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.
$\begingroup$
$\endgroup$
2
-
18$\begingroup$ "they" who? where did you read about this? $\endgroup$– FedericoSep 27, 2017 at 12:56
-
1$\begingroup$ 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. $\endgroup$– MNKYSep 30, 2017 at 22:53
Add a comment
|
4 Answers
$\begingroup$
$\endgroup$
4
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.
(A320 FCOM)
-
28$\begingroup$ TIL: airplane wings are actually complex rpg dungeons o.o $\endgroup$– WeaverSep 27, 2017 at 16:38
-
7$\begingroup$ @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. :) $\endgroup$– JulesSep 28, 2017 at 13:24
-
1$\begingroup$ Nice info! But what is the reason why all aircraft is that? What is the basis? $\endgroup$– MNKYSep 30, 2017 at 23:33
-
1$\begingroup$ @MNKY - if you mean, "Why does not this apply to all aircraft?", then I've added relevant information. $\endgroup$– user14897Oct 2, 2017 at 16:12
$\begingroup$
$\endgroup$
1
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.
-
1$\begingroup$ 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) $\endgroup$ Oct 3, 2017 at 6:24
$\begingroup$
$\endgroup$
The center tank pumps transfer fuel directly into the engines. When the slats are extended, the pumps are disabled to ensure that the wing tanks feed the engines rather than the center tank. The reason for this is to account for a possible tank pump failure. The tank pumps are electrically powered so there is a tiny bit of a chance of an electrical failure which can take the power off them. Unlike the center tanks the wing tanks can be used to gravity feed the engines.
With gravity fuel feeding active, the aircraft engines can still be supplied with fuel with no push from the tank pumps. The aircraft computers use the slat extension as a cue that the aircraft is preparing for a take off. Without gravity feeding, multiple tank pump failures can mess up fuel flow into the engine. This is the last thing you want to happen close to the ground. The center tank pumps are disabled for this reason. To allow for gravity fuel feeding by the wing tanks.
When the A321 came out Airbus made its fuel system simpler by replacing the center tank pumps with jet pumps (works with motive fuel) and put two transfer valves in the center tanks. So, in the A321 the fuel is pumped from center tank into the wing tanks which then transfer fuel into the engines. So, the engines are always supplied by the wing tanks which takes away the need for the previously mentioned logic. The post 2014 A320 models have the A321 system in place. Thus, it does not apply to them either.
A320 fuel tanks pre mod and post mod.
$\begingroup$
$\endgroup$
1
Its to provide an independent fuel supply to each engines. If both engines are fed from the Centre and its contaminated then both engines fail. If both are being fed from their respective wings then the supplies are independent, that's all.
-
4$\begingroup$ Welcome to Aviation.SE! Could you provide more details? This doesn't seem to add much to the existing answers. $\endgroup$– foootJun 18, 2021 at 21:02