Overview
The 747-400 fuel system stores fuel in seven tanks: four main tanks (one per engine), two reserve tanks (in the outboard wings), and a center wing tank. Some variants add a horizontal stabilizer tank for extended range. Boost pumps deliver fuel to a crossfeed manifold; four crossfeed valves route fuel to any combination of engines. Fuel can be jettisoned overboard through nozzles in the wingtips on most -400 freighters and combi variants.
Architecture
Components
| Main Tank 1 (Outboard L) | ≈ 4,400 USG |
| Main Tank 2 (Inboard L) | ≈ 12,300 USG |
| Main Tank 3 (Inboard R) | ≈ 12,300 USG |
| Main Tank 4 (Outboard R) | ≈ 4,400 USG |
| Reserve Tank 2 | ≈ 2,800 USG (in wing outboard of Main 1/4) |
| Reserve Tank 3 | ≈ 2,800 USG (mirror) |
| Center Wing Tank (CWT) | ≈ 17,200 USG |
| Horiz Stab Tank (optional) | ≈ 3,300 USG (range extension variant) |
| Total Capacity | ≈ 57,300 USG (≈ 384,000 lbs at 6.7 lb/gal) |
| Engine Feed Boost Pumps | 2 per main tank, AC-powered |
| Override / Jettison Pumps | 2 in CWT, 1 in each Reserve |
| Crossfeed Valves | 4 (#1, #2, #3, #4) |
| Jettison Rate | ≈ 2,500 lbs/min (both wings) |
Normal Operation
- Engine Start / Taxi: Main tank boost pumps ON; all crossfeeds CLOSED; engines feed from corresponding mains.
- Takeoff/Climb: Center tank override pumps ON if CWT contains fuel; CWT feeds all engines first (higher pressure than main boost pumps), saving main tank fuel.
- Cruise: CWT empties; reserves transfer to mains 1/4 when mains 2/3 reach trigger quantity; mains 2/3 then feed all four engines via crossfeed.
- Descent / Landing: Final feed from main tanks 2 and 3 with crossfeed selected to maintain balance.
Crossfeed Logic
Crossfeed valves are sequenced manually by the crew (or automatically in some configurations) to:
- Maintain lateral balance after asymmetric usage
- Transfer fuel from CWT before mains
- Isolate a leaking tank or pump
Crossfeed valves are color-coded green when OPEN on the overhead. Verify both halves of each valve indicate matching position.
Fuel Jettison
- Available on -400F / -400ERF and many -400 combi tails
- Two nozzles per wing, located outboard
- Jettison can be commanded down to a preset "fuel remaining" value
- Rate: ≈ 2,500 lbs/min total
Failures & Abnormals
- Identify leaking engine (compare consumption vs. EICAS fuel flow)
- Affected engine fuel control switch — CUTOFF
- Engine fire handle — PULL (isolates fuel at spar valve)
- Land at nearest suitable airport
Max imbalance between symmetric mains: 4,000 lbs. Use crossfeed to balance — open crossfeed to feed both engines from the higher-quantity side.
Limits
- Fuel imbalance (Main 1/4 ↔ 2/3): 4,000 lbs max
- Fuel temp minimum: −43 °C (Jet A) / −47 °C (Jet A-1)
- Fuel temp maximum (in tank): +54 °C
- Min fuel for takeoff: Per dispatch; typically 30 min reserve at destination