Definition
A turbine engine instrument that displays the rotational speed of the high-pressure compressor section (the N₂ spool) as a percentage of its rated maximum rpm. In a dual-spool turbine engine, N₂ refers to the high-pressure spool, which connects the high-pressure compressor to the high-pressure turbine and drives most engine accessories.
Plain English
A gauge that shows how fast the inner, high-pressure part of a turbine engine is spinning, shown as a percentage rather than actual rpm.
Context Anchor
Seen on turbine-engine instrument panels, especially during engine start, power checks, and engine monitoring.
Derivation
The 'N' comes from the standard engineering symbol for rotational speed. The subscript '2' identifies the second (high-pressure) spool in a multi-spool engine, distinguishing it from N₁, which is the low-pressure spool. This numbering convention helps pilots and engineers keep the two independent rotating sections clearly separate.
Why Pilots Care
It helps monitor engine performance and detect problems such as compressor stall or overspeed before they cause damage or power loss.
Intuition Check
Do not read N2 as the speed of the airplane or the speed of the entire engine. It is the speed of a specific rotating section inside a turbine engine, shown as a percent of its rated speed.
Example Sentence 1
During the start sequence, the pilot watched the N₂ indicator climb steadily and introduced fuel once it reached the manufacturer's specified percentage.
Example Sentence 2
During cruise the N2 indicator remained steady at 85 percent, confirming normal high-pressure spool operation.