In turbine engines, N2 refers to the rotational speed of the ...

• A. Low pressure turbine.
• B. Fan assembly.
• C. High pressure compressor.
• D. Shaft bearing.

Answer: (C)

N2 is the rotational speed of the high‑pressure compressor—the high‑pressure (core) spool of the engine. In a typical two‑spool turbofan or turbojet, there are two rotating assemblies: the low‑pressure spool (the fan and the low‑pressure compressor) driven by the low‑pressure turbine, and the high‑pressure spool (the high‑pressure compressor) driven by the high‑pressure turbine. The engine controls monitor N1 for the low‑pressure spool and N2 for the high‑pressure spool to regulate fuel flow and maintain stable operation, including compressor stability and surge margin. The other options don't fit because the low‑pressure turbine and the fan are part of the N1 spool, and a shaft bearing is a component, not a rotating speed.