According to the NNC an increase in fuel imbalance of 230 kg / 500 lbs or more in 30 minutes should be considered an engine fuel leak. However, bear in mind that
- not every fuel leak is an engine fuel leak ; a leak may also occur in pipelines and even in the tank itself, e.g. panel missing after maintenance.
- fuel leaks also occur without fuel imbalance, such as a rapid drop in fuel quantity in the center or auxiliary fuel tank.
- a major fuel imbalance is not always a fuel leak ; the imbalance can be caused by differences in fuel pump output pressure when flying with the crossfeed valve open, or by fuel pump, crossfeed valve or check valve malfunctions. A fuel imbalance immediately after takeoff may be caused by prolonged APU ground use.
As a general guideline, an engine fuel leak stops when the engine shutdown. If fuel quantity continues to drop after engine shutdown, you may be dealing with one of the leaks mentioned above. Refer to FCTM 8.x [Fuel Leak] for additional information.
FCOM 13.20 - Minimum fuel for ground operation of the EMDP is 760 kg / 1,675 lbs in the related main fuel tank.
! Although this limitation is for ground operations, the EMDP may also get overheated after extended operation in flight with the associated main fuel tank empty. So there is a risk losing the hydraulic system in combination with an engine shutdown (by checklist or by fuel starvation - fuel leak) ...
If the landing field cannot be reached within 20 to 30 minutes after fuel tank is empty, consider temporarily switching-off the associated EMDP in order to avoid an overheat. Re-engage the pump in approach to configure the aircraft for landing.