If you have a fuel leak, there might be things you can do to stop the loss of fuel. One of those things might be to shut an engine down. You might also need to move the fuel from where it is leaking, to where it is not. Unfortunately not all manufacturers address the problem methodically. I am offering the Gulfstream G450 procedure as an example, along with a few things learned from pilots who got it wrong.
Everything here is from the references shown below, with a few comments in an alternate color.
Figure: Fuel Pipe Crack, from Portugal Accident Investigation Final Report, Figure 4.
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. In 2018 a G650 test crew had a fuel leak from an engine, failed to shut the engine down, and landed with just minutes of fuel in the tanks. 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.
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.
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.
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.
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.
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.
[G450 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.
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.
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.
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.
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.
WARNING: DO NOT RESTART FAILED ENGINE AS A HIGH PROBABILITY OF ENGINE FIRE EXISTS.
See Section 01-03-80, Fuel Load Balancing.
If there continues to be and unexplained decrease of fuel from the affected side, proceed to Step 18.
See Section 04-08-30, One (1) Engine Inoperative Landing Procedure.
NOTE: Use of autothrottle during single engine approaches is prohibited.
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.
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.
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.
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
Gulfstream G450 Airplane Flight Manual, Revision 35, April 18, 2013
Portugal Accident Investigation Final Report, All Engines-out Landing Due to Fuel Exhaustion, Air Transat, Airbus A330-243 marks C-GITS, Lajes, Azores, Portugal, 24 August 2001
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