Honeywell's GTCP36-100 APU
Quit throwing parts at it!
By John Casey
Are you one of those mechanics that start throwing parts at the auxiliary power unit (APU) when it is not doing what you think it should? Usually if one changes enough parts, the law of averages says that you will eventually find the faulted component. I saw six fuel control units (FCU) changed for a start related fault, before finding that the aircraft fuel system was heavily contaminated. In another case, a mechanic changed 12 starters before further investigation revealed gearbox problems. APUs are removed for aircraft electrical problems and electronic control units (ECU) are swapped for fuel atomizer restrictions. Throwing parts at the APU and hoping to get the one that fixes the fault is expensive, time consuming, and very frustrating. If one takes time to learn the APU and its operation, troubleshooting will become easier. Troubleshooting is isolating faults, not changing parts.
Please note that this article contains tips and suggestions — always refer to the applicable maintenance manual before performing any maintenance tasks.
First, what makes the APU tick?
The 36-100 APU has been installed in nearly two dozen different applications including: Falcon 50, Gulfstream (GI, GII, GIII, GIV), BAe HS-125, Bae 146, Jetstream II, Canadair Challenger, Ground Carts and Military applications. The APU is a constant speed engine utilizing a single centrifugal compressor and a single radial turbine. The APU uses a single, duplex fuel atomizer installed in the center of a can style combustor and is equipped with a single igniter.
Control of the APU is through the action of an analog ECU. The ECU does not monitor for open circuits. If the component fails mechanically, the ECU cannot "see" the fault, forcing the mechanic to resort to conventional troubleshooting methodology. All signals to and from the APU must go through the ECU. When troubleshooting, do not forget the wiring harness, the cannon plug and the ECU.
Most of the 36-100 series APUs incorporate a surge control valve (SCV) to prevent the APU from surging (compressor stall) during acceleration, heavy shaft loads and in-flight operation. The installation is equipped with a load control valve (LCV) to allow selection of pneumatic (bleed) load and provide exhaust gas temperature (EGT) protection for the APU.
One needs to be aware of what happens at each point in the APU's operation. At 10 percent RPM, the fuel solenoid shut-off valve (FSOV), ignition unit and the SCV (if equipped) are energized. At 60 percent RPM, the starter is de-energized and, at 95 percent plus four seconds, the ready circuits are energized. The APU accelerates to governed speed (100 percent) and the ECU, in conjunction with the FCU, keeps the APU operating within one percent of 100 percent RPM. This governor action will control APU RPM during idle, electrical and bleed loading.
The APU is running - Why monitor?
APU monitoring should start when the APU is operating normally. Do some preliminary footwork; fill out an APU monitoring check-sheet with the basics. What is the start like? What is the acceleration time? What is the EGT at idle and at full load? How long does the APU take to stop rotating after you have pressed the stop button? When the APU is at idle, is the SCV open or closed? Is it in the correct position? Are you starting to get the idea?
If you do not know what is correct, how do you know when the APU begins to falter? Often there will be a change in the APU's operational characteristics before it really gives you problems. These will usually show up on an APU monitor check-sheet, indicating a malfunctioning system or component. One misconception with this APU is that all you have to do is operate it until it breaks. This is not correct, one should perform periodic maintenance inspections in accordance with the applicable maintenance manual and open their senses to what the APU is doing. A properly completed APU monitor check-sheet, filled out at regular intervals, can reduce operating costs by identifying problems early and allowing the operator to schedule maintenance.
APU systems are: Lubrication, Fuel, Pneumatic, and Electrical. Learning the systems will usually give a better picture of where to start when a malfunction occurs.
The lubrication system is comprised of an oil pump; oil pressure regulator; oil filter; low oil pressure (LOP) switch, and a high oil temperature (HOT) switch on some units. The oil pump and regulator infrequently cause problems. They are inside the gearbox and not accessible at line level. Both the LOP and HOT switches are normally closed. The LOP switch opens on increasing oil pressure and the HOT switch opens on fault (141 to 147 C oil temperature). The oil filter is located downstream of the regulator and if the filter becomes excessively contaminated, it can cause LOP faults. Oil pressure is seldom checked on this unit, but there is a test port located in the center of the oil filter cover. When the chips are down, connecting a gage to this port will tell you the regulated oil pressure. The oil pressure should be 45+/-10 psi before Service Bulletin 49-6149 and 40+/-5 psi post-bulletin.
An LOP fault will allow the APU to start and accelerate to 95 percent RPM plus 10 seconds before a protective shutdown is initiated. If experiencing LOP faults; check the oil level, then the filter. In the Challenger, a leaking generator adapter seal can result in oil migration from the generator adapter to the APU gearbox. This can over-fill the gearbox causing oil foaming as the gears whip the oil into froth, resulting in APU HOT and/or LOP faults.
A HOT fault will cause an automatic shutdown and prevent the APU from starting. The first thing to note when a HOT fault occurs is whether the APU is hot or not. If the APU is not warm, then the problem is an electrical fault, not oil temperature. If the HOT circuit is open (a faulty switch, loose pin, corroded connection, faulty ECU, high oil temperature) a shutdown is initiated and starter engagement prohibited. For Falcon 50 and BAe operators, the HOT switch is an option and usually not installed. The circuit is still there and the wire now forms a loop inside the harness, failure of this circuit can result in HOT shutdown.
Fuel system faults seem difficult for many to troubleshoot, but they should not be. The aircraft supplies somewhere from 10 to 30 psi fuel pressure to the APU FCU. The FCU regulates fuel flow to the FSOV valve and to the fuel atomizer. Knowing fuel pressure can help isolate faults; the metered fuel pressure (that going to the atomizer) generally runs about 200 to 250 psi at idle and increases by about 30 to 50 psi when fully loaded. These are not exact figures, but will give you an idea of what normal fuel pressure may be. The FCU has a minimum fuel flow setting and does not stop the fuel flow. The torque motor, inside the FCU, will increase fuel flow after the APU has accelerated to about 25 percent RPM. A failed fuel control torque motor, harness, cannon plug, or ECU (fuel control driver circuits) will still allow the APU to start and accelerate to approximately 50 percent RPM! At minimum fuel flow, connecting a pressure gage between the FCU and the atomizer should exhibit a fuel pressure of about 40 psi.
The FSOV valve is normally closed and is energized at 10% RPM. A leaking FSOV will allow fuel to flow into the combustor as soon as the boost pump is energized. As a start is attempted, this excess fuel can cause booming starts and high EGT. The leaking FSOV can also extend the APU's run-down time beyond the nominal 45 seconds and cause carboning/coking of the fuel atomizer.
The fuel atomizer is actually a duplex atomizer assembly that contains a fuel flow divider and both primary and secondary nozzles. The primary orifice is always open and the secondary opens when fuel pressure reaches 165 pgig. A malfunctioning atomizer can cause start and operational problems, as well as seriously damage turbine components. A partially obstructed atomizer, or a fuel flow divider that fails to open completely will cause fuel pressure (between FCU and atomizer) to exceed normal values. A completely blocked atomizer or flow divider that fails to open can push fuel pressure to the 525-psi-relief limit and the APU may never reach 100 percent RPM. An obstruction can range anywhere from partial to a complete blockage; when fuel flow is restricted sufficiently, the APU RPM will decrease as the load is increased. If the fuel flow divider does not fully close when the APU is shutdown, the next start can be affected. It can be either a hot/booming start or maybe no start at all.
Start troubleshooting, first from the cockpit, then from the APU.
Check EGT. If EGT was much lower than 665 C, the problem is most likely fuel related. If EGT is high (at or near 665 C), the problem is more likely load related (a worn or damaged APU, starter hung, inlet obstruction, cold soaked, etc.) High EGT indicates fuel is there, but the APU can not carry the load.
Most mechanics never check fuel pressure, but doing so can be a benefit. Caution: If you tee into the fuel line at the FSOV valve, ensure that the fitting you use doesn't jam against the valve seat. If the fitting jams the valve, the valve may open, but not close, or it may not open, or it may hang partially open.
The two components in this system, that need to be covered, are the load control valve (LCV) and surge control valve (SCV).
The LCV is normally closed, electrically controlled and pneumatically actuated. The load circuits are armed after the APU has accelerated to 95 percent plus four seconds and depending on the installation, the LCV can be either solenoid, or torque motor controlled. Generally, Gulfstream and BAe will incorporate the torque motor controlled LCV, while Challenger and Falcon use the solenoid controlled LCV. What effect will this have on the mechanic and operation? Not nearly as much as one would think. The ECU controls both valves, allowing a maximum continuous EGT of 665+/-10 C. The zero to 10 vdc torque motor controlled LCV modulates the butterfly valve by action of a torque motor and the 28 vdc solenoid LCV uses internal rate-time adjustments to regulate the valve opening and closing rates. Once the APU is on-speed and has stabilized for at least one minute, the LCV can be selected on. This allows bleed air extraction from the APU. A LCV that fails to close completely can cause high EGT during acceleration and possibly hung starts. A LCV that fails to open fully can result in lower aircraft duct pressure and lower EGT when operating with bleed load.
Honeywell's GTCP36-100 APU
Quit throwing parts at it!
By John Casey
continued page 2 of 2...
On Gulfstream or BAe, during bleed load, if EGT reaches the 665-degree limit, the LCV will modulate to a more closed position. On Challenger or Falcon, the LCV will be de-energized and energized causing the valve to cycle between the closed and open positions. In both cases, the result will be EGT limiting and the write-up is that the APU exhibits low performance.
The SCV is normally closed, solenoid-controlled, and pneumatically actuated. It is typically energized to open at 10 percent RPM, but the pneumatic pressure will actually open the valve at approximately 60 percent RPM. Normal idle EGT for the 100 series APU is generally between about 280 and 330 C. Challenger, Falcon, and BAe, for example, operate with the SCV open at idle. This increases EGT by approximately 60 C, or to approximately 340 to 390 C. An SCV that fails to close can cause EGT to increase by that same 60 C, or to the 665 C limit, whichever comes first. If EGT reaches the limit (665 C), the ECU will reduce the bleed load to protect the APU. A SCV that fails to open can cause APU surging during acceleration, electrical loading and in-flight operation.
There are only a few components that we have not covered; the speed monopole, the EGT thermocouple, starter, and hour meter.
As the APU rotates, the sends a speed signal to the ECU. The ECU must receive this signal to actuate the necessary switch points. The monopole is shimmed to obtain an air gap of .015+/-.003 inches and if shimmed excessively, the APU can experience no-start or intermittent shutdowns. On the other hand, if too close, the possibility of rub exists. Loss of the monopole signal will result in APU shutdown. A start attempt will motor the APU, but there will be no combustion and no RPM indication. The auto shutdown faults do not cover "loss of monopole;" however, without a signal, the ECU cannot "see" speed. No speed signal, no 10 percent switch, no fuel, no ignition, and no speed indication.
The APU uses a single EGT thermocouple placed in the exhaust/tail pipe and when heated, the thermocouple emits a millivolt signal to the ECU. The ECU, in turn, sends a zero to 1 vdc signal to the cockpit indicator. The electronic control interprets an open thermocouple circuit as a 732 C overtemperature fault. This will automatically shutdown the APU, prevent starter engagement, and the EGT indicator will display maximum EGT (1,000 degrees on an analog gage and, 9,999 degrees on a digital). An electrical short in the thermocouple or harness will actually create another thermocouple and display a much lower than actual EGT. The ECU will now control EGT to this faulty indication. For example, if the erroneously indicated EGT is 75 C, the ECU will allow the APU to operate at full load, or accelerate, with an actual EGT much higher than shown. This obviously will cause severe distress to turbine components.
Some installations use a DC starter motor and clutch assembly. A slipping or damaged clutch can cause high EGT during acceleration. A damaged clutch can seize, forcing the starter to be motored by the APU. If rotated by the APU, the starter will fail causing severe damage to the APU.
A caution in the maintenance manual tells us whenever a DC starter failure has occurred, the starter clutch needs to be checked in the overrun position. Once the starter is removed, the internal spline in the clutch is visible; by inserting a screwdriver or spline adapter into the clutch, one can rotate the drive. The clutch should rotate in one direction smoothly without the feeling of contamination or damage. When rotated in the opposite direction, the APU should start to rotate immediately without hesitation or slippage.
If the clutch needs replacement, use extreme caution when removing as there are several washers between the outboard bearing and the adapter. When the adapter is removed, the washers can fall into the gearbox and there is also a spacer between the two inboard bearings. Use care when removing the clutch from the gearbox, the back bearing sometimes sticks in the gearbox exposing this spacer. You guessed it — it is also easily dropped into the gearbox. After the clutch and starter have been replaced, removing the inspection cover from the rear of the starter will facilitate checking the starter and clutch. Insert the appropriate size screwdriver into the slot in the shaft to rotate the engine, starter and clutch. The starter should stop immediately upon removal of the screwdriver and the APU should rotate freely. There are clutch inspection and shimming requirements, so refer to the maintenance manual when replacing the clutch.
The hour meter is energized when the RPM reaches 95 percent plus four seconds and starts counting time. On the 2118474 series ECUs (Some Gulfstream installations), if the hour meter is shorted, an overcurrent protection shutdown is annunciated. Like many other components, disconnecting the hour meter is an easy way to troubleshoot a fault as the hour meter draws very low current.
Troubleshooting an overcurrent fault can sometimes be very difficult. Learning when the components are energized will be an asset to you. Think about it; what could cause a shutdown at approximately 10 to 20 percent RPM? If you guessed that the ignition unit, FSOV, or SCV might cause the problem, you are on the right track. The easy part about this ECU is that in most cases a cannon plug can be pulled to isolate a component.
Remember that every APU is an individual and will operate a little different from another. Accurate fuel pressure and temperature values can only come from your APU — so don't make comparisons to another unit.