Multiple choice

Machmeter displays the ratio of the true air speed to the

  1. local speed of sound

  2. winds aloft on a particular altitude

  3. ground speed of the aircraft

  4. none of these
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A Correct answer
Explanation

A Machmeter measures the Mach number, which is the ratio of an aircraft's true airspeed to the local speed of sound at that altitude. As altitude increases, temperature decreases and the local speed of sound decreases, so the same true airspeed yields a higher Mach number. The instrument helps pilots avoid exceeding critical Mach numbers where control problems begin.

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