When a fuel leak is detected, but not located, the fuel leak procedure requests the flight crew to set to OFF the center tank pumps. In the case of a leak from the APU feeding line, or a leak from the engine feeding line, the center tank pumps are set to OFF and then to ON again when one wing tank contains less than 3 tons.
~~What happens in the case of an important leak?~~
In the case of a leak from the engine feed line located in an inner tank, fuel would spill from the center tank into this inner tank.
Should the engine feeding line breakage be important, the flight crew would not detect a leak, as the engine would shut down and/or there would be a “CTR TK PUMP 1(2) LO
PR” caution triggered, leading to procedures that will result in setting to OFF the affected center tank pump, thus stopping this leak.
~~What happens in the case of a small leak?~~
The crew may detect the possibility of a leak in case of a less important breakage, if they notice that a fuel imbalance is developing.
On the affected side, the engine is still fed and no low pressure is detected on the center tank pump. Fuel spills from the center tank to the inner tank.
In this case, if the crew applies the fuel leak procedure, the center tank pumps are set to OFF. Both inner tanks will deplete at a similar rate (Case 2 of the fuel leak procedure).
When the fuel in one inner tank has depleted to 5 tons, the following will occur:
Per design, when one inner tank is less than 5 tons and there is fuel in the center tank, the “FUEL AUTO FEED FAULT” caution will trigger on the ECAM.
As the center tank pumps are OFF when the alert triggers, an action line on the ECAM will require the flight crew to set the center tank pumps to ON again.
~~Then, why is it necessary to have the “3 tons line” in the fuel leak procedure ?~~
As the crew has been told by the FUEL LEAK procedure to set the center tank pumps to OFF, it has been considered that some flight crews might be confused, and would disregard the “FUEL AUTO FEED FAULT” and adhere strictly to the FUEL LEAK paper procedure. So they would not start again the center tank pumps when the “FUEL AUTO FEED FAULT” caution triggers and leave them OFF.
The “3 tons line” in the FUEL LEAK procedure has been introduced to cover this case. This line guarantees that the flight crew will set the center tank pumps back to ON, even if they previously disregarded the “FUEL AUTO FEED FAULT” caution.
This will allow the consumption of the fuel remaining in the center tank.
~~Why wait for 3 tons?~~
The pumps could be set to ON earlier, but this value has been chosen to cover also the remote case of a subsequent failure of the full level sensor in the affected inner tank.
In such a failure case, if the center tank pumps are set to ON early, there could be fuel spilled overboard, due to the fuel going from center tank to the inner tank through the line breakage and the full level sensor being inoperative.
So a 3 tons conservative value has been chosen, because at that time there is sufficient space in the inner tank to allow a transfer of fuel from the center tank to the inner tank (through the breakage in the feeding pipe) with a reduced risk of overflow.