Definition
An engine instrument that displays the rotational speed of the low-pressure (gas generator) compressor and turbine assembly in a split-shaft or free turbine engine, expressed as a percentage of its maximum rated rpm.
Plain English
A gauge that shows how fast the engine's first spinning section is turning, shown as a percentage rather than actual rpm.
Context Anchor
Seen in turboprop cockpit engine instruments, especially during engine start, power changes, and engine limit checks.
Derivation
N' is the standard engineering symbol for rotational speed. The '1' identifies the first (low-pressure) spool in a multi-shaft engine. 'Tachometer' comes from the Greek 'tachos' meaning speed, plus 'meter' meaning to measure. So N1 tachometer literally means 'speed gauge for the first shaft.'
Why Pilots Care
Allows the pilot to monitor core engine operation, manage power settings, and avoid over-speed or under-speed conditions during start, taxi, and flight.
Intuition Check
Do not read N1 tachometer as the propeller rpm gauge. In this context, it shows gas generator speed; the propeller may be turning at a different speed.
Example Sentence 1
After advancing the power lever during start, the pilot watched the N1 tachometer climb to idle before introducing fuel.
Example Sentence 2
The pilot cross-checked the N1 tachometer with torque and fuel flow to confirm stable power output in cruise.