Left: G450 Fuel Synoptic, from FSI G450 Pilot Training Manual, figure 5-23
Right: G550 Fuel Synoptic, from FSI G550 Pilot Training Manual, figure 5-2
The G450 and G550 fuel systems are virtually identical, both borrowing heavily from the Classic GV which borrowed heavily from the GIV and all the way back to the GII. The G550 carries more fuel for longer, so it has a heated fuel return system that is not found on the G450. Some of the limitations are slightly different too. But if you learn the fuel system on one, you will be 99% there for the other.
The fuel system on both aircraft require very little crew interaction with just a few exceptions. We will look at the complete system from the time the fuel enters the aircraft to the time it leaves the fuel tanks:
What follows comes from the references shown below. Where I think it helpful, I've added my own comments in blue.
Single point fueling, from Eddie's aircraft.
[G450 Maintenance Manual, §28-21-00, ¶4.A.(1)] During pressure fueling, fuel enters through the pressure fueling / defueling adapter. The fuel then flows through the main fill line to a cross fitting. From the cross fitting, fuel then flows though separate lines to a pressure fueling shut-off valve where it enters the main wing tanks. The pressure fueling shut-off valves are part of the fueling shut-off control system. Fuel flow through the shut-off valves will continue until the valves are closed by the fueling shut-off control system.
Figure: Pressure Fueling/Defueling Adapter, from FSI G450 Pilot Training Manual, figure 5-11.
If you set a specific onload at the automatic refuel panel the fuel will eventually even itself out. If you ask for a specific amount without setting the panel in a G450, you will tend to load right wing heavy. This is the way it was in the GIV. For some reason the GV/550 don't have this problem.
Figure: Pressure fueling system operation, from FSI G450 Maintenance Training Manual, figure 28-25.
Do you need electrical power to refuel? No, the pressure-fueling and high-level shutoff valves do not require it. But you will need electrical power if you want to automatically shut off at a preset level. More about how to fuel the airplane: G450 Normal Procedures & Techniques / Fuel Servicing.
Figure: Pressure Fueling/Defueling Adapter Precheck Valves, from FSI G450 Pilot Training Manual, figure 5-10.
More about this: G450 Normal Procedures & Techniques / Fuel Servicing.
Figure: Pressure fueling shutoff control, from FSI G450 Maintenance Training Manual, figure 28-19.
Nobody does the precheck valve inspection for every refueling but you really need to do it if you are fueling close to capacity. You ought to consider doing this at least once a month even if you never get near full capacity, just to make sure the pressure check system works.
Figure: Fueling high-level valves, from FSI G450 Maintenance Training Manual, figure 28-23.
[FSI G450 Maintenance Training Manual, page 28-41] The fuel system contains two high-level pilot valves in each tank (Figure 28-23). The inboard and outboard high-level pilot valves will shut off pressure fueling when the wing tank is full. Both valves must be closed to stop fueling. When the fuel in the tank reaches the full level, it causes a float in the inboard and outboard high-level pilot valves to close. thus causing back pressure in the pressure sensing line to the pressure fueling shutoff valve, closing that valve and stopping the fueling process.
Figure: G450 ground service control panel, from FSI G450 Maintenance Training Manual, figure 28-60.
Despite notes to the contrary, the panel doesn't default to manual refueling. If you mistakenly leave the switch in AUTO REFUEL, the system could shut off your fueling at whatever value was left in the PRESEL.
For more about this, see G450 Normal Procedures & Techniques / Fuel Servicing.
Figure: G550 fuel system control panel, from G550 Aircraft Operating Manual, §2A-28-00, figure 5.
You can electrically shut off fuel to either tank when refueling using single point pressure fueling using the remote fueling shutoff switches. These are powered by the ground service bus. If you place one of these switches to off and turn the ground service bus off, the valve will reopen.
Figure: G450 gravity fueling adapter, from FSI G450 Maintenance Training Manual, figure 28-32
[G450 Aircraft Operating Manual, §2A-28-20 ¶2.B. and G550 Aircraft Operating Manual, §2A-28-20 ¶2.B.] An over-wing (gravity) fueling adapter assembly is installed on the top of each wing near the wing tip. The adapter has a locking fuel cap and a sleeve intake into the wing tank. The sleeve is fitted with a screen filter to prevent foreign objects from entering the tank during fueling. Because of the length of the sleeve intake required to accommodate the fueling nozzle, the amount of fuel loaded using this method is slightly less than that if pressure fueling were used.
[G450 Aircraft Operating Manual, §09-02-10, ¶3.B. G550 Aircraft Operating Manual, §09-02-10, ¶3.B.] If fueling facility is not equipped for over-the-wing refueling both wings simultaneously, it may be necessary to alternate refueling each wing incrementally provided maximum fuel imbalance between both wings does not exceed 2000 pounds at any time during refueling operation.
More about this: G450 Normal Procedures & Techniques / Fuel Servicing.
G450 integral wing fuel tank and flow points, from FSI G450 Pilot Training Manual, figure 5-1.
[G450 Maintenance Manual, §28-00-00, ¶3.A] Fuel is integrally stored in the left and right wings. Wing construction allows for most of the interior area of the wings to act as fuel storage tanks. Most areas between the front spar, rear spar, wing root and the wing tip is used for fuel storage. The wings, designed with a 3° positive dihedral, form a natural gravity feed system to the fuel hoppers located at the wing root. The fuel hoppers are isolated compartments within the tanks that are continuously supplied with fuel by gravity and a fuel ejector pump system. Fuel is removed from the fuel hoppers to supply the engines and APU. To prevent the outward flow of fuel during in-flight turns, ribs within the tanks are fitted with baffles and check valves hinged to open toward the wing root, thus ensuring the hoppers remain full at all times.
[G450 Aircraft Operating Manual, §2A-28-20 ¶1. and G550 Aircraft Operating Manual, §2A-28-20 ¶1.] The airplane wing fuel tanks are integral to the wing structure. Fuel is contained within most of the interior of the wing, with the tank dimensions defined by the front wing spar, rear wing spar and the upper and lower wing skin. The interior of the wing is coated with a sealant during manufacturing to prevent fuel leakage. The shape of the wing accommodates the installations necessary for efficient operation of the fuel system. The tank area near the wing root has the largest volume and houses the fuel boost pumps and fuel feed lines. With wing dihedral of 3°, fuel within the wing will always flow towards the wing root, ensuring that the fuel boost pump inlets will be adequately supplied until all usable fuel has been consumed. To prevent the outward movement of fuel during flight maneuvers involving turns, the five wing ribs within the tank that form the contour of the wing are fitted with baffles hinged to open only in the direction of the wing root. The wing ribs also contain a series of holes above the baffles to allow fuel to flow outboard during single-point pressure fueling, since the single-point access is located near the wing root. A second set of holes penetrate the wing ribs below the baffles to allow residual fuel and any accumulated water to drain inboard to the wing root as fuel quantity decreases.
[G450 Maintenance Manual, §28-10-00, ¶1.B.]
(1) Determine class of external leak as follows:
(e) Classify leak from the following:
NOTE: The size of the wetted area described for seep, slow seep or heavy seep is limited by evaporation of fuel. When fuel evaporates from a wetted surface at the leakage rate, the wetted area will not enlarge.
CAUTION: ANY LEAK MAY BE AN INDICATION OF A STARTING STRUCTURE FAILURE.
(2) Repair external leak as follows:
NOTE: Source of leak must be located and cause determined prior to resealing.
For more about fuel leaks, see: G450 Abnormal Procedures / Fuel Leak Inflight.
Figure: Wing fuel hopper heat exchanger, from FSI G550 Pilot Training Manual, figure 13-4.
[G450 Aircraft Operating Manual, §2A-29-20 ¶2.B.] After pressurizing aircraft subsystems and components, [left] system fluid enters a radiator type heat exchanger located in the left wing fuel tank. The hot hydraulic fluid is cooled by fuel in the tank (and tank fuel is slightly warmed, although the high ratio of fuel to hydraulic fluid results in a minimal temperature rise) then returned to the system reservoir, after passing through a system return filter located in the filtration module.
[G450 Aircraft Operating Manual, §2A-29-30 ¶2.B.] Right system pressurized fluid follows a similar path as the left system, through the same sequence of acoustic filter, filter manifold, system accumulator, dedicated thrust reverser, to the tail-mounted flight controls and forward to the landing gear wheel well for powering the flight controls installed on the wing, through a pressure relief valve, into the radiator cooler in the right wing fuel tank hopper, through a return filter and back to the reservoir. The right system also includes plumbing to the motor section of the PTU and return.
[G550 Aircraft Operating Manual, §2A-29-30 ¶2.B.] After pressurizing aircraft subsystems and components, [left] system fluid enters a radiator type heat exchanger located in the right wing fuel tank. The hot hydraulic fluid is cooled by fuel in the tank (and tank fuel is slightly warmed, although the high ratio of fuel to hydraulic fluid results in a minimal temperature rise) then returned to the system reservoir, passing through the return filter en route.
[G550 Aircraft Operating Manual, §2A-29-30 ¶2.B.] Right Hydraulic System] Right system pressurized fluid follows a similar path as the left system, through the same sequence of acoustic filter, dedicated thrust reverser, system accumulator, filter manifold, to the tail-mounted flight controls and forward to the landing gear wheel well for powering the flight controls installed on the wing, through a pressure relief valve, into the radiator cooler in the left wing fuel tank hopper, through a return filter and back to the aft equipment bay to be available to power the PTU and return to the reservoir
The purpose of the hydraulic/fuel heat exchangers is to cool hydraulic fluid when there is low demand from the associated hydraulic components. A residual benefit is heated fuel. The G450 places on-side hydraulics to the on-side hopper, the G550 crisscrosses.
[G550 Aircraft Operating Manual, §2A-28-30 ¶2.F.]
Figure: GV Backup heated fuel return valve, from GV Maintenance Manual, §28-16-00, figure 1.
There are a few things wrong with this paragraph:
Note that this is not "left over" fuel from the engine but fuel after warming by the oil/fuel heat exchanger on its way to the engine. Under situations of high fuel demand, greater than 2,250 PPH, the engine's demand will shut down the HFRS.
Figure: GV HFRS line installation, from GV Illustrated Parts Catalog, §28-16-00, figure 3.
The heated fuel is sprayed on top of the wing fuel throughout the tank. The illustration shows on of those piccolo tubes in the outboard section labeled "A" but there are also piccolo tube assemblies at positions "B" through "G" on both wings.
Left: G450 fuel quantity schematic, from FSI G450 Maintenance Training Manual, figure 28-28.
Figure: Fuel quantity probe locations, from FSI G450 Maintenance Training Manual, figure 28-47.
[G450 Aircraft Operating Manual, §2A-28-40 ¶2.A. and G450 Aircraft Operating Manual, §2A-28-40 ¶2.A.] Each wing tank has [G450: 20, G550: 19] fuel quantity probes positioned throughout the tank. The probes are placed in positions that allow determination of fuel quantity for the full range of tank capacity, from full to lowest usable level. The probes are connected through a wiring harness that enters the tank through the forward wing spar The harness leads from the wing inboard, through the fuselage and up to the FQSC unit in the LEER. The probes measure quantity as a function of electrical capacitance. The FQSC transmits a low voltage signal (nominally 5 volts) to each probe through a wire in the harness and measures the capacitance through another wire in the probe. The ends of the two wires are separated by a fixed distance so that the capacitance between the wires is dependent upon the characteristics of the medium between the wires. The capacitance is higher when the probe is immersed in fuel and lower when the probe is exposed to air. The FQSC receives the capacitance signals from all probes in the wing and converts the combined signal to fuel quantity.
Figure: Fuel quantity probes, from FSI G450 Maintenance Training Manual, figure 28-48.
[G450 Maintenance Manual, §28-41-00 ¶3.B.] The fuel quantity probes are capacitors. The capacitance of the probes changes as the dielectric constant of the insulating medium between the capacitor electrodes changes. When the fuel tanks are full, the capacitance of the probes is greater than the probe capacitance when the fuel tanks are empty. Changes in fuel level cause a change in probe capacitance. This capacitive signal is supplied to the FQSC.
What is a capacitor? It is two conductors separated by an insulator. By placing different electrical charges across the conductors you establish an electrical field. The capacitance probe is a cylinder (one conductor) surrounded by a dielectric (the insulator) which is surrounded by another cylinder (the other conductor). The capacitance between the two tubes varies depending on how much of the tube is submerged in fuel. The FQSC measures all 20 (G450) or 19 (G550) probes and turns that into fuel quantity. Note there are no moving parts but aircraft attitude can influence readings since more or less of the probe will be covered by air. More about this: Technical / Fuel Capacitance Probes.
[G450 Aircraft Operating Manual, §2A-28-40 ¶2.A.] A separate probe is installed within the wing to provide a low fuel level signal to the FQSC. The low level probe is installed in the hopper, near the lowest level of the tank. When the fuel level drops to expose the probe to air, the capacitance falls and a low level signal is provided to the FQSC. The low level probe is positioned so that the low fuel level signal will occur at approximately 650 lbs.
[G550 Aircraft Operating Manual, §2A-28-40 ¶2.A.] Two separate probes are installed within the wing to provide high fuel level and low fuel level signals to the FQSC. The high level probe is positioned in the vent plenum so that when fuel quantity has reached tank capacity, the probe will be immersed and provide a high capacitance signal. The high fuel level signal is provided to the external refueling panel at the right wing root in order to illuminate the high level warning light on the panel door. The low level probe is installed in the hopper, near the lowest level of the tank. When the fuel level drops to expose the probe to air, the capacitance falls and a low level signal is provided to the FQSC. The low level probe is positioned so that the low fuel level signal will occur at approximately six hundred fifty pounds (650 lbs)
What about the fuel sloshing from tank to tank? When you roll crisply after takeoff you may notice a growing fuel imbalance of 500 lbs or more. You can transfer the fuel but will end up having to transfer it back. Or you can wait five or ten minutes and it takes care of itself. With the intertank and crossflow valves closed, there is no connection between tanks so do you have a problem? No. A crisp turn can put more fuel in the top part of a probe and momentarily fool the FQSC into thinking there is more fuel. Just give it a few minutes to figure it out. And stop rolling the airplane like a fighter.
Figure: Fuel quantity compensating probe, from FSI G450 Maintenance Training Manual, figure 28-49.
[G550 Aircraft Operating Manual, §2A-28-40 ¶2.A.] To refine the fuel quantity reading derived from the capacitance signals of the tank probes, two other types of sensors are installed in the tank. Each tank has a densitometer that determines the density of the fuel in the tank to improve the weight calculation of the FQSC. The densitometers are electromechanical oscillators that vibrate at a frequency proportional to the density of the medium surrounding the oscillator. The FQSC translates the frequency to density. The left tank densitometer is located in the tank hopper to monitor existing fuel density and the right tank densitometer is located outboard in the wing tank to measure fuel density as it is loaded in the tank.
[G450 Aircraft Operating Manual, §2A-28-40 ¶2.A. and G550 Aircraft Operating Manual, §2A-28-40 ¶2.A.] Each wing also has a compensator sensor placed in the fuel hopper to calculate the dielectric constant of the fuel. The dielectric constant is dependent upon the chemical makeup of the fuel, and different fuel brands or types have different additives that effect the dielectric constant. The right wing has an additional compensator outside of the hopper to determine the constant as fueling is in progress. The FQSC adjusts the capacitance reading of the fuel quantity probes using the dielectric constant reading from the compensator sensor.
There isn't anything published about the accuracy of the fuel gauges. In the G-III we used to say any analog fuel gauge was only as accurate to 10% of the gauge's total indication, so plus or minus 1,500 lbs. Of course we made that up but it seemed safe. The G-450 has many more probes and a dedicated computer to keep tabs on everything. The smaller Gulfstreams do publish this: G-100 Fuel Quantity Accuracy. So, for lack of any other metric, we can use 2% of the indication plus 0.75% of the full scale indication. Most of us use 3,000 lbs total as an absolute minimum. So on a tank we are only as good as plus or minus 0.02 (1,500) + 0.0075 (14,750) = 140 lbs.
Left: G450 fuel hopper block diagram, from FSI G450 Maintenance Training Manual, figure 28-4.
[G450 Aircraft Operating Manual, §2A-28-20 ¶2.A. and G550 Aircraft Operating Manual, §2A-28-20 ¶2.A.] Each wing tank has a internal compartment at the wing root, termed a hopper, at the lowest point of the tank. Each compartment is formed by the rear wing spar, the rib at the centerline of the fuselage, an outboard wing rib and a wall located forward of the rear spar. The outboard wing rib and the forward wall have baffles directing fuel into the hopper, and drain holes for residual fluids.
The aft wing spar serves as the mounting surface for the boost pumps, fuel shut off valves, the crossflow and intertank valves and temperature sensors. The outside of the aft wing spar is accessible from the main landing gear wheel wells, and the fuel system components in the hoppers are mounted through openings in the spar, allowing the components to be replaced without emptying the fuel tanks.
Each hopper can contain 190 U.S. gallons or 1,283 pounds of fuel. The intake lines supplying the boost pumps are installed along the bottom of the hoppers ensuring that all possible fuel can be extracted from the tanks.
The ejector pumps are constantly overfilling the hoppers so they are always full, excess fuel goes back into the wing tank.
Figure: G450 Allen type drain valves, from G450 Maintenance Manual, §28-14-01, figure 401.
[G450 Maintenance Manual, §28-14-00, ¶1.A.] The fuel drainage system consists of 12 gravity water / fuel drain valves. They are flush mounted to the lower wing skin. Each is located at a fuel tank low point where water would collect after the aircraft remains stationary for a few hours. The drain valves provide points from where fuel samples can be taken, where water can be drained from the tanks without defueling and where residual fuel can be drained when completely defueling the tank. The aircraft is equipped with the following two types of drain valves:
There are four of these on each lower wing panel: 1 inboard in the hopper under the ejector pump outlet, 1 inboard forward of the hopper wall, 1 outboard of outer hopper wall, and 1 outboard in the vent plenum.
There are two of these on each lower wing panel just outboard from BL 0: 1 in fuel hopper aft of hopper wall and 1 in main tank forward of hopper wall
Figure: G450 Autovalve type drain valves, from G450 Maintenance Manual, §28-14-01, figure 402.
[G450 Maintenance Manual, §28-14-00, ¶4.A.] To drain water and residual fuel from tank, open poppet valve with a cross-slot (Phillips type) screwdriver. The poppet valve stem is pushed all the way in with the screwdriver to a hard stop. The poppet valve is rotated about 1/4 turn and then released. The poppet valve is locked in the open position. Holes in valve body allow water and fuel to flow out of tank. The poppet valve remains open until the stem is again rotated 1/4 turn and released. A spring closes the poppet valve and an O-ring engages the valve body to stop the flow.
If your operational rules permit you, the pilot, to do this, you really ought to practice it once or twice with a mechanic. The valves can be tricky to reseat and there are techniques to avoid getting yourself doused with fuel.
Figure: G450 Fuel ventilation system, from G450 Maintenance Manual, §28-15-00, figure 1.
[G450 Maintenance Manual, §28-15-00, ¶1.A.] The fuel ventilation system provides air and fuel vapor flow in or out of the fuel tanks. This prevents too much pressure or vacuum in the fuel tanks. The system relieves pressure when necessary by discharging fuel flow overboard via the fuel ventilation system vents. In flight, the ventilation systems lightly pressurizes each wing tank. The fuel ventilation system is fully automatic. In each tank, the system contains a forward and aft vent duct, a vent plenum, float-operated vent / relief valves, nonrelieving float vent valves, an overboard line and a flush vent inlet / outlet (vent / ram air inlet).
[G450 Maintenance Manual, §28-15-00, ¶4.A.] The two vent ducts extend parallel from the inboard vent valves into the vent plenum. Any fuel in the vent ducts and tubing drains back into the wing tank through drain valves installed in the ducts and tubing. The vent plenum allows air flow in and out of the tank and collects any overflow fuel from the vent ducts. If the overflow fuel fills the vent plenum, the excess fuel flows overboard through the overboard vent / ram air inlet. Air / fuel vapor flows in and out of the wing tank through the overboard vent / ram air inlet. During flight, air is pushed into the tank through the inlet.
Figure: G450 vent plenum, from FSI G450 Maintenance Training Manual, figure 28-14.
[FSI G450 Maintenance Training Manual, page 28-23]
Figure: Fuel system filtration, from FSI G450 Maintenance Training Manual, figure 28-43.
[G450 Aircraft Operating Manual, §2A-28-20, ¶2.E. and G550 Aircraft Operating Manual, §2A-28-20, ¶2.E.] The fuel system is protected from foreign particles by three filters in each tank. One filter prevents the entry of contaminants during over wing fueling. Two other filters are fitted into the fuel boost pump intakes at the bottom of the fuel hoppers. The mesh of the filters block the entry of particles into the fuel system but have no effect on liquid contaminants.
Figure: Fuel temperature sensor, from FSI G450 Maintenance Training Manual, figure 28-52.
[G450 Aircraft Operating Manual, §2A-28-30, ¶2.F.] The temperature of the fuel in the tanks is monitored to ensure that it remains within limitations. A temperature bulb mounted in the left fuel hopper contains an element with an electrical resistance that varies with temperature. The resistance in the bulb is monitored by MAU #1, and converted to a proportional voltage that is then translated into a digital format for display by the MWS.
[G550 Aircraft Operating Manual, §2A-28-30, ¶2.F.] The temperature of the fuel in the tanks is monitored to ensure that it remains within limitations. Two temperature bulbs, one in each tank, are mounted on the aft wing spar and penetrate into the fuel hoppers. The bulbs contain an element with an electrical resistance that varies with temperature. The resistance in the bulbs is monitored by the MAUs (left tank by MAU #1, right tank by MAU #2) and converted to a proportional voltage that is then translated into a digital format for display by the MWS.
Figure: Engine fuel shutoff valves, from FSI G450 Maintenance Training Manual, figure 28-40.
Figure: APU fuel shutoff valve, from FSI G450 Maintenance Training Manual, figure 28-41.
The engine and APU fuel shutoff valves are motorized and are not spring loaded in either open or closed positions; they are "fail as they die" valves. If you need to undo the action of the fire handle for any reason, you can ensure the engine fuel shutoff valve remains closed by pulling the associated fuel shutoff circuit breaker.
Figure: Fuel boost pumps, from FSI G450 Maintenance Training Manual, figure 28-36.
[G450 Aircraft Operating Manual, §2A-28-30 ¶2.E. and G550 Aircraft Operating Manual, §2A-28-30 ¶2.E.] The airplane fuel tanks are equipped with four identical and interchangeable boost pumps, two in each tank. The boost pumps are located within the tank hoppers to ensure a positive supply of fuel to the pumps. The boost pump intakes are covered with filter screens to prevent the ingestion of foreign objects or particles that could damage the pumps. The boost pumps are designated main or alternate, with the main pumps installed in the inboard position, and alternate pumps outboard. Both main and alternate pumps are required to be selected on at all times, unless pumps in a tank are switched off to balance fuel between tanks. The main and alternate pumps are selected on with MAIN and ALT pushbutton switches on the FUEL SYSTEM panel on the overhead. The amber OFF legend in a switch will illuminate if the pump is selected off or if the pump is inoperative. The pumps are equipped with pressure sensors to monitor pump performance. When a pump is initially powered, the OFF legend will be illuminated until an operating pressure of 16 psi is reached. The OFF legend will illuminate with the switch selected on if pump pressure drops 16 psi and a CAS message will identify the failed pump.
The main and alternate boost pumps are electrically powered from different sources:
The pressure produced by the boost pumps is also used to provide fluid flow through the ejector pump in each tank. The ejector pump incorporates a small diameter line from the pressurized boost pump manifold plumbed to extend forward in the tank to a position in front of the intake baffle to the tank hopper. The ejector directs a stream of high pressure fuel into the mouth of a wider opening plumbed back into the hopper. The velocity of the fuel ejected from the pump induces the flow of a larger volume of fuel into the hopper, thus assisting in the movement of fuel into the boost pump intakes. Should both boost pumps fail, a separate line in the boost pump manifold, termed the suction bypass line, enables the engine to siphon fuel from the bottom of the hopper using the engine mounted fuel pumps. If the crossflow valve is open, boost pump pressure from the opposite side tank will be higher than engine suction pressure and the engine will be fed from the opposite tank.
Figure: G450 crossflow and intertank valves, from G450 Maintenance Manual, §28-24-00, figure 1.
[G450 Aircraft Operating Manual, §2A-28-30 ¶2.C. and G550 Aircraft Operating Manual, §2A-28-30 ¶2.C.] The crossflow line and valve is plumbed between the manifolds housing the two boost pumps within the hoppers of each tank, and allows an engine to be supplied with pressurized fuel from the opposite side tank. In the event of an engine failure, the remaining engine can be fed alternately from the onside fuel tank, then from the tank on the side of the inoperative engine to maintain fuel balance. Feeding an engine from the opposite side tank is accomplished by first opening the crossflow valve, turning on the boost pumps in the opposite side tanks, and then turning off the boost pumps in the onside tank. The crossflow valve is powered by the left essential DC bus.
While the book says all four boost pumps are identical and interchangeable, they don't all put out the same pressure and volume. You can find yourself with two boost pumps unable to overpower one boost pump through the crossflow valve. Simply switch pumps on the single pump side. Remember that the engines may not adequately suction feed from the wing tanks above 20,000 feet so you should always have boosted pressure to the engine above that altitude.
[G450 Aircraft Operating Manual, §2A-28-30 ¶2.C. and G550 Aircraft Operating Manual, §2A-28-30 ¶2.C.] To enable balancing fuel between tanks in the event of dual boost pump failure within a tank, an intertank line and valve are installed between the two fuel tank hoppers. The valve is opened with the INTER TANK pushbutton switch on the FUEL SYSTEM panel on the overhead. Depressing the switch will open the valve and the white line legend will illuminate within the switch completing the diagram line between tanks shown on the panel. The intertank valve is also powered by the left essential DC bus. The fuel balancing procedure requires that the airplane be flown slightly out of trim in the yaw axis to induce fuel migration through the intertank valve. The out of trim condition will produce sufficient lateral force to cause fuel from the tank with the greater quantity to flow into the tank with the lesser quantity.
Though the crossflow and intertank transfer systems have been with us since the G-II, for the most part unchanged, nobody really had it figured out until 2006. The crossflow works, but we've gone from a +10°C to a 0°C prohibition against turning boost pumps off inflight, to no restriction at all, and back to the 0°C restriction. That left us with the intertank valve which sometimes worked very well but at other times not at all.
If you look at the diagram above all will be revealed. While the pipes connecting both hoppers to the crossflow valve are pretty straight, not so with the intertank. The pipe in the left hopper is forward of the pipe in the right hopper, so all other things being equal the flow favors left to right. But all other things are not equal. Since the hopper is constantly being overfilled, if you open the intertank valve before establishing a sideslip the side with more pressure is likely to start fuel flowing to the side with lower pressure. By the time you get the sideslip in, it may be impossible to overcome the existing flow.
We didn't know any of this until Gulfstream studied it and published an article, Fuel Transfer Using Intertank System, in the 23 June 2006 Breakfast Minutes. The key change was to initiate 2-3 units of rudder trim in the direction of desired fuel transfer before opening the intertank valve. There are training materials out there from FlightSafety and others that still (as of August 2014) say "Balancing via the Inter Tank Valve requires the aircraft to gently side-slipped and fuel will flow from one hopper to the other." This is not true and if the prevailing hopper pressure is opposite your desired intent, it could make matters worse. More about this: G450 Abnormal Procedures / Fuel Unbalanced.
Figure: Pressure fueling system components, from FSI G450 Maintenance Training Manual, figure 28-16.
Defueling is easy, depending on where you are. If you are at a Gulfstream service facility, they will be happy to defuel you while they work on the airplane and then give you your fuel back. But it isn't so easy anyplace else. Few venders can store your fuel and redispense it to you, or are set up to dispose of it.
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
FSI G550 PTM, FlightSafety International Gulfstream G550 Pilot Training Manual, Volume 2, Aircraft Systems, Second Edition, Mar 2006
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
Gulfstream G550 Aircraft Operating Manual, Revision 27, July 17, 2008
Gulfstream GV Illustrated Parts Catalog, Revision 24, August 31, 2005
Gulfstream GV Maintenance Manual, Revision 25, August 31, 2005