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.
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.
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.
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.
Miller explains, "The unfortunate part of taking care of an APU is there is typically little access for maintenance purposes. The APU is often enshrouded in tight areas, so maintenance personnel tend to wait until they have to before touching an APU. Many times, we receive APUs and none of the regular maintenance that you would expect, such as oil and filter changes, appears to ever have been done."