Final answer:
The Mach number is displayed on PFD and ND in aircraft instrumentation to indicate the speed of an aircraft relative to the speed of sound.
Step-by-step explanation:
In aircraft instrumentation, the Mach number indicates the speed of an aircraft relative to the speed of sound. It is commonly displayed on Primary Flight Displays (PFD) and Navigation Displays (ND) in the cockpit. The Mach number is typically shown when an aircraft is flying at or near supersonic speeds, as it provides crucial information about the aerodynamic characteristics and performance of the aircraft.
The Mach number is the ratio of flow velocity after a certain limit of the sound's speed. In simple words, it is the ratio of the speed of a body to the speed of sound in the surrounding medium.
A Mach number is the ratio of an object's speed in a given medium to the speed of sound in that medium. Mach 1, then, is the speed of sound, around 761 mph at sea level on a standard day. The term is also used as a metaphor for high speeds more generally.