GTCP85 APU Repair and Maintenance
By Greg Napert
The GTCP85 Series Auxiliary Power Unit (APU), manufactured by Allied-Signal Aerospace Company, is a very common APU that has matured over a period of 20-plus years in the industry. It is used on Boeing 707, 727, 737, DC9, DC10, MD-80, BAC 1-11, and C-130 to name a few.
Although there are dozens of variation on the GTCP85, the heart of the engine and the systems involved in operating the engine are quite similar.
Typically, different accessories, hardware and wiring harness are used from instillation to installation and these minor changes, for the most part, account for the large variety of model numbers. Service bulletin compliance can also affect the model number of the APU - which is important to know in the event of selecting a replacement unit for an aircraft.
The Forgotten Engine
APUs are very forgiving in most aspect of their operation, and because they are tucked neatly out of sight, regular maintenance is often skipped or delayed until a major maintenance event. Because the engine is not considered crucial for the safety of flight, it's not often considered a big deal to defer problems or scheduled maintenance - particularly if there are alternate sources for starting and electrical power while on the ground.
Unfortunately, this attitude means that the APU is often neglected and some operators simply choose to operate the unit until it fails. The result is that the costs to repair the APU and bring it back to serviceable limits are much higher than if it had been maintained properly.
But then again, the design of the APU is such that it is subject to very little foreign object damage (FOD) — and since the engine is pneumatically and electromechanically controlled, it's protected from events such as overspeeds and overtemps.
According to John Miller, APU shop supervisor for Kitty Hawk Turbines, a turbine overhaul/repair facility in Ypsilanti, MI, which specializes in GE CF700 and Garrett GTCP85 and GTCP660 Series APUs, "The damage an APU can sustain and still continue to run is really quite amazing."
Many APUs arrive, from their own fleet of aircraft or from outside customers, in a state where they are no longer operational. Compressors and turbines often exhibit signs of extreme wear and need to be either completely repaired or even overhauled.
The interesting thing about the APU is that there is no particular requirement to overhaul the unit at any specific time as "This is not a time-controlled unit," says Miller.
According to Miller, "Many commercial airlines schedule an APU hot section inspection at each C-Check and this can reduce repair costs significantly."
Due to the unpredictable condition of the units that arrive at Kitty Hawk, the first item to address as these units arrive at the repair station is whether the unit can be put on a test cell. "Many units that are operational are put on the test cell to document performance prior to disassembly. We do this based on customer requests," says Miller.
Often the customer wants to know the condition if they receive an APU from another source or they are unsure of the operational characteristics of the engine. "Another reason we run it in the cell is to give us an indication of what the engine needs. The engine may only be in for a hot section inspection or other type of repair, but the test cell will indicate if the compressor is good, or if there are other problems that need to be addressed such as vibration problems."
Many of the units, however, come in very poor condition and don't even run. In these cases, the technicians simply begin the teardown process and document the condition of each of the components.
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