Definition
In a multi-spool turbine engine, N2 is the rotational speed of the high-pressure spool — the assembly consisting of the high-pressure compressor and the high-pressure turbine connected by a shaft. It is displayed in the cockpit as a percentage of the manufacturer-specified maximum rpm rather than in actual rpm. In a two-spool engine, N2 is the spool that sets core engine power and drives the engine accessories.
Plain English
N2 is a gauge reading that tells the pilot how fast the inner, high-pressure part of a jet or turboprop engine is spinning, shown as a percentage of its maximum allowable speed.
Context Anchor
Seen on turbine engine instruments, especially during engine start, power changes, and engine limitation checks.
Derivation
The 'N' is the standard engineering symbol for rotational speed. The '2' identifies the second spool — the high-pressure one. N1 refers to the first (low-pressure) spool. Knowing this makes the N1/N2 pairing on the engine display intuitive.
Why Pilots Care
Pilots monitor N2 during engine start, takeoff, and cruise to confirm normal acceleration and to avoid exceeding temperature or mechanical limits.
Intuition Check
N2 does not mean engine number 2. It means the speed of a specific rotating section inside a turbine engine.
Example Sentence 1
During engine start, the pilot waited for N2 to stabilize at idle before releasing the starter.
Example Sentence 2
At takeoff power the N2 indication stabilized at 95 percent with all other engine parameters normal.