why are Aircraft Mach Indications considered to be unreliable below 25,000

[whitecross]

why are Aircraft Mach Indications considered to be unreliable below 25,000
a- becuase tempreture is used in computation
b-becuase altitude is used in computation not temperature

can someone explain why please

thanks

[SNS3Guppy]

Mach indications aren't considered unreliable below 25,000. However, we generally don't go fast enough to be concerned about mach effects until we climb above the "cross-over" point, typically around 27,000 to 29,000'.

At lower altitudes, one will typically bump up against Vmo before reaching Mmo. As one climbs, the mach values will occur at lower and lower indicated airspeeds, and a transition from using indicated airspeed as a reference, to using Mach as a reference, occurs in the area of 28,000' during the climb and descent.

For aircraft that are capable of exceeding mach 1.0 at lower altitudes, mach is still a reliable indication. For most airline, corporate, and general operations, however, coming close to those values at lower altitudes ins't going to happen, and airspeed limits take precedence. Once at higher altitudes, airspeed is no longer the maximum limiting factor, but mach.

[Agaricus bisporus]

why are Aircraft Mach Indications considered to be unreliable below 25,000


Considered by whom?

Hooey. Someone bin telling you tales fella.

[FA10]

however, Mach number is a function of temperature - not altitude.

[ElNull]

however, Mach number is a function of temperature - not altitude.

Would you please explain? Because as far as I know Mach depends on altitude, perhaps you mean the speed of sound (which depends only on temperature)?

Mach is used at high speeds (i.e at high altitudes). For exmaple, if you are climbing at constant IAS your TAS will increase up to those altitudes at which the increased TAS results in reaching the critical mach number of the wing.. at which point the increased TAS is ceased by continuing the climb at constant Mach.

[SNS3Guppy]

Because as far as I know Mach depends on altitude, perhaps you mean the speed of sound (which depends only on temperature)?

Mach is a measure of percentage of speed of sound, somewhat speaking; it's used in aircraft as a measure of compressibility in terms of speed.

Generally speaking, airspeed is useful at lower velocities when compressibility isn't a factor. Subsonic air, particularly at lower speeds, is considered incompressible for calculation purposes. As we approach the transonic region (roughly .75 mach on up to about 1.2 mach), we encounter compressibility effects, and it is approaching these speeds that we begin to consider mach. Below that, we consider speed as a function of airspeed, rather than mach.

This isn't a hard and fast rule, of course. Some aircraft are limited by mach speeds, especially at higher altitudes, at mach values lower than the traditional transonic range. These aircraft also reference mach, but only when mach becomes the limiting factor (during a climb above about 27,000' during normal operations), rather than airspeed.

At higher altitudes and lower temperatures, airspeed values continue to drop as we climb, with a corresponding constant value mach. Mach becomes of interest primarily as a cruise value and a limiting value. We generally calculate our minimum and maximum airspeeds during high altitude cruise as well, to determine "buffet margins," which tell us about high-speed buffet (onset of mach effects--tuck, buzz, etc) and low speed buffet (traditional aerodynamic stall and associated effects).

[parabellum]

however, Mach number is a function of temperature - not altitude.

When I did my ATPL at Avigation, in Ealing, London, many, many years ago I seem to remember Tommy telling us that Mach was a function of the square root of the absolute temperature but if that is wrong I claim immunity from abuse on the grounds of age and insanity!:)

[Tinstaafl]

Speed of sound is related to temperature. Temperature happens to decrease with altitude (until the tropopause) so one can understand how you could associate Mach with altitude. But the real relationship is to the air temperature.

Mach readings are not unreliable below 25,000'. It's just that below this altitude the temperature is high enough that Mach limits aren't usually a factor. Dynamic air pressure limits ie airspeeds, are more of a factor.

[john_tullamarine]

Mach is a function of pressure height and CAS - there will be several threads (as, for example (http://www.pprune.org/tech-log/314513-mach-airspeed-descend-climb-737-a.html)) around with more explanation. Temperature is not involved in Machmeter operation as far as I am aware ...

Sonic velocity is related to temperature.

As we approach the transonic region (roughly .75 mach on up to about 1.2 mach), we encounter compressibility effects

Fat Albert aircraft might be a tad lower. The lowest limiting Mach I can recall was for the AW650 Argosy which had a limit around M0.55 as I recall without digging out an AFM and that Type was the fattest Albert with which I have ever had an involvement ..

Mach was a function of the square root of the absolute temperature

Sonic velocity rather than Mach

I think that the OP is drawing an unsafe inference based on conventional instrument usage driven by limitations which vary with height ..

[Pugilistic Animus]

They really better start writing M and speed of sound explanation in the pilot training literature a lot more clearly--:ugh:
the local speed of sound is of PHYSICAL importance because all pressure disturbances propagate at the speed except explosions and other powerful shock waves---

The Mach meter is a fancy ASI with the same data inputs [Ps and q]

Edit: for those who like math--from a prior post of mine

Proof of independence of Mach on T
since Mn = TAS/a [a = lss]

and TAS = EAS *[d2/d1]^.5 [density ratio]--- no rho to keep down visual clutter
and lss =a0[T2/T1]

I can say that M = Eas* [d2/d1]^.5 /a0*[t2/t1]^.5,...expressing TAS in terms of EAS and T ,..one obtains EAS *[p2*d1*T1/p1d2*T2]^.5/a0 [T2/T1]^.5,...it is thereby shown that T WILL cancel

ok...what about if I write c in terms of density, since density is a function T you end up as M= EAS [d2/d1]^.5/a0[p2*d1/p1*d2]^.5 again all like terms dependent on T disappear d1 an d2 cancel here..
:)