infrequentflyer789
 

Taking the literal defition of jumbo as simply something that's big, and the 777 is by definition a jumbo jet. If they referred to it as a 747 that's a different matter ;)

Just out of interest, when did "the jumbo jet" become a standard phrase to define the 747?

I have some drawings but I have no way to post them here. Anyone who has a means of posting them is welcome to them.
The vent system has more than one channel on large transport aircraft and it is standard for the Boeing aircraft to have a pressure relief valve in the surge tank that works to relieve positive or negative pressure in the wing. This valve can be seen inboard of the NACA duct at the wingtip.

The pumps do not pressurize the tank. The fuel tank pumps separate the fuel and air and supply a positive flow of fuel to the engine. The engine fuel feed manifold is the only thing that is pressurized by the pumps. The vent system gives a slight pressure to the fuel in the tanks to minimize fuel sloshing and aeration.

Vacuum
 

Spot on NSEU. A relatively simple theory usually proves to be the correct one. Forget about all these shlush theories or EMI/ESP etc. The only problem is that we are unlikely to ever find out for sure.
Awaiting to be shot down in flames together with you.

Cheers,

Jerry B.:)

Any help?



http://img260.imageshack.us/img260/3...fuelpw3.th.jpg

Terrific - a great help, but do you have the bit that goes from the Engine Feed Manifold to the pylon or aircraft / engine interface? And what happens to the left of the spar valve?

Nevertheless, a great leap forward - Thanks

Can't find any reference to vent negative pressure relief regarding the surge tank vent valve in the Boeing 777 Maintenance Manual D&O. Is it mentioned in the (proper) AMM? Wouldn't the float valves normally provide negative px relief?

777FLY
the only other way in which the fuel system can be open to the air is via the fuel jettison nozzle valves. If these were to somehow be signalled to open, there would be a large diameter pipe, open to air, which is routed via the override/jettison pumps into the same area as the wing tank pumps.

Sorry to disappoint you but the Jettison system does not use the engine fuel feed lines. There is a Jettison pump in each wing tank that pumps fuel into a Refuelling/Jettison manifold. At each end of this is a jettison nozzle.
To Jettison from the centre tank, the normal centre tank pumps must be switched on manually (if not already on), then when jettison is armed two jettison isolation valves open to allow the centre tank pumps to pump centre tank fuel into the jettison manifold. So to get air from an open jettison nozzle into the centre tank the jettison isolation would need to be open as well.

NSEU
Can't find any reference to vent negative pressure relief regarding the surge tank vent valve in the Boeing 777 Maintenance Manual D&O. Is it mentioned in the (proper) AMM? Wouldn't the float valves normally provide negative px relief?

AMM 28-10-00 p17/19 shows the surge tank pressure relief valves.
Boeing says..If a pressure difference opens the valve.. I assume this means either way.

By the way, posters that ask for details of the fuel system. The Boeing manuals are copyright and I am not posting them here.

JerryB:
Indeed an interesting theory. In my last post I suggested a vacuum created by fuel restriction (waxing) could have been the reason for the cavitation, but as the actual fuel freezing temp was -57°C, it seems more logical that the vacuum was caused by another factor, such as a stuck pressure valve... But then again, in both main tanks at the same time... ?

How about the 'loose union' mentioned in the report? could it have introduced air in the fuel system?

The engine feed is pretty basic and I dont believe that the investigators have found anything wrong with the system.

http://img442.imageshack.us/img442/4...gineug1.th.jpg

That is the main feed down to the left engine

A couple of points:
1. The pressure relief valve is illustrated in the pressure refuel ref AMM 28-21-00 or storage ref AMM 28-10-00.
"An open pressure relief valve is a symptom of a blocked vent scoop or flame arrestor.
The pressure relief valve can also open to relieve air or fuel pressure if there is too much pressure during refueling."

2. Float valves in the vent system block the vents from filling with fuel as the fuel level increases and drain the vent channels of fuel that gets in there.
The only air getting in the tank is from the vent system and this is normal.

3. The jettison manifold is connected to the eng feed manifold thru the jettison manifold isolation valves. It is not open to the tank itself. The main tank jettison pumps feed the jettison manifold itself and the center tank override/jettison pumps feed the jettison manifold through the engine feed manifold through the isolation valves and into the jettison manifold.

We don't know
 

JerryB:
Not a particularly unusual post on this long long long thread JerryB, and I don't mean to pick you out in particular, but paraphrasing what it you (and lots of other posters) say "We don't yet know the answer, we may never know the answer, but I have an idea in my head and I want to dismiss all the other possible answers".

AAIB has given NO indication in any of their reports as to the cause of the [in/ac]cident. They have noticed a safety issue regarding engine shutdown after the event, and have issued advice. They have noticed cavitation on the pumps, but also said "The manufacturer assessed both pumps as still being capable of delivering full fuel flow." (page 4 of S1/2008). They have noted all computer/control systems appeared to be functioning normally. In short they don't know (or at least haven't given any opinions in the publicly available reports).

Occams Razor (simplest explanation is the correct one) does often apply, but in safety investigations - particularly those where the cause is not found quickly - it is often a case of multiple events leading to the outcome.

If we are going to have a discussion about the possible causes, of which the fuel flow discussion is highly valuable, then let's talk about all the possible causes, unless there is evidence to the contrary.

So please let's all stop all these "let's all stop talking about X" posts! :O

(and yes, the oxymoron of this statement was intentional in an effort to inject some humour, but the sentiment was genuine!).

Never assume, Steve ;)

Neither should anyone else.

Even if it did, it would only introduce air to one side of the aircraft ;)

NSEU

Quote:
"If a pressure difference opens the valve.. I assume this means either way."
Never assume, Steve

Yes good idea, but the B777 AMM are very sketchy on a lot of descriptions, and its all I have to go on. I would have thought that if it only opened on surge tank overpressure, it would have said that, and not pressure difference.

By the way the B777 B1 course notes are lifted 100pc from the AMM Pt 1. So no good looking in there!

Thanks JET11 for the fuel diagram, very helpful but notable that it does not show the position of the wing tank jettison pump. It is a fair bet that its location and pickup point is close to the fwd and aft boost pumps pickup points.
NSEU I am not suggesting at all that the jettison system can in some way get air into the fuel by surface mixing. What my suggestion is, is that with a jettison nozzle valve open, significant air could be drawn down the jettison manifold by the wing tank boost pump suction and introduced into the pump inlets. ( Provided the non-running jettison pump allowed reverse airflow)
MILT: Why don't you read the AAIB update? The pump damage was either due to a reduction in fuel flow, or fuel aeration, according to AAIB.
SWEDISH STEVE: I was not suggesting, in any way, the centre tank or its pumps are involved. I know that the jettison sytem is not part of the engine supply manifold but if you have one, look in the Boeing tech manual 12.20.07 ( jettison schematic) imagine the jettison valves open and see where air could go if all tank pump inlets are close together. The scenario I suggest is valid with wing tank to engine feed, cross feeds closed and the centre tank empty, with pumps off.
JETDOC: The wing tank boost pumps may very well filter air or vapour, but if a relatively huge amount of air was introduced at the pump inlets, the filter system could be overwhelmed and the result would be loss of fuel flow to the engine HP pumps.

Standing by for more flak, but I am simply suggesting a way that aeration of the fuel supply manifold could occur. I wonder if the AIMS or any recording system notes the jettison valve position, if the system is not activated.

B777fly

All of the boost and ovrd/jett pumps on the aircraft are centifugal pumps. The air and fuel separation occurs as a result of pump action. The inlet to the pump is screened at the pump pickup point.
The outlet of the pumps have check valves installed. Fuel goes out of the pump but nothing can return through the pump.

Jet II - Many Thanks for that - I'm sure it will be very helpful to all.

NSEU and Swedish Steve and anyone else with sanctimonious attitudes to essential material, please do try to get out a little more.

JetDoc,

Thanks for the tech info. I am not sure that the pump inlet screens would be relevant to my argument , but if the three wing tank pumps ( fwd/aft/jettison) all have a fuel AND airtight NRV at the outlet, my suggested possible scenario is invalid.

Why would the wing tank jettison pump need an NRV? Everything is going overboard anyway............

Can you show the schematic with the NRV in the line?

I don't think the AAIB reports have said that the cavitation observed was as a direct result of anything related to this incident. The cavitation damage could have occurred at any time.

Blues&Twos Cavitation 'could' have occurred at any other time? Maybe, but why would it during years of normal operation? If 2 engines lose power due to lack of fuel to the HP pumps, it is almost certain that the pumps were damaged, in this particular way, during this incident.