PLT036: Machmeter Readings
What code PLT036 covers, why the FAA tests it, and how to study Machmeter Readings before your checkride.
Interpret a MACH meter reading
What this means for your checkride
A Machmeter shows true airspeed as a fraction of the local speed of sound, the Mach number. Because the speed of sound falls with temperature, the same true airspeed reads as a higher Mach number the higher and colder you fly. Critical Mach is the free-stream Mach number at which airflow somewhere on the wing first reaches the speed of sound.
High-altitude airplanes are limited by Mach number, not indicated airspeed, so the FAA tests reading a Machmeter and understanding critical Mach and the compressibility effects that begin near it.
A fixed indicated airspeed does not mean a fixed Mach number. As you climb into colder air the Mach number for the same true airspeed rises toward the critical value.
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.