Definition
An instrument that displays the rotational speed of the propeller shaft in revolutions per minute (rpm). On a free-turbine or split-shaft engine, the propeller turns on a shaft that is mechanically separate from the gas-generator section, so its speed is measured and shown on its own dedicated gauge.
Plain English
A gauge that shows how fast the propeller is spinning, measured in rpm.
Context Anchor
Seen on turboprop engine instrument panels when checking, setting, or monitoring propeller speed during start, taxi, takeoff, climb, cruise, and landing.
Derivation
Tachometer comes from the Greek 'tachos' meaning speed, plus '-meter' meaning to measure. So the word literally means 'speed-measurer.' The 'propeller' qualifier specifies that this particular tachometer is reading propeller shaft speed, not the speed of an internal turbine or compressor.
Why Pilots Care
It allows the pilot to monitor and control propeller speed during start, takeoff, cruise, and landing to ensure safe and efficient engine operation.
Intuition Check
Do not assume this is simply the same as the engine tachometer in every airplane. In a free-turbine turboprop, propeller speed is measured separately from the speed of the engine’s gas-producing section.
Example Sentence 1
After setting takeoff power, the pilot cross-checked the propeller tachometer to confirm the propeller was turning at the rpm called for in the checklist.
Example Sentence 2
In flight the propeller tachometer reading guided the adjustment of torque to maintain the desired RPM.