FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsMedium
A technician is explaining the operation of a Machmeter to a pilot. Inside the instrument, an altitude aneroid capsule senses absolute static pressure while a differential pressure capsule senses the difference between pitot and static pressure. What ratio of these pressures does the Machmeter compute to display Mach number?
- ADynamic pressure divided by outside air temperature
- BImpact pressure (pitot minus static) divided by absolute static pressure
- CStatic pressure divided by cabin differential pressure
- DTotal air temperature divided by static air temperature
Show answer & explanationAnswer & explanation
Correct answer: B. Impact pressure (pitot minus static) divided by absolute static pressure
Mach number is essentially the ratio of true airspeed to the local speed of sound, which can be derived from the ratio of impact pressure (pitot minus static) to absolute static pressure, since static pressure correlates with the local speed of sound at altitude.
Why the other options are wrong
- A. Outside air temperature is not a mechanical input to a standard pneumatic Machmeter.
- C. Cabin differential pressure is unrelated to Mach number computation.
- D. Temperature ratios are not the pressure inputs used mechanically inside a Machmeter.
Machmeter
An instrument that computes and displays Mach number using the ratio of impact pressure (pitot minus static) to absolute static pressure.
- Combines an altitude aneroid and a differential pressure capsule
- Ratio of impact pressure to static pressure yields Mach number
- Static pressure correlates with local speed of sound at altitude
Memory trick: Mach = impact pressure riding on the back of static pressure.