Revolutions per minute (RPM) as displayed by the RPM indicator in the cockpit, is an indication of the amount of power provided by the aircraft’s power plant. As a general rule of thumb, higher the RPM, more the power and more the fuel consumption.

That said, more the number of cylinder’s in the aircraft, lesser the RPM indicated at the same speed on the RPM indicator. This is obviously due to engine efficiency. A lower powered engine e.g. a 4-cylinder Aircraft Sensor Systems has to put out more RPM to generate the same speed as say an aircraft with a 8-cylinder engine.

Apart from the amount of fuel the pilot feeds into the engine, another factor that controls propeller RPM is something called the ‘Propeller tip speed’ and on most single engine aircraft, it is usually below 2,700 RPM. Aircrafts with longer propellers e.g. king air, the RPM would be around 2,000 because it would generate more thrust and therefore require RPM i.e. have better economy.

Again, as a general rule of thumb, higher RPM generally means more maintenances visits due to wear and tear on the oil seals, bearings and also breakdowns caused due to transmission fluid leaks.

Bigger (e.g. 8-cylinder) aircraft engines have lower RPM by design. Running a higher MP/lower rpm results in better cylinder compression, reduces frictional losses, improves prop efficiency at lower rotational velocities and, allows the valves to run cooler resulting in lower EGTs and TITs.

For all the above reasons, the RPM indicator and the health of the RPM sensor as also the Bendix Mag is important. As the aircraft ages, the RPM sensor could get dust coated, damaged and so forth. With age, the accuracy of the RPM being displayed on the RPM indicator might be suspect. We’ve seen RPM accuracy off by 100 to 200 RPM in 15-year old aircraft.

If your aircraft is due for an overhaul and the RPM sensor, indicator and Bendix mag is due for replacement, you might want to consider a better-quality RPM sensor for pressurized Bendix series.

Simple in construct, the Bendix Magneto RPM Gauges is easy to maintain, is cost effective and pairs well with the J.P. Instruments manufactured RPM Sensors; which is highly accurate.

In relation to the Bendix mag, there is a rotating magnet inside it and it is important that it rotates at a speed that is within the prescribed range. If this does not happen, the voltage generated will be erratic and the spark plugs will misfire. This is where the RPM Sensor comes in. The RPM Sensor plugs into the magneto and transmits the data to the RPM indicator / display unit in the cockpit.

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