GTCP85 APU Repair and Maintenance

GTCP85 APU Repair and Maintenance By Greg Napert October 1999 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...


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Miller says, "Each APU is treated as an individual and we work closely with our customers to determine the workscope for the engine. In general, there's a lot of flexibility with an APU. Some customers just want the unit to be operational again, and others want a complete overhaul — still, it's not very common to perform a complete overhaul on a unit — it's more a matter of inspect-and-repair-as-necessary. Many times, however, these repairs are very heavy repairs which result in rebuilding most of the engine.

"It depends on the current condition of the APU," he continues. "Commercial airlines typically want to go as long as possible between overhauls/repairs, so they invest more in the engine, tending not to cut corners."

"It is not uncommon that a customer sends a unit for service based on a perceived problem only to find that the engine requires other repairs. For instance, we find many times that the customer expects a hot section inspection, but the hot section may not be what is causing the engine to perform poorly. So we have to add to the workscope and do the additional maintenance to bring the engine up to acceptable standards."

Costs under control
Although the typical repair on this particular APU can range from $10,000 to $40,000, those costs are being reduced over time. Materials used to manufacture replacement parts are improving and we are seeing less damage to the engine components during the typical inspection. Additionally, many customers are learning how to manage the maintenance of these engines to reduce costs.

Further, repair schemes developed over the years have helped reduce the cost of replacement components significantly. Repair facilities, in the past, had to replace most components with new, but major components, like the compressor impeller, can now have blades welded and recontoured onto the impeller under an approved process specification.

Going the extra mile
According to Scott Davis, marketing manager for Kitty Hawk Turbines, the company takes many extra steps to ensure the APU will last out in the field. For instance, "When we decide we are going to repair a unit, we remove and inspect all the accessories and harnesses. We emphasize thorough repair procedures on all of the accessories as they play a large part in the engine operating properly, particularly since the APU is a self contained self-sustaining unit. Our philosophy is ÔIf your going to do it, do it right the first time.' It doesn't make sense to fix half of the problem and have to troubleshoot from the field throwing exorbitant amounts of money at the unit for ÔBand-Aid®' repairs."

"Also, we balance all rotating components prior to installing them into the engine, then we do a vibration analysis in the test cell to insure that all vibrations are within acceptable limits," he says.

Preventative maintenance
Edgar Daggett, a lead mechanic for Kitty Hawk, provides most of the over-the-phone and field troubleshooting for Kitty Hawk, explains that one of the most critical components to maintain on the engine is the fuel atomizer. Daggett gained most of his experience troubleshooting Kitty Hawk's own fleet of APUs from the company's cargo and charter operation.

"The 85 model APU has only one atomizer, which makes it much more critical to keep it operational and clean. A good operator will clean this atomizer frequently. If it starts streaking, it will damage interior components by producing hot spots on the burner can or on the turbine nozzle.

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