Banging and hot
Inside the engine scroll, after combustion, a deflector shield deflects the hot gases down into the turbine nozzle. If this shield is cracked, it will allow air to bleed over into the cold side and be recombusted again, and you are combusting dirty air. This develops carbon, which contaminates the engine. To determine that this is the problem, open the load control valve to bleed off the pressure. If doing this stops the engine from surging, you know you have a cracked shield.
This engine is designed to run close to what is called the "surge line." Anytime you have FOD damage, it will take it over the surge line. The sound coming from the engine will be similar to a machine gun or like a dog barking. If you try to reduce this by bleeding air from the load control valve (turn on the air conditioning), and the engine still is making the noise, you either have FOD on the compressor, or you have carbon blocking the flow of air.
Low air with high EGT
If you have low air with High EGT, you have a problem with the compressor turbine nozzle. The leading edges of the nozzle can get overheated and damaged, and the result is low air coming from the APU. For example, you hit the start button for your main engine, and the pressure drops to 6 psi. Again, this would result in a hot start for your main engines. Shutdown problems: The key to determining APU shutdown problems is as follows: No sign of life If you hear the APU stop operating and the gauges in the cockpit immediately drop to zero, you've lost power to the ECU. This could be a problem with the door switch, a cowling switch, or the pilot may have hit the generator switch on the Falcon 50. In any of these cases, it would have to be something that would have occurred for more than 50 milliseconds in order for the ECU to recognize the problem and shut down the engine.
RPM drops to zero
There is a monopole on the engine that uses the teeth on the bull gear of the engine to measure the frequency and determine the speed of the engine. If this fails, the RPM gauge will drop to zero and the EGT will roll back. You can check the monopole by measuring resistance. It should read about 20 ohms. Again, you need more than a 1/2 second loss of signal before the ECU recognizes it. If you have metal on the end of the monopole, you will fool it, and the signal will be erroneous, which will cause the engine to shut down. If you pull it out of the engine, and you notice metal filings on it, you know you've found a problem with the gearbox.
EGT drops to zero and the rpm rolls back
If the EGT drops to zero and the rpm rolls back, a thermocouple may be opened. Sometimes it will only be a flicker, then they shutdown, and then they both roll back. This can be something as simple as one of the thermocouple nuts being loose, or even one of the posts being loose.
For troubleshooting this problem, if you go from the ECU cannon plug up to the gauge and put in 1VDC, the gauge should read 1,000C. This is good to know because if you put in .5 VDC, you should get 500C in the cockpit. If you're reading 400C or 600C, you've got a problem with the gauge.
If you have a foul smell coming from the engine at the first start of the day, you are smelling burned synthetic oil. The impeller sits near a seal, so there's a lot of vacuum there, and if there is any problem with the seal, the impeller will suck oil out of the engine pretty quick. Whenever the carbon seal does crack, it mists the entire cabin with a synthetic oil. When the seal fails, it will suck 2 1/2 quarts in 3 1/2 minutes, and that's pretty much all the oil in the APU. By the time you realize you have a bad one, and it fogs, it will have already sucked all of the oil out of it. It will count to 10 and shut down.
That's why I emphasize that if you're smelling a foul odor, you need to see if your oil consumption is going up. If you're adding oil to it all the time, you've got a problem. You may also see brown syrupy oil coming out of the APU. But you're going to smell it first.
Don't be alarmed
Many people are alarmed when they see cracks in the labyrinth seal. Cracks in the labyrinth seal are not a very big problem as long as they are within maintenance manual limits. That is, six cracks maximum that are nonadjacent. Essentially, if you have large pieces cracked off, they are not going anywhere — they are contained by the diffuser. Even large gaps are okay as long as there are larger pieces.