As air density changes the RPM of a gas turbine engine will change. How is RPM kept at a constant speed?

• A. It is not.
• B. Automatically by a simple engine device.
• C. Manually by the pilot.
• D. By varying fuel flow only.

Answer: (A)

Air density changes how much air the engine can ingest at a given rotor speed. If density drops (as with higher altitude), the same RPM draws less mass, so the fuel–air balance shifts and the speed tends to change unless something automatically compensates. In a simple gas turbine without a speed governor or automatic fuel-control system, there isn’t a built-in mechanism that continuously holds RPM constant as density varies. Therefore, the RPM is not kept constant automatically; it would require deliberate adjustment of fuel flow (or throttle) to try to maintain a desired speed. Modern engines with full authority governors or FADEC can do this automatically, but a simple engine setup does not inherently keep RPM constant.