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I stumbled over this sentence in a training material about fuel leak:

"If the fuel quantity symmetrically decrease in both wing tanks and the fuel quantity in the center tank decrease, the fuel leak comes from the center tank or the APU feed line."

Regarding the APU feedline part, what I don't understand is if the APU is feeding from the left tank always regardless if it's by the fuel pumps or it's own pump (in case the fuel pumps are off) how can the fuel decrease evenly from both wing tanks?

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If you look at the fuel leak checklist you will see that you will come to this point only after you have confirmed that there are no leaks from the inner wing tanks and engine fuel feed lines.

According to the checklist if there are no visual indications of fuel leaking near the engines, you are to ensure the fuel cross feed is maintained closed. Then you turn off the center tank pumps. When the center pumps are off the engines will be only supplied by the inner tanks. Then you are to monitor the fuel in the tanks for a total of 30 minutes. If one tanks depletes faster than the other tank by 300 kg you can assume an engine leak. So, you shut off the associated engine. If this stops the leak, engine leak is confirmed. If it continues to leak you can assume an inner wing tank leak.

Now, the checklist says, if both the inner tanks deplete at a similar rate, you can suspect a center tank leak (remember the pumps are off) or a leak from the APU feed line. I have highlighted the word similar, because the checklist says to check for similar readings, not same or symmetrical readings. Yes, with the APU on, the left wing inner tank will deplete faster than the right tank, but it should not bring a massive difference to the figures. The A320 APU burns about 130 kg of fuel per hour. The checklist says to monitor the fuel for 30 minutes. This means, at the APU fuek line a leak would happen at a lower rate. That is why it says to look for similar rates, because an APU fuel line leak is quite insignificant. But you cannot still assume no leaks at the APU fuel line. The fuel leak checklist is a very conservative checklist. It is all about trying to close off any possible areas where a leak might be possible.

The checklist also says that if you have a fuel smell in the cabin, to turn off the APU to prevent additional fuel loss. It uses the word additional because the rate of leak at APU fuel is insignificantly low.

enter image description here

The checklist says similar not symmetrical (highlighted).

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I have had a situation like this while on ground during a long transit halt. As mentioned in your question how can fuel decrease evenly from both tanks with all the fuel pumps off.

The answer is to open the cross feed valve. As with the fuel pumps off the APU has its own fuel pump which provides feed only from the left hand side fuel tank. Keeping the cross feed open will negate this as fuel can now be fed to the APU feed line from the right tank also. Refer the below diagram enter image description here

The fuel cross feed valve can also while on ground be used to transfer fuel from one tank to the other. Say in this case for example if we want to transfer fuel from the right tank to the left tank we have to follow the below procedure.

On the refuel control panel we have to put the mode selector to defuel/xfr position to open the defuel/xfr valve this in turn connects it to the refuel gallery. Post this the recieving sides refuel valve has to be opened and the non recieving sides refuel valve has to be closed. Once this is finished we switch on the fuel pumps of the tank from which we want to transfer fuel and keep the other sides fuel pumps off. The same diagram above can be used for understanding.

However for a more detailed diagram refer to the attached diagram below.

enter image description here

From this diagram you can clearly see the APU feed line and it's fuel pump along with the cross feed valve. Now even thought the fuel pump is located in the centre tank it takes its feed from the left tank only. This is with the cross feed valve closed. The moment the cross feed valve is opened the APU feed line is now also supplied by the right tank.

Hope this clarifies

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  • $\begingroup$ Just out of curiosity. Is it actually called a cross-feed valve or a cross-flow valve. In my mind, a cross-feed system takes fuel from a different tank than normal and directly supplies it to the engine. In other words, the fuel doesn't flow from one tank to another for the engines to burn it... that would be a cross-flow system. $\endgroup$
    – wbeard52
    Commented Sep 3, 2020 at 18:44
  • $\begingroup$ @wbeard52, the diagram (see the linked question) calls it a cross-feed valve and it does seem to be a cross-feed valve, connecting the left and right feed lines, not the tanks themselves. I am not sure how it works for transfer (either it can flow back through the pumps when they are off, or makes a detour through the engines and return lines) $\endgroup$
    – Jan Hudec
    Commented Sep 3, 2020 at 19:45
  • $\begingroup$ @JanHudec as far as the system function goes it's very simple you need some form of pressure to move the fuel from either sides tanks while on ground with the engines shut off. So in this case if we want to transfer fuel for example from the right tank to the left tank we will first open the cross feed valve and then switch on the right hand side tanks fuel pumps on. While running the pumps will now provide sufficient fuel pressure to allow the fuel to flow through the cross feed valve to other sides tanks. I have also had this situation while on ground during the refueling. We followed this. $\endgroup$
    – Jai
    Commented Sep 4, 2020 at 4:22
  • $\begingroup$ @Jai, but how does the fuel actually get into the tank when you do that? The cross-feed valve connects to the feed line, which is connected to the tanks via the pumps on both sides, so the fuel can't flow directly into the tank. Either it has to be able to go back through the pumps when they are off, or flow around the engine and through the return lines. $\endgroup$
    – Jan Hudec
    Commented Sep 4, 2020 at 5:21
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    $\begingroup$ @janhudec hope that answered your question. The fuel doesn't pass over the fuel pumps but goes through the refuel gallery to the open refuel valve the fuel inturn goes to the outer tank which spills over to the inner tank. I'm cass of the A321 there are only wing tanks and no outer and inner tanks $\endgroup$
    – Jai
    Commented Sep 4, 2020 at 15:55

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