Review of Helicopter Engine Instruments 

by:  Paul D. Faltyn – Commercial Pilot and A&P Mechanic

A common misleading scenario concerning engine instrumentation among piston engine helicopter operators is that you do not repair or calibrate them unless they fail or there accuracy is suspect. All mechanically driven instruments have a ( TBO ) ” time between overhaul “, most manufacturers recommend that there instruments be overhauled between 1500 to 2500 hours in service.

Engine and Rotor Tachometers: 

The engine and rotor tachometer have bearings, motors, springs, pivots, and mechanical linkages that are subject to vibration, temperature changes, and wear. As the internal lubrication break downs, spring tension changes, parts wear, the pointers can become sluggish, sticky, or loose calibration.

I would suggest that at every engine overhaul or 1200 hours the calibration should be verified and overhauled at 2500 hour intervals per most manufacturers specifications for TBO.

Tach Generators:

Tach Generators are basically an electrical motor producing power to drive the tach indicators. The generator drive shaft should be checked for excessive play or worn splines, as this can cause the shaft to shear and you will loose your indication on the instrument.

Some of the older tach generators magnets will dissipate there charge over time causing low readings on the indicator. A common indication is where you are able to maintain standard manifold pressure but unable to maintain normal RPM.

The tach generator consists of 2 bearing, an oil seal, drive shaft, and a armature. They are cheap to overhaul and will last in excess of 3500 hours, but I would strongly recommend a TBO of no more than 2500 hours.