The procedure shown in FCOM PRO-ABN-28 Fuel, starts with the assumption of an ENG FAIL.
Otherwise there are two other procedures available: FUEL L (R) WING PUMPS LO
PR (THE FAILURE IS
NOT DUE TO AN ENGINE FEED LINE RUPTURE), or the FUEL L (R) WING PUMPS LO
PR (THE FAILURE
IS DUE TO AN ENG FEED LINE RUPTURE).
(My bold)
This is a very serious situation as the pilots must apply their best judgment when analyzing the failure.
The best way to detect a fuel leak is to keep constant record of the Fuel Used vs AFOB. The sum of those two numbers has to be consistent with the actual FOB at departure.
Another way to keep a closer look at the Fuel System is to keep in mind the Fuel Over Destination figures. To get consistent numbers on FOD, winds and expected LVL Changes have to be inserted on the FMGEC no later than a couple of minutes after the TOC.
When we notice more than 300kg of difference on our estimated FOD it is normally because there is a TRIM AFT/FWRD or OUT TK FUEL XFR going on..after completion of the procedure we'll get the right number again. (Fuel being transferred - actually flowing in the lines - is not measured by FCMC).
I think the best way to manage the FUEL SYS is to be aware of what's going on.