FUEL BALANCE, CROSSFEED AND LEAK INTERPRETATION
Fuel monitoring needs a common comparison point. Planned fuel at a waypoint, actual fuel on board, fuel used and tank distribution answer related but different questions. An apparent discrepancy can be created by comparing the wrong route point or by leaving a revised loading value unaccepted in the management system. Establish units, the flight actually flown and the time of each observation before diagnosing a system fault. A correct destination prediction is not independent evidence if it was calculated from the same wrong starting quantity. [72, PDF pp. 9–10, 18–21] [66, PDF pp. 123–124] [23, PDF pp. 1078–1081]
The crossfeed valve connects the feed sides so a defined pump/source arrangement can support the required engine feeds. It is not a general instruction to equalise tanks by moving fuel directly from one wing into the other. Pump configuration, engine consumption and the applicable procedure determine how a balance changes. A crossfeed indication alone therefore proves neither a completed balance correction nor a closed-off leak. Follow the path and remaining sources on FUEL, and preserve the quantity trend rather than replacing it with an assumption about valve selection. [72, PDF pp. 3–6, 11–12, 18–20]
A leak, abnormal consumption, transfer problem and quantity-indication fault can produce overlapping symptoms. Compare left/right fuel used, tank changes, total fuel, actual engine indications and the wider aircraft state. The location of a leak relative to shutoff valves and crossfeed paths matters because feeding the affected side can change the loss. The purpose of this architecture review is to recognise those dependencies; the isolation sequence comes from the applicable procedure. A seemingly helpful redistribution can otherwise obscure the source or place additional fuel in the wrong path. [72, PDF pp. 3–10] [21]
The APU adds another consumer and feed path, and fuel used by that unit should not be confused with a discrepancy between the two main-engine flow indications. After a loading or servicing change, reconcile the actual aircraft with the latest loadsheet and MCDU entries. The dated V3 known-issues record distinguishes live mass/trim transfer from loadsheet values. That software transfer issue is separate from an actual fuel-system leak. Preserving that separation helps diagnose whether the discrepancy is physical quantity, displayed sensing, entered data or a prediction based on different assumptions. [72, PDF p. 7] [83, 24 October 2026, edited25 October; Send to MCDU live weights/trim behaviour]