FAA Powerplant Written Practice Test

In what units are turbine engine tachometers typically calibrated?

1. Revolutions per minute

2. Percent of engine rpm

3. Knots

4. Degrees of rotation

The correct answer is: Percent of engine rpm

Turbine engine tachometers are typically calibrated in percent of engine rpm to provide a relative measure of the engine's performance. This calibration is crucial because turbine engines operate efficiently within specific performance ranges, and the percentage format allows for better understanding of engine output compared to just the raw revolutions per minute. Since turbine engines can vary significantly in design and optimal operating conditions, expressing performance in percent helps technicians and pilots quickly assess how close the engine is operating to its design limits, rather than just focusing on raw speed readings. Using percentages allows for easier comparisons between different engines and flight conditions. Unlike the other options, which do not provide the nuanced performance metrics necessary for turbine engine operations, percent of engine rpm indicates how much of the maximum capacity the engine is utilizing at any given time, making it a more versatile and informative measurement for monitoring performance during various phases of flight.