Why do subsonic airliners fly a mach number?

Question

I don't understand why airliners change from airspeed to Mach.

Could someone explain in simple terms? Thanks

Answer 1

It's easier to track constantly at altitude than indicated airspeed.  Since Mach is a ratio, you don't have to deal with temperature/pressure issues, indicated errors, etc. (the old ICET; indicated, calibrated, equivilent, true airspeeds).

Answer 2

The speed of sound varies according to the density of the medium.   Consequently when air becomes thiner speed of sound decreases.

Almost every aspect of aircraft control and operation is influenced by air density.  Even light aircraft change VNE (Velocity Never Exceed) with density altitude.  http://www.auf.asn.au/groundschool/flutt...

Also remember Indicated Air Speed via the pitot is also influenced by Air Density.  The higher you go the lower the reading.

I hope this gets you closer to understanding

Answer 3

Mach is detected through the pitot tubes. And the static ports. Then whirly gigs mix up a bunch of electrons and such and the result is the mach number. Something like that anyway. Glad nobody ever asked me that one on a check ride.

Answer 4

Mach is a variable speed - the associated airspeed to any Mach number changes with temperature.  Jets have what's called Mmo, or maximum operating Mach speed.  Since a Mach number will be a different indicated airspeed at different temperatures and Mmo is the limiting speed, airplanes fly Mach numbers at high altitudes.

Even flying lower speeds than Mmo, Mach is still used because it is a ratio rather than an absolute speed, so it's easier to chart performance as a ratio of the maximum speed instead of constantly varying indicated airspeeds.

Indicated airspeed becomes somewhat useless at high altitudes.  Since the air is so thin, there are extreme errors between indicated airspeed and true airspeed.  A very small difference in indicated airspeed may be a large difference in both Mach number and true airspeed.

EDIT: Machmeters work by taking input from both the pitot-static system and the air temperature sensors.  On modern airplanes, this is most often done by a function of the Air Data Computer.