PLT315: Machmeter Principles
What code PLT315 covers, why the FAA tests it, and how to study Machmeter Principles before your checkride.
Recall Machmeter - principles / functions
What this means for your checkride
The Machmeter displays the aircraft's speed as a fraction of the speed of sound, called Mach number, rather than as knots. This matters at high altitude and high speed where the local speed of sound changes with temperature. A given true airspeed can represent a higher Mach number as altitude increases, which is why high performance aircraft monitor the Mach meter.
Pilots of high speed aircraft must track Mach number to stay below compressibility and high speed limits.
Students assume a fixed knots limit is enough, but the critical limit at altitude is Mach number, which the Machmeter shows directly.
Where to study it
A code on your test report is an area of deficiency. An authorized instructor must review it with you and endorse that review before you take the practical test, and the examiner may revisit it during the oral. Study it until you can explain it out loud, not just recognize the answer.