Leaking 747 Wingtip

[TheGreatLeslie]

After refuelling at a stop in the persian gulf we were taxing out to the runway. I was looking out at the starboard wingtip. There appeared to be a constant flow of liquid, I presume aviation fuel coming from the wingtip. Is it normal for 747's to dump fuel onto a taxiway or was this a potential diaster that failed to manifest itself?

[A and C]

All large aircraft have a tank in each wing tip that is normaly empty ,this vent/surge tank gives the fuel a place to go when it expands ,if the tank then fills the excess fuel is vented out of the tank in this case onto the floor !.

The tank also has a NACA duct and this via the vent surge system puts a positive air pressure into all the fuel tanks to assist the fuel to the engines in the case of failure of both the boost pumps.

[gas path]

With a high outside air temp. and wings full of fuel it will expand with the heat, the fuel has got into the vent system filled the surge tank and spilt overboard. More than likely there is a broken vent valve in one of the tanks.

damn ....beat me to it :)

[ 16 January 2002: Message edited by: gas path ]</p>

[QAVION]

"Is it normal for 747's to dump fuel onto a taxiway or was this a potential disaster that failed to manifest itself?"

Any fuel spillage is cause for concern, both for environmental concerns and for safety reasons. If there was a continuous flow, then this suggests that something was amiss.

There is an air vent system fitted to 747 tanks which allows for two things:

1. a way of getting rid of air when the tanks are being fuelled. If the air (normally) above the fuel is vented overboard, then it is easier to fuel the aircraft... i.e. there is less back pressure.
2. a way of providing a positive air pressure on top of the fuel when the aircraft is in the air. Air is scooped up from outside via a Naca scoop into the venting system and it is used to pressurise the tanks. A small positive air pressure in a fuel tank allows for better fuel feed and, I believe, helps prevent fuel vapour building up above the fuel (i.e. there is less risk of an explosion).

A network of vent pipes links the fuel tanks to small "Surge Tanks" in the wingtips. Although float valves are fitted to the ends of the pipes (in the tanks) to prevent fuel entering the pipes and flowing to the Surge Tanks, fuel sloshing around in the tanks, say, during taxy, may get past the float valves. Small quantities of fuel which get into the surge tanks simply drain back into the inboard tanks via different pipes (with non-return valves in them). However, if large quantities of fuel get into the Surge Tanks, eventually a limit is reached and the fuel spills out of the Surge Tank onto the tarmac (or wherever) via the Naca scoops.

If, on the other hand, the fuel being dumped from your aircaft was coming from the Fuel Jettison pipes on the trailing edges of the wings (towards the wingtips), then you have, perhaps, even more serious problems.

Rgds.
Q.

[boofhead]

Same system. I try to stop my FO from setting the dump fuel during preflight on the 744 because it is a. not in the book, b. not necessary, c. a worthless procedure since any dumping after takeoff will take 45 minutes or more and so an early dumping is a waste of time, d. dumping cannot take place until the flaps are up (ar at least above flap 1), e. it is not a recall action and the setting of the dump figure is part of the checklist procedure (and you don't know whether you will dump to max landing weight, min landing weight or some other value determined by the emergency you are handling), and, e. (the one I want your advice on please) if there is a fault in the dump system, any residual fuel in the dump lines might leak onto the tarmac.

I am amazed that so many pilots I fly with think that setting the dump figure is necessary. Calculate it, by all means, but why set it?

Apologies it this is off topic, but some of the replies here show that there is knowledge out there and I want it!

[TheGreatLeslie]

Addidional information for those that want to know.

I don't believe that this was a simple dump process. 'cos
a. The fuel flow was consistent and heavy until the flow was dispersed into spray as the a/c gathered speed on the runway.
b. On arrival at Singapore, maintenance crews decended onto the aircraft.
c. We did not make the next scheduled leg to Brunei instead passengers for Brunei left the a/c and we had to wait for the Brunei-Oz passengers to arive at Singapore.

[Cornish Jack]

Boofhead
Allowing that the Capt can decide exactly what he wants his FO to do or not do, may I make a point or two about your post.
a. Not in your book perhaps but certainly Birdseed used to (maybe still do?) preset 40 tonnes as Fuel To Remain during the pre-flight.
b. Discuss with the Continental(?) Classic crew out of Gatwick re. the F/E's instant fuel dump after engine loss and the ground clearance crossing Russ Hill !!
c. See above !!
d. Not so - it CAN take place once above Flap 10 (2&3 X-feeds re-open); is ADVISED not to be done between F5 and F1 (for obvious aerodynamic reasons)and the jettison gallery opens with either of the jettison controllers selected.
e. Possible, maybe, but with the nozzles where they are, I would have thought it would need a fair head of fuel in the gallery.
However, see opening sentence <img src="smile.gif" border="0">
One point in passing, re. pre-setting the Fuel to Remain - Having set the amount and switched the controller selector to OFF, please bear in mind that the Fuel to Remain selector is STILL ACTIVE, even with the Jettison selectors OFF. In other words, if someone moves it, unintentionally, the Fuel to Remain figure WILL change.

[boofhead]

Never ceases to amaze me how so many people build these little platforms and climb up on top, claiming to know more about Boeing airplanes than Boeing does, but I guess I will never be able to stop that.

Boeing does not have the procedure in their manuals and do not teach it.

The Classic FE calculates the figure, he does not set it.

In both airplanes there is nothing to stop the FE/pilot from starting to dump immediately after takeoff, it does not matter what the residual is set to. There is plenty of time to change the figure once dumping has started, since it takes a long time to get down to any figure that may be already set.

The amount of fuel that can be dumped in the first few minutes is not significant and will not affect performance, but will present a potential fire source, especially if the engine is burning. In fact, if you have a fire you might choose to not dump and land overweight. Dumping immediately after takeoff is a "feel good" exercise.

I tried to continue dumping one time in the sim while setting flap, and the dump stopped as the flap went past 1, started again after reaching 5. Dunno if the real airplane does the same. There is a good reason for not dumping with this config, something to do with airflow over the flaps. Maybe it sucks fuel into somewhere you may not want it to go.

Good luck and thanx.

[ballpoint]

Leslie and others,

Fuel leaking from a surge tank vent is in 90% of the cases caused by overfilling one of the main tanks.
This can be caused by expanding fuel during a hot day, by towing the aircraft and 'rocking'the wing or by a malfunction in the volumetric topoff system.
In the other 10% of the cases it is likely to have a mechanical failure in the fuel distribution system inside the tanks, for example a ruptured fuel feeder tube.
As soon as you switch on the pumps you get a fuel transfer from one tank to another, causing an overflow.
Normal procedure is to return to the gate, have the surge tank drained (takes 30 minutes), switch on the pumps again for 15 minutes or so and see what happens.
If the fuel starts flowing again you're grounded for at least a day or two.

Regards, Ballpoint, LAE 743/744.

[ 20 January 2002: Message edited by: ballpoint ]

[ 20 January 2002: Message edited by: ballpoint ]</p>

[Dan Winterland]

Boofhead, I doubt there is any pilot in this country who knows more about the 744 than CJ. He did my groundschool last year, truly encyclopaedic! And because something isn't stated in the manuals doesn't necessarily mean it should or shouldn't be done. The manuals were written with lawyers sitting on the authors shoulder - a quick read of the Boeing disclaimer in the leading pages stating to the effect that it is assumed the pilot of a Boeing is expected to have a sound level of knowledge and skill and use his judgement accordingly tells you that.

My company's last 5 744s delivered have a 'Max Land Weight' selection on the dump selector. No maths involved - a brilliant idea.

As for the manuals being scetchy, a good example of this is the omission of the fact the VHF radios can have the squelch facility overidden by pressing down and holding the select switch on the relevant RTP - useful in areas of weak VHF transmissions.

. .Dan W. A member of the Cornish Jack fan club <img src="smile.gif" border="0">

[boofhead]

If it is a good idea then Boeing will know about it and incorporate it in the manuals. That they do not take up the vast majority of changes introduced by the airlines shows that the changes are of no benefit. Dumping fuel, as I tried to show, is not a Recall action and there is always plenty of time to do it using the QRH. The level set will always vary, and choosing the max landing weight is only one of the choices. I am still waiting for someone to shoot me down over the possibility of inadvertant loss of fuel on the ground while setting the dump value.