At altitude, idling RPM is.

• A. Higher than at sea level
• B. Same as at sea level
• C. Lower than at sea level
• D. Cannot be determined

Answer: (A)

When a gas turbine engine idles, the fuel control keeps the rotor spinning at a low, stable speed so that the combustor can stay lit with the minimum air flow. At higher altitudes the air is thinner, so for the same rotor speed there is less mass flow through the compressor and less pressure rise. To compensate and maintain stable idle operation (avoiding surge and flameout), the fuel control tends to allow and often require the engine to spool up to a higher idle speed. In other words, the engine must rotate faster at altitude to ingest enough air and maintain a stable idle condition. That’s why idle RPM is higher than at sea level. Lower or unchanged idle speeds wouldn’t reliably support stable idle operation in thinner air, and “cannot be determined” isn’t correct because the engine’s idle schedule is designed to adapt with altitude.