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Recently flew a 767 that needed to turn back after about 45 minutes b/c, according to the pilot, fuel was only being pumped from one tank, which could lead to an imbalance if left to continue.

Generally in small GA aircraft one of the first things you do on the ground is switch fuel tanks to ensure proper flow from both. Is this also standard procedure on a 767? If so, what’s likely to have happened in-flight?

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  • $\begingroup$ Just to clarify, you're asking if the pre-flight checks on a 767 include testing that the fuel transfer pumps are working? $\endgroup$
    – Ben
    Nov 30, 2023 at 10:07
  • $\begingroup$ Yes, and if so, what could cause them to not work within 45 minutes of take off $\endgroup$
    – cph2117
    Nov 30, 2023 at 14:19
  • $\begingroup$ You said they squawked 7700, what was the airline and flight number? And how did you know they declared an emergency? There’s likely more information available. $\endgroup$ Feb 21 at 13:11

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TL;DR: Large aircraft normally isolate the left and right fuel systems, so if engines start, you have enough accessible fuel to take off and divert or return. There is also a cross-feed mechanism to rebalance fuel in flight, so most failures can be corrected.

Generally in small GA aircraft one of the first things you do on the ground is switch fuel tanks to ensure proper flow from both.

Small GA aircraft have one engine and two wing tanks, so there is a selector valve and potential latent faults, e.g. a blockage affecting one tank. A jet transport, like a 767 has at least two engines and at least two wing tanks. In normal configuration, each engine solely feeds from its own wing tank, so if the engines start and run, it is a sign that each side is functioning properly.

Aircraft also have a cross-feed system, which is not normally used, but is important during an engine failure. In particular, a stuck-open crossfeed valve may cause fuel to preferentially feed from one side, leading to an imbalance. This isn't an uncorrectable problem, as you can use the cross-feed to rebalance the fuel via switching on and off fuel pumps, but a return is likely warranted.

Where the isolation breaks down is the center tank(s). Here, you have one center tank and separate pumps to transfer the fuel to each engine. Potentially, one of the center tank pumps failed, so only one engine could be fed from the center tank, which would eventually cause an imbalance. This could be problematic if the cross-feed valves were to fail.

Airbus in particular inhibits center tank feeding on takeoff for isolation purposes, so you can take off and then realize your center tank fuel is unusable. Boeing does let you feed from the center tank at takeoff. In both cases, this isn't a safety issue, as you burn center tank fuel first, and you have more than enough wing tank fuel for a return or diversion.

Another issue is fuel leaks. The fuel system consists of a complex series of piping, motor-operated valves, and check valves. Some failure modes can cause fuel to flow in an incorrect direction. Fuel piping is, to the greatest extent possible, contained inside tanks, so a leak won't result in a hazardous spill, but may manifest as a unwanted fuel transfer.

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  • $\begingroup$ With a failed center tank pump & an operative crossed valve, you can still feed both engines from the center tank without causing any imbalance. $\endgroup$
    – Ralph J
    Dec 2, 2023 at 5:52
  • $\begingroup$ @RalphJ Right, I see the procedure for that. I changed the text a bit. $\endgroup$
    – user71659
    Dec 2, 2023 at 7:23
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    $\begingroup$ If a center tank pump fails, you immediately open the crossfeed valve so you're not creating an imbalance. If that fails to open, you have 2 unrelated failures at once, and you'd turn off the other center tank pump (to maintain balance) anf you'd land. But, if the crossfeed opened, you could continue (ETOPS scenarios aside) since the crossfeed valve's failure mode is a failure to move, not an uncommanded closing. $\endgroup$
    – Ralph J
    Dec 2, 2023 at 16:02
  • $\begingroup$ Would any of this necessitate squawking 7700? My flight did, but based on your answer I wouldn't have expected the pilot to do so. $\endgroup$
    – cph2117
    Dec 4, 2023 at 17:12
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    $\begingroup$ @cph2117 The issue would be the early return. An overweight landing requires an emergency declaration. $\endgroup$
    – user71659
    Dec 4, 2023 at 20:29
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This, and several similar incidents like it, sounds like the Boeing 767 fuel imbalance anomaly, which sometimes occurs when there enough fuel is on board to require putting fuel in the center tank.

Boeing normal procedures for the fuel system require using center tank fuel first, so center tank pumps will be on from the point of engine start.

The problem manifests itself when one of the center tank pumps is not producing enough pressure to override the pumps in one (or both) of the wing tanks, resulting in a simultaneous decrease in center tank fuel level and in one (or both both) wing tanks. When it's just one wing tank involved, that results in a fuel imbalance.

It is not known why this anomaly occurs, and it is not predictable, but it appears to be related to an Airworthiness Directive from 2001 which superseded a 1997 AD that was issued after the Swiss Air accident.

The developing fuel imbalance presents two potential problems:

  1. There is a fuel imbalance limitation of 1500 to 2500 pounds in the 767, depending on wing fuel quantity. Continued flight in the anomalous condition could result in exceeding that limitation, although I am told that as a practical matter, the 767 flies just fine with one wing full and one wing empty.

  2. Continued flight in the anomalous condition could eventually result in completely emptying the affected wing tank, resulting in an engine flameout on the affected side.

My company's AOM (Aircraft Operating Manual) directs the crew to an appropriate supplemental normal or non-normal procedure when the anomaly occurs. Presumably, other 767 operators have similar instructions in their manuals.

I have seen this anomaly at least once, maybe twice in my experience. It was corrected by using the aforementioned procedures. If that had failed to correct the problem, then certainly the most conservative course of action would be to get on the ground and figure it out there.

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  • $\begingroup$ Welcome to Av.SE! Thanks for adding to this thread - real world experience is always a great addition. Please continue contributing here. $\endgroup$
    – Ralph J
    Feb 21 at 13:43

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