Learn how the Boeing 747-400 fuel system uses boost pumps, pressure priority, crossfeed valves and its main, centre and reserve tanks.
The Boeing 747-400 normally pressure-feeds each engine from its corresponding main wing tank through electric boost pumps. Crossfeed valves can connect all four engine feed lines to a common manifold, while higher-pressure centre-wing or override pumps take priority. Reserve and stabiliser fuel must first transfer into feed-capable tanks.
In our Aviation & Real-World Flying coverage, the key distinction is between tanks that feed engines and tanks that only transfer fuel. The airframe layout is also closely related to the structural and operational reasons fuel is stored in aircraft wings.
Which tank feeds each 747-400 engine?
In normal tank-to-engine operation, main tanks 1 through 4 feed engines 1 through 4 respectively. Engine and tank numbers run from the left outboard position to the right outboard position.
| Fuel tank | Normal destination | How it supplies fuel |
|---|---|---|
| Main tank 1 | Engine 1 | Directly through its main boost pumps |
| Main tank 2 | Engine 2 | Through main pumps, with higher-pressure override pumps available |
| Main tank 3 | Engine 3 | Through main pumps, with higher-pressure override pumps available |
| Main tank 4 | Engine 4 | Directly through its main boost pumps |
| Centre wing tank | Any or all engines | Through high-pressure pumps and the crossfeed manifold |
| Reserve tanks 2 and 3 | Main tanks 2 and 3 | Transferred first; they do not feed engines directly |
| Stabiliser tank, if fitted | Centre wing tank | Transferred forward before reaching the engines |
Each main tank has two electrically driven main pumps. The duplicated pumps provide pressure and redundancy, while check valves prevent fuel from flowing backwards into a tank.
How does fuel reach the combustion chambers?
Fuel reaches each combustion chamber through a pressure-fed chain of aircraft pumps, valves and engine-mounted equipment.
- The tank pumps pressurise the fuel. Main boost pumps draw from their respective main tanks. Centre-tank and main-tank 2 and 3 override pumps can supply the manifold at higher pressure.
- Pressure determines the active source. When a higher-pressure source is connected, it supplies the flow while check valves isolate lower-pressure pump outlets. The lower-pressure pumps may still be running without contributing much fuel.
- Crossfeed valves establish the route. Opening an engine's crossfeed valve connects its feed line to the common manifold. Closing it isolates that engine for normal tank-to-engine feeding.
- Shutoff valves protect the airframe. Fuel passes the engine's spar shutoff valve when the fuel-control and fire-shutoff logic permits flow.
- The engine meters the final flow. Engine-driven pumping and fuel-control equipment raise the pressure and meter fuel to the burners. Details differ between the CF6, PW4000 and RB211 installations, but the aircraft-side tank and manifold principle is the same.
The autothrottle and engine electronic controls determine required engine fuel flow; they do not select the supplying tank.
Why does centre tank fuel feed first?
Centre wing tank fuel normally feeds first because its pumps discharge at a higher pressure than the ordinary main-tank boost pumps. With the necessary crossfeed valves open, that pressure advantage allows the centre tank to supply multiple engines through the manifold.
At a heavy fuel load, the broad sequence is centre-tank use, scheduled transfer of any stabiliser fuel into the centre tank, use of surplus inboard-main-tank fuel, and eventually balanced tank-to-engine feeding. Reserve fuel transfers into main tanks 2 and 3 at the prescribed stage rather than feeding an engine itself.
The exact pump and crossfeed changes depend on fuel quantity, operator procedure and aircraft configuration. This is not a computer simply choosing the fullest tank: pump selection, valve position and discharge pressure establish which tank is feeding.
The pumps in the centre tank and main tanks 2 and 3 are often called override/jettison pumps. Running one for normal feed does not dump fuel; jettison requires the separate jettison valves and outlets to be configured.
What do the crossfeed valves actually do?
Crossfeed valves let a pressurised tank source feed engines other than its normally associated engine. This supports centre-tank operation, fuel balancing and continued engine feed after certain pump or tank problems.
A common mistake is assuming that opening crossfeed moves fuel between tanks. It does not. Crossfeed changes which tank the engines consume from; dedicated transfer valves or pumps physically move reserve or stabiliser fuel into another tank.
To correct an imbalance, the system is configured so more engines consume fuel from the fuller side while the lighter side supplies less or none. The precise configuration must follow the applicable checklist because opening every valve and leaving every pump on can allow a higher-pressure source to dominate unexpectedly.
What happens if a fuel pump fails?
One failed boost pump does not normally stop an engine because the second pump or another manifold source may maintain pressure. A pump low-pressure indication therefore reports a pump or source problem, not necessarily an immediate loss of engine fuel flow.
- If both pumps for a main tank are unavailable, the corresponding engine may have limited suction-feed capability. Suction feed is not equivalent to normal pressure feed and becomes less dependable with altitude, temperature and high fuel demand.
- The usual abnormal strategy is to provide a pressurised source through the crossfeed manifold, following the aircraft checklist.
- The centre wing tank is not a normal suction-feed source. Losing both of its pumps can leave much of its fuel unavailable through the normal engine-feed path.
- After a tank empties, dry pumps should not simply be left running. EICAS messages and the applicable checklist identify which pumps to switch off.
How should the fuel feed look in a flight simulator?
In a high-fidelity 747-400 simulation, the fuel synoptic should show pump status, valve positions, tank quantities and the active manifold path. A simplified aircraft may automate transfers or ignore pressure priority, so its behaviour can differ from the real aeroplane.
- Centre fuel is not decreasing: check that the centre pumps are operating and that a crossfeed path to the engines exists.
- The wrong tank is decreasing: look for an active centre or override pump whose higher pressure is taking control of the manifold.
- An imbalance remains after opening crossfeed: crossfeed alone is insufficient; the pump configuration must make the engines consume from the fuller source.
- An engine has no fuel flow: verify tank quantity, pump pressure, crossfeed position, the spar-valve indication and the engine fuel-control position.
For cockpit preparation, our cold-and-dark 747 start guidance places the pump and fuel-control setup in its proper sequence. FSX users can also compare their configuration with the manual and checklist for the default 747-400, while allowing for the default aircraft's simplified systems modelling.