What kind of tachometer is used on most large multi-engine airplanes?

• A. A mechanical tachometer
• B. A magnetic drag tachometer
• C. A digital tachometer
• D. A three-phase AC electric tachometer

Answer: (D)

In large multi-engine airplanes, engine speed is read using an electric tachometer driven from the engine’s accessory gear drive. The tachometer generator produces a three-phase AC electrical signal whose frequency is proportional to the engine RPM. The instrument in the cockpit reads that signal and displays the RPM. Using a three-phase AC output gives a stable, accurate signal over a wide RPM range and is less susceptible to wear, drift, or mechanical issues than a direct mechanical drive. It also adapts well to multiple engines since each engine can feed its own tach signal consistently to the cockpit indicators. So, the three-phase AC electric tachometer is the standard approach for these aircraft.