Definition
On a three-spool turbine engine, N3 is the rotational speed of the high-pressure spool, expressed as a percentage of its maximum rated rpm. The three-spool design has three independently rotating shafts: N1 (low-pressure/fan spool), N2 (intermediate-pressure spool), and N3 (high-pressure spool).
Plain English
In engines built with three separate rotating shafts, N3 is the speed of the innermost, fastest shaft, shown as a percentage rather than actual rpm.
Context Anchor
Seen on some turbine engine instruments, engine displays, aircraft manuals, and engine limitation charts.
Derivation
The 'N' stands for the rotational speed of a shaft, and the number identifies which shaft. N1 is the slowest, outermost spool; N2 is the middle spool; N3 is the innermost, highest-pressure spool. Most turbine engines have only two spools (N1 and N2), so N3 only appears on three-spool designs.
Why Pilots Care
N3 is a key parameter for monitoring engine health, thrust setting, and avoiding overspeed conditions in three-spool engines.
Intuition Check
N3 is not a navigation term or a model number here. It is an engine speed indication for the third rotating section of certain turbine engines.
Example Sentence 1
After start, the crew checked that N3 stabilized within limits before advancing the thrust levers.
Example Sentence 2
A sudden N3 drop can indicate a problem in the high-pressure turbine section.