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.
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