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.
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