When the unit stops, the confusion shouldn't start

April 1, 1998

When the unit stops, the confusion shouldn't start

AlliedSignal GTCP 36-100 Series APU troubleshooting

By Tim Coggin

April 1998

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.

I'm talking ECU-listening?
A basic understanding of the ECU is very important because it controls everything on the engine, and all inputs to the engine go through it.

A review of the communication inputs and outputs related to the ECU are as follows:

The four inputs are really the pilot (through the master switch), the stop switch, the generator switch, and the air switch (and you can control that).

Then there are two inputs that control the ECU: the EGT probe and the RPM. You have to have a failure of either of these controls for more than a half a second before the ECU reacts to it.

There are also eight outputs from the ECU which include the starter relay, surge valve, fuel shutoff, FCU, ignitor, LCV, hourmeter, and generator relay.

Finally, there are five fault trips: speed, temperature, current, hot oil, and low oil.

On the Falcon 50 aircraft, the hot (high oil temperature) indicator is not used. The wire for the hot (optional) sensor simply comes out of the box and goes back in. I have seen this wire cut. The result is that the ECU will shut down, even though there is no sensor on the Falcon 50.

Start with a simplified schematic
You can really get bogged down if you try to troubleshoot the electronics inside the ECU. It's best to start troubleshooting with a basic schematic so that you don't get lost in the ECU. The schematic I recommend basically separates all airframe-related components from APU-related components. Everything on the left is airframe-related, and everything on the right is APU-related. The latches in the center are for fault indication only. One thing that's critical to understand is that the overspeed is always latched by the ECU whenever any other problem occurs. If you find the overspeed latched along with another indicator, it means you had a problem with the item that latched along with the overspeed. It doesn't mean you had a problem with the overspeed. There was an overspeed problem only if the overspeed is the only item that was latched.

When the unit stops, the confusion shouldn't start

AlliedSignal GTCP 36-100 Series APU troubleshooting

By Tim Coggin

April 1998

continued from page 1....

Other items before you start
Other good-to-know items about the engine that you should review before you start troubleshooting are airflow (refer to the airflow illustration) and items that are line replaceable. There are 17 line replaceable units (LRUs) on the -100 APU. They are shown in the following illustration. Don't forget that the ignition lead itself can be cause problems as well.

Also, before you start, get to know where the most problematic areas on the engine are. One typical example on the -100 is a recurring problem with the hourmeter which inevitably results in an overcurrent shutdown. Overcurrent problems are hard to troubleshoot because it is basically some item shorting in the system. To troubleshoot it, you have to start isolating each item. Also, overcurrents are often erratic. For some reason, it's quite common for the hourmeter to short out. You may want to just disconnect the wires to the hourmeter, and then if the APU starts, you're on your way. This is something wouldn't normally condone, yet it may be necessary in order to get you back to a maintenance base where the problem can be addressed properly. The hourmeter isn't cheap — it's roughly $5000 — and this creates another problem. People are removing these and putting in a Hobbs meter, which is around $250. The problem with this is that it draws more current that the AlliedSignal hourmeter, and it eventually burns out the circuit in the ECU. So now you're having to replace a $5000 hourmeter and a $17,000 ECU (exchange for $8,000 or $9,000). All this for an hour meter problem.

Some maintenance facilities have gotten around this by installing a Hobbs and rewiring it so that it gets its power from a separate source, instead of through the ECU. But remember, you just don't arbitrarily go putting equipment on your engine that isn't certified. You've got to go through the legal hoops for whatever modifications you make.

Although it's not a complete list of troubleshooting problems, the following is a list of the most common items you will need to troubleshoot on the -100 engine.

Starting problems:
Not enough battery power
You need a minimum of 16 volts before you can even turn on the ECU. After you hit the start button, the ECU allows the voltage to drop to 12 Volts because the starter draws quite a bit of current. It will allow this drop up to 25 percent engine speed, then after that, you've got to be back to 16 Volts.

Many people have this problem: the APU climbs to 25 percent and then dies because there isn't enough battery voltage. They think that it's a problem with the APU and not the battery. However, you need sufficient battery power in order to start.

In reality, though, the manufacturers have installed better, larger batteries in the aircraft, so this doesn't happen often. There was a time when the pilots had to remove the batteries from the aircraft and bring them inside the hangar so they stayed warm; the batteries were just too small.

Relays and switches
On the Falcon 50, a common problem occurs with the inlet air door relay. Power has to go through the door relay and the door switch on the GIII and IV before it goes to the ECU. If you don't get power through that, your ECU will not have power. Quite frankly, it's easy to tell if you've got power to the ECU because if you look at your EGT gauge, it's will read ambient temperature. On the Challenger, there's no door.

Some GIIIs and GIVs have what are called "hidden switches" or "stealth" switches. These are switches which prevent the APU from starting when the cowling is open by disarming power to the APU. This switch is a problem because if the switch fails, or if the cowl isn't closed correctly, the APU won't work. The switch is there because otherwise, the exhaust would be pointing at the cowling with the cowling down, and then the cowling would burn. Many people remove them, however, because they fail so quickly.

Starter resistance Another problem would be starter resistance. This is easy to check for — you just check for less than one ohm. It's about a 1/2 ohm between the positive and negative terminal. If you're getting more than one ohm, you want to exchange it for a new one.

You can also check the starter clutch. On the back of the starter, there is a slot for a common screwdriver behind a triangular plate. If you take a screwdriver, you should be able to turn it one way and notice that the APU doesn't rotate. If you turn it the other way and it should engage the APU. If it turns both ways, your clutch has failed, and you have to pull the APU because you have probably contaminated the gearbox with metal.

EGT probes
The EGT probes can be a problem at times. Older style probes were set in a potting compound that would break down, and the connection would become bad. Newer style probes are better and don't have that problem. If you have an open, or if one of the leads is off (or even loose nuts), it will sense very high temperatures and won't start due to the loss of the EGT signal.

Also, if the posts are shorted together, you would read ambient temperature; no matter what it's reading inside, the fuel control will keep dumping fuel in. In this case, you can literally burn up the engine.

Acceleration problems (engine hanging up):
Engine hangs at 28 percent
The starter can turn the engine up to around 28 percent, so if you're at 28 percent with no EGT, and fuel is draining from the APU, you can pretty much assume that you have an ignition problem. If you look at the ignitor connection, it should be nice and white. If you notice, however, that at either end, you have a brown-colored ignition lead, as if it is burnt, you can suspect that either the lead or the ignitor box is shorting out. The washer on the end of the ignitor plug is very important, it insulates the heat from the combustor and protects you from damaging the plug. It's primary job is as a heat sync.

Engine hangs at 30 - 40 percent with low EGT
If the engine is hanging at about 30 - 40 percent with low EGT, you know you have at least some ignition. An electrical connector on the fuel control should be disconnected at this point. Inside the electrical connector on the fuel control unit is a small torque meter. This torque meter is a small coil that receives a milliamp signal from the ECU. This milliamp signal pulls a small arm up and down. The more amperage it receives, the more it opens, and as it opens, it allows more fuel to flow to the engine. You need to check to see if you have roughly 39 ohms at this point with the engine at rest. A value of between 30 - 42 ohms is really close enough because what you're looking for is an open or really high resistance. If your torque motor is not getting a milliamp signal from the ECU, it will just produce a minimum fuel flow to the engine. Also, tap on the fuel control unit slightly and look for a deflection of the needle on your analog meter. If you get any kind of a deflection, you know your torquemeter is bad; it's intermittent.

The last item to check is the output pressure of the fuel control. With the electrical connector disconnected and the ignition system disabled, place a gauge between the fuel control and the fuel nozzle. Then motor the engine up to 28 percent. Refer to a chart in the maintenance manual, and compare your fuel flow against the chart's minimum and maximum pressures. Then repeat the procedure with the electrical connector on the fuel control, and you should get a much higher setting. If either of the readings are off the chart, replace the FCU.

Engine hangs at 30 - 40 percent with high EGT
Another scenario is the engine hanging at 30 - 40 percent with a high EGT. In this case, you should check your oil level as well as the oil filter. It will be hard to rotate the gearbox if there is metal contamination. These models have a ten-second loss of oil shutdown delay. If you have a low oil level, the oil indicator light will flash on due to no oil pressure. The pilot continues to run the engine, and he or she doesn't realize the APU is starving for oil until the ECU senses more than ten seconds of continuous oil loss. There again, if you're having to add a quart of oil every time the engine comes back from a flight, this should be a strong indication that you have a problem. The thing only holds two and a half quarts!

Another problem that is quite common is a weak or erratic ignition. This is typically caused by people installing the ignitor plug improperly. You can rotate the combustor cap until it's hard against the plug, and after about 50 hours, the plug will fail. Instead of firing at the tip, the current will leave the ignitor half way down the plug and go to ground. This will give you very poor combustion.

The ignitor plug is on condition, but you do have an inspection every 600 - 1,200 hours. However, if you look at it and notice chafing halfway down the plug, you need to change it as it is probably misfiring.

When the unit stops, the confusion shouldn't start

AlliedSignal GTCP 36-100 Series APU troubleshooting

By Tim Coggin

April 1998

continued from page 2....

Engine runs great, but it's not producing:
The APU can be running just fine but still not producing enough air to do things like start your main engine. This is a different class of problem that is often related to the load control valve. The load control valve controls the flow of air to the engine for starting or for air conditioning. Most problems with the load control valve are related to the bushing for the butterfly valve. This bushing is pulling out, which pushes the butterfly into the magnesium body. This causes the butterfly valve to chafe, and it will eventually only partially open. The result is that the valve will reduce the volume of air needed for a start. This may not be apparent right away because you will still have good pressure, just not the right volume. Main engine starts off the APU are more predicated on volume, not pressure. Therefore, if you are not getting enough volume, your engine may only spin up to 14 percent (where you should get 18 percent), and the result will be a hot start on your aircraft engine.

Also, the valve can be stuck in the open or closed position.

Another common problem with the load control valve, which doesn't affect power, but can still be a problem, is related to a small steel filter on the APU side of the butterfly valve that is shaped like a small eraser. This filter is made of steel, and the body is magnesium. The steel tends to corrode, which allows the filter to fall out into the bottom of the combustion can. Most of them sit down in the can; however, they do have the potential to go through the can and the turbine, causing a lot of damage. Many people find the filter and call me, not knowing what it is.

Running problems:
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.

Foul odors:
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.

Don't prematurely pull the APU
Not all -100 units need to be pulled at 1,500 hours for hot section (HSI). Learn as much about your model engine as possible, and find out when it needs to be pulled for HSI. If you've got one of the higher mod standard turbine nozzles and one of the three higher mod standard turbine wheels, you don't schedule the APU at 1,500 hours anymore; you can schedule it at 2,100 hours.

To determine this, you have to look at turbine nozzle and turbine wheel part numbers, which should be recorded in the log book. Compare them to the manufacturer's service bulletins and maintenance manuals.

On all corporate -150 models, regardless of the Service bulletins, they went from 3,000 to 4,500 hours.

What should you expect for service life
Based on our experiences working with these engines, the amount of life service you can expect to get out of each engine varies among engine models and the type of operating you do. Corporates typically operate 100 to 300 hours a year, while Regionals can operate 225 hours a month. You should expect to get approximately the following life out of the following models of APUs (the Turbine Nozzle is the determining factor since it's exposed to the highest temperatures):
- 100E: 1200-1500 hours; used on the Challenger.
- 100G: 1400-1800 hours; used on the Gulfstream.
- 100A: 10 years (FOD can be a problem due to where the inlet is located — the inlet for the APU is behind right wheel well); used on the Falcon 50.
- 150F: 22 years (FOD can be a problem due to where the inlet is located — the inlet for the APU is behind right wheel well); used on the Falcon 900.

Get the service bulletins complied with
As a final thought on this engine: AlliedSignal has hundreds of bulletins out there. What we did is look at the different bulletins that will really extend the life of the engine and are worth it. We came up with the following three that we recommend as being critical:

• 6081, Improved Bearings.
There are six bearings that really should be replaced. For all six bearings it is less than a thousand dollars.

• 6114, Improved Clutch Shaft.
This is critical because if the shaft fails, it will destroy your gearbox.

• 6121, Improved Idler Gear.
There is a bearing that's failing, and this also can take out your gearbox.