FU/FOB Disagree
Just for info:
The Fuel Used/FOB disagree message is calculated by the FCMC and is contained in FCMC9.0 and above. A FWC change is required in parallel to 'activate' the warning.
This is active on all new deliveries and Service Bulletins are available to apply the change retrospectively.
The Fuel Used/FOB disagree occurs when there is over 5t mismatch between the departure fuel minus fuel used, and the FOB figure.
This warning was conceived about 6 years ago, as an A340 operator and an A320 operator both lost significant amounts of fuel from the engine HP fuel filter cover which came off due to the helicoil inserts in the housing giving away. The A340 lost about 20 tonnes of fuel (xfeed valves were not opened), the A320 lost a lot more because the pilot did exactly as per the Air Transat pilot and opened crossfeed. The A320 landed with something like 200kg FOB.
There was resistance in some of the operators contacted (including the two operators, A320 and A340 cases above). The resistance was due to the fact that the Flight Ops departments considered en-route fuel checks to be basic airmanship, and they were concerned that the existance of such a warning could cause complacency.
Airbus eventually decided to inplement the function, as it was considered that it was beneficial to alert crew as soon as the threshold is exceeded. In theory if the leak is from one wing tank or engine, the fuel imbalance message should come first (in most cases).
There was one case about 9 years ago when an A330 had an excessive transfer to the trim tank due to a leaking vent, a pipe disconnected in the trim and pushed about 5 tonnes overboard (trim transfer continued as the tank was not reaching the trim tank fuel quantity requried for the aft CG target). This problem was due to incorrect pipe installation.
I think the FU/FOB Disagree function is a good thing, but hopefully it will be the last defense.