Figure: G450 APU control schematic, from G450 Maintenance Manual, §49-60-00, figure 1, sheets 1 and 2.

Eddie Sez:

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.

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 Malfunctions


References

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