snanceki

What prevents the CWT pumps from restarting due to aircraft attitude change once they have initially shut down due to the 900kg limit being reached. Presumably the answer is nothing.

Thanks for your reply Swedish Steve which of course totally destroys my argument.
However, elsewhere I have read that the CWT pumps are controlled automatically by a "float" valve. Maybe the float just initiates the warning.
Not wishing to doubt your input (I'm sure you are more informed than I) BUT...are you sure!
Why wouldn't such an operation occur automatically even if some form of optional manual override exists for emergency management.
Hmm!!

bsieker

snanceki,

The scavenge pumps have float-operated shutoff valves, to keep them from pumping (a) out of a full centre tank and (b) into a full wing tank.

The boost pumps have no such valve.

tanimbar

Warning: I'm non-professional; not crew, not engineer - just guest here, thanks.

I'm back, not to bang-on about fuel stratification, but to ask a question of those who know.

An article on smartcockpit, written by an Airbus employee, on Low Fuel Temperatures has a table listing the Minimum Inlet Temperatures for engines from various manufacturers. RR has temp of 3C (This figure seems generalised for RR).

The question, for those that really do know the answer, is what happens next if the fuel heat exchange systems cannot raise the fuel temperature to, or above, 3C?

Thanks in advance.

Regards, Tanimbar

jackharr

Has the AAIB indicated when the next interim report will be published? Are Boeing saying anything?

As someone said recently in this thread, unless there is some satisfactory explanation in the near future, the public's confidence in Boeing equipment and/or British Airways will be seriously eroded. Maybe little will happen until the share prices fall significantly and only then will the shareholders ask the real questions.

Jack Harrison

Jetdoc

Quote:
When you shut an engine down using the spar valves it runs for 30-50secs on the fuel in the pylon, so from the boost pimps it will probably run for about 2 minutes.


I fully disagree with this. As I stated previously in my post 378, the main tanks have fuel pump bypass valves which you will find on any Boeing aircraft if anyone has access to schematics. The engine driven pumps are capable of suctioning fuel out of the tanks. Shutting off the fuel tank pumps will not shutdown the engines. Furthermore, the scavenge pumps take forever to move the fuel. They hardly would have created a problem of flooding the tanks with water and not only that, jet engines are capable of operating with a certain percentage of water in the fuel at the optimum icing conditions. The airframe side of the fuel system must supply 100% of the required fuel flow(vapour free) to a gas turbine engine under all normal operating attitudes. These are legal requirements in the engine and aircraft design. Additionally, as I stated before, there would have been several cockpit warnings related to the fuel system that may have given the crew notice that there was a fuel problem. Surely these faults would have appeared in the central maintenance computers but not necessarily the DFDR or QAR.

wilyflier

So if they might run on a pipe full of fuel for up to 2 minutes, what cockpit actions were taking place 2 mins before touchdown?Say around 1500ft?

Add ref post 422

Jetdoc

This is from a previous post I made:

I am not sure if it has been mentioned before, but in addition to the 2 fuel pumps in each main tank, there is a pump bypass valve in the left and right tanks that allow the engine driven fuel pump to suction fuel from the tanks in the event the boost pumps are not working. I believe that this situation would be highlighted by a level B ENG FUEL PRESSURE warning which includes an audio warning.
Additionally, if the boost pumps were not providing flow, they would also give a similar LOW PRESSURE warning. And if all that ice was heading down to the engine, I am sure that the FUEL FILTER clogging warning would have come on.

I can't imagine anything else going on in the cockpit prior to touchdown under normal conditions. The aircraft would be already configured for tank to engine feed (legal requirements) long before the approach and the crossfeed valves would or should be shut. The crossfeed valves are not normally operated on the B777 other than to correct an inflight fuel imbalance.

Baron rouge

more probably because the autopilot coul not anymore follow the glide path, the A/C beeing fully trimmed.

avrflr

From the report:
"On examination, both of the engine spar valves were
found to be OPEN, allowing the fuel leak evident at the
accident site."

Can someone please explain how a fuel leak occurs if the spar valves are open? What path does the fuel take and where does it exit?

Perhaps this could help advance the theories involving water in the fuel, as it may help to explain how the fuel system was "purged" of water.

taufupok

If the fuel boost pumps were still running , the fuel control switches were still in the run positions and also if the engine fire switches were not pulled, pressurised fuel in the fuel manifold and lines can leak through when these were punctured or severed in the crash. If the passenger evacuation drills were accomplished with power still available, the spar valves would close but fuel could still leak from the manifold prior to the spar valves.

wilyflier

Jetdoc,
You say you"cant imagine anything going on prior to touchdown" before landing .I cant believe you fly.I`m talking about all the check list items ,many of which are done without a physical cross check by the other pilot.
Ive seen enough errors and ommissions to know they can happen with familiarity, tiredness or stress or even lack of knowledge* .We know for a fact that one such error took place the during the close down checks* .( no criticism implied here)
I dont know the standard 777 check lists and SOP, but sure as hell some change which occurred took place on this flight ,either in technical or flight conditions or in standard actions which triggered the whole sequence of double failure .
Think of ALL the things that happened in the whole flight and before. We can only pick them out a few at a time, there must be thousands.

NSEU

The spar valves are contained within the wing tanks Any leaks prior to the spar valve would possibly see fuel flow back into the wing tanks.

The leakage would have occured after the spar valves, perhaps in the strut area, prior to the fuel pumps and FMU: The FMU has a high pressure shut off valve, so if this was closed, the leakage would have to be between the spar valve and the HP shutoff. Even if the FMU HPSOV was open, the fuel would have a bit of a stuggle to get throught the engine pumps, especially with the tank pumps off and the engines not turning.

Rgds.
NSEU.

Jetdoc

Quote:
Jetdoc,
You say you"cant imagine anything going on prior to touchdown" before landing .I cant believe you fly.

Actually, you are right. I do not fly. I am an aircraft maintenance instructor. When I mentioned that I can't imagine anything going on prior to touchdown, I was referring to the fact that the fuel panel would not be disturbed under normal operating conditions at that point in the flight. Actual crew procedures and matters concern the flying of the aircraft are best commented on by pilots themselves.
I am only presenting some basic facts about the fuel system and possible warnings that could have occured.

PBL

The 8086 was a 16-bit architecture in which all processing and all data channels were 8-bit. It means you had to do everything twice: once for the lower half and once for the upper half of the 16 bits. The 80186 had full 16-bit data paths and processing.

The option to develop a "mathematically correct" chip was there only in theory. To find out what happened when the Brits tried to do that, Google on "Viper RSRE". You'll see Brian Randell's submissions to Risks about what happened commercially with the Viper chip at
http://catless.ncl.ac.uk/Risks/10.15.html#subj4
http://catless.ncl.ac.uk/Risks/11.73.html#subj1
and you can follow the story by searching through earlier Risks issues.

The points are that (1) you have to get it right; (2) other people have to believe you have gotten it right; (3) you then have to fabricate and sell quite a few of them. Fabricating a special chip to put in a few thousand Airbi was not then a commercially viable proposition, and a risky proposition even if the entire development costs were written off (read: government subsidised). Things are somewhat different now.

There is, however, a problem with supply. It costs a lot of resources to keep chips in production over decades, and manufacturers don't do it, with processor capabilities doubling every 18 months (Moore's "Law") and SW writers making full use of it.

Parts of FAA ATC ran until recently on mainframe computers that were many decades old. The manufacturers had stopped supporting them; the FAA had to build its own fabrication labs, and then it had a hard time keeping the boffins around who knew how to fabricate and repair the processing elements after retirement age. It became a crisis on which the NTSB produced a special report.

I know one major ATC en-route system designed in the 90's (weren't they all?) based on banks of commercially-available desktop computers. For a number of years, the service provider has been stockpiling replacements by buying up all stocks of these machines wherever they surface in the world (ebay and so on). This business of keeping processors around for the projected lifetimes of aviation systems is not a trivial problem.

PBL

Swedish Steve

NSEU
The leakage would have occured after the spar valves, perhaps in the strut area, prior to the fuel pumps and FMU: The FMU has a high pressure shut off valve, so if this was closed, the leakage would have to be between the spar valve and the HP shutoff. Even if the FMU HPSOV was open, the fuel would have a bit of a stuggle to get throught the engine pumps, especially with the tank pumps off and the engines not turning.

Look at the state of the engines. They were badly damaged. The AAIB report specifically mentions that although the spar valves were turned off they did not close because the wiring was damaged by the undercarriage failure. So the fuel in the engine feed lines was leaking out through the damaged engines onto the ground.

Spaz Modic

Notwithstanding the B777 fuel system having been rebuilt several times over, the odds are still, it ran outa gas

cats_five

The AAIB say there was 'adequate' fuel on board. Are you failing to be clear about if you mean the tanks were empty or fuel wasn't reaching the engines, or do you think the AAIB are lying?

overthewing

The engines didn't stop, therefore they were still receiving fuel.

whiowhio

Quote:
Notwithstanding the B777 fuel system having been rebuilt several times over, the odds are still, it ran outa gas

Fuel STARVATION as opposed to fuel exhaustion

PAXboy

JackharrI doubt it. I really would not expect that to happen - and I am generally a pessimist where these matters are concerned!!

1. No deaths.
2. It was a 1st world airline with an extremely high safety record.
3. The public often have zero idea what type of a/c they are on. Any wide body can automatically be dubbed a 'jumbo'.
4. Boeing has, in the eyes of the public, a fine safety record. Did they lose any business over the 737 rudder problem?? In that series of accidents (some of which, IIRC, are not fully explained) many people died.
5. Post crash, the TV carried reports from smiling pax saying 'I did not realise that we had crashed'.

That is not to say that some pax will be concerned but the stats of 777 operations over the past decade, make it clear that this is an unusual and rare event. To those that take an interest, that makes it even more worrying but, I suggest for most pax? Not a jot.