Definition
In a split-shaft or free turbine engine, N1 is the rotational speed of the gas generator section — the compressor and the turbine that drives it — displayed in the cockpit as a percentage of its maximum rated rpm rather than as actual revolutions per minute. In a turbofan, N1 refers to the low-pressure spool, which includes the fan, low-pressure compressor, and low-pressure turbine on a common shaft.
Plain English
N1 is a gauge reading that shows how fast the first (low-speed) section of a turbine engine is spinning, given as a percentage of its top speed. So 95% N1 means that section is turning at 95% of its maximum allowed speed.
Context Anchor
Seen on turbine engine instruments, especially during engine start, power changes, and checks of a split-shaft or free-turbine engine.
Derivation
The 'N' is the standard engineering symbol for rotational speed. The '1' identifies the first (lowest-pressure) spool in the engine. Engines with a second spool use N2 for that one, and three-spool engines add N3. Numbering runs from the front of the engine inward by pressure stage.
Why Pilots Care
N1 is the primary power reference used to set and verify engine output during takeoff, climb, and cruise.
Grounding Statement
In a free-turbine engine, N1 can change even though the propeller speed is controlled separately.
Intuition Check
N1 does not mean “number one engine.” Here it means the speed of a specific rotating section inside the turbine engine.
Example Sentence 1
During the start sequence, the pilot waited until N1 stabilized before advancing the power lever.
Example Sentence 2
A drop in N1 during flight prompted the pilot to check for a possible compressor stall.