To ensure an engine maintains self sustaining speed.

• A. Idle increases with density decrease.
• B. Idle remains same for any density.
• C. Idle increases with density increase.
• D. Idle decreases with density increase.

Answer: (A)

Air mass flow is tied to air density, so at a fixed engine speed a drop in density means less air is entering the compressor. That reduces the pressure rise the engine can generate and the power available to drive the compressor-turbine system. To keep the engine able to sustain rotation and stay in stable operation, the idle speed must increase to draw more air and restore the balance between compressor demand and turbine supply. So, as density decreases, idle tends to rise.

If density increases, more air is available for the same speed, so the engine can maintain self-sustaining operation without needing to raise idle as much. The ideas that idle stays the same with density or that it increases with density are not consistent with the way mass flow and the power balance affect self-sustaining speed.