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Is there a tank priority sequence for using the fuel tanks, or does the pilot decide?

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    $\begingroup$ Welcome to Av.SE! $\endgroup$
    – Ralph J ♦
    Commented Oct 13, 2018 at 17:39
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    $\begingroup$ Your question is a really good one (I'm surprised it has not been asked before). You can narrow your question to some few aircraft model as it is aircraft specific (e.g some aircraft transfer fuel from one tank to another to adjust CG in flight, some such as fighters have less tanks and possess accumulators,...) $\endgroup$
    – Manu H
    Commented Oct 19, 2018 at 14:26
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    $\begingroup$ @ManuH Thank you for your comment. I was wondering this topic. But I don't have more information aircraft models and technical properties so I couldn't add aircraft models. $\endgroup$
    – TestUser
    Commented Oct 20, 2018 at 12:10

6 Answers 6

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Which fuel tanks are used in order of priority in aircraft?

What you're talking about, in large aircraft, is often referred to as the fuel burn schedule. Light aircraft generally do not have a fuel burn schedule though they may have have minimal requirements.

The fuel burn schedule is dictated by the aircraft design and is thus different for different aircraft designs.

Is there a sequence of priority?

Yes, and if you don't observe the fuel burn schedule you risk exceeding the structural limitations of the aircraft, and in particular wing bending moments.

Or the pilot decides?

No, the pilot does not decide usually. While there may be some leeway in the fuel burn schedules of large aircraft, the pilot or a flight engineer (old aircraft) or an automated system does not decide but rather implements a preset protocol.

As an example, here's a general outline of the fuel burn schedule for 747-100/200 aircraft. This is from memory so might not be entirely correct but is sufficiently so for example purposes:

  1. On start, taxi, and takeoff burn tank to engine. In other words, route the fuel from main tank 1 to engine 1, main tank 2 to engine 2, etc.
  2. Once you're established in your cruise climb, all engines burn out of the center tank if there is fuel in the center tank until the center tank fuel is exhausted (but don't burn any fuel in the center tank that is there for ballast).
  3. Burn engines 1 & 2 from main tank 2 and engines 3 & 4 from main tank 3 until the amount of fuel in main tank 2 matches that in main tank 1 and the amount of fuel in main tank 3 matches that in main tank 4. Main tanks 2 & 3 are larger than 1 & 4. When parity is reached, burn tank to engine.
  4. At some point transfer the fuel in reserve tank 1 to main tank 1 and reserve tank 4 to main tank 4. Likewise, if you have them, from reserve tank 2 to main tank 2 and reserve tank 3 to main tank 3. Most 747-100/200 aircraft didn't have reserve tanks 2 & 3. You have to get the fuel out of the reserve tanks before landing.

Be aware that just as there is a fuel burn schedule, there may also be a fuel loading schedule. In the 747-100/200 that schedule dictated that no fuel goes into the center tank until all other tanks are full. As I remember, the largest center tank available held 114999 lbs of fuel at 6.7 lbs/gal fuel density. Center tank size varied as ordered by the airline and changed through the production run of the aircraft.

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    $\begingroup$ Fortunately, it doesn't need to be committed to memory because it will be on the checklist. $\endgroup$
    – GalacticCowboy
    Commented Oct 14, 2018 at 12:22
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    $\begingroup$ "No, the pilot does not decide usually." I assume though there are overrides possible in case of an emergency? $\endgroup$
    – Mast
    Commented Oct 14, 2018 at 15:44
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    $\begingroup$ @Mast I can't speak to automated systems or other aircraft, but in the case of 747-100/200 aircraft the system was very flexible (and was considered complicated) with the exception that once you move fuel out of a reserve tank, you can't put it back as I remember. It's not a matter of overriding but of the flight engineer throwing switches in an order other than would normally be done. $\endgroup$
    – Terry
    Commented Oct 14, 2018 at 17:55
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    $\begingroup$ That makes perfect sense, thanks for adding that. $\endgroup$
    – Mast
    Commented Oct 14, 2018 at 18:40
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    $\begingroup$ And some craft, like the Concorde, had the additional complexity of needing to actively adjust the aircraft CG, by pumping fuel betwen the tanks, to maintain trim in different phases of flight. Here you had to not only worry about the fuel schedule, but you also had to be periodically moving the remaining fuel around to keep the correct attitude during flight. $\endgroup$
    – J...
    Commented Oct 15, 2018 at 15:16
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There is a sequence of priority and it is spelled out in the Aircraft Operations Manual (or similar). For example, in a B757/767 fuel is loaded into the wing tanks first and then if more fuel is required fuel is added to the center tank. During the flight the fuel is used from the center tank first and then the wing tanks.

Some light aircraft, e.g. Cessna 310, 402, etc., may have auxiliary fuel tanks in addition to main tanks that are designed to be used in accordance with procedures in the aircraft's flight manual/pilot operating handbook.

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    $\begingroup$ I was going to bring up the Cessna 310 as an example too. The backup fuel pumps can only pull from the Main tanks IIRC, so you burn off the Aux fuel first, BUT fuel return from the engines is to the main tank so you need to burn enough off the mains that they don't overfill. Fun ship. :) $\endgroup$
    – Erin Anne
    Commented Oct 14, 2018 at 1:36
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    $\begingroup$ @ErinAnne - Good comment. Great airplane. Trivia question: Which circuit breaker controls the "tip tank transfer pump?" $\endgroup$
    – user22445
    Commented Oct 14, 2018 at 1:43
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    $\begingroup$ @757toga I did all my multi time in 2006, but I found a POH online and looked it up because I'd forgotten: the landing light circuit breaker, lol. (In the 310K POH I found at myhra.org/Cessna310KOnePageperPage.pdf it's on page 2-1, under "Fuel System". $\endgroup$
    – Erin Anne
    Commented Oct 14, 2018 at 1:47
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My G35 Bonanza (and many other, older aircraft) used a pressure carburetor that would return excess fuel to the left main fuel cell.

This meant that no matter what fuel cell the engine was consuming fuel from, the carburetor would always be sending the fuel it did not feed to the intake manifold back to that particular fuel cell.

If the left main fuel cell were full of fuel, the returned fuel from the carburetor would be vented overboard.

The workaround for this behavior of the fuel system was to consume fuel from the left main fuel cell for at least the first hour of the flight before consuming fuel from the other fuel cells/tanks. This would leave space in the left main fuel cell for the the carburetor return unused fuel without it being vented overboard.

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As others have already alluded, this really depends on the aircraft. Your question does not specifically state fixed wing - so here is my $.02

I know from flying Robinson model helicopters, specifically the R22 and R44, the main and auxiliary fuel tanks are actually tied together - you burn from both at all times; there is only a master shutoff, there is no tank selector.

diagram

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    $\begingroup$ It strikes me as quite odd that the tanks are different sizes. Wouldn't you empty the left tank (from the perspective of this drawing) before emptying the right tank and thus be left with an imbalance? $\endgroup$
    – FreeMan
    Commented Oct 15, 2018 at 14:45
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    $\begingroup$ Yep! And that is by design! When you solo, you fly from the right seat. The imbalance offsets a solo pilot (and some other forces related to anti-torque). $\endgroup$
    – Matt Clark
    Commented Oct 15, 2018 at 14:51
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    $\begingroup$ Got it. I thought about that, but thought a solo sat on the left (my IANAP is showing...). What do you do when you've got a passenger (he asks as a whole new question in the comments)? $\endgroup$
    – FreeMan
    Commented Oct 15, 2018 at 14:54
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Yes there is often a prescribed sequence. Sometimes it has to do with wing bending moments; often it has to do with the fore-and-aft location of the CG. For example my understanding is that the P-51D (and C?) Mustangs were rather unpleasant to fly while the aftmost tank was full of fuel, so some fuel from that tank would typically be consumed even before using any fuel from the drop tanks.

And sometimes it just has to do with the plumbing of the fuel system. For example if only tank A is directly connected to the engine, you wouldn't want to forget to transfer some fuel from tank B to tank A before tank A runs dry. Sometimes one tank feeds more reliably than others due to where the boost pump is located so it is wise (and prescribed in the manual) to use that tank for take-off. Etc.

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Yes! When you are filling up the aircraft and you use the centre tanks, it then means that the pilot has to use the centre tanks first before the 1 or 2 wing tanks are selected, and to keep checking to make sure the tanks are balanced according to the manufacturer’s procedure.

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    $\begingroup$ Welcome to Av.SE. Your answer is correct & it does answer the question, but you're getting down-votes because it doesn't really add to the other answer here, doesn't explain "why", and doesn't reference any sort of authoritative source. So it ends up looking like "somebody on the internet said..." which isn't particularly useful. Can you add a reference or an explanation "why" we do it this way? That would improve the answer considerably. $\endgroup$
    – Ralph J ♦
    Commented Oct 14, 2018 at 4:37

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