number at higher levels, namely -
.1. Mach Number defines the aerodynamic limiting speed of the aircraft, and
.2. Mach Number completely modifies the Total Drag Curve of the aircraft, thus all PERFORMANCE is predicated upon Mach.
In closing (because I felt like it), I reinforce earlier posters remarks that Temperature has
NOTHING to do with the EAS/CAS to Mach Number relationship.
PRESSURE HEIGHT is the determining factor for the EAS/CAS to Mach Number relationship.
As for DAS, throw it in the waste bin where it belongs!
Regards,
Old Smokey