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 required checks become impossible. Read more about this under G450 Systems / APU, and G450 Systems / 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 G-V 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.
There was a 2016 Gulfstream Journal report of a crew that dispatched with an inoperative left battery charger as permitted by the MEL. The APU started okay and they flew to their destination. For the next start the APU never made it past 12% and shut itself down. Incredibly, Gulfstream Technial Ops could only advise them to get an air cart and arranged for the Field and Support Team (FAST) to bring a new charger. The FAST team replaced the charger, charged the battery, and they were good to go. What else could they have done? Unlike the GV and G550, in the G450 they could have simply turned the left battery switch off to start the APU using the right battery. Gulfstream says they will add this to the AFM, AOM, QRH, MEL, and MOPP. A little systems knowledge would have saved the day.
Most of this comes from the G450 Aircraft Flight Manual, with a few comments in blue.
[G450 Aircraft Flight Manual, §2-02-10]
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:
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.
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.
CAUTION: PRIOR TO APU START ALLOW BATTERIES TO FULLY CHARGE WITH EXTERNAL AC CONNECTED.
[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.
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.
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.
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.
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.
Technique: leave the pump off until after the Ready Light is illuminated the second time, to save battery wear.
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.
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.
WHEN APU IS STABILIZED AT 100% RPM FOR NINETY (90) SECONDS:
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.
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.
Gulfstream G450 Aircraft Operating Manual, Revision 35, April 30, 2013.
Gulfstream G450 Airplane Flight Manual, Revision 36, December 5, 2013