The TruTach II Optical Digital Tachometer (part number 10-01021) is a handheld aviation instrument used to measure aircraft propeller or rotor RPM using optical sensing. It does not require any connection to the aircraft system—instead, it reads rotational speed by detecting blade passage as the user points the device at the propeller.

It is commonly used in general aviation maintenance and flight operations to verify or cross-check mechanical tachometers and engine RPM readings.

Its primary features include:
Optical, non-contact RPM measurement system
Measures propeller speeds from approx. 240 to 70,000 RPM
Supports 2–5 blade propeller configurations
High accuracy digital signal processing (±1 RPM resolution)
Portable, cockpit- or ground-use instrument (no panel installation)
Powered by standard 9V battery (~150 hours operation)
Used for engine performance verification and calibration checks
Not FAA/EASA panel-certified (for reference use only)

This instrument is widely used by pilots and mechanics because it provides a quick, independent verification of engine RPM without modifying aircraft systems, making it especially useful for troubleshooting and performance checks.