Mach Meter

Mach number is defined as the speed of the aircraft relative to the speed of sound. Unlike TAS, Mach number has no dimension. In air- navigation, this new unit of speed is applicable above 300kts. The Mach meter is the DR reference for the Mach number. According to equation (8.18), the Mach meter derives from two orthogonal air capsules: the dynamic- pressure capsule, and a static- pressure capsule. The instrument is available on super- sonic aircrafts only.

The Mach meter equation (8.18) is a quadratic in . A turning point (known as the critical mach number), therefore, exists and it is given by:

Question 1.

State precisely how the speed of sound varies with temperature. An aircraft is at F.L.350, TAS 462kts and mach 0.79 when the temperature deviation is 9 C relative to the jet atmosphere. Without using a computer, give the temperature deviation at F.L.310, which at mach 0.79 would give the same TAS 462.

C.P.L. [10]

Question 3

If a decrease of 0.13 in Mach number results in a decrease of 72kts in the TAS, what is the local speed of sound?

A.T.P.L. [10]

Solution:

Drg. 11.3 Speed of sound in terms of equation 8.12