Auxiliary Power Unit

Gulfstream GVII

Eddie sez:

For a quick refresher or a good intro, be sure to see Ivan Luciani's notes: G500 Auxiliary Power Unit Notes.

There are also flash cards available at: G500 APU and Quizlet.

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


Photo: G500 APU Drawing, Eddie's notes
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How it works . . .

The Auxiliary Power Unit (APU) is a Honeywell Gas Turbine HGT400 [G]. The HGT400 [G] provides an alternate source of electrical power on the ground and in flight to 45,000 feet. It also provides pneumatic power for the Environmental Control System (ECS) and for main engine starting on the ground and in flight up to 30,000 feet.

The Components in greater detail . . .

Limitations and Abnormal Procedures . . .

Last revision:


How it Works


[PAS, p. 3-2]

  • Nomenclature
  • The official name is HGT400[G], where HGT is Honeywell Gas Turbine and [G] is Gulfstream

  • Purpose
  • The APU is designed to supply an auxiliary source of bleed air and electrical power. On the ground it provides electrical power as well as air to start main engines and provide cabin air conditioning. In flight it provides auxiliary electrical power and air for assisted air starts. APU air cannot be used for pressurization.

  • 2 Main Sections
  • The powerplant is a 2-stage turbine which turns a drive shaft connected to an accessory gearbox. This gearbox powers a 40 kVA generator and an oil pump.

  • Location
  • The APU is mounted sideways on stainless steel, anti-vibration pallet in the tail compartment. It is completely enclosed in a titanium, fireproof enclosure for aircraft protection in case of an APU fire. Access to APU components is accomplished through various access panels on APU enclosure, as well as a fireproof APU access door on left side of aircraft.


[PAS, p. 3-2]

  • The APU is highly automated and requires very little crew input. It is the normal source for AC power and air while on the ground. The APU Generator switchlight is always left in the “pressed IN” position; therefore, AC power will come online as soon as the APU is up to normal operating RPM. This switch configuration allows for immediate power from the APU should it need to be started in flight.
  • The APU provides a higher volume of air than the main engines can provide at idle thrust. Thus, when taxiing out in hot conditions, powering the ECS packs with APU air generally provides more cooling air than using engine bleed air. Additionally, having the engine bleed air selected OFF causes the engine idle speed to be reduced by approximately 5% HP RPM, reducing idle taxi speed. The last step of the Taxi checklist directs you to return bleed air to normal configuration prior to takeoff.
  • The APU will automatically shut down for any number of faults and system problems detected by the ECU depending on which mode the APU is operating in at the time. The APU operates in two modes: non-essential mode on ground and essential mode in flight. In non-essential mode, the APU will automatically shut down for certain faults. In the essential mode, the APU will continue to run, and one or more CAS messages will alert the crew that the ECU has revealed a fault that would shut down the APU if the aircraft was on the ground. It’s then up to the crew to decide whether or not to shut down the APU. If the APU is operating in essential mode upon landing, it will continue operating in essential mode on the ground until the START/STOP switch is pressed, or the MASTER switch is selected off, or 15 minutes have elapsed.

[FAQs, p. 10] The APU burns approximately 250 pounds of fuel per hour on the ground with APU bleed on and the engines off.

The fuel flow inflight is considerably less, on the order of 145 lbs/hr and lower, depending on altitude. The FMS fuel burn predictions will include the APU if it is running.

The Components in Greater Detail

Air Inlet Door


Photo: G500 APU inlet door, PAS, p. 3-5
Click photo for a larger image

[PAS, p. 3-3]

  • Air enters APU inlet duct through here and enters APU air intake

  • The variable position door is moved by an electric door actuator controlled by the APU ECU. The door will stop moving in transit if electric power is lost.

  • The door opens when the APU master switch on overhead panel selected on. On the ground it fully opens, in the air it is fixed at a 45% setting. If the motor fails on the ground, the door can be moved manually.

  • The door closes when the APU is shut down. Don’t turn main batteries off until the APU door is closed because the door stops moving when electric power is removed.

  • The door position is one of the parameters the ECU needs for an APU “Ready” light.

Control Panel


Photo: G500 APU control panel, PAS, p. 3-8
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[PAS, p. 3-8] Master Switch

  • When selected ON:

    • ECU performs Power-Up Built-In Test (PBIT). If failure detected, ECU inhibits APU start (except for low oil temp), sends CAS message, reports fault(s) to Central Maintenance Computer (CMC), and displays amber “FAULT” in MASTER switch. It is OK to cycle MASTER switch and give it another try. The ECU commands APU air inlet door to open immediately on the ground, or after APU RPMs increase when in flight. The ECU commands APU fuel shutoff valve open.

    • DCN auto selects Nav lights ON and left main fuel pump ON.

    • Cyan “READY” light illuminates indicating all prestart conditions are passed and APU ready to start. On the ground starting mode doesn’t accept START input w/o READY light.

  • When APU MASTER switch selected OFF:

    • Switch legend extinguishes

    • APU fuel shutoff valve closes

    • Immediately shuts down if running

    • Air inlet door close to 10% open below 63% RPM, fully closed < 40% RPM

    • Shutting down APU with MASTER switch strongly discouraged → Component damage likely

  • Normal shutdown: Select MASTER switch OFF < 70% RPM, wait until 0% RPM before turning main batteries OFF.


Photo: G500 APU start/stop switch, PAS, p. 3-9
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[PAS, p. 3-9] Start Sequence

  • Commences when switch depressed

  • ON legend illuminates

  • If READY light illuminated

    • ECU transitions to start mode

    • Commands starter engagement for 12 to 50 secs, depending on oil temperature

    • 5% RPM: opens fuel control unit, energizes ignition system, begins to control fuel flow

    • Starter cuts out at about 46% on the ground, up to 60% in flight

    • Ignition commanded off at 50% RPM on the ground, 98% RPM in flight

    • Blue “ON” legend remains illuminated indicating APU is running

  • If APU in cool down mode when switch depressed ramps APU speed back up to 100% RPM

[PAS, p. 3-10] Normal APU Shutdown

  • Stop sequence commences when switch depressed if APU running

  • ON indication in switchlight goes out

  • APU loads are shed

  • Enters cool-down mode: On ground and in flight < 20,000’ the RPM decreases 1/2% per sec for 60 secs (to 70% RPM) and then shuts down. In flight > 20,000’ the RPM is kept at 100% RPM for 60 seconds before shut down. If the START/STOP switch depressed anytime during the cool-down period, the APU is commanded to 100% RPM and the on indication in switchlight illuminates.

  • After the cool-down period is complete, the APU shuts down by performing an overspeed test. Any faults detected during shutdown testing are sent to CAS and / or CMC. The ECU starts 5 min timer to supply power to EGT and RPM indicators and to the air inlet door to close it. You must recycle the MASTER switch to restart.

  • APU Master On indicates APU MASTER is selected ON and either an APU start has not been attempted or the APU is no longer running.


Photo: G500 APU overhead panel, PAS, p. 3-11
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[PAS, p. 3-11] APU Gen Switch

  • ON position → Pressed in. The switch is normally left in ON position unless checklist or maintenance directs otherwise. It indicates dark until generator is activated. APU generator activates ON when APU reaches operating speed 99% RPM plus 2 seconds. Indicates amber “ON” representing abnormal in-flight condition. It is never intended to be required in-flight unless malfunction exists. Use at pilot’s discretion also authorized. Switchlight dark → Indicates pressed in but APU below operating speed.

  • OFF position → Deselected out. APU generator power removed even if APU running → Switchlight dark.

[PAS, p. 3-11] APU Bleed Air Switch

  • ON position (pressed in) → Opens APU load control valve and isolation valve when APU on speed. Connects bleed air to system via right manifold only, but is available to left side via isolation valve. There is a one minute delay if EGT < 149° and WOW in ground mode. No delay if WOW in air mode for an engine airstart.

  • OFF position → Deselected out, switchlight dark.

Load Control Valve


Photo: G500 APU load control valve, PAS, p. 3-6
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[PAS, p. 3-6]

  • Allows flow of bleed air from APU into aircraft bleed air system.

  • APU bleed air used for ground engine starting and air conditioning, as well as in flight engine starting.

  • Powered by APU BLEED AIR switch on overhead panel. When selected on the ECU opens the Load Control Valve and Isolation Valve to allows flow of air to both ECS PACKS.

  • ECU controls amount of airflow, allowing APU to run at 100% efficiency without over-temperature. In flight air is available immediately for engine airstart. On the ground, air available immediately if EGT > 149°C, otherwise there is a 60 second delay. The valve closes if RPM drops too low or EGT gets too high.

Lubrication System


Photo: G500 APU lubrication system, PAS, p. 3-5
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[PAS, p. 3-5]

  • Lube pump is used to lubricate and cool APU powerplant’s: Gearbox, Bearings, Shafts

  • Cooling for oil system provided by external air from the oil cooler duct. If oil temp gets too high on the ground, the ECU auto shuts down APU.

  • Servicing. Check oil level 10-30 mins after shutdown to prevents over-servicing. Oil level can be checked via TSC # 1-5 or the Fluid Quantity Indicator (FQI) in tail compartment. There is no historical data for APU oil, it only shows current level. Add oil via oil replenisher in tail compartment.


Photo: G500 APU remote oil replenishing system, PAS, p. 3-20
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[PAS, p. 3-20]

Located on left side of tail compartment at top of ladder, replenishes main engines and APU oil.



Photo: G500 APU OHTPS, PAS, p. 3-13
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[PAS, p. 3-13]

  • Fire Test Switch is removed when APU MASTER switch is selected.

  • Powerplant instruments: EGT, RPM. Red and amber indications appear after APU MASTER selected.

  • DU Ctrl / Test island shows the Fire Test Switch.

Photo: G500 APU OHPTS fuel, PAS, p. 3-14
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[PAS, p. 3-14] The fuel island shows fuel flow to the APU.



Photo: G500 APU AC synoptic, PAS, p. 3-18
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Photo: G500 APU bleed air synoptic, PAS, p. 3-10
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Limitations and Abnormal Procedures


[PAS, p. 3-23]

  • Can be operated on the ground, during takeoff, in flight, and during landing.

  • Maximum guaranteed start altitude → 37,000’

  • Maximum operating altitude → 45,000’

  • Maximum altitude for APU assisted engine airstart → 30,000’

  • Maximum TAT for inflight operation → 47°C (Except in an emergency)

  • Cannot be used to supply pressurization airflow in flight

  • Takeoff with APU operating prohibited with OAT > 45ºC

  • Use of external DC power to start APU → Prohibited

  • Maximum EGT: Start → 1050°C, Running → 732°C

  • Max rotor speed → 106%

  • Continuous operation of APU starter, when powered by airplane batteries: Maximum of 3 consecutive start attempts with a 1 hour cool down period before next full starter cycle commenced.

  • Successful consecutive starts: 6 starts with 10 min intervals between starts.

  • When ≤ -15ºC → Refer to Supplemental Procedures: Cold Wx Start and Ops.

  • APU Generator Electrical Load: On ground or in flight from sea level to 35,000’ → 100% (40 kVA). If load > 55% → Descend ≤ FL350

See Also:

Gulfstream GVII-G500 Airplane Flight Manual, Revision 3, July 16, 2019

Gulfstream GVII-G500 Production Aircraft Systems, Revision 3, July 15, 2019