When the unit stops, the confusion shouldn't start
AlliedSignal GTCP 36-100 Series APU troubleshooting
By Tim Coggin
he AlliedSignal GTCP 36-100 series APU can appear as though it is a relatively complex engine. The Electronic Control Unit (ECU) on the engine coordinates many inputs from the aircraft and from the engine itself to produce a smooth running, relatively trouble-free engine that should go many hours between failures. Because of this, many technicians are not exposed to it on a day-to-day basis and don't become intimately familiar with the specifics, and therefore, trying to dig into the details of how the ECU works can be quite intimidating.
Fortunately, successful troubleshooting for this engine does not require a detailed understanding of electronics. Instead, what is needed is a broad view of how each of the systems interact on the engine. Once committed to a broad understanding, you can then refer to known problematic items to repair the engine in an expeditious manner.
The Difference in models
There isn't much difference between the -100 and the -150 in terms of their construction. Really the only difference is that electronically, the -100 is an analog, and the -150 is a digital. In terms of troubleshooting, the -150 will give you many more false trips. What I mean by that is that since the digital is more sensitive or more accurate, it will react to an overspeed or overtemp much more readily. It's more prone to shutting down for inexplicable reasons. This is actually good for the engine, however, because it protects the APU much more thoroughly. It can be more troubling to the technician, though, as many problems seem to disappear into thin air. In fact, the -150 APU has a TBO of over 4,000 hours, whereas the -100 is under 2,000 hours.
I'd rather fight than swap
One problem with this APU is that just about everyone is changing out the electronic control unit (ECU) at the drop of a hat. You should determine that the ECU is the problem before ordering a new one. Many times, the ECU is fine. Just about every Challenger and Gulfstream is equipped with two of these ECUs flying around at a cost of around $17,000 each, so it gets expensive to swap them.
For aircraft operating with two ECUs, don't wait until one of them fails before swapping to the other unit. The best approach is to use the two ECUs by swapping back and forth between them every other month. Then you know that they're both operating. This is better than waiting until one unit fails away from home and finding that your backup doesn't work. If you're going to spend that kind of money, at least use it to make sure it's serviceable. It's easy to swap out — it's a matter of swapping cannon plugs.
Is it really broken?
Your first approach to troubleshooting should always be to review when things are suppose to be operating, and when they aren't. One of the examples I give is ignition. Ignition doesn't start until 10 percent and turns off after 95 percent. If you have someone who is complaining about not having ignition above 95 percent, they don't know what they're doing. The system is operating the way it is suppose to.
Another example is the old start trim limits versus the new start trim limits. What happens is that on newer units, the ECU looks at the time as well as temperature for how much fuel is delivered to the APU at start. On older models, the ECU only looked at temperature, and the system was too slow to shut down when the temperatures got too hot, so operators were overheating the turbine wheels and nozzles. Now the ECU only allows about a 30-second start with a metered amount of fuel, and the typical starting temperature has been reduced from 857C down to 538C. You have a much cooler start and a longer engine life. The actual temperatures are not all that critical. There will often be a 100C spread from unit to unit, but you should only be concerned if the temperature variations are dramatic, and if the change in temperature within a short period of time is dramatic. For instance, I just saw an APU where it started at 500C and shot up to over 900C by the time it hit 25 percent. Just about everyone now has upgraded to the newer ECUs which reduce start temperature.
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