Upper limit of gravity feed.

[lion-g]

Hi guys,

Season greetings !!!

I am very confused with a fuel problem. Can some kind soul out there explain to me the concept behind a upper limit for gravity feed into engines ?

How come if crusing at a higher altitude for a long period of time increases the upper limit of gravity feed? I assumed it has got something to do with FUEL TEMP as it gets colder the density increases and thus weight increases therfore easier for it to be gravity-fed.

Thanks a lot for your help guys .....

Cheers
lion-g

[rudderrudderrat]

Hi,

Fuel contains dissolved air which will bubble out of solution when pressure is reduced. With gravity feed, there is a lower pressure in the fuel line due to the "suck" of the engine driven fuel pump - causing cavitation.

After some time at high altitude, the dissolved air will naturally come out of solution allowing gravity feed.

[Microburst2002]

I have read somewhere (I will try to find where) that there is a risk of structural damage (in the wing tanks) if you feed the engines by gravity unless sufficient time passes after leveling off at cruising altitude.
I think it has to do with the venting of the tanks. The pressure of the air in the tanks is not the same as the atmospheric pressure when the airplane is climbing. Apparently it takes time to be the same. Although I don't quite understand it very well, to be honest. Why does not the same problem occur with the pumps running?

We need an engineer for this one.

[alatriste]

Lion-g, you can use the search tool and try to find the thread titlte "Deaerated fuel". It could be helpfull.

[john_tullamarine]

unless sufficient time passes after leveling off at cruising altitude

While there is bound to be a system somewhere to prove me wrong, I don't think the statement is reasonable. Maybe another OWT at play ?

The whole idea of a venting system is to keep the in/out pressures as near to the same as practicable (noting FE Hoppy's comment re slight positive diff, below).

If that doesn't happen, undesirable things can happen if the diff gets too high (forces on a bit of tin relate to pressure diff across the area of the bit of tin).

Plenty of examples around ..

(a) tanks crushing due to vent obstructions

(b) a C130 incident I can recall where an ill-considered short term mod (the problems of "it's got to be done now" can lead to checks and balances in the decision process being omitted) left the external tank unvented and, you guessed it, all crinkled up at the end of the flight (read "embarrassment" and "dollars").

etc.

[FE Hoppy]

Nice description of "suction feed" tests carried out during engine certification in this article:

Rolls considers keeping 747 for future engine trials: AINonline (http://www.ainonline.com/news/single-news-page/article/rolls-considers-keeping-747-for-future-engine-trials/)

For the OP, the reason engines flame out under suction feed is vapourisation in the feed lines. As has been stated, after prolonged flight there will be a certain amount of deaeration allowing a relight at lower ambient pressure.

Most vent systems are designed to put a slight positive pressure in the tanks but it's only small. One of the requirements of a tank system is it must not fail with a 3.5psid pressure (CS25).

Just to give an idea of what can go wrong. Embraer had to modify the naca fuel vent inlet on the 190 after it was found that at certain climb speed and weight combinations the vent works the wrong way round and creates a syphon effect which combined with a flap valve failure in the vent tank baffle can lead to venting a lot of fuel very quickly!

[lion-g]

Wow, thanks for the all the replies.

Here's what I got after searching for deaerated fuel:-
TO MEMO the previous answer is right but I guess that your are looking for a more operational info regarding GRAVITY FUEL FEEDING when engine fuel pumps are inop. (FCOM 3.02.28 p10).
If flight time above FL 300 is greater than 30 min, it is time enough for all the air bubbles to be gone cause of the decreased pressure in the fuel tank (like a coke bottle when opened), so the fuel can be considered as deaerated (with no air diluted) and engines can be feeded by suction.
http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/statusicon/user_offline.gif http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/buttons/report.gif (http://www.pprune.org/report.php?p=3306862)

Thank you very much

[Mechta]

A similar problem occurred when a rather long aircraft with a tailplane tank was introduced. Under certain circumstances there was no gravity feed from the tail to wing tanks at all.

A full-size ground test rig was built using a combination of aircraft parts and transparent tubes. This rig allowed a variety of angles of attack & altitudes to be simulated by virtue of a tilting frame and the tanks being connected to vacuum pumps.

Two problems were identified. Firstly, when the transfer pipe was opened, the fuel instantly degassed from sea level to whatever altitude was simulated, resulting in very sudden airlocks.

The other problem was that the shape of the exit from the tailplane tank to the transfer pipe caused cavitation, which resulted in what transfer that did occur to go in a series of 'slugs' of fuel.

The shape of the waves on these 'slugs' would have been a surfer's dream!

As far as I recall, a number of changes were made to components and operating procedures to overcome the problem.

[Microburst2002]

Please
Disregard my previous post. I mixed two different aspects (venting and gravity feeding). If venting is obstructed, then we can have structural damage. Nothing to do with gravity feeding. But now I have learnt what I am looking everytime I look at the cross in the wing lower surface during walk around (A320). That's why I love this forum.

By the way, in some airlines with A320 they note the time they pass FL 300 or start the chrono. In others they count the 30 min for the gravity feeding procedure after reaching any cruising level above FL300, but not after passing FL300 if still climbing.

Which do you think is correct?