Boeing 737 fuel transfer explained: centre-tank priority, crossfeed balancing, automatic scavenging and simulator faults that stop fuel flow.
On most Boeing 737s, pilots do not transfer fuel directly between tanks in flight. Higher-pressure centre-tank pumps feed the engines first; once centre fuel is depleted, each wing tank normally feeds its own engine. A lateral imbalance is corrected by crossfeeding both engines from the fuller wing tank, not by pumping fuel across.
For Aviation & Real-World Flying readers, the critical distinction is between fuel feed, which sends fuel to an engine, and tank-to-tank transfer. The 737 flight-deck controls primarily manage fuel feed.
How does normal 737 fuel feed work?
The 737 uses pump pressure and check valves to determine which tank supplies each engine.
| Operating state | Pump and valve configuration | Fuel source |
|---|---|---|
| Centre fuel available | Centre pumps selected as required, main pumps on and crossfeed normally closed | The higher-pressure centre pumps supply the engine manifolds, preserving wing fuel |
| Centre pumps off or centre tank empty | Main pumps on and crossfeed closed | Main tank 1 feeds engine 1; main tank 2 feeds engine 2 |
| Lateral imbalance correction | Crossfeed open and pumps in the lower tank off | The fuller main tank feeds both engines |
The main-tank pumps normally remain on while centre fuel is being used. Centre-pump output pressure overrides the main pumps without forcing fuel into either wing tank. When the centre pumps are switched off according to the applicable procedure, the main tanks take over without a separate transfer selection.
If one centre pump stops producing pressure before the other, the associated engine may begin drawing from its wing tank first. That can create a developing left-to-right imbalance even though there is still fuel in the centre tank.
Simulator pilots can see the correct switch logic in our 737 cold-and-dark fuel-panel sequence, including when centre pumps should remain off.
How do pilots correct a Boeing 737 fuel imbalance?
A 737 fuel imbalance is corrected by making the fuller wing tank supply both engines temporarily.
- Rule out a fuel leak. An unexplained or rapid quantity loss calls for the aircraft's fuel-leak procedure, not blind rebalancing.
- Open the crossfeed valve. This connects the left and right engine fuel manifolds; it does not itself move fuel between tanks.
- Switch off the pumps in the lower-quantity tank. The pumps in the fuller tank can then feed both engines through the open crossfeed.
- Handle the centre pumps as directed. Their higher output pressure can prevent the engines from consuming wing fuel, so the model-specific checklist may require them to be off during balancing.
- Monitor both quantities. Once the tanks are within the prescribed tolerance, restore the low-side pumps and close the crossfeed before the imbalance reverses.
A mistake we see constantly is opening the crossfeed while leaving every pump on. With both wing tanks still pressurising their respective manifolds, little useful balancing may occur. Another common error is switching off the fuller tank's pumps; it is the lower tank that must stop feeding.
Does the 737 move centre fuel into the wing tanks?
Normal centre-tank operation sends fuel to the engines, not into the wing tanks.
On suitably equipped later 737s, including many 737NG aircraft, an automatic scavenge jet pump recovers residual centre-tank fuel into main tank 1 after that tank falls to roughly half full. It uses motive flow from a main-tank pump and has no flight-deck transfer switch. This is residual-fuel recovery rather than the aircraft's main fuel-management method.
Scavenge arrangements differ between 737 generations and modification standards. Ground crews can also move or remove fuel through approved refuelling and defuelling equipment, but opening the cockpit crossfeed valve on the ground will not make one wing tank refill from the other.
Why is fuel not transferring in a 737 simulator?
When simulated fuel appears not to transfer, the usual cause is an incorrect expectation about what the crossfeed and pumps actually do.
- Crossfeed open, all main pumps on: both tanks remain pressurised, so the fuller side may not feed both engines.
- The wrong pumps are off: for imbalance correction, switch off the pumps in the lower-quantity tank, not the fuller tank.
- Centre pumps are still dominating: their higher pressure can keep both engines supplied from the centre tank instead of burning down the fuller wing.
- The centre tank is empty: a
LOW PRESSURElight indicates inadequate pump output, commonly because the tank is nearly empty. It is not a transfer-in-progress indication. - The engine is suction feeding: a main-tank engine may continue running with electric pumps off under some conditions. That does not prove the crossfeed configuration is correct, and suction feed should not be treated as a substitute for pump pressure.
- The aircraft model simplifies the system: some add-ons do not reproduce pressure priority, scavenge operation or every warning accurately. Hardware bindings can also override an on-screen pump or crossfeed switch.
Fuel transfer or crossfeeding changes the source being consumed; it does not change total fuel aboard. If the total quantity is dropping faster than expected, compare it with planned consumption using our breakdown of realistic 737 hourly fuel burn and check for a leak or loading error.
Variant and add-on differences
737 Classic, NG and MAX aircraft can differ in centre-pump protection, scavenge details, warning logic and permitted imbalance. Real crews must use the approved FCOM or QRH for that aircraft; simulator pilots should follow the documentation supplied with their specific 737 model rather than applying one add-on's procedure to every version.