Definition
A cockpit gauge that displays the rotational speed of the high-pressure (second) compressor spool of a turbine engine, shown as a percentage of its rated maximum rpm rather than as actual rpm.
Plain English
A dial that shows how fast the engine's high-pressure inner shaft is spinning, given as a percent of its top speed.
Context Anchor
Seen on turbine engine instrument panels, especially during engine start, power setting, and engine monitoring.
Derivation
In turbine engine notation, 'N' stands for the rotational speed of a spool (shaft with its compressor and turbine). The number identifies which spool: N1 is the low-pressure spool, N2 is the high-pressure spool. In a three-spool engine, N3 would be the highest-pressure spool. Knowing this naming convention makes the whole family of turbine gauges easier to read.
Why Pilots Care
Correct N2 speed confirms proper engine operation, prevents overstress, and ensures adequate thrust and fuel efficiency.
Intuition Check
N2 does not mean engine number two. It means the speed of the engine’s second rotating section, usually the high-pressure section.
Example Sentence 1
During the start sequence, the pilot watched the N2 indicator climb through idle speed before introducing fuel.
Example Sentence 2
At takeoff power the N2 indicator stabilized at 95 percent, showing the engine was producing the required thrust.