In a twin-spool engine, N1 denotes the speed of which component?

• A. High-pressure compressor
• B. Low-pressure turbine
• C. Low-pressure compressor
• D. Auxiliary power unit

Answer: (C)

In a twin-spool engine, there are two rotating shafts: a low-pressure spool and a high-pressure spool. N1 is the speed of the low-pressure spool, which includes the low-pressure compressor and the fan. The high-pressure spool speed is N2. The auxiliary power unit is separate from the engine spools, so it isn’t what N1 measures. So the motor speed you monitor and control as N1 corresponds to the low-pressure compressor/fan assembly.