Figure: G450 APU control schematic, from G450 Maintenance Manual, §49-60-00, figure 1, sheets 1 and 2.
The APU is a fairly reliable piece of magic that has a finite life. It works great until it comes time to replace it. If you do have a problem, remember it is computer controlled and often a good reboot (OFF then ON) will cure what ails you. The APU will shut down before it damages itself, unless it is operating in "essential" mode, as detailed below.
About the APU itself: G450 Systems / APU.
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
APU Essential versus Non-essential Mode
[G450 Aircraft Operating Manual, §2A-49-20 ¶2.A.] The ECU control authority for the APU has two operating regimes: non-essential and essential.
- The non-essential regime is for normal ground operations when the APU is suppling electrical and/or pneumatic bleed air to aircraft systems. While operating in this regime the ECU will provide a full range of protective shutdowns to prevent damage to the APU. The protective logic is based on the assumption that for ground operations other sources can be used to supplant the APU to meet aircraft requirements, such as an external AC electrical supply or a pneumatic air cart.
- The essential control mode is confined to APU operation while the aircraft is airborne. The essential regime is initiated by the aircraft combined Weight On Wheels (WOW) switch transitioning to the in the air state. In the essential control mode the ECU blocks some of the protective shutdowns of the APU since the control logic assumes that starting the APU while airborne would only be necessary during a malfunction or failure. Abnormal circumstances might require that the APU electrical power be essential to recovering from the condition, therefore the safe condition of the aircraft takes precedent over any possible damage to the APU. NOTE: If the APU is operating in essential mode while the airplane is airborne, the APU will continue to operate in essential mode for fifteen minutes (15 min.) after landing in order for the flight crew to accomplish necessary securing procedures.
In the Essential Mode, the APU will not perform a protective shutdown for several items that would prompt a shutdown in the Non-Essential Mode: High EGT, high oil temperature, low oil pressure, loss of oil pressure or EGT sensor, reverse air flow, under speed, and loss of APU inlet door RVDT. If that happens, you would get an amber APU ESSENTIAL CAS message.
- APU FIRE — G450 Abnormal Procedures / APU Fire
- APU DOOR — [G450 Quick Reference Handbook, page MB-15]
- APU door is open and APU RPM is greater than 4%.
This appears to be a mistake in the AFM and QRH. The AOM [G450 Aircraft Operating Manual, §2A-49-30, ¶3.B.] say the APU Door CAS message comes on amber when "APU Air Inlet Door position incorrect." It makes more sense that you would get this CAS message if the door had failed closed.
- At 300 KCAS or less, cycle APU MASTER switch ON, then OFF.
You have some control of the APU door through the CMC as detailed in the G450 Maintenance Manual, §49-12-01. If you need a copy, I've made one for you here: G450 APU Air Inlet Door — Manual Override.
- APU ESSENTIAL — [G450 Quick Reference Handbook, page MB-16]
- APU is running while in flight and a fault is detected which would normally cause APU to shutdown if not in the Essential mode.
- If the APU is not required for the remainder of flight, shut down APU by selecting APU MASTER OFF. Monitor RPM and EGT. APU will not start after landing. If APU is required or desired after landing, leave running. Shut down after landing.
- APU EXCEEDANCE — [G450 Quick Reference Handbook, page MB-16]
- Central Maintenance Computer (CMC) has recorded an Auxiliary Power Unit (APU) exceedance.
- Check APU EGT and RPM. If out of limits and APU is not needed for an essential function, shut down APU and log for maintenance action.
- APU FAIL — [G450 Quick Reference Handbook, page MB-16]
- An uncommanded APU shutdown had occurred.
- Check the APU rpm less than 5% then select APU MASTER OFF. If there are no other abnormal indications, a restart may be attempted.
- APU FIRE DETECTOR FAIL — [G450 Quick Reference Handbook, page MB16]
- APU fire detection system has detected a fault. NOTE: This message normally illuminates during the APU fire test.
- Notify maintenance, do not use APU.
- APU GENERATOR OVERLOAD — [G450 Quick Reference Handbook, page MB-16]
- APU generator is overloaded.
- Reduce electrical load as required.
- APU OIL PRESSURE LOW — [G450 Quick Reference Handbook, page MB-17]
- APU is in essential mode and low oil pressure has been detected.
- If APU is not required for the remainder of flight, shut down the APU. If APU is required, continue operation. Maintenance action will be required prior to any subsequent flight requiring the APU. If APU is not required, dispatch with reference to the MEL.
- APU POWER FAIL — G450 Abnormal Procedures / APU Generator Fail
- APU POWER FAULT — G450 Abnormal Procedures / APU Generator Fail
- APU SHUTOFF VALVE FAIL — [G450 Quick Reference Handbook, page MB-17]
- APU fuel shutoff valve has failed.
- Cycle APU MASTER OFF then ON. If message clears, continue. If message does not clear, log for maintenance action. Dispatch with reference to the MEL
- APU STARTER ENGAGE — [G450 Quick Reference Handbook, page MB-17]
- APU starter still engaged after scheduled starter drop out RPM.
- Press APU STOP then select APU MASTER to OFF.
FSI G450 PTM, FlightSafety International Gulfstream G450 Pilot Training Manual, Volume 2, Aircraft Systems, October 2008
FSI G450 MTM, FlightSafety International Gulfstream G450 Maintenance Training Manual, August 2008
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013
Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013