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Why are the center tank pumps deactivated when slats are extended on the A320? I researched about this and all I can see is hints. They say it is because of regulations or certifications.

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    $\begingroup$ "they" who? where did you read about this? $\endgroup$ – Federico Sep 27 '17 at 12:56
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    $\begingroup$ I read it at aviation forums my friend, they say it's some kind of certifications. I also read part 23 but it is not directly stated. $\endgroup$ – MNKY Sep 30 '17 at 22:53
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You're right. But it's not because of the slats. It's because of what the plane is doing when the slats are being used.

The slat position is used in some system logic because it's indicative of a takeoff and approach. For example, on the MD-11 the engine ignition is automatically put into continuous mode when the slats are operated, because that's indicative of slow speed.

The same for the A320, slats extended means a takeoff / climb is about to commence, or the plane is on approach with a potential go-around.

Those conditions require high fuel pressure.

The center tank has 2 pumps, while each wing tank has 2 pumps. If a center tank pump fails, it will be 1 pump supplying 2 engines, compared to 2 pumps for each engine (or 1 for each in a rare double pump failure—one in each wing).

If a pump fails, there is no automatic command that can be sent to the sequence valves to change the sequence, or to engage the crossfeed valves.*

Usually the center pumps overpower the wing pumps, but what is omitted from that explanation is that there are sequence valves. If one center pump fails, the pressure may remain high enough to keep the valves closed on the good side, but not high enough to supply two engines during high demand.

Also note there is no suction (gravity) feeding from the center tank. I attached an A320 fuel schematic below, number 13 is the sequence valve.


RE: Why does not this apply to all aircraft?

The 737 Classic faced issues (see footnote). Other types have more than 2 pumps for the center tank, and/or have more powerful pumps. Another option is to use the center tank to fill the other tanks (like on the MD-11).

The 747-1/200 also required using the wing tanks during takeoff, see this answer by @Terry.

A speculation on my part, is that it was cheaper to certify the plane like that, or problems were found and that was the cheapest solution.

A fuel tank that is not designed to feed the engines under all flight conditions need be tested only for the flight regime for which it is designed to do so (e.g., cruise conditions).

(FAA Flight Test Guide For Certification Of Transport Category Airplanes)

* A fuel imbalance may quickly develop when each engine is consuming +70 kg of fuel per minute (an issue that affects the 737 Classic).

On the classics, when departing with less than 1,000kg of fuel in the centre tank, an imbalance may occur during the climb. This is because the RH centre tank pump will stop feeding due to the body angle so number 2 engine fuel is drawn from main tank 2, while engine 1 is still drawing fuel from the centre tank. When this “runs dry” the scavenge pump will also transfer any remaining centre tank fuel into main tank 1, thereby exacerbating the imbalance.

enter image description here
(A320 FCOM)

enter image description here

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    $\begingroup$ TIL: airplane wings are actually complex rpg dungeons o.o $\endgroup$ – StarWeaver Sep 27 '17 at 16:38
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    $\begingroup$ @StarWeaver - when I was somewhat younger, I wrote a D&D dungeon crawl based around a map produced by sketching over a diagram of fittings in a toilet cistern... any system of plumbing or wiring of sufficient complexity can be an RPG dungeon. :) $\endgroup$ – Jules Sep 28 '17 at 13:24
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    $\begingroup$ Nice info! But what is the reason why all aircraft is that? What is the basis? $\endgroup$ – MNKY Sep 30 '17 at 23:33
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    $\begingroup$ @MNKY - if you mean, "Why does not this apply to all aircraft?", then I've added relevant information. $\endgroup$ – ymb1 Oct 2 '17 at 16:12
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I believe it's an interpretation of an airworthiness requirement in 14 CFR 25.953:

Each fuel system must meet the requirements of § 25.903(b) by -

(a) Allowing the supply of fuel to each engine through a system independent of each part of the system supplying fuel to any other engine;

where 25.903(b) reads:

(b)Engine isolation. The powerplants must be arranged and isolated from each other to allow operation, in at least one configuration, so that the failure or malfunction of any engine, or of any system that can affect the engine, will not -

(1) Prevent the continued safe operation of the remaining engines; or

(2) Require immediate action by any crewmember for continued safe operation.

When the center tank is feeding the fuel on the (typical) A319/A320 feeds directly into the fuel manifold, acting as a single tank outlet for both engines (see also 14 CFR 23.953). Additionally, takeoff with crossfeed open is prohibited for the same reason.

The A321, newer A319/A320, and other designs like Boeings, feed fuel from the center into a wing tank first, avoiding this requirement. Indeed, if you have an A321 or the A319/A320 with the new system, the limitation is deleted.

It would not be for purposes of high fuel pressure, since when center tank feeding, all pumps run, just the center tank pumps overpower the wing tank pumps.

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    $\begingroup$ The 737 has similar sequencing valves built into the fuel pumps, the wing pump pressure relief is set lower than the center tank pumps. So there is no difference with the A319/A320. (By the way, the A321 has a center tank by default, you may be thinking of the additional center tank) $\endgroup$ – user71659 Oct 3 '17 at 6:24

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