Fuel System Abnormals
- James Albright (a former G450 driver)
Updated: 2017-03-12
The G450 fuel system is fairly simple, taking the lessons learned from earlier Gulfstreams all the way through the GV (without the heated fuel return system). If you are in CG without a drop of fuel, you are in CG with full tanks and vice versa. Marvelous. Fuel upload is fairly easy but prone to become out of balance if the airplane isn't on a level surface. Balancing fuel can also be a mystery until you've figured out the secret to using the Intertank Valve. Secret? Yes, until lately. All that being said, you still need to be careful about fuel leaks and other issues common to most jet aircraft.
Everything here is from the references shown below, with a few comments in an alternate color
Fuel Leak
Fuel belongs in the tanks, the engines, and out the tail pipes. If you have a fuel leak, you at the very least will land with less than you thought or you may end up in a rubber life raft. But there is more to it than that. The airplane is crawling with electricity and is emanating electrons from just about every surface. Not the thing you want to mix with fuel.
I've seen a few fuel leaks over the years, the worst was in a Boeing 707, retold here.
The Gulfstream series don't have a history of this, but it has happened. In 2000, a GV suffered a leaking O-Ring to the engine fuel pump and lost 4,000 lbs in just 1.5 hours. You need to take the possibility of a fuel leak seriously.
The crew of Air Transat Flight 236 did not and turned their Airbus 330 into a glider.
Symptoms:
Your first indication will more than likely be a fuel imbalance that you will attempt to correct using the fuel crossflow valve, and when that fails, the intertank valve.
Whenever you initiate a fuel transfer, you should ensure the fuel leaving one tank arrives in the other tank. Do not take your attention off the fuel system until this happens.
You could also detect the odor of fuel in the cabin if the leak is between the fuel tanks and the engines, or perhaps just notice fuel consumption is exceeding the expected 4, 3, 3, 3, 2 pattern.
You should check the FMS fuel quantity indication on an hourly basis and compare that to the actual fuel on board. If the actual quantity is much less than what the FMS predicts, you could have a fuel leak.
Get to know how well your FMS predicts fuel usage. You shouldn't see more than a couple hundred pounds difference in an hour. If the difference is negligible, zero out the difference entering DELETE into the scratch pad and onto the fuel total. That will make the next hour's comparison more accurate.
Analysis:
The obvious problem is possibly running out of fuel before finding a place to land. If you are over water, pointing the nose toward a runway and stopping the leak will be your focus.
Another problem will be lateral imbalance. You can find yourself unable to keep the wings level to keep the aircraft flying.
There are two possible sources of fuel leaks: the tanks or the engines and their feed systems.
Troubleshooting:
Inspect the wings by looking out the cabin windows and checking the exterior cameras. A fuel leak will probably look like mist shooting backwards. A trail of mist from one wing that isn't present on the other wing is credible evidence of an exterior leak.
An odor in the cabin could be a leak from a tank or in a fuel line from the tank to the engines. Unless you have evidence of an exterior leak, you should trouble shoot as follows:
Descend below 15,000 feet
Reduce power on the affected side engine to idle, add power on the other engine to maintain steady flight.
Ensure the crossflow and intertank valves are off, then turn the boost pumps for the side of the engine at idle to off.
Wait two minutes. If the engine fails, runs erratically, or continues to run with extremely high fuel flow indications, you can assume the leak is in the fuel feed system between the boost pumps and the engine. If the engine continues to run without change, you can assume the leak is from the tank.
Fuel Feed System Leak
If the leak is between the main tank boost pumps and the engine, you will have to shut the engine down and transfer that fuel using the intertank valve to the other side as required to maintain lateral balance. Don't restart the engine in case the leak is within the engine cowl. Consider getting fire coverage on landing and getting out of the aircraft as soon as possible.
Fuel Tank Leak
You need to measure the loss rate versus the amount of flight time you need to get the airplane on the ground. Your best option maybe to transfer as much fuel to the opposite side before it is leaked off the aircraft. Gulfstream has demonstrated up to 4,000 lbs fuel imbalance is possible.
Procedure
[G450 Airplane Flight Manual, §04-14-10 and G550 Airplane Flight Manual, §04-14-10]
A suspected fuel leak will most likely manifest itself as an increasing difference in fuel quantity that cannot be explained by normal differences in fuel flows between engines. It can also be identified by fuel smell in the cabin, abnormally high fuel flow or fuel flow out of range indications on one of the engines, lateral trim changes, and/or an increasing difference between the FMS-calculated fuel quantity and the quantity indicated on the fuel gauge.
CAUTION: DO NOT PERFORM THIS PROCEDURE ON THE BASIS OF ERRATIC FUEL GAUGE INDICATIONS ONLY OR DURING ASYMMETRIC TRIM REQUIREMENTS FELT DURING ACCELERATING PHASES OF FLIGHT SUCH AS INITIAL CLIMB OR NOSE HIGH OR NOSE LOW PITCH ATTITUDES.
FUEL CROSSFLOW Valve . . . CLOSED
FUEL INTERTANK Valve . . . CLOSED
As the crew of Air Transat Flight 236 discovered, cross flowing a leak downstream of the tank can easily send that fuel off the airplane where it does you no good.
FMS Single Engine Performance / Range . . . CHECK
Monitor fuel tank quantities and lateral trim requirements. Identify leak by observing one wing fuel tank quantity decreasing faster than the other, and/or slowly increasing lateral trim force.
IF FUEL LEAKAGE IS VISIBLE FROM WING TANK, PROCEED TO STEP 18.
IF FUEL LEAKAGE IS NOT VISIBLE FROM WING TANK, IT MUST BE DETERMINED IF THE FUEL LEAK IS COMING FROM THE WING TANK OR FROM THE ENGINE / ENGINE FEED SYSTEM. DESCEND TO 15,000 FEET AND PROCEED AS FOLLOWS:
Non-affected Side Power Lever . . . SET MCT
NOTE: Set power to Maximum Continuous Thrust (MCT). Refer to the G450 Operating Manual, Section 11-05-00: Engine Out Drift Down, or G450 Quick Reference Handbook, Section EB, for Engine Out Drift Down charts. For ETOPS data, see drift down speeds from the ETOPS manual.
I'm not sure why MCT is specified, it seems to be "as required" may be more advisable.
Steady Flight . . . ESTABLISH
Affected Side, Engine Power Lever . . . IDLE
CAUTION: DO NOT START THE APU UNLESS IT IS NECESSARY FOR CONTINUED SAFE FLIGHT AND LANDING.
NOTE: The flight crew shall agree on the correct (affected) FUEL CONTROL prior to placing it in the OFF position.
Affected Engine FUEL CONTROL . . . OFF
MASTER WARN . . . DEPRESS
Affected Engine FIRE HANDLE . . . PULL (DO NOT ROTATE)
Inoperative Engine GEN . . . OFF
Inoperative ENG BLEED AIR . . . OFF
ISOLATION Valve . . . OPEN
PWR XFR UNIT (If Left Engine Failed) . . . ON (IF REQUIRED)
TCAS . . . TA ONLY
WARNING: DO NOT RESTART FAILED ENGINE AS A HIGH PROBABILITY OF ENGINE FIRE EXISTS.
Fuel Load Balance . . . MONITOR / MAINTAIN
See Section 01-03-80, Fuel Load Balancing.
Fuel Tank Quantities . . . MONITOR
If there continues to be and unexplained decrease of fuel from the affected side, proceed to Step 18.
Nearest Suitable Airport . . . LAND
See Section 04-08-30, One (1) Engine Inoperative Landing Procedure.
NOTE: Use of autothrottle during single engine approaches is prohibited.
FUEL TANK LEAK:
Affected Side, MAIN and ALT Boost Pumps . . . ON
The flight crew must now evaluate the fuel loss rate and distance to nearest suitable landing airfield. Consideration should be given but is not limited to the following: two-engine versus single-engine climb to altitude in order to achieve greater range, maximum transfer of fuel to good tank versus lateral controllability, and affected side engine failure due to fuel starvation.
NOTE: Up to 4,000 lbs. of fuel imbalance between tanks has been safely demonstrated in flight under abnormal conditions. For approach and landing with abnormal fuel imbalance and an engine shut down, be prepared to use full rudder and aileron as necessary.
FUEL CROSSFLOW . . . AS REQUIRED
INTERTANK Valve . . . AS REQUIRED
WARNING: IF INTERTANK VALVE IS USED FOR MAXIMUM FUEL TRANSFER RATES, ENSURE PROPER SIDESLIP IS MAINTAINED TO PREVENT FUEL FROM FLOWING BACK INTO LEAKING TANK. APPLY RUDDER TRIM IN THE DIRECTION OF LEAKING TANK.
Fuel Transfer to Non-Affected Side . . . INITIATE AS REQUIRED
Autopilot . . . DISCONNECT
Airspeed . . . AS REQUIRED TO MAINTAIN LATERAL CONTROL
CAUTION: IF IT BECOMES APPARENT THAT PRIOR TO LANDING ENGINE ON LEAKING SIDE WILL FAIL DUE TO FUEL STARVATION, SHUTDOWN ENGINE. SEE 04-08-10, ENGINE SHUTDOWN IN FLIGHT. OBSERVE AFM MAXIMUM FUEL IMBALANCE LIMITS PRIOR TO LANDING.
Nearest Suitable Airport . . . LAND
See Section 04-08-30, One (1) Engine Inoperative Landing Procedure, if required.
WARNING: DO NOT USE THRUST REVERSERS ON LANDING.
WARNING: DO NOT START APU AFTER LANDING
Fuel Boost Pump Failure
All four boost pumps are theoretically identical and any one pump can get to either or both engines. Losing one should not be a big deal and the procedure is simple.
But if this happens, you probably ought to review the next most likely scenario: Failure of Two Boost Pumps on One Side.
Symptoms
You may have a CAS message:
Alt Fuel Pump Fail, L-R
- or -
Main Fuel Pump Fail, L-R
You may have an "OFF" indication on the fuel boost pump switch or a non-green synoptic:
Analysis
[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:
Left Main: Left Essential DC
Left Alternate: Left Main DC
Right Main: Right Essential DC
Right Alternate: Right Main DC
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.
Procedure
[G450 Airplane Flight Manual, §03-14-20 and G550 Airplane Flight Manual, §03-14-20]
L / R Main and Alternate Boost Pumps . . . ON
X Flow . . . OPEN
Inoperative Pump . . . OFF
The pump has a high electrical draw and if it is inoperative it could very well be higher. You should look at the DC synoptic page and check for a load drop when you shut the pump off. It could be that it has already failed and there will not be a drop. If there isn't a drop, I would consider pulling the pump's circuit breaker, just in case.
Fuel Load Balance . . . MONITOR / MAINTAIN
See Fuel Unbalanced.
Failure of two boost pumps on one side
All four boost pumps are theoretically identical and any one pump can get to either or both engines. Losing one should not be a big deal and the procedure is simple. Losing two fuel boost pumps on one side is a big deal but it doesn't mean you will necessarily lose an engine or experience an uncontrollable fuel imbalance.
Symptoms:
You may have several CAS messages:
Fuel Pressure Low, L-R
Alt Fuel Pump Fail, L-R - and - Main Fuel Pump Fail, L-R
You may have an "OFF" indication on both fuel boost pump switch and a non-green synoptic:
Additionally, above 20,000 feet, the engine may run erratically or flame out.
Analysis:
[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:
Left Main: Left Essential DC
Left Alternate: Left Main DC
Right Main: Right Essential DC
Right Alternate: Right Main DC
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.
Procedure
[G450 Airplane Flight Manual, §03-14-30 and G550 Airplane Flight Manual, §03-14-30]
To deliver fuel under pressure to each engine:
X Flow . . . OPEN
Operative Pumps . . . ALL ON
CAUTION: FLYING IN THIS CONFIGURATION WILL PRODUCE A GRADUALLY INCREASING FUEL UNBALANCE, AS FUEL FROM ONLY ONE TANK WILL BE USED.
CAUTION: THE ENGINE WILL ONLY RUN ON SUCTION FUEL FEED AT OR BELOW 20,000 FT. ABOVE 20,000 FT, THE ENGINE WILL RUN ERRATICALLY AND FLAME OUT IF THE CROSSFLOW IS NOT OPEN WITH AT LEAST ONE BOOST PUMP ON.
When fuel balancing is required:
Inter Tank . . . OPEN
Sideslip . . . TRIM RUDDER TO ESTABLISH LIGHT WEIGHT WING DOWN
When fuel balancing is completed:
Inter Tank . . . CLOSED
Normal Trim . . . RE-ESTABLISH
Fuel balancing shall be accomplished using the intertank valve and establishing a small sideslip (approximately ½ trapezoid). Move the rudder trim arrow in the direction of the "heavy" tank, which will create a slight wing down condition toward the "light tank".
Low Fuel State
An age-old rule of thumb is to never let a fuel gauge go less than 10% of its full reading. I've heard of many flameouts with pilots who ignored this rule. So, using half of the G450's 29,500 or G550's 41,300 lb fuel capacity as a starting point, you should never see less than 1,475 lbs in either tank of a G450 or 2,065 lbs in either tank of a G550. If you wait for the Fuel Level Low, L-R CAS message and you know the fuel isn't super-cooled, the airplane may soon become a glider.
Symptoms:
You may have a CAS message:
Fuel Level Low, L-R
Analysis:
[G450 Aircraft Operating Manual, §2A-24-40, ¶3.B and G550 Aircraft Operating Manual, §2A-24-40, ¶3.B]
Area Monitored: Fuel Quantity in Hopper 650 lbs or less
CAS Message: Fuel Level Low L-R
[G450 Airplane Flight Manual, §03-14-30 and G550 Airplane Flight Manual, §03-14-30] CAUTION: PROLONGED FLIGHT AT ALTITUDES WITH TEMPERATURES COLDER THAN -70°C WITH FUEL TANK TEMPERATURES COLDER THAN -30°C CAN CAUSE THE L-R FUEL LEVEL LOW MESSAGE TO BE DISPLAYED. IF THIS OCCURS, DESCEND WITHOUT DELAY TO AN ALTITUDE WHERE STATIC AIR TEMPERATURE (SAT) IS -60°C OR WARMER AND MAINTAIN MACH 0.80 MINIMUM SPEED. NORMAL FUEL QUANTITY INDICATIONS SHOULD RETURN IN APPROXIMATELY 10 TO 15 MINUTES.
[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)
If the fuel temperature gets too cold, the capacitance of the fuel itself can fool the probes into thinking they are exposed to air, triggering a fuel low signal. If the hopper is actually full, warming the fuel should extinguish the message.
Procedure
[G450 Airplane Flight Manual, §03-14-30 and G550 Airplane Flight Manual, §03-14-30]

IF EITHER OR BOTH FUEL QUANTITY INDICATORS TRIGGER THE "FUEL LEVEL LOW" MESSAGE:
Airplane . . . PROCEED TO NEAREST SUITABLE AIRPORT AND LAND
Boost Pumps . . . ALL ON
X FLOW . . . OPEN
Extreme Nose High Or Nose Low Attitudes/Excessive Forward Acceleration/Uncoordinated Flight Maneuvers . . . AVOID
Go-Around From Approach . . . DO NOT ATTEMPT WITH LESS THAN 600 LB (272 KG) OF FUEL INDICATED IN EITHER TANK
10° Pitch Up Attitude . . . DO NOT EXCEED WITH "L-R FUEL LEVEL LOW" MESSAGE ACTIVE
Fuel Tank Temperature
A high fuel tank temperature is unlikely when in flight, 54°C is 129°F, but should be a cause for concern, especially on the ground. A low fuel tank temperature should also be a cause of concern because of the possibility if fuel jelling. (Jet A has been known to freeze as high as -40°C.) If your fuel temperature is cold enough to generate a CAS message, you should monitor the temperature all the way to landing. Fuel tends to adhere to its surroundings best at -12°C and the increasing fuel flow demands once the aircraft is configured for landing has been known to clog fuel filters.
Symptoms:
[G450 Aircraft Operating Manual, §2A-28-30, ¶3.B and G550 Aircraft Operating Manual, §2A-28-30, ¶3.B]
Fuel Tank Temperature
Area Monitored: Fuel tank temperature below -37°C or above +54°C
CAS Message: Fuel Tank Temperature
Analysis:
[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.
Procedure
[G450 Airplane Flight Manual, §04-07-40 and G550 Airplane Flight Manual, §04-07-40]
FUEL Synoptic Page . . . SELECT
Fuel Tank Temperature Indication . . . MONITOR
IF ON GROUND, AND FUEL TANK TEMPERATURE IS GREATER THAN +54°C:
Takeoff . . . DELAY
Do not dispatch until this condition is corrected.
IF IN FLIGHT, AND FUEL TANK TEMPERATURE IS -40°C OR LESS:
L / R MAIN and ALT Boost Pumps . . . ON
Altitude . . . DESCEND TO ALTITUDE WHERE SAT IS -60°C OR WARMER
Keep in mind that the warmer air could be at a higher altitude.
New Altitude . . . REMAIN AT NEW ALTITUDE UNTIL MESSAGE EXTINGUISHES
Fuel Tank Temperature Indication . . . ALLOW TO INCREASE ABOVE -40°C
Optimum Altitude . . . RETURN IF REQUIRED
Diversion To Alternate Airport . . . CONSIDER IF UNABLE TO COMPLETE MISSION AT LOWER ALTITUDE
Fuel Unbalanced
Figure: G450 crossflow and intertank valves, from G450 Maintenance Manual, §28-24-00, figure 1.
The crossflow valve works but you can find yourself in a situation where one pump operating from the low wing can still overpower two pumps operating from the high side. Switching pumps on the low wing will usually fix things. The problem, of course, is you can't always use the crossflow valve.
What about the intertank valve? It is one of the most misunderstood systems on the airplane, even though it is just a pipe and a valve! 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: if the ejector pump on one side is stronger than the other, there will be more pressure in one hopper than the other. 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.
Symptoms
[G450 Aircraft Operating Manual, §2A-28-40, ¶3.B and G550 Aircraft Operating Manual, §2A-28-40, ¶3.B]
Fuel Imbalance
The fuel quantity indicators may also reflect an imbalance:
Figure: Fuel quantity pointers, from Eddie's notes.
If you start with balanced fuel and operate both engines fairly matched, getting a fuel imbalance in a G450 or G550 is unlikely. The only avenues for unintended fuel transfer between tanks are (1) a fuel leak between hoppers, (2) a fuel leak in the crossflow valve, or (3) a fuel leak in the intertank valve. You could also have a fuel leak off the aircraft that will first manifest itself as a fuel imbalance. When diagnosing a fuel imbalance, check the total fuel versus the FMS predicted fuel to rule out a fuel leak. Also check the aircraft's lateral trim. A more likely cause of a fuel imbalance indication would be momentary fuel probe "confusion." Often waiting a few minutes is all it takes to extinguish the indications.
If you believe a fuel imbalance is not a fuel leak or not an indication problem, proceed with care. Always ensure fuel leaving one tank arrives at the other tank. Do not "set and forget" any transfer scenario. My technique: I take my wrist watch off and hold it in my hand. This is uncomfortable and a bit inconvenient, making it hard to forget something unusual is happening.
Analysis
Figure: G450 permissible fuel unbalance, from G450 Aircraft Operating Manual, §2A-28-30, figure 6.
[G550 Aircraft Operating Manual, §2A-28-40, ¶4.B]
Maximum fuel imbalance for takeoff is 1000 lb.
Maximum fuel imbalance in flight is 2000 lb.
Note that the G450 fuel imbalance is only 400 lbs when at higher fuel quantities but gradually expands to 2,000 lbs as the total weight decreases to 55,000 lbs. Because of the wing's 3° dihedral, the impact of the imbalance is greater at higher quantities since the extra fuel is closer to the wing tips.
Procedure
[G450 Airplane Flight Manual, §02-08-300 and G550 Airplane Flight Manual, §02-08-300]
CAUTION: THE ENGINE WILL ONLY RUN ON SUCTION FUEL FEED AT OR BELOW 20,000 FT. ABOVE 20,000 FT, THE ENGINE WILL RUN ERRATICALLY AND FLAME OUT IF THE CROSSFLOW VALVE IS NOT OPEN WITH AT LEAST ONE BOOST PUMP ON.
TO BALANCE FUEL USING THE CROSSFLOW VALVE PROCEED AS FOLLOWS:
When balancing fuel through use of the crossflow valve, ensure tank temperature is above 0°C and boosted fuel pressure is always available to the engines. If fuel temperature is 0°C or below, use the intertank valve to balance fuel.
Crossflow Valve . . . OPEN
Verify valve position using Fuel synoptic display.
Boost Pumps on "Light Tank" Side (one at a time) . . . OFF
Fuel Balancing . . . MONITOR
ONCE DESIRED BALANCE HAS BEEN ACHIEVED:
Boost Pumps (one at a time) . . . ON
Verify boost pump operation using Fuel synoptic display.
CAUTION: SELECT ONLY ONE BOOST PUMP ON AT A TIME. WAIT AT LEAST FIVE (5) SECONDS BEFORE TURNING THE SECOND BOOST PUMP ON. FAILURE TO DO SO MAY CAUSE A FAILURE IN THE FUEL PRESSURE SWITCH.
Crossflow Valve . . . CLOSED
TO BALANCE FUEL USING THE INTERTANK VALVE PROCEED AS FOLLOWS:
There are minor differences in the presentation of this procedure in the G450 and G550 AFM's. The G450 procedure is shown.
Balancing fuel using the intertank valve requires the airplane to be placed in a sideslip condition. Use of autopilot is recommended.
Rudder Trim in the Direction of "Heavy Tank" . . . APPLY
Apply sufficient rudder trim to create a "light tank" wing down condition and allow fuel to flow toward the "light tank".
Intertank Valve . . . OPEN
Verify valve position using Fuel synoptic display.
A Gulfstream study presented in the Gulfstream Breakfast Minutes, 23 June 2006 says 2 to 3 units of sideslip may be necessary or even as high as 5 units.
Fuel Balancing . . . MONITOR
If you did not apply enough sideslip you may actually see the fuel imbalance get worse. Adding sideslip with the intertank valve open may not overcome the prevailing flow. You should close the intertank valve, increase the sideslip, and reopen the intertank valve.
AS DESIRED FUEL BALANCE IS APPROACHED (WITHIN 200 LB):
NOTE: Due to fuel indicating system inaccuracies while in sideslip flight, balancing process should be stopped just prior to both tanks indicating equal. Stopping the process 200 lb early will result in balanced indications once sideslip has been removed.
Intertank Valve . . . CLOSED
Verify valve position using Fuel synoptic display.
Rudder Trim . . . AS REQUIRED TO MAINTAIN ZERO SIDESLIP
Fuel Balance . . . RE-CHECK
IF UNABLE TO ACHIEVE DESIRED FUEL BALANCE, USE "CROSSFLOW VALVE" METHOD.
If you were using the intertank valve, chances are the fuel is too cold to use the crossflow valve. You should carefully consider your options, to include: descending to warmer temperatures or turning off one only boost pump with the understanding it may fail to restart once your balance is reestablished.
References:
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
Gulfstream G450 Maintenance Manual, Revision 18, Dec 12, 2013
Gulfstream G450 Operating Manual Supplement, G-450-OMS-02, Extended Operations (ETOPS) Guide, Revision 2, April 2, 2009
Gulfstream G450 Performance Handbook, GAC-AC-G450-OPS-0003, Revision 20, November 30, 2011
Gulfstream G450 Quick Reference Handbook, GAC-AC-G450-OPS-0003, Revision 34, 18 April 2013
Gulfstream G450 Weight and Balance Manual, Revision 3, March 2008
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