Definition

A machmeter is an instrument which provides an indication of the Mach Number, (M), which is the ratio between the aircraft true air speed (TAS) and the local speed of sound (LSS). This ratio, which equals one when the TAS is equal to the local speed of sound, is very important in aircraft operating at high speed.

The machmeter uses the aircraft pitot-static system to generate M and usually portrays this on a simple needle and dial instrument, such as that shown below.

Machmeter

Alternatively, the machmeter may be combined with the Air Speed Indicator (ASI), in which case it is often referred to as a Combined Speed Indicator (CSI).