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APU Start Checklist

Gulfstream G450 Checklists

The APU Start Checklist is straight forward, though there are a few checks to be accomplished along the way. Once the APU is providing AC electrical power, the entire airplane will be powered and many of the required checks become possible. Read more about this under G450 APU, and G450 AC Essential Relay Check.

Before you start this jet engine, it might be helpful to consider the APU starter places a tremendous electrical load on your batteries. Unlike the other aircraft in the GV series, the battery switching is quite clever for the G450 APU. The starter load is normally taken by the left battery but the right battery powers the ECU. For a video of the electrical panel during an APU start, see G450 APU Start / Electrical Panel.


Batteries On

AFM Procedure

[G450 Aircraft Flight Manual, §2-02-10]

  1. Emergency Power . . . ON
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    Photo: from Haskel's cockpit


    Photo: from Haskel's cockpit

    The emergency batteries are now powering the aircraft. Check for the following: pilot's audio control panel, pilot's clock, MCDU #1 shows standby engine instruments, MCDU #3 shows radios, SFD, EBDI, landing gear indicators, and the copilot's clock:

  3. Left / Right Batteries . . . ON
  4. images

    Photo: from Haskel's cockpit

    Note: Wait until the POWER UP TEST IN PROGRESS is no longer displayed on the Display Controllers before proceeding.

    The "ON" capsules will illuminate if there is no other power source for the DC ESS, which should be the case at this point.

  5. Batteries Volts . . . Check (22V DC MIN)
  6. Note: If the left battery is below 22V and the right battery is at or above 22V, the left battery must be deselected to utilize the right battery for APU start.

    For APU starts following prolonged cold soaks if external AC power is available:

  7. External AC . . . ON

  9. DU 1 . . . NORM
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    [Gulfstream G450 Aircraft Operating Manual, §07-01-20 5.C.(6)] Cockpit systems power up should be delayed until cockpit, cabin, and Electronic Equipment Rack (EER) temperatures have reached approximately +65°F (+18°C). These systems include Display Units (DUs) and Inertial Reference Systems (IRSs). Extreme cold-soaking can cause spurious failure messages and indications on the Crew Advisory System (CAS) until core temperature of control units have reached normal operating temperatures.

    If the temperature is less than 65°F, leave the DUs off, conduct the fire test, start the APU, use the bleed air to heat the cockpit, and activate cockpit electrical systems once the temperature is above 65°F.

    Scan the list of CAS messages and get to know what you are looking for in general. We get a stack of red and amber followed by one blue. There have been times we saw more ambers which pushed the blue off the list. The aircraft powered up okay but the avionics didn't work correctly and we had to "reboot." We could have done the reboot earlier knowing to check this list first.

  11. APU Fire Test . . . PEFORM
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    Depressing the APU Fire Test switch should cause a three-chime aural warning tone, both Master Warning glare shield switches to illuminate, an exterior fire bell in the nose wheel to sound (ground only), the FIRE light on the APU control panel, a red APU Fire and an amber Fire Detector Fail CAS message annunciation. If the DUs are not powered because of cold temperatures, the requirement to view the CAS messages is understood to be "not possible," though nothing really says this.

  13. Fire Bottle Discharge, L-R CAS Message . . . VERIFY NOT DISPLAYED
  14. If the DUs are not powered because of cold temperature and the fire bottle does discharge, the Fire Bottle Discharge message will remain once the DU is powered.

    You really need to read through the list of amber CAS messages to check for this item. If you had a thermal discharge of the bottle prior to the test, it will not be flashing.

  15. Essential AC-Bus Fail CAS Message . . . VERIFY NOT DISPLAYED
  16. Why are you doing this? You are checking the REAC relay to the AC ESS bus that can only be checked when both AC main buses are unpowered. This check cannot be made if using external AC power since those buses will be powered and can be waived until the next APU test without external AC power.

    If the DUs are not powered because of cold temperature, the check can still be made by ensuring the Cabin Pressure Controller panel does not indicate "FAIL," which would happen if the E-Inverter was unable to provide power to the pressurization system because the REAC relay was frozen in position.

    You really need to read through the list of amber CAS messages to check for this item. If the REAC relay had failed, it would have been failed prior to the test and would not be flashing at this point.

  17. DU 1 . . . OFF
  18. MFD Display Switching . . . PILOT / COPILOT = NORM
  19. APU AIR . . . OFF
  20. The APU air load control valve is inhibited OFF until 90 seconds after the APU reaches 95% so leaving this switch ON doesn't impact the start.

  21. NAV Light . . . ON
  22. L MAIN Boost Pump . . . ON
  23. Technique: leave the pump off until after the Ready Light is illuminated the second time, to save battery wear.

  24. APU Master Switch . . . ON
  25. The READY light will come on momentarily and then go out as the APU ECU performs a Built-In Test, opens the fuel shutoff valve, and opens the APU inlet door (65° if the engines are not operating, 27° if they are). This should take about 30 seconds. Here's what that looks like from the outside: APU Door Open.

    • Hold the Start Switch for at least a few seconds, the ECU has been known to become confused if the switch is immediately released.
    • At 5% the fuel control shutoff valve opens and the igniters are energized.
    • At 7% the READY light extinguishes.
    • At 50% the ECU disengages the starter and the START switch extinguishes.
    • At 95% ignition is deactivated.
    • Four seconds after 95% the APU generator is available and will come on line if the APU Generator switch is in the ON position and the engine generators are off line.
    • Ninety seconds after 100%, bleed air is available and will be provided if the APU bleed switch is IN. Much of the AOM says 95% but these are starting to change to the more accepted 100%


  28. APU GEN . . . ON
  29. NOTE: If the APU GEN switch is already in the ON position when the APU start sequence is initiated, the APU generator will automatically power the bus four (4) seconds after the APU achieves 95% RPM.

    This is the normal state for the system, with the APU GEN switch in the ON position.


  32. APU AIR . . . ON
  33. NOTE: The APU air load control valve is inhibited OFF until the APU is stabilized for ninety (90) seconds.

    NOTE: When operating in high ambient temperatures (greater than +35°C / 95°F), reduce cabin temperature by selecting full cold in manual mode. Utilize both air conditioning packs, close the main and the external baggage doors, and lower window shades to expedite cooling. Minimize electrical loads until the airplane has cooled down by selecting items such as Display Units, Boost Pumps, and Galley Equipment OFF.

  34. L and R Total Air Temperature (TAT) Probes Airflow . . . CHECK
  35. NOTE: Check for proper aspirated airflow at the aft end of the probe. Steady airflow should be present with the APU Air ON. If the airflow is not steady or there is some doubt, maintenance action is recommended prior to flight.

  36. External AC (if used for start) . . . OFF


Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.

Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013

Revision: 20160701