Let us discuss the working of a Machmeter, its uses, and applications. We will also see how to differentiate a Machmeter from an airspeed indicator.

A machmeter is an instrument that measures the ratio of an aircraft’s true air speed (TAS) in relation to the speed of sound. When the TAS is equal to the local sound speed, the ratio becomes one. This ratio is denoted as Mach number, a dimensionless quantity. This means that an aircraft with Mach 1 means it is travelling at the speed of sound.  

The machmeter is a pressure sensitive instrument which displays the Mach number on a needle-and-dial instrument. Additionally, the machmeter may be used to measure airspeed, and this is known as a Combined Speed Indicator (CSI). The Mach numbers for high-speed aircraft, such as airliners and business jets, must not be intentionally exceeded. Aerofoils may be buffeted or tucked if the aircraft is permitted to fly faster than its limitation mach number, whether on purpose or by accident. Here, the limitation Mach number,  is a dimensionless number that is significant in fluid dynamics and is defined by Ma, where a denotes the sound speed in meters per second.

Working Principle of a Machmeter

If you want to know about the working principle of machmeter, look no further. Two capsules and linkages are used in conjunction with a Machmeter to show the aircraft’s True Airspeed (TAS) as a percentage of the local speed of sound (LSS) This is the first capsule, which is an Airspeed Capsule, and it will expand and contract as a consequence of variations in dynamic pressure when it is introduced. When the static pressure within the instrument box changes, the second capsule, which is a sealed Altimeter Capsule, will expand and contract in response to the change. The information from an air data computer system is used to conduct calculations utilising inputs from a pitot-static system in modern electronic Machmeters. Pitot-static pressure is converted to a Mach number in certain older mechanical Machmeters using an aneroid for altitude and an airspeed capsule.

How does Machmeter differ from an airspeed indicator?

Machmeters are more accurate than airspeed indicators for displaying airspeed. It is unaffected by changes in the surrounding temperature or density. The position error (Due to the fact that it employs the same pitot and static pressure sources as the ASI, it is susceptible to position inaccuracy caused by disrupted airflow at the pitot head and/or static vent. The ASI does not suffer from this problem) is still there, thus it must point to a phenomenon that lies somewhere in the range between IAS (Pressure measured at the Pitot-tube) and CAS (Constant Airspeed). Due to the fact that dynamic pressure is related to square of speed, both the airspeed indicator and the Machmeter display square root of the difference (since dynamic pressure is proportional to square of speed). As a result, all faults have the same effect on both, and in electronic instruments, all errors are compensated to the same amount as before.

Pressure gradients are used in the pitot-static instrumentation. In order to determine speed and altitude, it measures pressures or pressure differential. When determining airspeed, the pitot pressure is used for the static pressure.

How to know the Mach number?

To get the vehicle’s Mach number (M), just divide the vehicle’s velocity (V) by the speed of sound (S). Mach 0.8 is 80 percent of the speed of sound, while Mach 3 is three times the speed of sound.

Uses of Machmeter

The uses of machmeter are discussed below:

The IAS (pressure measured at pitot-tube), the speed indicated by the airspeed indicator of an aeroplane is normally constant throughout ascent; if this is the case, the True airspeed will rise as a result of the continually decreasing air density as altitude is gained. Because the speed of sound correlates directly with temperature, the speed of sound decreases as altitude increases. Now, take a look at the formula: TAS (True Airspeed of the aircraft) grows, LSS (Local Speed of Sound) lowers, and what happens to the Mach number is unknown. Isn’t it true that it will rise? If the Mach number continues to rise, there is a strong possibility that it will surpass the MMO (Maximum Operating Mach Number). As a result, at a certain altitude, often 28000ft (although this varies depending on the aircraft), the pilot transitions from rising at a constant Indicated airspeed to climbing at a constant Mach number. As a result, a Machmeter is a very vital indicator for ensuring that the aircraft is flown within its capabilities.


When we fly at high altitudes or at high speeds, a Mach metre provides us with an indicator of our speed that is more precise and hence applicable for a wider range of altitudes and speeds that we may travel at. It is simple to convert our Mach number to a desired true air speed (TAS) at any point, for example, when we need to compute our ‘over the ground’ (GS) speed or estimate the wind direction based on our ground speed (GS) and true air speed (TAS). Our standard Air Speed Indicator (ASI) would need us to apply a large number of correction factors at various altitudes in order to determine our TAS, and those adjustment factors would all be different from one another. There would also be a significant increase in the number of crucial speeds that pilots would need to memorise, since they would be appropriate for certain altitude ranges.