FAA Powerplant Written Practice Test

In what units are turbine engine tachometers typically calibrated?

1. Revolutions per minute

2. Percentage of engine rpm

3. Liters per hour

4. Temperature in degrees Celsius

The correct answer is: Percentage of engine rpm

Turbine engine tachometers are typically calibrated in percentage of engine RPM because this measurement provides an efficient way to convey engine performance relative to its maximum operating speed. Using a percentage allows operators and technicians to easily assess the engine's performance in relation to its limits, which is crucial for safe operation and maintenance. Turbine engines have a wide range of operating conditions and can vary significantly in their operational characteristics. By expressing the speed as a percentage, it simplifies the interpretation of the engine's status, especially during takeoff, climb, and other critical phases of flight where monitoring engine performance is essential. In contrast, revolutions per minute could be a direct measurement but might not provide the necessary context regarding the engine's operational limits. The other options, such as liters per hour and temperature in degrees Celsius, are related to fuel consumption and thermal measurements, respectively, and are not relevant to measuring the speed of turbine engines. Hence, the use of percentage of engine RPM becomes the preferred choice for tachometer calibration in turbine engines.