Helicopter Piston Engine Maintenance

Helicopter Piston Engine Maintenance By Greg Napert April 2000 For the most part, piston-powered helicopters are a relatively rare breed. Turbine-powered helicopters are more popular because of their increased utility and also for what many...

The Graphic Engine Monitor (GEM) allows you to monitor the cylinder head temperature of each cylinder, plus the exhaust gas temperature. One example of the use of the GEM is if a spark plug goes out, for instance. If a plug is inoperative, you can observe that the temperature of that cylinder will be hotter than the rest with both magnetos running. If you then switch from one magneto to the other, you will see that one cylinder is cooler than the other with one of the magnetos switched off. This will tell you exactly which cylinder the plug is in, and which magneto the problem plug is associated with.


Another example of using the GEM for troubleshooting is through monitoring the EGT on all cylinders and this will tell you, for instance, you may have a bad fuel control. If you have a bad magneto, you can tell by abnormal temperature indications across all cylinders while the engine is switched to one magneto. Or, these abnormal temps may be that you have one of the magnetos incorrectly timed, whether it's retarded or advanced. Advanced timing will actually give you lower EGT readings. For example, a low reading of 1,350 degrees across all cylinders (which is 100 degrees too low) will tell you that the magneto is probably one or two degrees out of time. This can result in detonation or pre-ignition and depending on if the magneto is advanced or retarded.

"In general," says Burdue, "we like to think of all items on a helicopter engine as being critical. For instance, in addition to the timing, we like to make sure that you always have good compression and good plugs on each cylinder. The reason that you need this is that it takes most of the 225 horsepower for the helicopter to run well. So, if you lose one cylinder or even one spark plug on a four-cylinder engine, that represents a large portion of the available horsepower. We also recommend frequent differential compression checks to monitor the condition of the engine and don't recommend any more than 10 percent difference from the highest to the lowest cylinder in terms of compression.

TBOs on the engines used in Enstrom helicopters are 1,500 hours and a bit lower than their fixed-wing counterparts. "We occasionally have received requests for TBO extensions," says Burdue, "but since Lycoming is the manufacturer, they are the ones who you need to go to for the extension. I have not seen them grant many extensions, but when they do, they typically require you to do more frequent inspections, oil changes, etc."

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