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  Electrical System  

Gulfstream G450 Systems

The GV electrical system was as near perfection as any electrical system I've ever seen. The G450 has basically the same system with just two drawbacks: a lower output HMG and a lower altitude APU. Other than that, it is very good indeed.


 

There is a lot to the electrical system, but it helps if you break it into a few pieces:

AC Power Sources

AC Electrical Power System Description

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G450 AC electrical power system schematic, (G450 Aircraft Operating Manual, §2A-24-00, figure 3)

[G450 Aircraft Operating Manual, §2A-24-10 ¶2.A.] AC electrical power is the primary electrical system on the G450. Three integrated drive generators (IDGs), one mounted on each engine high speed turbine, and one mounted on the APU produce three phase (3Ø) 115/200V AC at 400 Hz. The IDGs are rated at 40Kva. The APU generator is normally only used for ground operations but is capable of full AC output in the air if necessary, supplanting an inoperative engine generator.

If engine and APU powered generators are not available, a standby generator rotated by a hydraulic motor using pressure from the left hydraulic system (or using left hydraulic system fluid pressurized by the right hydraulic system through the Power Transfer Unit) will produce up to 5KVA three phase (3Ø) 115/200V AC power for aircraft systems.

If no AC power can be generated, an emergency inverter powered by DC from the aircraft batteries through the left and/or right essential DC bus can supply AC power at a single phase (ØA) to operate equipment on the essential AC bus.

On the ground, an external AC power supply may be plugged into a receptacle in the access panel on the right forward lower fuselage to power aircraft systems for maintenance or preflight inspections.

Engine-mounted Integrated Drive Generators (IDGs)

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Figure: G450 IDG Block diagram, (FSI G450 MTM, Figure 24-11)

[G450 Aircraft Operating Manual §2A-24-20 ¶2.A.] The aircraft engines each turn an IDG though a transmission mounted on the engine accessory gearbox at the compressor section of the engines. The IDGs are driven at a constant speed of twelve thousand (12,000) rpm regardless of engine speed through a hydromechanical transmission coupling the generator to the engine. A constant rotational speed enables the Generator Control Unit (GCU) to maintain AC power parameters of three phase (3Ø) 115/200 volts at 400 cycles-per-second (Hz) and deliver up to forty thousand volt / amps (40kVA). The IDG rotates whenever the engine is operating. There is no provision for disconnecting the IDG from the engine while in flight.

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Figure: G450 generator air cooled oil cooler, (G450 Maintenance Manual, §24-12-01, figure 401)

[G450 Maintenance Manual, §24-10-00 ¶3.A.] The engine generator oil cooling subsystem accepts hot oil from the IDG and cools it with air from the engine bypass duct. The oil cooling system consists of an air cooled oil cooler and interconnecting oil transfer tubes and fittings.

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Figure: G450 combined fuel cooled oil cooler, (G450 Maintenance Manual, §79-22-01, figure 402, sheet 1)

[G450 Maintenance Manual, §73-00-00 ¶4.A.(1)] The Integrated Drive Generator (IDG) requires oil cooling of its of its independent integral oil system and has a dedicated fuel cooled oil cooler supplied with LP fuel located to the engine fuel cooled oil cooler. At lower fuel flows particularly combined with high generator load the IDG oil heat transfer can be excessive therefore a surface air cooled oil cooler is located in the bypass duct. Fuel filtration and system protection from ice particles is provided by a non-cleanable filter. Additional heating to prevent filter ice formation is given by HP return fuel directed around the housing.

[G450 Airplane Flight Manual, §1-24-20] The Integrated Drive Generator (IDG) electrical load is limited to 45% (18 kVA) when ambient temperature is greater than 110°F / 43.5°C in order to maintain steady state fuel temperatures below 95°C.

We rarely see a generator exceed 25% of its capacity so operating above 110°F ambient temperature should not be a problem with both engines running or the APU with or without either engine.

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G450 AC synoptics, both IDGs and APU powered, (FlightSafety G450 Pilot Training Manual, figure 2-51 sheet 1)

With both IDG's providing electrical power, everything on the airplane that needs power will have power. In the example shown above, the APU GEN switch light is amber because the APU generator is on line but not connected due to normal system priorities. The HMG is not providing power to the AUX TRU which is not providing power to the DC essentials, neither of which is powering the E-Inverter; which is why they are all gray on the AC page..

APU-mounted Generator

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Figure: APU Generator, (G450 Maintenance Manual, §24-22-01, figure 401)

[G450 Aircraft Operating Manual, §2A-49-20 ¶2.G.] An oil-cooled generator mounted on the APU gearbox provides a source of AC power for use on the ground or in flight if an engine-driven generator is not available. The APU generator produces three phase (3Ø) 400Hz 115/120v and can deliver up to 40kVA. The producing capacity of the generator is identical to those mounted on the aircraft engines - the only difference is that the APU generator does not have an Integrated Drive transmission since the APU operates at a constant speed (rpm). The generator can provide a full 40kVA output up to the limit of the APU operating envelope of 37,000 ft.

When the APU has reached an operating rpm of 95% for at least 4 seconds, the APU is capable of driving the generator. The APU generator has a GCU that monitors the voltage output and phase sequencing of the generator. If the output of the generator is within acceptable parameters, and the APU is at operating rpm, the ECU determines that the APU generator is ready to assume aircraft electrical loads. The APU generator can be selected on or off using a switch button labelled APU GEN on the ELECTRICAL POWER CONTROL panel on the cockpit overhead. The amber ON legend within the switch button will illuminate when the APU generator is in use. The APU generator GCU will drop the generator off line if the quality of power produced by the generator falls outside of normal parameters.

With the APU running, you have everything you need on the ground. When in flight with just the APU, the galley will shed under most installations.

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Figure: G450 AC synoptics, APU only powered, (FlightSafety G450 Pilot Training Manual, figure 2-50_sheet_1)

The APU is powering everything the two IDG's normally power on these synoptic pages.

Standby Electrical System and Hydraulic Motor Generator (HMG)

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Photo: G450 HMG, (FlightSafety G450 Pilot Training Manual, figure 2-11)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.E.] If failures and/or malfunctions preclude the use of engine and APU generators for electrical power, a hydraulic motor generator (HMG), located in the tail compartment, may be used as an AC electrical source. The HMG and associated buses are termed the Standby Electrical System and are controlled with cockpit overhead push button switches. When the HMG is selected ON with the STANDBY ELECTRICAL POWER MASTER switch, a solenoid valve opens, allowing pressurized left system hydraulic fluid to rotate the variable piston drive of the generator. The generator has a GCU that is initially powered by 28V DC from the left essential bus or the right battery bus, depending upon system availability.

Once the HMG reaches normal operating speed of 8,000 rpm, the GCU uses generator PMG power for control functions. The HMG is capable of 5kVA of three phase (3Ø) 115V 400 Hz power, however actual power generation level is dependent upon hydraulic system output that is in turn a consequence of engine power setting. The GCU controls voltage to 115V ±2, and provides standard protective functions (over / under voltage, frequency and speed). A time delay of up to 6 seconds prior to tripping the generator accommodates fluctuations in left hydraulic system pressure associated with the actuation of other hydraulic system components. The overspeed trip is not delayed, since this condition would not be caused by a hydraulic pressure decrease. Thermal overload sensors will signal the GCU to drop the HMG off-line if high demand results in overheating.

NOTE: An HMG overload trip requires a manual reset, cycling the STANDBY ELECTRICAL POWER MASTER switch OFF, then ON.

The primary purpose of the HMG is to provide an AC power source to the AUX TRU. The two AC Standby buses will also be powered by the HMG if main AC is lost, and feed a limited number of AC installations, including pitot heat, TAT heat, flap / stab power and emergency battery chargers. The AUX TRU provides a DC power source to energize the DC essential buses powering the flight instruments, navigation and communication radios necessary to maintain flight with the loss of AC power.

Operation of the HMG GCU may be tested without starting the HMG by activating a test switch on the OBSERVER AND TEST MONITOR panel. Pushing the button will illuminate a blue TEST legend in the top half of the button for the duration of the test, approximately 5 seconds. A successful test is indicated by a green GCU OK legend illuminated in the bottom half of the button. After the conclusion of the test, the button must be selected off.

[G450 AFM 1-24-10] When the Standby Electrical Power System (HMG) system is in operation, speed brakes may be used provided operation is slow (approximately 3 seconds for full range movement). Why? See G450 HMG Limitation Explained. Do not operate the Standby Electrical Power System (HMG) system when normal AC power is available, except as provided for in Section 3-04-30: L (or R) ESS DC Bus Failure.

The HMG has a big advantage over the APU as an emergency source of AC power: it isn't altitude limited. You can select it immediately while thinking about descending to 37,000' to start the APU. It also has a big disadvantage: 5kVA. While the APU, with 30kVA, can run everything, the HMG is only connected to a few things, albeit very important things.

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Figure: G450 AC synoptics, HMG only powered, (FlightSafety G450 Pilot Training Manual, figure 2-50_sheet_1)

Whenever the HMG is powered, it provides power to the standby AC busses and to the AUX TRU. With the appropriate switches, it can also power the Left and Right Essential DC busses. As is clear on the DC synoptic, much of the DC system is left unpowered.

HMG Limitation Explained

The G450 Hydraulic Motor Generator is rated at 5 kVA, half the HMG on the GV/G550, and has the same limitation when using the speed brakes: When the Standby Electrical Power System (HMG) system is in operation, speed brakes may be used provided operation is slow (approximately three (3) seconds for full range movement).

Why? The HMG is sensitive to changes in hydraulic pressure and volume. The G450 HMG is identical to the GV/G550 HMG but is rated at half the power because the G450 engine-driven hydraulic pumps produce about half as much volume of fluid.

G450 and GV/G550 HMG are Identical

G450 Engine-Driven Hydraulic Pumps are Weaker

[G450 Aircraft Operating Manual, §2A-29-20 ¶2.A.] Hydraulic pump output is rated at 3,000 psi at a pump rpm of 3,750 producing a flow of 25 gpm. Actual flow rates vary between 10.5 gpm at engine idle and 21.3 gpm at maximum engine thrust.

[G550 Aircraft Operating Manual, §2A-29-20 ¶2.A.] Hydraulic pump output is 3,000 psi at flow rates between 18 gpm at engine idle and 28 gpm at maximum engine thrust.

Emergency Inverter

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Figure: G450 E-Inverter operation, (FlightSafety G450 Maintenance Training Manual, figure 24-22, sheet 2)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.F.] An Emergency Inverter (E-Inverter) powered by the DC essential buses will provide single phase (ØA) AC power to the essential AC bus with the loss of all AC power sources. (The essential DC buses are powered by either the HMG, or if not available, aircraft batteries in this instance.) The E-inverter is controlled with the E-INV push button switch on the cockpit overhead. The switch is normally positioned to the AUTO selection so that the E-inverter will be automatically powered by essential DC if all generators are lost. The OFF switch position will remove power from the E-inverter if a malfunction prompts a nuisance activation.

Single phase (ØA) versus three-phase (ØA, ØB, and ØAC)? For an explanation of three-phase see: Three-phase Electrical Power. What you need to know here is that the E-Inverter does not power the entire Essential AC bus, only a third of it. What it gives you, if you lost power to the AC Essential is: Cabin Pressure Channel 1, the right pitot heaters, and the standby pitot heaters.

Phase A of the AC Essential normally gets its power from the Left Main AC bus through the LEAC relay. If no power is available, the LEAC switches to the Right Main AC bus. The only condition that allows you to check the REAC relay is when both main AC busses are without power. That's why you check for this following the APU fire test.

More about: AC Essential Relay.

External AC Power

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Photo: External AC and DC power connections, (Eddie's aircraft)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.I] An external AC power source may be connected to the aircraft for ground servicing and/or maintenance. The AC receptacle is located on the lower right side of the aircraft fuselage, forward of the right wing. When external AC power is plugged into the receptacle, the left BPCU first monitors the power source for quality parameters, and when requirements are met, illuminates the AVAIL legend on the EXT PWR push button on the ELECTRICAL PANEL. (External power parameters may be visually monitored on the AC Power 2/3 synoptic page or the AC / DC Power Summary 1/6 system page.) Selecting external AC power ON with the EXT PWR switch will power all buses of the aircraft electrical system. The left BPCU continues to monitor external AC for fault protection, and will drop external AC from the aircraft electrical system if a malfunction occurs. External AC power will be dropped after a short time delay for any of the following fault conditions:

  • Overvoltage - more than 125.5 (±1.5) volts
  • Undervoltage - less than 102.5 (±2.5) volts
  • Underfrequency - less than 380 (+5/-0) Hz
  • Overfrequency - more than 420 (+0/-5) Hz
  • Overcurrent - more than 122 (+5) amperes
  • Phases out of sequence
  • Open or short in Current Transformer
  • Failure of the Central Processing Unit (CPU) in the right BPCU
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The external power switch light will illuminate "AVAIL" when AC or DC external power is plugged into the aircraft and the corresponding BPCU senses the power's parameters are acceptable.

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If the "AVAIL" caption is illuminated and the switch light is pushed in, the plugged in power source will be applied to the aircraft and the "ON" caption illuminates. If only one source is plugged in, the "AVAIL" caption extinguishes.

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If both sources are plugged in, the AC is connected and the "AVAIL" remains illuminated.

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Figure: G450 AC synoptics, HMG only powered, (FlightSafety G450 Pilot Training Manual, figure 2-48_sheet_1)

External AC is just as good as having the APU running.

DC Power Sources

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Figure: G450 Electrical system component locations, (G450 Aircraft Operating Manual, §2A-24-00, figure 1)

DC Electrical Power System Description

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Figure: G450 DC System Diagram, (G450 Aircraft Operating Manual, §2A-24-00, figure 13)

[G450 Aircraft Operating Manual, §2A-24-10 ¶2.B.]

  • Direct current is obtained by transforming and rectifying aircraft AC power into DC. Five transformer / rectifier units (TRUs), powered by the aircraft AC buses, produce DC power at a nominal 28 volts (voltage of the TRUs is not regulated and varies with load - normal voltage range is 26V to 29V).
  • If all AC power is lost, the TRUs will be inoperative. DC power for essential flight functions is furnished by two aircraft batteries. The lead/acid batteries are rated at 45 amps/hour and supply 24 volts. The aircraft batteries are normally used only for starting the APU or for powering the Ground Service bus in preparation for flight. Each battery has a high capacity charger to return the battery to full capacity after use.
  • Two emergency lead/acid type batteries, rated at 9 amps/hour, provide DC power for critical flight functions if the aircraft NICAD batteries are exhausted of capacity. Emergency battery power will enable limited navigation and communication radio use and power the Inertial Reference Units (IRUs). Two additional lead / acid batteries provide emergency interior and exterior lighting to illuminate emergency escape paths and exits.
  • An external DC power supply may be connected to a receptacle on the forward right lower fuselage for APU starting or powering the ground service bus for servicing, maintenance or preflight inspections.

Aircraft Batteries

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Photo: G450 aircraft batteries, (FlightSafety G450 Pilot Training Manual, figure 2-30)

[G450 Aircraft Operating Manual, §2A-24-30 ¶2.D.]

  • Two (left and right) lead / acid main aircraft batteries are located in the unpressurized equipment compartment in the tail of the aircraft. The batteries are rated as 24V DC, 45 amp/hour. During normal operations, the main batteries are used to power the essential DC buses prior to starting the APU to provide APU fire detection and fire extinguishing. (The APU start contactor is powered only by the left battery during the starting process unless the left battery is manually selected off or not connected or installed, in which case the right battery will power the APU start contactor).
  • Main aircraft batteries can also power the auxiliary hydraulic pump in order to pressurize the parking brake, close the main cabin door and perform ground maintenance. When the aircraft is on the ground (weight-on-wheels), the right main aircraft battery is also used to power the ground service bus for aircraft servicing and lighting.
  • Two battery chargers, one for each battery, are also located in the tail equipment compartment. The battery chargers operate as TRUs, converting 115V, 400 Hz, 3Ø AC into 28V DC to maintain main battery charge. The left charger is powered by the left main AC bus, the right charger by the right main AC bus. The chargers will begin to operate if battery voltage drops to 23V and will replenish an almost discharged battery in approximately 90 minutes. The main battery chargers will also aid in starting the APU or powering the AUX hydraulic pump by providing an extra 40 amps in addition to battery power, provided the main AC buses are powered. This feature could also be employed in the unusual circumstance of loss of TRU input to the essential DC buses, requiring the batteries to power essential DC with main AC TRUs powering the main DC buses. Protective features in the battery chargers will prevent damage to the aircraft batteries.

[G450 Airplane Flight Manual, §2-02-10] NOTE: If the airplane has been exposed to an overnight cold soak at temperatures lower than -20°C (-4°F) and the ambient temperature is -20°C (-4°F) or colder at the time of APU start, the batteries should be warmed by external means to raise battery temperature prior to attempting an APU start. Use an external AC power cart, if available, to charge and warm batteries prior to attempting APU start.

3. BATTERIES VOLTS...CHECK (22V DC MIN)

The airplane has difficulty coming to life with poor batteries so it is important to keep them well charged and warm when it is especially cold. If you need to charge them while still installed in the aircraft, you will need an AC power cart, DC won't do it. You press the battery switches in for APU start and will see the "ON" caption illuminate when starting the APU or operating the AUX pump. The only other time those "ON" capsules should illuminate is when the voltage on the DC essential buses fall below 20V. If you see that, you have 90 minutes to get an AC source up to charge the batteries or face having to rely on the E-batts.

Transformer Rectifier Units (DC)

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Figure: G450 TRU component locations, (FlightSafety G450 Maintenance Training Manual, figure 24-35)

[G450 Aircraft Operating Manual, §2.A.] Five identical and interchangeable Transformer-Rectifier Units (TRUs) provide DC power to the electrical power system. Each is capable of up to 250 amps of power at a nominal 28 volts (dependent upon DC load). Four of the TRUs are dedicated to specific bus outputs, with the fifth AUX TRU as a standby unit:

  • Left Main TRU - converts left main 115V AC to 28V DC to power the left main DC bus
  • Right Main TRU - converts right main 115V AC to 28V DC to power the right main DC bus
  • Left Essential TRU - converts left main 115V AC to 28V DC to power the left essential DC bus
  • Right Essential TRU - converts right main 115V AC to 28V DC to power the right essential DC bus
  • Auxiliary TRU - converts 115V AC from either the left main, right main or standby system (in that order) into 28V DC to automatically power any single DC bus with an inoperative TRU. If neither main AC bus is powered, the AUX TRU converts 115V AC from the standby system to 28V DC to power the left and/or right essential DC buses (DC power to the essential buses must be manually selected in this instance)

The AUX TRU will automatically supplant a malfunctioning TRU to power the associated DC bus in a programmed priority:

  • Left Essential DC Bus
  • Right Essential DC Bus
  • Left Main DC Bus
  • Right Main DC Bus

The Main, Essential and AUX TRUs are located in a rack beneath floor panels in the vicinity of the aircraft entrance door. The TRUs are cooled with ducted airflow from the EER two-speed cooling fans and PSU fan air that is drawn over the units prior to discharge overboard though the Thrust Recovery Outflow Valve (TROV). All of the TRUs have a wire discrete connection to I/O modules in the MAUs (left main, left essential and AUX to MAU #1, right main and essential to MAU #2) for over temp signal input to the MWS.

The TRU's rarely work harder than 30% of their capacity and you have a "hot running spare," ready to step in and take over for a bad TRU. There is a definite pecking order to where that AUX TRU will step in but there are a few work arounds at your disposal.

Emergency Batteries

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Figure: G450 E-Batts, (Eddie's notes)

[G450 Aircraft Operating Manual, §2A-24-30 ¶2.E.] In the event that the main aircraft batteries become depleted during a failure of all AC generators, two emergency lead/acid batteries are available to power emergency navigation and communication resources. (The equipment powered by the emergency batteries is tabulated in the Electrical Power Distribution Buses section.) The left and right emergency batteries are located in the respective electronic equipment racks (LEER and REER) and are rated at 24V and 9 amperes per hour. The emergency batteries each have a dedicated charger, powered by the respective left and right standby AC bus. The emergency battery chargers will replenish 80 - 90% of battery charge in an hour, and fully charge the batteries in one and one-half hours. The operation of the emergency batteries is controlled by three switches on the overhead panel labelled EMERGENCY POWER. The three switches select emergency battery power ON, OFF or ARM for automatic supply of DC power should the main batteries fail.

NOTE: The three control switches have split legends, one for the emergency lighting batteries (LIGHTS) and one for avionics power (AV PWR). A dual emergency battery installation provides power for emergency exit and egress lighting if needed, and is controlled with the same switches as the avionics emergency power batteries. The EMERGENCY POWER switch labels and functions are:

  • ON - when depressed, powers the left and right emergency buses and essential flight instrument bus from the emergency batteries and illuminates the AV PWR amber legend - the AV PWR legend will also be illuminated if the emergency and flight instrument buses are automatically powered by the emergency batteries during main battery failure.
  • ARM - when depressed, arms the emergency batteries to power the emergency and flight instrument buses if essential DC bus power drops below 20V indicating main battery failure. When the switch is depressed, the AV PWR legend is not illuminated - the legend illuminates only if the switch is not armed.
  • OFF - the emergency batteries may be selected off provided that there is more than twenty volts (20V) present on the essential DC buses. If the switch is depressed, the AV PWR legends of the OFF and ARM switches will illuminate amber.

The avionics Emergency Batteries are called "IRU Backup Batteries" in the Minimum Equipment List (MEL). [G450 MMEL, page 24-7 Item 16] Gulfstream told me the FAA would not approve an MEL item to anything called "emergency," so they changed the name in the MEL because they wanted to be able to dispatch with a bad E-Batt. But do you really want to go without one? The MEL says you can go without one E-Batt:"May be inoperative provided that the affected battery is located in the #2 or #3 IRU back up battery position." Of course if you want to play that game, that would be the Right E-Batt and going without that means you don't have a back up for your lading gear position indicators, fuel quantity, #1 MCDU, ATC #1, and VHF #1. Your call.

Emergency Floor Path Lighting

[G450 AOM, §2A-33-60, ¶2.C.] Floorpath lighting is installed to indicate paths to emergency exits should the cabin visibility be reduced by smoke, flame or fumes. The floorpath lighting is powered by a dedicated system of three batteries that provide overlapping power sources to the lights ensuring that a visible exit route is always available.

[G450 MM, §33-54-00, ¶1.A.] The emergency floorpath lighting system, installed on the aircraft during the completion process, provides passengers and crew an illuminated evacuation path providing guidance to exits in event of an emergency or loss of aircraft power. In darkened or smoke filled cabin, white aisle lights illuminate the exit path, red lights signify the need to stop and look for an exit and illuminated exit arrow signs point to the nearest emergency exit. The 28 Vdc power for the floorpath lighting is provided by three batteries dedicated to the system. The batteries are charged through the FLOOR PATH circuit breaker anytime the main cabin dc bus is powered. The circuit breaker is located in the INTERIOR LIGHTS section of the cabin circuit breaker panel.

These are installed during completion and any manuals should have been provided at that time. This is a trivia question that is often answered incorrectly twice:

  • "How many emergency batteries do we have?"
  • "Four (the four eBatts).
  • "Wrong! There are also three emergency floor dedicated batteries so the correct answer is seven!"

Well ponder this. There is a D-cell battery behind each emergency exit sign over each overwing emergency exit. You can see a drawing of them in the G450 MM, §33-51-00, figure 402. This manual doesn't explain how these batteries get their charge and they aren't mentioned at all in any of the pilot manuals. So that raises the count to eleven. But wait! What about the batteries in each flashlight? Yes, the entire series of questions is silly.

Power Control

The airplane is run by computers, that much we know. The electrical system is run by several Generator Control Units (GCUs) and two Bus Power Control Units (BPCUs). The GCU's protect the generators and the BPCU's protect the buses. I took the time to draw the diagram above when I started flying the GV, thinking a Gulfstream's electrical system bears close watching. As it turns out, this system requires almost no care and feeding. About the only time you touch the panel as a pilot is when you are at the simulator.

Generator Control Units (GCUs)

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Figures: G450 left and right GCU's, (G450 Maintenance Manual, §24-21-03, figures 401 and 402)

[G450 Aircraft Operating Manual § 2A-24-20 ¶2.C.] 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. The GCUs are initially powered by 28V DC from the essential DC buses (L GCU and APU GCU from the left essential DC, R GCU from the right essential DC) until the generators reach normal operating speed: the GCUs then convert generator three phase AC from the permanent magnet generator into 28V DC with an internal diode transformer / rectifier. The GCUs protect the AC buses by monitoring the quality of power produced by the associated generator. If generator performance does not meet specified parameters, the GCU will not permit power to be supplied to the AC buses. GCUs provide the following protection: overvoltage, undervoltage, overfrequency, underfrequency, overcurrent, phase sequence, differential fault, inadvertent paralleling trip, open phase protection, servo valve protection, GCU processing unit failure and underspeed protection (engine IDG less than 4,500 rpm or APU power ready signal not present within 100 milliseconds).

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 pushbutton on the overhead ELECTRICAL POWER CONTROL PANEL.

If an aircraft engine is shut down for a malfunction or fire, the GCUs will de-energize the associated generator if a fire handle is activated or if a fuel control switch is selected off.

[G450 Maintenance Manual, §24-25-01 ¶2.A.] The GCU can be reset from a fail-safe condition when power is removed from the GCU or when the left/right generator control switch is operated from off to on. This reset causes the software to re-initialize. If the conditions that caused the microprocessor fail-safe are still present, the microprocessor will fail-safe again.

In a nutshell, the purpose of the GCU is to protect its generator. If the current of any phase of generator power supplied to a bus exceeds 116 -0/+5 amps, the GCU attempts current reduction by removing power first from the main AC bus cross-tie relays, secondly by changing the power source of the essential bus, or finally by opening the contactor between the generator and the normally assigned main AC bus, thus locking out the generator from all buses. An APU generator overcurrent will result in the generator first dropping power to the right main AC bus, then left main AC bus before locking out the APU generator from all buses. (If another power source for the main AC buses is operating, that source will pick up and power the buses after APU generator power is dropped.)

Bus Power Control Units

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Figure: G450 bus power control units, (G450 Aircraft Operating Manual, §2A-24-00, figure 5)

[G450 Aircraft Operating Manual § 2A-24-20 ¶2.D.] Generator AC power is applied to the aircraft AC buses through two identical and interchangeable Bus Power Control Units (BPCUs) located in the respective Left Electronic Equipment Rack (LEER) and Right Electronic Equipment Rack (REER), that provide power and control logic for the contactors connecting power sources to the aircraft buses. The left BPCU monitors AC power to the left main AC bus and provides quality oversight of external AC power. The right BPCU monitors power to the right main AC bus, the essential AC bus and provides quality oversight of external DC power.

The BPCUs close and open contactors to power AC (and DC) buses in response to the cockpit overhead panel pushbutton switch positions. BPCUs also illuminate the appropriate legend in the switch, indicating electrical system performance. With the right and left bus tie switches (L BUS TIE, R BUS TIE) in the normal AUTO position, the BPCUs provide automatic switching for the left and right main AC buses. In a standard aircraft engine starting sequence, the APU generator would first power both main AC buses. When the right engine is started and correct power is available from the right generator, the right BPCU switches the right main AC bus to right engine generator power, and the APU generator continues to power the left main AC bus. When the left engine is started and correct generator power is available, the left BPCU switches the left main AC bus to left engine generator power, and the APU generator reverts to a standby power source until the APU is shut down.

When the BPCUs transfer main AC buses to available power sources, the BPCUs first ensure that the power source parameters match in terms of frequency and phase angle. The BPCUs select one of the power sources as a reference for comparison, then signal the GCU to adjust IDG frequency and/or phase angle so that both power sources are congruent. This process is termed paralleling, and enables the BPCU to achieve a No-Break Power Transfer (NBPT) that maintains power on the buses while switching power sources. All automatic power transfers will be NBPTs except those between an external AC power source and the APU generator, since the APU generator is not an IDG and the GCU cannot adjust frequency or phase angle without a hydromechanical drive unit.

The BPCUs provide monitoring and bus protection when external sources are providing electrical power to the aircraft. The left BPCU monitors external AC power and the right BPCU tests external DC power. The BPCUs act in a manner similar to the GCUs, checking external AC for under / over voltage, under / over frequency, phase sequence and overcurrent. External DC is monitored for over / under voltage, overcurrent and polarity. If external power quality is within limits, the BPCU will signal the acceptable quality by illuminating the AVAIL section of the EXT PWR pushbutton on the cockpit overhead panel. The AVAIL light will extinguish when external power is selected ON (external power quality can be monitored on the synoptic and system window displays). The BPCUs will trip the external power contactor if power quality falls outside of usable parameters.

The main purpose of each BPCU is to protect the buses. They also function to monitor external power and to provide no break power transfers between IDGs and any other AC power source.

AC Power Distribution

Most power in this airplane begins at an engine or APU generator in the form of four very large cables, three of which carry 115V AC, 40,000 volt-amps and a fourth that carries the opposite ground. Each of the three live wires alternate their current between positive and negative 400 times every second (400 Hertz). These power sources are in three phases, meaning as one is increasing in voltage from -115 to +115, the other two are in other states where all three, added together, become zero. This allows the voltage to "cancel," reducing the size requirement of each cable which are still very large.

More about this: Three Phase Electrical Power.

These cables eventually end up in Power Distribution Boxes (PDBs) which is where, for sake of explanation, most of the buses reside. (A bus can be thought of as nothing more than a very large strip of metal, but of course it would be three large strips of metal in a three-phase system.

The buses divide the massive power into smaller cables that end up at other electrical equipment racks (EERs) which further divide the power into yet smaller cables to individual circuit breakers. From there the power goes to individual components. Simple, eh?

AC Electrical Power System Description

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G450 AC electrical power system schematic, (G450 Aircraft Operating Manual, §2A-24-00, figure 3)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.G.] AC electrical power is distributed throughout the aircraft by the following buses:

  • Left main AC
  • Right main AC
  • Essential AC
  • Left standby AC
  • Right standby AC
  • 60 Hz AC

Left Power Distribution Box

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Photo: Left PDB, (Eddie's aircraft)

[G450 Maintenance Manual, §24-51-00 ¶4.A.] The left main AC bus resides in the left PDB.

[G450 Maintenance Manual, §25-52-00 ¶4.A.] The essential AC bus is contained in the left PDB.

Right Power Distribution Box

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Photo: Right PDB, (Eddie's aircraft)

[G450 Maintenance Manual, §24-51-00 ¶4.A.] The right main AC bus resides in the right PDB.

Power Distribution: From an IDG

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Figure: Electrical control panel, generator switches, (G450 Aircraft Operating Manual, §2A-24-00, figure 8)

[G450 Maintenance Manual, §24-25-01 ¶1.A.] The GCU monitors its respective respective IDG and electrical system to provide control functions, system protection, indicator functions and BIT capability.

[G450 Maintenance Manual, §24-25-01 ¶2.A.] The GCU can be reset from a fail-safe condition when power is removed from the GCU or when the left/right generator control switch is operated from off to on. This reset causes the software to re-initialize. If the conditions that caused the microprocessor fail-safe are still present, the microprocessor will fail-safe again.

The generator switch light is normally left selected (in) under which conditions it illuminates "ON" when the IDG is producing power and is permitted to send power by the GCU, or it illuminates "OFF" if the IDG is not producing power. If the switch light is not selected (out), the IDG is not permitted to connect by the GCU. Deselecting the switch (out) and reselecting it (in) serves to reset the GCU.

Power Distribution: From an APU

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Figure: Electrical control panel, APU generator switch, (G450 Aircraft Operating Manual, §2A-24-00, figure 9)

[G450 Maintenance Manual, §24-25-05 ¶1.A.] The APU GCU and IDG GCUs are physically identical and completely interchangeable..

[G450 Maintenance Manual, §24-25-05 ¶2.A.] The GCU can be reset from a fail-safe condition when power is removed from the GCU or when the APU generator switch is operated from off to on. This reset causes the software to re-initialize. If the conditions that caused the microprocessor fail-safe are still present, the microprocessor will fail-safe again.

The APU generator switch light is normally left selected (in) under which conditions it illuminates "ON" and will connect to one or both main AC buses as a matter of priority. If the switch light is not selected (out), the APU generator will not be permitted to connect to any bus. Deselecting the switch (out) and reselecting it (in) serves to reset the GCU.

Power Distribution: From External Power

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Figure: Electrical control panel, external power generator switch, (G450 Aircraft Operating Manual, §2A-24-00, figure 10)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.I.] When external AC power is plugged into the receptacle, the left BPCU first monitors the power source for quality parameters, and when requirements are met, illuminates the AVAIL legend on the EXT PWR push button on the ELECTRICAL PANEL. (External power parameters may be visually monitored on the AC Power 2/3 synoptic page or the AC / DC Power Summary 1/6 system page.) Selecting external AC power ON with the EXT PWR switch will power all buses of the aircraft electrical system.

The external power switch light will illuminate "AVAIL" when AC or DC external power is plugged into the aircraft and the corresponding BPCU senses the power is acceptable. If the "AVAIL" caption is illuminated and the switch light is pushed in, the plugged in power source will be applied to the aircraft and the "ON" caption illuminates. If only one source is plugged in, the "AVAIL" caption extinguishes. If both sources are plugged in, the AC is connected and the "AVAIL" caption remains illuminated.

Power Distribution: From the HMG

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Figure: Electrical control panel, HMG switches, (G450 Aircraft Operating Manual, §2A-24-00, figure 11)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.E.] The primary purpose of the HMG is to provide an AC power source to the Auxiliary Transformer / Rectifier Unit (AUX TRU). The two AC Standby buses will also be powered by the HMG if main AC is lost, and feed a limited number of AC installations.

The HMG, an AC generator, is used to power one or both DC essential buses through the AUX TRU. This is done by selecting the appropriate bus on the Standby Electrical Power panel. The HMG also powers the two standby AC buses, which provide power for pitot heat, TAT heat, flap/stab power and emergency battery chargers.

Power Distribution: From the Emergency Inverter

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Figure: Electrical control panel, E-Inverter switch, (G450 Aircraft Operating Manual, §2A-24-00, figure 10)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.F.] The Emergency Inverter provides single phase (ØA) AC power to the essential AC bus with the loss of all AC power sources. The switch is normally positioned to the AUTO selection so that the E-inverter will be automatically powered by essential DC if all generators are lost. The OFF switch position will remove power from the E-inverter if a malfunction prompts a nuisance activation.

Single phase (ØA) versus three-phase (ØA, ØB, and ØC)? For an explanation of three-phase see Three-phase Electrical Power. What you need to know here is that the E-Inverter does not power the entire Essential AC bus, only a third of it. What it gives you, if you lost power to the AC Essential is: Cabin Pressure Channel 1, the right pitot heaters, and the standby pitot heaters.

Power Distribution: To the Main AC Buses

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Figure: G450 electrical schematic, power to main AC buses, (Eddie's notes)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.G.] Left and right bus tie switches on the cockpit overhead panel can prevent a bus from receiving power from another generator source. Selecting the R BUS TIE or L BUS TIE switches to ISLN (isolated) allows only the respective right or left engine IDG to power the associated bus.

The Bus Tie switches are normally left selected to the "AUTO" position where they permit normal Main AC Bus power by priority. Each Main AC bus prefers power in the following order: On-side IDG, APU generator, external AC, and cross-side IDG. In the "ISLN" position, the particular main AC bus will only accept power from its IDG.

Power Distribution: To the Essential AC Bus

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Figure: G450 electrical schematic, power to essential AC bus, (Eddie's notes)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.G.] The left main AC bus normally powers the essential AC bus. Redundancy is provided by bus tie relays in the electrical system that allow any of the engine or APU mounted generators to power the main AC and essential AC buses. One phase of the essential AC bus may also be powered by either the HMG or by the aircraft batteries through an emergency inverter that will supply single phase (ØA) AC power to the bus if all generators are inoperative.

The AOM isn't exactly right when it says any of the generators can power the essential AC bus. The essential AC bus is connected to the left main AC bus by default through a single relay called LEAC. If the left main AC bus goes unpowered, that relay flips to another called REAC which is connected to the right main AC bus. If that bus goes unpowered, the second relay flips to allow one phase to be powered by the E-inverter. The APU and external power can power the essential AC bus, but they do so through one of the main AC buses.

The REAC, by the way, is the relay we are checking during the APU Fire test.

More about this: G450 AC Essential Relay Check.

Power Distribution: To the Standby AC Buses

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Figure: G450 electrical schematic, power to standby AC buses, (Eddie's notes)

[G450 Aircraft Operating Manual, §2A-24-20 ¶2.G.] The left main AC bus normally powers the left standby AC bus. The right main AC bus normally powers the right standby AC bus. The standby buses may be powered by the HMG if the main AC buses cannot be powered by an aircraft generator.

Other than the HMG, there is no other redundancy to these buses which provide power to TAT probe heaters, battery chargers, left pitot heat, and to the secondary STAB motor. If you lose a main AC bus the QRH doesn't tell you this, but you can get the standby AC buses back by simply selecting the standby electrical power master switch to ON.

AC Essential Relay

We look for a Cabin Pressure Control FAULT light during the APU test, we are told, because the system checks for power from the E-Inverter at this time. That isn't exactly right.

The LEAC and REAC relays are checked during the APU Start Fire Test because that is the only time you will normally have both Left and Right Main AC buses unpowered with a DU powered to check for a Essential AC-Bus Fail CAS message. If the REAC relay has failed, there are no other opportunities, since the LEAC relay is normally energized to the L Main AC Bus, bypassing the REAC relay.

The Left ESS AC Bus powers Channel 1 of Cabin Pressure Control, the left lower and right upper pitot heats, and flaps/stab if both standby busses are dead. If the Left ESS AC Bus is unpowered at the time of the APU fire test, the Cabin Pressure Control FAULT light will be illuminated.

[G450 Maintenance Manual, §24-23-00, ¶1.A.] The emergency ac generation subsystem is used in the event of Integrated Drive Generators (IDG) and APU generator failure. The emergency ac generation subsystem uses the emergency inverter to provides 1 Kva, single-phase, 115 Vac, 400 Hz power to the ac essential bus phase A through the de-energized contacts of Right Essential AC Contactor (REAC) and Left Essential AC Contactor (LEAC).

Normal: Three Phases from Left Main AC

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Figure: LEAC normal / REAC normal, from Eddie's notes.

The AC Essential Bus is normally powered by the L Main AC Bus:

Normal: Three Phases from Right Main AC

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Figure: LEAC switched / REAC switched, from Eddie's notes.

If Left Main AC is not available (locked out by GCU or BPCU, or for any other reason unpowered), the LEAC and REAC relays switch to provide power from the Right Main AC Bus:

Normal: One Phase from E-Inverter

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Figure: LEAC switched / REAC normal, from Eddie's notes.

If the Right Main AC is also not available, the REAC relay switches to provide power for the A Phase from the Emergency Inverter, provided the E-INV switch is in AUTO. Note: the two remaining AC Essential phrases will be unpowered.

DC Power Distribution

Distribution: Transformer-Rectifier Units

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Figure: G450 electrical system component locations, (G450 Aircraft Operating Manual, §2A-24-00, figure 1)

[G450 Aircraft Operating Manual, §2A-24-30 ¶2.A.]

  • Five identical and interchangeable Transformer-Rectifier Units (TRUs) provide DC power to the electrical power system. Each is capable of up to 250 amps of power at a nominal 28 volts (dependent upon DC load). Four of the TRUs are dedicated to specific bus outputs, with the fifth AUX TRU as a standby unit:
    • Left Main TRU - converts left main 115V AC to 28V DC to power the left main DC bus
    • Right Main TRU - converts right main 115V AC to 28V DC to power the right main DC bus
    • Left Essential TRU - converts left main 115V AC to 28V DC to power the left essential DC bus
    • Right Essential TRU - converts right main 115V AC to 28V DC to power the right essential DC bus
    • Auxiliary TRU - converts 115V AC from either the left main, right main or standby system (in that order) into 28V DC to automatically power any single DC bus with an inoperative TRU. If neither main AC bus is powered, the AUX TRU converts 115V AC from the standby system to 28V DC to power the left and/or right essential DC buses (DC power to the essential buses must be manually selected in this instance),
  • The AUX TRU will automatically supplant a malfunctioning TRU to power the associated DC bus in a programmed priority:
    • Left Essential DC Bus
    • Right Essential DC Bus
    • Left Main DC Bus
    • Right Main DC Bus
  • The Main, Essential and AUX TRUs are located in a rack beneath floor panels in the vicinity of the aircraft entrance door. The TRUs are cooled with ducted airflow from the EER two-speed cooling fans and PSU fan air that is drawn over the units prior to discharge overboard though the Thrust Recovery Outflow Valve (TROV). All of the TRUs have a wire discrete connection to I/O modules in the MAUs (left main, left essential and AUX to MAU #1, right main and essential to MAU #2) for overtemp signal input to the MWS.

[G450 Aircraft Operating Manual, § 2A-24-30 ¶2.D.] Two (left and right) lead / acid main aircraft batteries power the auxiliary hydraulic pump in order to pressurize the parking brake, close the main cabin door and perform ground maintenance. When the aircraft is on the ground (weight-on-wheels), the right main aircraft battery is also used to power the ground service bus for aircraft servicing and lighting.

In a normal state, all DC power on the airplane should be coming from four TRU's, each dedicated to a main or essential DC bus, left and right. The aircraft batteries provide a few exceptions. The batteries are used during normal conditions to power the AUX hydraulic pump and start the APU.

Distribution: Ground Service Bus

[G450 Aircraft Operating Manual, § 2A-24-20 ¶2.F.] The ground service bus provides a means to service the aircraft without powering other aircraft equipment. On the ground, the bus may be powered by external DC power or from the aircraft right main battery. Ground service bus power is distributed to the aircraft fueling panel, the engine oiler panel, wheel well lights, bottom anti-collision light and utility lights. Additional lighting is also provided to illuminate the interior of aircraft service panels. When the aircraft is airborne, the ground service bus is powered by the right main DC bus, enabling in-flight use of wheel well, utility and the lower anti-collision light. The operation of the ground service bus is controlled with three switches, only one of which is accessible in-flight

The ground service bus switches are "momentary contact" switches, they do not latch on. Activating any one switch while on the ground will turn the ground service bus on but at least one of the three service doors have to be open, if all three doors are closed the bus will deenergize.

More about this below: Ground Service Bus.

Abnormal DC Electrical Power System Distribution

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Figure: G450 electrical system schematic, TRUs abnormal, (Eddie's notes)

[G450 Aircraft Operating Manual, § 2A-24-30 ¶2.C.] The four major buses each have a dedicated TRU and are supported by the AUX TRU if the corresponding TRU fails. The TRUs supplying the left and right main DC buses may be manually switched to the other main AC bus for a source of 115V power should the normal supply bus fail. Two switch lights on the overhead panel select L MAIN TRU to R AC and R MAIN TRU to L AC. The left and/or right essential DC buses may be powered by the standby system through the AUX TRU if engine and APU-driven generators fail. Switch lights on the STANDBY ELECTRICAL POWER panel are used to manually connect the left and right essential buses to AUX TRU output once the standby system hydraulic motor generator (HMG) has reached normal operating parameters.

Under normal conditions all DC power on the aircraft comes from the two main and two essential DC TRUs. If any of those should fail, the AUX TRU automatically replaces one in the priority (1) left essential, (2) right essential, (3) left main, and (4) right main to power their dedicated buses. You can force the AUX TRU to power either or both essential DC buses by activating the standby electrical power HMG and selecting the L ESS and/or R ESS switches on that panel.

If you lose power TO either main TRU, you can manually provide power to that TRU using the overhead panel TRU switches. If you lose the left main AC bus, for example, the left main TRU will be unpowered. You can power the left main TRU using the right AC main bus with the L MAIN TRU switch.

Emergency DC Electrical Power System Description

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Figure: G450 electrical system synoptics, battery power only, (FlightSafety G450 Pilot Training Manual, figure 2-47)

[G450 Aircraft Operating Manual, § 2A-24-30 ¶2.C.] In addition to the major DC buses, battery powered buses will support essential and emergency equipment with 28V DC power for a limited time if all AC power and associated TRU supplied DC power is lost. Two (left and right) main aircraft batteries are connected to the Battery Tie Bus with push button switches on the overhead panel. Even though the left and right main battery push buttons are depressed to select each battery ON, the ON legend in the push button will not illuminate unless the batteries are actually supplying power to one or both of the essential buses. The dual battery output of the Battery Tie Bus is then available to power all the normal loads of the left and right essential DC buses should a malfunction cause a loss of all TRU supplied DC power. The left essential DC bus normally powers the left emergency DC bus and the flight instrument bus. The right essential DC bus normally powers the right emergency DC bus.

The main aircraft batteries are capable of powering the essential buses for approximately 30 minutes, including two attempts to start the APU (the start attempts must be less than thirty (30) seconds each). If the main aircraft batteries are depleted, two (left and right) emergency batteries will power the left and right emergency buses and the flight instrument bus.

[G450 Aircraft Operating Manual, § 2A-24-30 ¶2.D.] Two battery chargers operate as TRUs, converting 115V, 400 Hz, 3Ø AC into 28V DC to maintain main battery charge. The left charger is powered by the left main AC bus, the right charger by the right main AC bus. The chargers will begin to operate if battery voltage drops to 23V and will replenish an almost discharged battery in approximately ninety (90) minutes. The main battery chargers will also aid in starting the APU or powering the AUX hydraulic pump by providing an extra 40 amps in addition to battery power, provided the main AC buses are powered. This feature could also be employed in the unusual circumstance of loss of TRU input to the essential DC buses, requiring the batteries to power essential DC with main AC TRUs powering the main DC buses.

If you lose all AC power on the airplane (Both IDGs, the HMG, and the APU Generator), you will lose the TRUs and all normal DC power. The batteries will power both DC essential buses for 30 minutes and if, for some reason, you do have the battery chargers working, you can go longer than that.

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Figure: G450 E-Batt Schematic, (Eddie's notes)

[G450 Aircraft Operating Manual, § 2A-24-30 ¶2.E.] In the event that the main aircraft batteries become depleted during a failure of all AC generators, two emergency lead/acid batteries are available to power emergency navigation and communication resources. The left and right emergency batteries are located in the respective electronic equipment racks (LEER and REER) and are rated at 24V and 9 amperes per hour. The emergency batteries each have a dedicated charger, powered by the respective left and right standby AC bus. The emergency battery chargers will replenish 80 -90% of battery charge in an hour, and fully charge the batteries in one and one-half hours.

The avionics emergency batteries are your last line of defense when everything else goes bad. How long will they last? The only thing in writing I could find is the Securaplane manufacturer's data sheet which says the 24V 9 amp/hour capacity was rated at one hour, hardly definitive.

The MEL calls them "IRU Backup Batteries" (MEL 24-7 Item 16) though they are clearly more than that. The MEL also says you can go without the right one. I wouldn't.

Ground Service Bus

A whole section dedicated to the ground service bus? Well, yes. The G450 operating manual says the ground service bus is powered in the air to provide wheel well, bottom anti-collision, and utility lights; and yet the other components are not powered. So that unleashed a search for the reason. Conclusion: the ground service bus is indeed powered when airborne but several of the components are only powered weight on wheels. The bottom anti-collision light goes through a relay with multiple inputs. (See schematic below.)

General Description

[G450 Aircraft Operating Manual, §2A-24-30, ¶1.] The aircraft is equipped with a DC Ground Service bus to provide a source of power for operation of aircraft servicing, lighting and doors prior to the application of AC power.

[G450 Aircraft Operating Manual, §2A-24-30, ¶2.D.] When the aircraft is on the ground (weight-on-wheels), the right main aircraft battery is also used to power the ground service bus for aircraft servicing and lighting.

[G450 Aircraft Operating Manual, §2A-24-10, ¶2.B.] An external DC power supply may be connected to a receptacle on the forward right lower fuselage for APU starting or powering the ground service bus for servicing, maintenance or preflight inspections.

Switching

[G450 Maintenance Manual, §24-60-00, ¶3.C.]

  • The ground service bus is controlled by three ground service bus switches located at the following locations:
    • Tail compartment ground service panel
    • Nose wheel switch panel
    • Left system monitor panel
  • These switches are disabled when the weight on wheels switch is in air mode. These switches are also disabled when all three doors are closed.
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Photo: Ground Service Bus Switch in Cockpit, (Eddie's aircraft)

[G450 Aircraft Operating Manual, §2A-24-30, ¶2.F.]

  • The ground service bus provides a means to service the aircraft without powering other aircraft equipment. On the ground, the bus may be powered by external DC power or from the aircraft right main battery. Ground service bus power is distributed to the aircraft fueling panel, the engine oiler panel, wheel well lights, bottom anti-collision light and utility lights. Additional lighting is also provided to illuminate the interior of aircraft service panels. When the aircraft is airborne, the ground service bus is powered by the right main DC bus, enabling in-flight use of wheel well, utility and the lower anti-collision light. The operation of the ground service bus is controlled with three switches, only one of which is accessible in-flight: [see the photo and illustrations].
  • An ON indicator, labelled GND SVC BUS and located on the cockpit overhead panel will illuminate on the ground when the ground service bus is powered by external DC or main battery power. The external nose service panel and tail compartment bus switches have indicators adjacent to the switches. Ground service bus status may also be monitored on the DC Power 2/3 synoptic page or AC/DC Power Summary 1/6 system page.
  • When the right main DC bus is not powered, three door interlock switches are incorporated to automatically de-energize the ground service bus to prevent right main battery depletion. The bus is de-energized when all of the following doors are closed:
    • Aft equipment (tail) compartment door
    • Main entrance door
    • Forward external switch panel door
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Photo: Ground Service Bus Switch in Cockpit, (Eddie's aircraft)

[G450 Maintenance Manual, §24-64-00, ¶3.C.] The ground service bus indicators come on when ground service control relay is energized. The ground service bus indicators are connected in parallel. All ground service bus indicators come on when ground service bus is enabled. The ground service bus indicators are located in the following locations:

  • COP
  • Forward external switch panel
  • Tail compartment ground service panel

As pilots we never really have a reason to use the ground service bus from this switch. The external battery switch gives us access to the aux pump, which the ground service bus does not.

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Photo: Ground Service Bus Switch in Aft Equipment Compartment, (Eddie's aircraft)

[G450 Maintenance Manual, §24-64-00, ¶3.B.]

  • The door switches are connected in parallel. The door switches enable activation of ground service bus when at least one door is open. If all doors are closed, ground service bus control switches are disabled.
  • The three momentary ground service bus switches are connected in parallel. When momentary switches are enabled, pressing any momentary switch once will activate ground service bus by closing ground service bus control relay. Pressing any momentary switch a second time will deactivate ground service bus by opening ground service bus control relay. The momentary switches are located in the following locations:
    • Tail compartment ground service panel
    • Forward external switch panel
    • LEER system monitor panel

Bottom Rotating Beacon

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Figure: Flashing Beacon Schematic, (G450 Wiring Diagram, §33-42-00, Figure 1, Sheet 3)

You often hear that the bottom rotating beacon is on the ground service bus, well that might be true. Or it might be that it is on the Right Essential when the external battery switch is on and everything else goes through a weight on wheels relay.

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 GV, GV-SP, GV-SP (G550), GV-SP (G500), GIV-X, GIV-X (G450), GIV-X (G350) Master Minimum Equipment List, Revision 07, February 4, 2010.

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

* Fair Use Statement

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—Eddie's Lawyer

My objective is to give pilots the tools they need to fly more safely. If you are a copyright holder and see your material here, that means I think your work can help to that end. Still, if you would rather your materials not appear on this website, please "Contact Eddie" using the link below and your material will be removed. It will be our loss, but I understand the need to protect creative rights.

To answer another frequent question: yes I live in fear of law suits and have reached out for permissions where possible. Gulfstream has been very good about all of this provided I add the note shown below. Let me be clear about this: I think the world of Gulfstream Aerospace. There is no prettier wing in existence than what you see on a GV or G550 and the best cockpit I've ever touched is in my trusty G450.

—Eddie

Revision: 20150722
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