Indicated Mach versus True Mach Number

[Faster]

I need some help understanding Mach Number............

Is there a valid term 'true mach number', as opposed to 'indicated mach number' ?

As I understand it, Mach number is the ratio of true airspeed to the speed of sound..........Why would there be an indicated value of mach versus a true mach number.

Would it have something to do with the Ct (i.e. value of .8 or 1.0)of the temperature probe?

This question started with a query as to whether the mach value displayed in our cockpit (Honeywell Primus 2000 avionics suite) was a 'true mach number' or an 'indicated mach number'.

As for flying across the North Atlantic airspace, we are instructed to fly a constant mach number a.k.a. Mach Number Technique to maintain adequate spacing in trail of other aircraft. I have found reference material stating that this is to be a 'True Mach Number'

Thanks in advance

[Genghis the Engineer]

All instruments contain errors, and the Machmeter is no different in that respect. So, yes, there is a correction between IMN and M.

The main error terms in the machmeter calibration equation (which rather defeats my efforts to reproduce as plain text in Pprune) are pitot error (due primarily to shockwave formation across the front of the pitot and high subsonic, transonic, and if you happen to fly Concorde, supersonic speeds), and static error, which tends to be proportionally greater nearer to M=1 (from either direction).

Rather surreally, the (military) course notes that I have describing this also say that North American NATO, and European NATO use different calibration equations for Machmeters.

I would have thought that if you are flying an aircraft that routinely spends time above M=0.7 either the Machmeter is sufficiently accurate that you could treat it as IMN=M, or there should be a correction chart in the operating data somewhere.

G