A320 fuel tanks
 

Had a stuck transfer valve the other day and ended up cycling the CBs which put all the fuel in the inner tanks. The manuals don't actually say WHY it's necessary to have outer tanks but the general concensus is that it is necessary for structural integrity. Any "sharp pencil" types out there want to enlighten me?

Thats correct, it's for structural integrity. Actually, the A320 has 2 fuel tanks in each wing, an inner and an outer tank. The outer (At the wing tip) holds, I believe 1400 or 1500 lbs. of fuel and is to remain full at all times for structural integrity.
Someone please correct me if I'm wrong concerning the amount of fuel in the outer tank.

flynverted

In any large aeroplane, to reduce wing root bending moments, it is desirable to keep as much fuel outboard as possible. However, there's no point in having such tanks 'full at all times' as otherwise you may as well have lead weights at the wingtips! I suspect that the outboard tanks will probably be kept full for most flights but would be partially used in the event of diverting to the alternate.

The OUTER wing tanks on A320 only hold 690kg/1500lb, and was designed into the system to reduce wing tip flutter at some high Mach Nos. & FL's.

It remains there in the cruise until the INNER tank capacity reaches 750kg/1650lb, the transfer valves then open allowing this OUTER fuel into the INNER tanks, and becomes available for use.
This usually occurs just as the aircraft pitches nose down to start the descent, but obviously depends upon initial fuel load.

Note, the A321 does not require this feature, and only has a single tank in each wing, very simple.

Roger Miller.

I think you will find structural integrity doesn`t come in the the equation. Airbus were merely trying out the concept for the larger aircraft that came after the A320.

Gracious!
 

You sure about that?! Think I'll go with the S.I. angle for now. As king says, the valves seem to open just as descent is started and I quite often get an ECAM Lo fuel level warning simultainiously. We always seem to be running around on min fuel, but that's another topic. Thanks to everyone for the inputs.

The transfer valves often open when pitching down for decent however this is only if the inner cell quantity is low enough. Ie about 1000kgs.

Silky,
The nose down pitch momentarily gives a fuel qty under-reading, thus triggering the valves to open.

AIRBUS - has a about 140Lbs of Gust Load Alleviation clap trap to relieve wing root bending moments - I gotta believe they are either worried about flutter OR ultimate loading .. Boeing has a key lead over Bus in two areas - engine -to-wing integration & aeroelastics .. Boeing historically does not use Gust Load Alleviation systems - except to improve ride comfort ..

Load Alleviation hasn't been fitted to A320/1 for years now, at least six to my knowledge, maybe more.

Roger Miller.

KingoftheRoad - you are right - AIRBUS has a service bulletin to remove it from all existing aircrafts prior to its baseline deletion - however, they crowed like hell about it ( white papers ) when it was rolled out - I think Boeing only uses it for ride comfort to relieve sharp motions.. However, the A321 has "lift dumpers" - I would imagine, since the A321 is pushed to its absolute limits from a wing design standpoint - It probably has "it" - I do think it was very clever of AIRBUS to extend the aft loading of the A321 wing by re-profiling/extending the aft supercritical cusp using new - super extended & profiled split fowler flaps - These Airbus folks are almost wily in certain areas ..

Silky,

"the transfer valves automatically open when the inner tank reaches the low level (about 750 kg)"

"Note: The 750 kg value is based on a level aircraft attitude with no acceleration. During steep descents or accelerations/decelerations, the transfer tank valves may open with more than 750 kgs of fuel in each inner tank and the low-level warning may be triggered"

Courtesy of A320 Vol 1.

I guess that would include pitching down for descent.