Honeywell's GTCP36-100 APU
Quit throwing parts at it!
By John Casey
Are you one of those mechanics that start throwing parts at the auxiliary power unit (APU) when it is not doing what you think it should? Usually if one changes enough parts, the law of averages says that you will eventually find the faulted component. I saw six fuel control units (FCU) changed for a start related fault, before finding that the aircraft fuel system was heavily contaminated. In another case, a mechanic changed 12 starters before further investigation revealed gearbox problems. APUs are removed for aircraft electrical problems and electronic control units (ECU) are swapped for fuel atomizer restrictions. Throwing parts at the APU and hoping to get the one that fixes the fault is expensive, time consuming, and very frustrating. If one takes time to learn the APU and its operation, troubleshooting will become easier. Troubleshooting is isolating faults, not changing parts.
Please note that this article contains tips and suggestions — always refer to the applicable maintenance manual before performing any maintenance tasks.
First, what makes the APU tick?
The 36-100 APU has been installed in nearly two dozen different applications including: Falcon 50, Gulfstream (GI, GII, GIII, GIV), BAe HS-125, Bae 146, Jetstream II, Canadair Challenger, Ground Carts and Military applications. The APU is a constant speed engine utilizing a single centrifugal compressor and a single radial turbine. The APU uses a single, duplex fuel atomizer installed in the center of a can style combustor and is equipped with a single igniter.
Control of the APU is through the action of an analog ECU. The ECU does not monitor for open circuits. If the component fails mechanically, the ECU cannot "see" the fault, forcing the mechanic to resort to conventional troubleshooting methodology. All signals to and from the APU must go through the ECU. When troubleshooting, do not forget the wiring harness, the cannon plug and the ECU.
Most of the 36-100 series APUs incorporate a surge control valve (SCV) to prevent the APU from surging (compressor stall) during acceleration, heavy shaft loads and in-flight operation. The installation is equipped with a load control valve (LCV) to allow selection of pneumatic (bleed) load and provide exhaust gas temperature (EGT) protection for the APU.
One needs to be aware of what happens at each point in the APU's operation. At 10 percent RPM, the fuel solenoid shut-off valve (FSOV), ignition unit and the SCV (if equipped) are energized. At 60 percent RPM, the starter is de-energized and, at 95 percent plus four seconds, the ready circuits are energized. The APU accelerates to governed speed (100 percent) and the ECU, in conjunction with the FCU, keeps the APU operating within one percent of 100 percent RPM. This governor action will control APU RPM during idle, electrical and bleed loading.
The APU is running - Why monitor?
APU monitoring should start when the APU is operating normally. Do some preliminary footwork; fill out an APU monitoring check-sheet with the basics. What is the start like? What is the acceleration time? What is the EGT at idle and at full load? How long does the APU take to stop rotating after you have pressed the stop button? When the APU is at idle, is the SCV open or closed? Is it in the correct position? Are you starting to get the idea?
If you do not know what is correct, how do you know when the APU begins to falter? Often there will be a change in the APU's operational characteristics before it really gives you problems. These will usually show up on an APU monitor check-sheet, indicating a malfunctioning system or component. One misconception with this APU is that all you have to do is operate it until it breaks. This is not correct, one should perform periodic maintenance inspections in accordance with the applicable maintenance manual and open their senses to what the APU is doing. A properly completed APU monitor check-sheet, filled out at regular intervals, can reduce operating costs by identifying problems early and allowing the operator to schedule maintenance.
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