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.

More Drawings
 

Courtesy of Jetdoc, here are some more drawings.

These are not specifically for the B777, but from other Boeing types, but they are probably very similar to the 777 variants.

Click the thumbnails for bigger images.

1/ A Typical auto-sumping installation, or water scavenge system.

http://panchromat.org/.misc/B777-doc...llation_th.jpg


2/ A typical Boeing vent system

http://panchromat.org/.misc/B777-doc..._system_th.jpg


3/ A typical boost pump and override/jettison pump installation

http://panchromat.org/.misc/B777-doc...llation_th.jpg


Thanks again for making these available, also to Jet II and all the others who post diagrams, images and other information!

Bernd

B777fly

From the B777 AMM
A discharge check valve is installed in the pump housing for each fuel boost pump, override/jettison pump and jettison pump. The discharge check valve prevents the flow of fuel back through the applicable fuel pump.

I wish I could show you a good schematic. The B777 AMM is not like the previous AMMs that Boeing has produced and really good schematics are lacking.

Thank you for the illustration of the (non B777) tank pump. Can JetDoc or anyone produce the same for a B777 Main Tank Jettison pump?

Bsieker

Thanks for posting those drawings. I hope they give some people an understanding of the components in the fuel system. They give some details of what the pump installations, pickup points and ejector pumps look like.
Also, I wanted to add that the vent system runs along the top of the wing.
Just out of curiosity, I was wondering how long damage from cavitation takes to become noticeable. Are there any pump experts out there?

Chris Scott,

3db would be happy to meet the aforementioned PPRUN’ers in the UK. Do you think we would increase the useful knowledge on the subject, remembering it’s the AAIB who have all the real information? That said, a face to face meeting may prove valuable for the “shadow AAIB”. Chris, I also hope it (EMI) is not still awaiting investigation by the “real AAIB”.
Regards
3db

JetDoc, my last entry crossed with yours. If the main tank jettison pumps do indeed have NRVs, I retire my argument.

One last question: are the NRVs fitted to these pumps on G-YMMM? Worth a look, maybe. It might have been Friday at the factory...

B777fly

The drawings I gave to bsieker represent typical installations on Boeings. They are actually from my B757 notes. If you look at the pump installation, thats how the jettison pump would look as well and I doubt that they could forget the NRVs.

I forgot to add that the jettison manifold also serves as the refueling manifold. A missing NRV would be noticed.

Fair Use of Copyrighted Materials
 

Thanks to all who have posted photos, diagrams, images, excerpts, reports, articles, etc., from a wealth of varied sources. It is vital to the quality and progress of our discussions and debate.


Even the most litigious nation allows for such fair use of copyrighted materials:
.
.

From the U.S. Copyright Act of 1976, 17 U.S.C. § 107

"... the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—

1. the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
2. the nature of the copyrighted work;
3. the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
4. the effect of the use upon the potential market for or value of the copyrighted work.

.
.
Let's keep the excellent information and discourse flowing!

-=MachacA=-

Bienvenue
 

Quote from Bis47 [Feb26/21:42]:
More likely, the flow fuel restriction problem was there well before the demand for higher engine output occured on short final. Maybe the fuel flow started to be restricted (cold related - any kind fuel of contamination, make you choice : ice, wax, bioslush ...) much earlier in the descent. The restriction built up slowly, somewhere between the LP pump in the wing fuel tank and the engine HP fuel pump. In a very cold spot? In a bend? Idlle fuel flow and very low temps at TOD might be a factor. Everything almost symetric (same cause, same effects). The restriction went unnoticed as long as only minimum fuel flow was required. Then ... when more fuel was required, i-e when the valves downstream the HP pump opened, the engines initialy accellerated, burning all the available fuel in the line, until the fuel flow became regulated by the fuel restriction upstream.
[Unquote]

Bien ecrit en Anglais, Monsieur/Madame... I certainly would not want to test my French in argument on this forum.

The problem we all have is that we do not have access to the FDR/QAR. :{ And the AAIB is not telling us if thrust above idle was used at any time during the descent. All they have said is that "the A/C entered the hold at Lambourne at FL110; it remained in the hold for approximately 5 minutes, during which time it descended to FL90."

So they descended only 2000 ft in 5 mins (average of 400 ft/min), presumably at a speed below 250 kts. At that speed at idle power, you would expect, say, 2000 ft/min. But we don't know if they were slowing down at the same time, in which case they might not have needed extra thrust.

What happened after they left the hold is also for us to guess. But at LHR at midday it is fairly unusual to fly at idle thrust all the way from leaving the Lambourne hold to the intercept of the ILS glide-slope. [Once you are on the glide-slope, you have a better chance of avoiding thrust above idle, as you are - to oversimplify - generally slowing down.]

To sum up: we don't know.

Chris Scott

Good post Chris.

But there is some room for some presumption

If the missing info (FDR/QAR .... etc.) was to have confirmed any speculations along these lines then surely the AAIB/Boeing would have acted on it by now in a heads up to the operators.

back to reading between the lines

Jetdoc, I had forgotten that the jettison manifold also acted as the refuel manifold. If there are NRVs in the pumps at the tank end of the manifold in each tank, where does the fuel get into the tank? Does it enter the tank anywhere near the tank pump inlets? I am still thinking in terms of open fuel jettison nozzles allowing air into the tank pump inlets.

Quoting 3db, post #439:
"An electromagnetic pulse may have caused the logic states to reverse, at least until everything re-set itself – however, I would have expected that to be recorded in the FDR somewhere or some other anomaly would make the AAIB think “what caused that reading” on the FDR?"

Fuel control switch position is a DFDR parameter. If spar valve position is a DFDR parameter i don't know. If it is, then it would be interesting to know which position is recorded. Is it the moment the valve is in transit from the open to the closed position or only when fully closed?

If only recorded when the valve reaches the fully closed position but not when in transit, then there may be no record of the valves moving to a partially closed position if reverse logic lasted shorter than it takes the valves to fully close. At the moment that the system re-sets it takes an equivalent amount of time for the valves to reach the fully open position again. For example, if a valve moves from fully open to fully closed in say, 15 seconds but the reversed logic lasted for only 12 seconds (valve almost closed) it would have meant the valve was in transit for a total of 24 seconds in which fuel flow to the engines would have been restricted.

Would such an unscheduled (perhaps only partial) closing of the valves generate an EICAS status message? When reviewing the system logic regarding the spar valve fault indications and projecting this on the condition of reversed logic to the control relay, i don't think so (see AMM: Functional Description of Engine Fuel Spar Valve - Fault Indications).

Next question to answer is, and i stress again it is only theory, if EMI would have corrupted the spar valve control system, how much transmitting power (radiated field emission) would be required to make a 28 Vdc relay move from run to cutoff position? I estimate that to be very large.

Regards,
Green-dot

Quoting 3db, post #439:
See previous post, somehow posted twice.



Regards,
Green-dot

Jet II,

Thanks a lot for the diagrams!

Is the in-engine fuel flow diagram actually for the RR engines?

I've seen different simplified schematics, one for the GE90 engine, which corresponds to this one:

LP pump -> FOHE -> Filter -> HP pump -> FMU -> Engine Valve,

as opposed to another simplified schematic posted earlier, which has:

LP pump -> HP pump -> FOHE -> Filter -> FMU -> Engine Valve.

Can anyone definitively say which is correct for the Trent engines?


Bernd

blocked vents
 

First post on R&N. Son of RB-211 chief engineer. Suspect spotter, but not stupid.

Blocked vents are a problem in cars and motorbikes. First hand experience. However, a boat experience leads me to suggest that evidence of this would have been immediately available to the AAIB.

Brand new boat fitted with a £30 freshwater distibution pump. Unfortunately the tank breather had tape over it. When we ran out of water I went to refill the tanks. Wouldn't take more than a few gallons into at least 40 gallons capacity. Investigate. Two stainless tanks almost completely crushed. Amazing what a cheap pump can achieve. Bet a 777 pump can get closer to 0 atm than a cheap ITT pump.

The tank structure is unlikely to to have been designed to cope with 'negative' pressure, and would display obvious signs of such.

Chris Scott
If I'm correct, there is effectively already a 250kt speed restriction 12 before LAM, so I don't see a sufficient speed excess potential that could prevent any additional thrust requirement somewhere in that hold ... but not a word from AAIB !?

It’s what they said on Jan 23, but in the initial report from Feb 18 they were more cautious:
“the total fuel on board was indicated 10500kg” and even if “both of the eng spar valves were found to be open, allowing the fuel leak evident at the accident site” they avoided to mention how much fuel they drained from the main tanks.
I don’t know which rate a spar valve leaks but it must be measurable and an estimated leaked fuel amount must have been determined … but not a word from AAIB !?

BA pilots kind enough to answer
For a typical PEK-LHR T7 flight plan:
- What is the planned fuel allowance for taxi ?
- Is it correct to say that planned arrival fuel is departing fuel minus (planned taxi fuel + planned en route flight fuel) ?
- Which quantity or en route percentage is allowed to reserve fuel ?
- What does include the planned arrival fuel at LHR (in this case 6900kg) ?

AAIB … it doesn’t hurt asking
What was the planned en route time ?
What were the planned and actual ZFW ?

The "tank structure" is the wing, and as has already been said, there is a relief valve as well as a vent, furthermore, there would have been a serious number of messages relating to low fuel pressure, which apparently did not happen.

To try and distill this matter to avoid lengthy repetition of questions answered pages ago:

1. Occams razor then suggests that unless the fuel pressure sender, EICAS, DFDR etc. was broken or incapacitated, there was sufficient fuel pressure at all relevant times.

2. If this is true, then that only leaves the supposition that unless nozzles or pipework upstream of the sender were blocked, that what was sprayed through the nozzles into both engines was something that produced less temperature/pressure (and hence less power) than what it was supposed to produce.

I do not know enough to speculate more, but I would like to ask someone with actual knowledge of the aircraft and its engines two things:

(a) Is engine fuel flow recorded? Where is the sender, and at what time interval?

(b) Is turbine inlet temperature/pressure (or similar parameter) recorded and at what interval?


If the fuel flow responded according to autothrottle and pilot inputs, yet turbine inlet temperature/pressure did not, then absent any other possible cause, what went into the engine was not pure Jet A1.

And that of course raises the question of the probability of both engines being affected virtually simultaneously.

As for EMI, I'm not an electronics person, but I would have thought that EMI capable of affecting one part of the aircraft system would also have affected a hell of a lot of other systems, and the damage would have been obvious.

Over to the speculators until we hear more from the Board.

Quoting Sunfish:
"As for EMI, I'm not an electronics person, but I would have thought that EMI capable of affecting one part of the aircraft system would also have affected a hell of a lot of other systems, and the damage would have been obvious."

I would have thought so as well . . . .

But as i have addressed before, i have been confronted with a situation where an aircraft type encountered unscheduled closing of the fuel shutoff valves without damage to any of the other electrical or avionics systems on the aircraft. I admit, it was in the early years of FBW technology, yet it happened and was later corrected, hence the introduction of an EMI filter to the valve assembly.


Regards,
Green-dot

Is anyone claiming that the crew opened the thrust levers to max and kept them there? This is one thing I would have expected to read if it actually happened. If It did not happen, all the speculation about engine access to fuel is diverting attention away from whoever would be responsible for ensuring it did.

autoflight
 

The engines initially responded
but, at a height of about 720 ft, the thrust of the right engine
reduced. Some seven seconds later, the thrust reduced on
the left engine to a similar level. The engines did not shut
down and both engines continued to produce thrust at an
engine speed above flight idle, but less than the commanded
thrust. The engines failed to respond to further demands
for increased thrust from the autothrottles, and subsequent
movement of the thrust levers fully forward by the flight
crew


it's in the report.