Engine hangs at 30 - 40 percent with low EGT
If the engine is hanging at about 30 - 40 percent with low EGT, you know you have at least some ignition. An electrical connector on the fuel control should be disconnected at this point. Inside the electrical connector on the fuel control unit is a small torque meter. This torque meter is a small coil that receives a milliamp signal from the ECU. This milliamp signal pulls a small arm up and down. The more amperage it receives, the more it opens, and as it opens, it allows more fuel to flow to the engine. You need to check to see if you have roughly 39 ohms at this point with the engine at rest. A value of between 30 - 42 ohms is really close enough because what you're looking for is an open or really high resistance. If your torque motor is not getting a milliamp signal from the ECU, it will just produce a minimum fuel flow to the engine. Also, tap on the fuel control unit slightly and look for a deflection of the needle on your analog meter. If you get any kind of a deflection, you know your torquemeter is bad; it's intermittent.
The last item to check is the output pressure of the fuel control. With the electrical connector disconnected and the ignition system disabled, place a gauge between the fuel control and the fuel nozzle. Then motor the engine up to 28 percent. Refer to a chart in the maintenance manual, and compare your fuel flow against the chart's minimum and maximum pressures. Then repeat the procedure with the electrical connector on the fuel control, and you should get a much higher setting. If either of the readings are off the chart, replace the FCU.
Engine hangs at 30 - 40 percent with high EGT
Another scenario is the engine hanging at 30 - 40 percent with a high EGT. In this case, you should check your oil level as well as the oil filter. It will be hard to rotate the gearbox if there is metal contamination. These models have a ten-second loss of oil shutdown delay. If you have a low oil level, the oil indicator light will flash on due to no oil pressure. The pilot continues to run the engine, and he or she doesn't realize the APU is starving for oil until the ECU senses more than ten seconds of continuous oil loss. There again, if you're having to add a quart of oil every time the engine comes back from a flight, this should be a strong indication that you have a problem. The thing only holds two and a half quarts!
Another problem that is quite common is a weak or erratic ignition. This is typically caused by people installing the ignitor plug improperly. You can rotate the combustor cap until it's hard against the plug, and after about 50 hours, the plug will fail. Instead of firing at the tip, the current will leave the ignitor half way down the plug and go to ground. This will give you very poor combustion.
The ignitor plug is on condition, but you do have an inspection every 600 - 1,200 hours. However, if you look at it and notice chafing halfway down the plug, you need to change it as it is probably misfiring.
When the unit stops, the confusion shouldn't start
AlliedSignal GTCP 36-100 Series APU troubleshooting
By Tim Coggin
continued from page 2....
Engine runs great, but it's not producing:
The APU can be running just fine but still not producing enough air to do things like start your main engine. This is a different class of problem that is often related to the load control valve. The load control valve controls the flow of air to the engine for starting or for air conditioning. Most problems with the load control valve are related to the bushing for the butterfly valve. This bushing is pulling out, which pushes the butterfly into the magnesium body. This causes the butterfly valve to chafe, and it will eventually only partially open. The result is that the valve will reduce the volume of air needed for a start. This may not be apparent right away because you will still have good pressure, just not the right volume. Main engine starts off the APU are more predicated on volume, not pressure. Therefore, if you are not getting enough volume, your engine may only spin up to 14 percent (where you should get 18 percent), and the result will be a hot start on your aircraft engine.
Also, the valve can be stuck in the open or closed position.
Another common problem with the load control valve, which doesn't affect power, but can still be a problem, is related to a small steel filter on the APU side of the butterfly valve that is shaped like a small eraser. This filter is made of steel, and the body is magnesium. The steel tends to corrode, which allows the filter to fall out into the bottom of the combustion can. Most of them sit down in the can; however, they do have the potential to go through the can and the turbine, causing a lot of damage. Many people find the filter and call me, not knowing what it is.
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