APU System Abnormals

Gulfstream G450

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 APU.

Everything here is from the references shown below, with a few comments in an alternate color.


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

Last revision:


G450 APU Abnormals

APU Malfunctions

  • APU FIRE — See the procedure below: 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, but Gulfstream denies it. 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. But Gulfstream says this is how the message was intended.

    • 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.
    • Whenever the APU is operating when the aircraft is airborne, it enters the "essential mode," and will remain in that state until 15 minutes after landing.

      More about this: G450 APU / Essential Mode.

  • 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 — See the procedure below: APU Generator Fail
  • APU POWER FAULT — See the procedure below: 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.

APU Fire

The APU compartment is monitored for excess temperatures by a sealed tube of helium that should detect a fire. If that happens, the APU shuts down automatically, shutting the fuel off at the APU itself. It is up to the pilot to shut the fuel off upstream of the APU compartment and discharge the fire extinguisher. While some early G450's had a separate APU fire bottle, the vast majority plumb the number one bottle to the APU. Discharging this fire bottle into the APU means you have one bottle left for the engines.


Photo: G450 APU control panel, from Eddie's aircraft.


APU Fire

The APU should shut down automatically if a fire is detected.


[G450 Aircraft Operating Manual §2A-26-20, ¶2.C.]

  • The APU fire detector is a continuous element routed around critical areas within the APU container. The element consists of a tube, approximately seven (7) feet in length, filled with helium gas and a stabilizing chemical. The tube is sealed at both ends, resulting in a container with a constant internal pressure. Two sensors are installed in the end of the tube: one sensing high pressure and the other sensing low pressure. If the gas within the tube is heated, molecular motion within the tube increases, resulting in a rise in pressure. A pressure increase above a preset threshold is indicative of a high temperature level in one or more areas monitored by the detector. The threshold corresponds to a pressure increase caused by a temperature of approximately one thousand degrees Fahrenheit (1,000°F) over a small section of the sensor tube or by a temperature level of four hundred fifty degrees Fahrenheit (450°F) over the length of the sensor. When sensor pressure exceeds the threshold, a fire signal is sent to I/Os in MAUs #1 and #3 for initiation of MWS CAS visual and aural fire warnings. Hard wire signals are generated to illuminate the FIRE legend on the APU overhead panel (shown in Figure 2), the red master warning light on the cockpit glare shield and if the aircraft is on the ground (weight on wheels), to the APU fire warning horn in the nose wheel well. The APU Electronic Control Unit (ECU) automatically shuts off fuel to the APU if a fire is detected, bypassing the cool-down mode. (Testing the APU fire warning system while the APU is operating will not result in an automatic shutdown.)
  • The second tube sensor monitors low gas pressure in the fire detector. If an APU malfunction or other failure causes a rupture in the tube structure allowing the escape of the gas within, the sensor will detect the resulting loss of pressure and signal a failure of the APU fire detector to I/O modules in MAUs #1 and #2 for MWS initiation of a CAS annunciation. Failure of the fire warning system will not automatically shut down the APU.

If a fire is sensed, the APU ECU automatically shuts down the APU. There will still be fuel pumped into the enclosure until the APU Master Switch is selected OFF. It is up to the pilot to discharge the fire extinguisher. Doing so discharges the left fire extinguisher.

See G450 Auxiliary Power Unit for more information.

[G450 Aircraft Operating Manual §2A-26-30, ¶2.C.]

  • Only the left fire extinguisher bottle is plumbed into the APU compartment. If a fire is detected within the APU enclosure, fuel is automatically shut off at the APU fuel control. The flight crew may discharge the contents of the left bottle into the APU by pushing in the guarded FIRE EXT DISCHD push button above the APU FIRE indicator light. When the left bottle is discharged, the bottle low pressure sensor will signal the activation of the amber L Fire Bottle Discharge CAS message and the illumination of the amber FIRE EXT DISCHD legend in the push button
  • NOTE: After the fire extinguishing agent is discharged into the APU compartment, the left fire bottle is no longer available for engine fire protection.


[Airplane Flight Manual, §4-10-40]

NOTE: The APU will automatically shut down it the APU Fire Detection circuit detects a fire except during use of the APU Fire Test switch.

  1. APU MASTER . . . OFF
  3. APU FIRE Indicator . . . MONITOR
  5. If APU FIRE indicator remains illuminated, or if APU fire test is abnormal, begin descent to lower altitude and consider diversion to the nearest suitable airport. Then proceed with Step 5.

    If on ground or after landing, and APU FIRE indicator remains illuminated, or if APU fire test is abnormal:

  6. Airplane . . . STOP
  8. Both FUEL CONTROL Switches . . . OFF
  10. NOTE: When the Main Batteries are selected OFF, the Emergency Power system will activate. If emergency lighting is not required for evacuation, select the Emergency Power Off switch to OFF before selecting the Main Batteries to OFF.

  11. Airplane . . . EVACUATE

APU Generator Fail

An APU Power Fail message can mean a bad generator or just a voltage spike of some sort that put it beyond tolerances. The APU generator has a dedicated generator control unit (GCU) which is nothing more than a computer. If the generator output returns to specs or if the GCU just needs a reboot, you can make this happen with the AC/DC RESET switch.


APU Power Fail

APU Power Fault



Figure: G450 electrical power AC/DC reset switch, from G450 Aircraft Operating Manual, §2A-24-00, figure 9.

[G450 Aircraft Operating Manual §2A-24-20, ¶2.]

  • The APU mounted generator is directly driven by the APU accessory gearbox without a hydromechanical transmission. The APU generator operates in a manner similar to the engine driven IDGs, and is capable of producing the same 40kVA of 115/200 volt 400 Hz 3φ AC power up to the limit of the APU operating envelope.
  • APU speed is critical for pure electrical power. The load control valve should protect APU RPM by reducing air loads. APU speed could suffer if the load control valve falters or for any number of reasons. If you hear or see the APU rpm degrade momentarily followed by an APU Power Fail CAS message, simply resetting the GCU should restore APU electrical power.

  • The GCUs provide control, monitoring, test and indication functions for the generators. The GCUs are electrically isolated from the generators in order to perform control and monitoring functions.
  • Some generator protective features incorporate a time delay after the generator is removed from the aircraft AC buses. When a GCU determines that the power output from the generator is again within acceptable parameters, the generator is made available to power the aircraft buses. Normal generator to bus configuration may be restored by cycling the AC RESET push button on the overhead ELECTRICAL POWER CONTROL PANEL.


[Airplane Flight Manual, §3-03-40]


  1. APU GEN . . . OFF
  2. AC / DC RESET Switch (Lighted or Unlighted) . . . PUSH
  3. APU GEN . . . ON

  5. APU GEN . . . OFF
  6. The Standby Electrical Power System (HMG) is available as an alternate inflight power source. If not required for APU AIR, shut down the APU.

See Also:

Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013

Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013

Gulfstream G450 Operating Manual Supplement, G-450-OMS-02, Extended Operations (ETOPS) Guide, Revision 2, April 2, 2009

Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011

Gulfstream G450 Quick Reference Handbook, GAC-AC-G450-OPS-0003, Revision 34, 18 April 2013

Gulfstream G450 Weight and Balance Manual, Revision 3, March 2008