In turbine engines, N1 refers to the rotational speed of which component?

• A. In turbine engines, the rotational speed of the high pressure compressor.
• B. In turbine engines, the rotational speed of the low pressure compressor.
• C. In turbine engines, the rotational speed of the turbine rotor.
• D. Auxiliary drive speed of the engine.

Answer: (B)

N1 is the rotational speed of the low‑pressure compressor spool. In most turbine engines, there are multiple spools that can rotate at different speeds: the low‑pressure spool includes the fan and its compressor, and the high‑pressure spool includes the high‑pressure compressor. Each spool has its own shaft and its own speed indication. The high‑pressure compressor’s speed is tracked separately as N2. The turbine rotor that powers each spool is tied to its respective spool, but the flight instrument labeling uses N1 specifically to indicate the LP spool. Auxiliary drives have their own indicators. So the best-fit description is that N1 measures the low‑pressure compressor (fan) speed.