When does engine idle speed increase to approach idle?

When does engine idle speed increase to approach idle?

• A. Flaps in takeoff configuration
• B. APU running
• C. Flaps in landing configuration or engine anti-ice on
• D. High altitude operation

Answer: (C)

Idle speed isn’t a fixed number; it rises when the engine has to supply more air or power for other systems. When the flaps are set to the landing configuration or the engine anti-ice is on, the engine must deliver extra bleed air and support additional air-system loads. To meet those demands while staying stable and avoiding compressor surge, the engine control increases the idle speed toward the approach-idle range. That higher idle keeps enough air flowing and maintains stable operation with the added loads from anti-ice and extended flaps. Flaps in takeoff configuration, APU running, or high altitude operation don’t impose the same increased bleed-air or system demands that push idle up, so they don’t trigger the same rise toward approach idle.

Idle speed isn’t a fixed number; it rises when the engine has to supply more air or power for other systems. When the flaps are set to the landing configuration or the engine anti-ice is on, the engine must deliver extra bleed air and support additional air-system loads. To meet those demands while staying stable and avoiding compressor surge, the engine control increases the idle speed toward the approach-idle range. That higher idle keeps enough air flowing and maintains stable operation with the added loads from anti-ice and extended flaps.

Flaps in takeoff configuration, APU running, or high altitude operation don’t impose the same increased bleed-air or system demands that push idle up, so they don’t trigger the same rise toward approach idle.