Definition
A cockpit gauge that displays the rotational speed of the high-pressure (inner) spool of a turbine engine, expressed as a percentage of its maximum rated rpm. In a two-spool turbine, the N₂ spool consists of the high-pressure compressor driven by the high-pressure turbine, and N₂ is the primary indicator of core engine speed used for starting and power management.
Plain English
A gauge that shows how fast the inner, high-speed shaft of the turbine engine is spinning, shown as a percentage of its top speed.
Context Anchor
Seen on turbine-engine instrument panels and in engine power-setting procedures.
Derivation
The 'N' is the standard engineering symbol for rotational speed. The subscript '2' identifies the second (inner, high-pressure) spool. N₁ is the low-pressure spool; N₂ is the high-pressure spool. Knowing the numbering helps you read the gauges correctly at a glance.
Why Pilots Care
Used to set thrust, monitor engine health, and avoid exceeding RPM limits that could damage the engine.
Intuition Check
Do not read N1 as an engine number or a simple label. Here, N1 means the speed of a specific rotating section inside the turbine engine.
Example Sentence 1
During engine start, the pilot waited for N₂ to reach the specified percentage before moving the fuel control to run.
Example Sentence 2
At takeoff power the N2 reading reached 95 percent before the pilot advanced the thrust levers.