Definition
A small electrical generator driven by a rotating shaft (typically an engine accessory drive) whose output voltage or frequency is proportional to the shaft's rotational speed. The signal is sent to a cockpit indicator that displays RPM. Tachometer generators are commonly used on turbine engines and on aircraft where a remote, electrically driven RPM indication is preferred over a mechanical cable drive.
Plain English
A small generator attached to the engine that produces an electrical signal whose strength changes with how fast the engine is turning. That signal drives the RPM gauge in the cockpit.
Context Anchor
Seen in engine instrument systems, maintenance troubleshooting, and checks involving an inaccurate or failed RPM indication.
Derivation
From Greek 'tachos' meaning 'speed' and 'metron' meaning 'measure,' combined with 'generator' (a device that produces electricity). So a tachometer generator is literally a 'speed-measuring electricity producer' — it turns shaft speed into an electrical signal that a gauge can read.
Why Pilots Care
Accurate RPM indication is essential for setting power, avoiding engine overspeed, and complying with operating limitations.
Intuition Check
Do not confuse this with the aircraft’s main electrical generator. A tachometer generator is mainly a signal source for the RPM instrument, not the unit that powers the airplane’s electrical system.
Example Sentence 1
The N2 tachometer generator on the accessory gearbox sends a signal to the cockpit RPM indicator.
Example Sentence 2
During climb the pilot cross-checked the tachometer reading against manifold pressure to confirm the generator was functioning correctly.