A320 Fuel leak
We run this sceaniaro on this years AQP/Maneuvers Observation period. The situaition is you get an ecam stating Check fuel balance or something of that sort and you procede with the QRH or quick reference handbook.
The important thing is to determine where the leak is, but if you cannot, do not open the crossfeed. This will prevent the occurance of the Air Transat situaition. You will loose the engine on that side with the leak but not the other engine and it's associated wing fuel.
For purposes of the simulation, the wing empties and the engine is lost. The wing leak is really fast and you don't get to go all the way through all the available trouble shooting the checklist provides before the engine flames out.
The airplane is easily controlled with one wing full and the other empty, the trim system compensates nicely. There are no hard and fast rules I can think of except don't open the crossfeed unless you are sure of the leaks location.
There are situations where a leak in the pylon could be stopped by turning off fuel pumps in the wings and shutting down the engine. There are many different posibilities, but if you are not sure, just don't open the cross feed and loose all the fuel.
I hope you are now confused at a higher level.