The aircraft RPM Sensors we refer to in this article are the ones used to measure RPM of the Bendix Magneto or the Slick Magnetos. There's also the Dual Magneto but the popular ones in use are the Bendix Magnetos and the Slick Magnetos.
The Bendix Magneto is easily overhauled and therefore extremely cost effective. Companies such as J.P. Instruments manufacturers highly accurate aircraft RPM sensors for both - Bendix Magnetos and Slick Magnetos.
For the uninitiated, rather than rely on the aircraft battery, the power to the spark plugs comes from the magneto – think of it as a mini power generator. The Bendix Magneto is a small generator with a transformer, breaker switch and a distributor to guide the high voltage to the spark plugs.
A power generator obviously means there is a rotating magnet inside the generator. It is important that this magnet rotate within the prescribed range and to confirm this, you need the aircraft RPM Sensor which is a small cylindrical device that plugs into the magneto and provides a feedback to the RMP display unit in the cockpit.
Why Aircraft RPM Sensor is a big deal
A sudden RPM drop could indicate a serious problem with the spark plug. The magneto cam follower (or cam in the case of Slick magneto) generally wears at the same rate as the point faces. This intentional design feature ensures that the magnetos internal timing are at the correct angular opening point for long periods of time.
If there is inadequate lubrication of cam follower (or cam), it will result in accelerated wear leading to a domino effect inside your aircraft engine. To start with, it results in late engine timing i.e. a late spark which in turn reduces the sustainable RPM which in turn means fuel is not burned properly and there is a energy kick-back to the piston. Energy burned in the exhaust system results in abnormally high EGT. This is why it is important that the aircraft RPM always be within specified limits and to ensure this is so, your aircraft needs an accurate RPM sensor.
Replacing your Keyspan USB is easy
Attaching a new RPM sensor is easy; just remove the vent plug from the port on the magneto that covers the portion of the magneto that contains the rotating magnet. Next, you insert the correct EGT Gauges (depending on whether your aircraft has the Dual, Bendix or Slick Magneto) into the vent port and tighten.
Route the wiring bundle back towards the firewall taking care to not attach the bundle directly to the ignition, harness or magneto p-leads. Ensure there is sufficient slack. Plug in the connector or connect the wires to the corresponding colour wires in the instrument harness.
