In fact, almost every operator at some point (B or C Check), will remove and replace the combustion assembly, which includes the combustion cap, combustion liner, and fuel atomizer (nozzle). This extends the life of the APU considerably and it plays a big role in the long-term health of the engine. These components usually come as an assembly requiring disassembly and inspection of individual component parts. The atomizer is overhauled and all components are then reassembled and reinstalled into the unit. It is critical that the atomizer is properly shimmed based on the dimensional requirements of the combustion can."
By changing the combustion liner and atomizer, you ensure you're getting a more even combustion and it is easier on the combustion liner, turbine nozzle and turbine wheel, also known as the unit's "hot end."
Another reason to make sure you have a good nozzle and a good combustion can is reliability of starting.
According to Daggett, the most common problem in the field is that the APU won't accelerate beyond 30 to 35 percent at startup.
"The first thing we do if an engine won't start is to instruct the customer to listen for popping of the igniters. If the igniters aren't working, the starter will run the APU to around 30 to 35 percent and then it won't start."
"If the igniters are working properly, I typically tell the customer to check and inspect the P3 plugs and the P3 lines. Inability to accelerate is usually a result of the P3 line leaking. With a leak in the P3 line, you won't get the APU started. You will also notice a rise in exhaust gas temperature on the gauge in the cockpit."
Another reason for the engine not accelerating beyond 30 to 35 percent might be a damaged thermostat. The thermostat controls the amount of pressure in the P3 line, so if all P3 connections are OK, check the thermostat on the load control valve. A good way to check if the problem is in the thermostat is to cap off the P3 line from the thermostat. This will isolate the thermostat from the P3 line and allow the pressure to build. If the problem was in the thermostat, the engine will start and quickly accelerate. A word of caution — do not apply bleed load at this point as an "overtemp" condition may result. If the APU still doesn't start, you'll need to check the load control."
Daggett explains a less likely cause related to not being able to start the APU is if the atomizer is not shimmed properly. This will typically only occur following an atomizer change or inspection. If you have too many shims installed, the atomizer won't be protruding into the combustion liner far enough and it won't be getting the full benefit of the spray. The same can happen if it is sticking too far into the liner. If this is the case, you won't get proper atomization of the fuel. Finally, if the unit still doesn't start, the next item to check would be the fuel control unit.
GTCP85 APU Repair and Maintenance
The general APU condition can typically be tracked by monitoring the exhaust gas temperature (EGT) of the engine. As it deteriorates, the EGT rises. The APU will try to compensate for this somewhat by reducing output of the engine to keep the EGT within specifications. The overall effect of this is that there will be less and less air available for starting the main engines. In fact, the APU will actually cut off some bleed air of the APU. It will, however, continue to supply full power for electrical requirements of the generator. EGT is a good way to monitor the health of the engine. The required EGT specifications will vary based upon the engine model and age of the individual unit.
A word of caution from Miller — don't try to adjust the fuel pressures by changing the "crack" pressure settings or "governor" pressure settings. These adjustments are made to the fuel control unit in the test cell and should not be changed in the field without proper training and manufacturer specified equipment. Changing these settings can increase starting and operating fuel flows and result in damage to the internal components of the engine.
In addition to monitoring the EGT, it is a good practice at B and C checks to inspect the compressor for general damage. This can be done by removing the intake screens and feel the compressor blades for FOD damage, if possible, based upon the location of the APU within the specific aircraft.
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