Mach Number (MN) = (Total Pressure - Static Pressure)/ Static Pressure

or MN = (Pt - Ps)/Ps

 

MN = Dynamic Pressure/Static Pressure = CAS/Ps

In this formula you do not need to take account of temperature and only need to take account of changes in altitude, ie. the Ps component.

 

If the Mach number remains constant and the aircraft is at a constant altitude then CAS must also remain constant.

 

If the aircraft is climbing then the Ps component decreases and therefore CAS also decreases.

Conversely if the aircraft is descending then the Ps component increases and CAS also increases.

This formula also works for an aircraft flying at a constant CAS and determining the affect on MN.

Furthermore if an aircraft is flying at a constant TAS use the following formula:

MN = TAS/LSS In this case you only have to take account of temperature changes and the affect on the LSS to solve the questions.