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rondun
18th Feb 2008, 14:56
A special bulletin has been released today by the AAIB:

here (http://www.aaib.gov.uk/publications/special_bulletins/s1_2008___boeing_777_236_er__g_ymmm.cfm)


And the update (http://www.aaib.gov.uk/publications/special_bulletins/s3_2008___boeing_777_236_er__g_ymmm.cfm) of 12th May 2008

Interim report 1 (http://www.aaib.gov.uk/publications/interim_reports/boeing_777_236er__g_ymmm.cfm) 09/08

Interim Report 2 (http://www.aaib.gov.uk/publications/interim_reports/interim_report_2___boeing_777_236er__g_ymmm.cfm) 03/09

Rolling-Thunderbird
18th Feb 2008, 15:14
Interesting 'snippets' from the report


"Detailed examination of both the left and right engine high
pressure fuel pumps revealed signs of abnormal cavitation
on the pressure-side bearings and the outlet ports. This
could be indicative of either a restriction in the fuel
supply to the pumps or excessive aeration of the fuel. The
manufacturer assessed both pumps as still being capable
of delivering full fuel flow."

"Investigations are now underway in an attempt to
replicate the damage seen to the engine high pressure
fuel pumps, and to match this to the data recorded on the
accident flight. In addition, comprehensive examination
and analysis is to be conducted on the entire aircraft and
engine fuel system; including the modelling of fuel flows
taking account of the environmental and aerodynamic
effects."

shoey1976
18th Feb 2008, 15:29
http://www.aaib.dft.gov.uk/cms_resources/S1-2008%20G-YMMM.pdf

what does everybody think?
ian shoesmith
bbc news
[email protected]

shoey1976
18th Feb 2008, 15:42
can somebody give an explanation of what these valves are, and how they play an important role, please?

Hand Solo
18th Feb 2008, 15:43
I think the previous poster who suggested fuel starvation is reading something into the report that isn't there. There's nothing I can see in this report that gives an indication of the cause but it does rule some things out. As to the debris found in the tanks, it's undesirable but not uncommon or necessarily dangerous. Aircraft have been leaving the Boeing plant with bigger bits of FOD than those in the tanks for years.

Shoey - the spar fuel valve is in the fuel tank and the fuel passes through this first on the way to the engine. It then passes through various fuel pumps, heat exchangers and other paraphernalia before it arrives at the engine fuel valve, through which is passes into the engine to be burned. It would appear in this instance that there was a rupture somewhere between the engine and the spar fuel valve, allowing fuel to leak through the open spar fuel valve and out of the system even though the engine fuel valve was closed.

Feathers McGraw
18th Feb 2008, 16:08
So, with the exception of the HP fuel pump damage, which might be due to insufficient fuel supply or aerated fuel (or something else which escapes me just now) everything was working normally. The FOD in the tanks is probably not sufficiently large to block any of the exit screens/pipes, so what else could cause fuel starvation?

I can almost hear the sound of heads being scratched from here.

mono
18th Feb 2008, 16:17
Though I must say that every a/c I have worked on. Pulling the fire handles activates the LP (spar valves) and HP (engine fuel valves)

You could have knocked me over sideways when I read that they weren't on the 777!!!???

lomapaseo
18th Feb 2008, 16:21
Shoey - the spar fuel valve is in the fuel tank and the fuel passes through this first on the way to the engine. It then passes through various fuel pumps, heat exchangers and other paraphernalia before it arrives at the engine fuel valve, through which is passes into the engine to be burned. It would appear in this instance that there was a rupture somewhere between the engine and the spar fuel valve, allowing fuel to leak through the open spar fuel valve and out of the system even though the engine fuel valve was closed.


isn't this secondary to the ground impact and not in the causal chain of the accident?

Hand Solo
18th Feb 2008, 16:26
Yes it is secondary, although it does have survivability aspects for the accident. For clarification I wasn't intending to suggest they contributed to the crash.

BoeingMEL
18th Feb 2008, 16:30
............ at least all that garbage about EMI can be buried..... as can fuel exhaustion, windshear, finger trouble, Gordon Brown's heavies... (IMHO you understand) Cheers bm:hmm:

SILENT_BADGER
18th Feb 2008, 16:31
Yes it is secondary. The reason the report brought it up is because of the fuel that leaked during the evacuation.

The evacuation checklist aims to shut off things that could lead to fire or other dangers. BA's evacuation checklist has the captain switching off the fuel valves whilst the F/O does the engine fire handles which meant that if the F/O did his bit before the captain then the electric relay controlling the fuel valve would be isolated and the valve didn't close. So the report has issued a sensible recommendation that operators make sure their checklists do things in the right order. It also mentions that Boeing were already in process of changing all 777s so that the fire handles close the fuel valves as well as all the other disconnections so you wouldn't need to do switch the fuel valves separately.

Most other aircraft already have the fire handles/buttons doing all the isolation in one fell swoop already.

Cheers,
SB

booke23
18th Feb 2008, 16:50
The report also mentions a planned descent to FL315 due to "extreme cold"....which did not happen because of an ATC request.

Also mentioned was that later in the flight at FL380 over the Ural mountians the oat was -76. The met office decscribed this as "unusually low compared to the average, but not exceptional"

They seem to be considering the possibility of fuel icing, however the fuel never got below -37.

Puzzling indeed.

MungoP
18th Feb 2008, 16:53
What occurs to me when reading this report is just how intensive and detailed an accident investigation has to be.. and should humble some of the 'contributers' here who are repeatedly whinging about the lack of results... additionally, the AAIB is dealing with an a/c for the most part in one piece... just how awsome their task must be when dealing with scattered/burned wreckage begins to become apparant..

Hand Solo
18th Feb 2008, 16:56
I would hazard a guess that they had ruled out fuel icing based on the information in the report. The fuel never got lower than 20C above freeze point and I'm sure the inclusion of the information is to clarify that fuel freezing was highly unlikely to be a factor, just like they included the information about water in the tanks.

misd-agin
18th Feb 2008, 17:00
A/P kicked off at 175'. Airspeed down to 108Kts by 200' as the a/p tried to maintain the g/s.

Thank god the a/p kicked off.

nhs
18th Feb 2008, 17:04
MungoP

Couldn't agree more. I hope people remember also that this is not a complete investigation report, but one published rapidly due to public interest. The fact that they have already come up with advisory notices without finding a cause shows their attention to detail.
As a SLF I'm grateful they are there looking out for all of us.

I Just Drive
18th Feb 2008, 17:08
So the only manual flying part of the adventure was from < 200' then?

Mr @ Spotty M
18th Feb 2008, 17:21
Yes l was surprised about the Auto-pilot, not being a Pilot, l thought that you would disconnect ASAP, would the A/P keep raising the nose if it was not getting any more thrust.
I would like someone to explain if l am wrong please, l know hindsight is a great thing.

glad rag
18th Feb 2008, 17:22
Well thank heavens it went in on the undershoot. The combination of ruptured fuel tanks, ruptured O2 cylinders and an ignition source from contact with the runway are just too horrible to contemplate.

I Just Drive
18th Feb 2008, 17:24
Yes, the auto-pilot would try and keep the glide slope at the expense of the speed. (With no thrust)

infrequentflyer789
18th Feb 2008, 17:27
Yes it is secondary, although it does have survivability aspects for the accident. For clarification I wasn't intending to suggest they contributed to the crash.

I think this might end up needing to be clarified a lot :uhoh:, even though the report also specifically says "This was not causal to the accident". Unfortunately it says that in normal type right before the big highlighted box with the safety recommendation in.

Since this is about the only clear conclusion and the only recommendation in the report, I expect it will get a fair bit of press attention and become causal in some peoples minds.

In terms of the rest of it, I'm with the poster who commented on the sound of head scratching. Even the comments on the cavitation damage to the pumps don't sound very confident (as if it might turn out to be a non-issue).

One thing that suprises me slightly is that the report says essentially nothing about flight crew actions (beyond moving the thrust levers) - there's been lots of speculation about whether they changed flap settings etc., none of which is answered in this report. I wonder if they think the crew actions are just not relevant to reveal at this point, or if there is still some uncertainty in that area.

lomapaseo
18th Feb 2008, 17:57
I wonder if they think the crew actions are just not relevant to reveal at this point, or if there is still some uncertainty in that area.

One doesn't talk about the crew until you have exhausted all possible contributions by the hardware design and environment that the crew were operating with.

Flight Safety
18th Feb 2008, 18:01
Fuel pump cavitation is a complex issue, it may take a while to sort that one out.

Bearcat
18th Feb 2008, 18:04
from the accident report...

After the aircraft crossed the Ural mountain range in Russia it climbed further to FL380 where the ambient temperature dropped to as low as minus 76°C


What is the enviornmental envelope re min temps for the 777. I know the airbus 320 is -70c

Elastoboy
18th Feb 2008, 18:14
Please can someone help me with the type of HP fuel pumps referred to; centrifical or positive displaced (lobe) type.
Thanks

RMC
18th Feb 2008, 18:30
FS, I agree that establishing the cause of fuel pump cavitation may prove to be complex. The bottom line however is that the only way cavitation happens is if the local fuel pressure drops upstream. The pressure at which this occurs is always between 0 and -30 inches of mercury (-14.7 PSI Gauge).

This pressure gets closer to zero at altitude.It is normally associated with G/A (high fuel flow) situations.

As niether of the above appear to apply then fuel would only cavitate if a restriction in the upstream fuel supply developed. One of the problems with cavitation is that, once the trigger pressure is reached, your fuel supply remains an excited mass of air bubbles until pressures significantly higher than trigger pressure.

As has been said it may be that this evidence of cavitation is eventually found to be unrelated to the incident...who knows?

wwIIace
18th Feb 2008, 18:52
maybe nothing but has no one thought about the fact that TOGA wasnt selected, or was it? seems to me only the thrust levers moved forward. trivial maybe but with TOGA, alot of things are then triggered so maybe a change of tact for the computers etc etc?

Avionista
18th Feb 2008, 18:52
With regard to the evidence of cavitation in the area of the fuel pumps, as cited in the AAIB interim report:

"This could be indicative of either a restriction in the fuel supply to the pumps or excessive aeration of the fuel."

What could cause excessive aeration of the fuel, particularly when, by all accounts, it had been subjected to cold temperatures for an extended period of time?

misd-agin
18th Feb 2008, 18:59
I just drive - at 175' and 108 kts you're not doing much flying. AOA will be limited by the aircraft, you're in charge of roll, and that's it. Guessing that AOA limit, or stickshaker, kicked it off based on the wording of the report. It states the a/p "disconnected" without any inference of human intervention.

Mr Spotty M - Yes, a/p off and maintaining airspeed is critical. Below L/D speed(basically approach airspeed) gliding distance goes down rapidly, and at stickshaker speeds, or close to that, there will be no airspeed available to flare the a/c to reduce vertical impact loads.

Fargoo
18th Feb 2008, 19:00
HP fuel pumps on T7
Please can someone help me with the type of HP fuel pumps referred to; centrifical or positive displaced (lobe) type.
Thanks

"The gear-type HP pump gets fuel from the:
* LP fuel filter
* Servo return
* FMU bypass.
The HP pump supplies fuel to the FMU and servo fuel to engine
components."

From the 777 AMM for RR Trents.

zukini
18th Feb 2008, 19:03
They have just said "Damage to the fuel pumps caused the crash"

I'm certain I read in the report that the pumps would not stop working in this condition.

Hand Solo
18th Feb 2008, 19:11
Thats rather annoying given that on BBC1 News Tom Symonds was clear that the cause remained unknown. Perhaps Shoey will be along to comment.

bsieker
18th Feb 2008, 19:16
Mono, Silent Badger, ...

If all things are still in one piece, the fire handle does close the spar valve on the B777 (see, e. g. the diagram in Section 6.7 of the Flight Manual. I only have seen it for the GE90 variant, but someone posted the equivalent for the Trent in a previous thread).

The report is quite clear on that, just read it carefully. In B777 aicraft not modified according to SB 777-28-0025 there are two separate wiring paths to the spar valve, one from the cutoff lever, and another one from the fire handle. The fire handle, in addition, also isolates the wire path from the cutoff lever, so that the cutoff lever will no longer be able to shut the valve.

Hence the original Boeing checklist for evac and engine fire specified to close the cutoff lever first, and only then activate the fire handle. The BA checklist split the two tasks between the Cpt and F/O, thus creating, as we would say in computer science, a "race condition".

In the case of BA038 the path from the fire handle to the spar valve was damaged, and although the path from the cutoff lever to the valve was ok, it was isolated by the fire handle, which on this occasion was activated before the cutoff lever.

This left the spar valve open, which, in combination with other damage, caused fuel to leak from the aircraft, which, luckily, in this case, did not cause a fire.

With the desirable sequencing (cutoff first, then fire handle) the spar valve would have closed. Althought not causal to this accident, it would have reduced the risk of fire, and therefore merits the safety recommendation.


Bernd

Pinkman
18th Feb 2008, 19:18
I was surprised to see the reference in several places in the report to Jet A-1, my understanding was that Chinese aviation kerosene was called Jet Fuel Number 3 (used to be RP-3). I thought initially that maybe what they were trying to say was that it met at least the spec of Jet A-1 (with which the world is more familiar), but they bang on about Jet A-1 spec of -47deg C and the fact that the actual fuel had a spec of -57 deg C (which is more or less what you would expect from RP-3/JF #3). Unless of course its actually RP-3 marketed as Jet A-1. Doesnt really matter in terms of causation I suppose.

How can you tell if cavitation has occurred post event? Does it actually physically damage the HP rotor/vanes? I thought that once the pressure had been equalised the cavitation stopped and flow resumed?

The debris findings are interesting, particularly the red scraper under the right suction screen. Just supposing one wing fuel suction is suddenly inhibited, notwithstanding the fact that the autothrottle demanded an increase from both engines, in the configuration/mode that the aircraft was in, does the engine management software also limit the differential thrust between the engines. You can see what I am getting at - the seven second difference - could it be that the right engine was starved and the left engine reduced to match, under some limiting algorithm, independant of commands from the autothrottle.

I'm an oilhead so tell me if I am talking cr*p. It wouldnt be the first time.

Pinkman

zukini
18th Feb 2008, 19:22
"It was damage to fuel pumps that caused a BA plane to crash land at Heathrow last month. 136 passengers and the crew on board were safely evacuated"

Self Loading Freight
18th Feb 2008, 19:23
BBC newsroom has been contacted by a pal, and further bulletins should not repeat the fuel pump error (also heard on R4 here).

R

bsieker
18th Feb 2008, 19:35
How can you tell if cavitation has occurred post event? Does it actually physically damage the HP rotor/vanes? I thought that once the pressure had been equalised the cavitation stopped and flow resumed?

(To correct a possible misunderstanding first: this is about the high-pressure fuel pump, not the HP rotor of the engine proper.)

Cavitation is the formation and subsequent collapse of small cavities (fuel vapor, dissolved air, ...) in a liquid due to low pressure, e. g. on the suction side of a pump. The collaps can be so violent that it damages the material. It is one of the limiting factors in the design of ship propellers.

does the engine management software also limit the differential thrust between the engines.
The aircraft is designed to handle TOGA power on one engine and zero thrust (shut down/windmilling) on the other. The rudder will compensate. Also, the engine management of each engine is independent of the other.

Would be quite interesting during an engine failure at takeoff or go-around, when the computer would limit the other engine, too, depriving the aircraft of full power when it most needs it.

You can see what I am getting at - the seven second difference - could it be that the right engine was starved and the left engine reduced to match, under some limiting algorithm, independant of commands from the autothrottle.

No. On the contrary, the autothrottle, trying to maintain speed, would increase thrust on the other engine to compensate for the failed one.


Bernd

barrymung
18th Feb 2008, 19:37
"What could cause excessive aeration of the fuel"

AFAIUI, it can be caused by the fuel flow being restricted, especially to a fast running pump.

tiggerm0th
18th Feb 2008, 19:40
Here's my theory then:
(bearing in mind I have no knowledge of the 777 fuel system - but that won't stop the D*ily M*il from quoting it):

Some seals/joints were deformed during the extreme low temperatures earlier on in the flight. The seals however had lost flexibility and still maintaining a seal, or at least weeping only slightly. As the atmospheric pressure was low at altitude, the weeping would be from the fuel system to the outside.

As the aeroplane descended, the seals warmed up, regaining flexibility then flicked back, but this caused an inleakage of air into the fuel lines. The inleakage of air caused the pump cavitation observed.

By this point also the water that would have been thawing in the tank at this point - perhaps by putting the landing gear down it caused more condensation to drop from the walls of the fuel tanks.

Anyhow, going back to the air. As the air was drawn into the inlet side of the high pressure fuel pump, this induced sudden foaming exacerbated by the presence of the water in the fuel lines. This choked the fuel pumps and disrupted fuel supply.

Tiggermoth

If this had been the case, then the way to have recovered from the situation would have been to stop demand on the fuel system, allowed the fuel to fill the suction line, then slowly put an increased demand on the pumps while throttling back the discharge side. As the autopilot just put a greater demand on the fuel system, it just made it worse.

barrymung
18th Feb 2008, 19:41
BoeingMEL wrote: "............ at least all that garbage about EMI can be buried..... as can fuel exhaustion, windshear, finger trouble, Gordon Brown's heavies..."

To be fair, the report doesn't appear to rule any of that out...

Until the investigation is concluded anything is a possibility!

Fargoo
18th Feb 2008, 19:44
A fuel leak from the British Airways Boeing 777 which crash-landed at Heathrow could have engulfed passengers and crew in a fireball, a report has revealed.

Here was me thinking the Telegraph was a respectable newspaper :yuk:

golfyankeesierra
18th Feb 2008, 19:53
Reading the report, specially this part:
The airspeed reduced as the autopilot attempted to
maintain the ILS glide slope and by 200 ft the airspeed
had reduced to about 108 kt. The autopilot disconnected
at approximately 175 ft, the aircraft descended rapidly
and its landing gear made contact with the ground some
1,000 ft short of the paved runway surface
they owe a lot to keeping the autopilot on (and its design as well)!
That way all possible energy was used up to its full extend, bringing the aircraft within the airport perimeter.
I don't think a human could have done it better, probably not, as a pilot would have unconsciously lowered the nose.

Airways B
18th Feb 2008, 19:57
Can someone explain how 'evidence of Cavitation' in the HP fuel pumps is being considered here?

Is it that the pump is trying to pump from an obstructed source causing bubbles (cavities) in the fuel or that there is evidence that the pumps are physically damaged?

What will be the evidence of cavitation? assuming THIS (http://www.mcnallyinstitute.com/01-html/1-3.html) is a good explanation of such?

:confused:

Paradise Lost
18th Feb 2008, 20:00
As observed in an earlier post, all aboard were unbelievably lucky that BA038 ran out of lift where it did, landing on the only bit of waterlogged lawn within miles of their approach path.
I am sure that the 2 pros flying that sector were trying to get it onto 27L, which in the circumstances would have likely had a much more injurious outcome.

armchairpilot94116
18th Feb 2008, 20:01
http://edition.cnn.com/2008/WORLD/europe/02/18/heathrow.incident.ap/index.html?iref=mpstoryview


What was it then if not mechanical?

tiggerm0th
18th Feb 2008, 20:04
Evidence of cavitations would be small pits on the tips of the impeller blades of a certifugal pump, and in other key places of the pump (depending entirely on the design).

The pits are caused by the rapid collapse of bubbles.

Bubbles can be caused not only entrained air (such as a seal leak) but also when the fuel vapourises (ie. 'boils') at partial vacuums.

green granite
18th Feb 2008, 20:10
The report does not say that cavitation caused the problems, nor does it say when it occurred, It could have occurred at any time from new, up to the time of the crash.

tiggerm0th
18th Feb 2008, 20:13
Cavitation in itself does not cause a problem unless there is failure of the components themselves as a result of it, or unless the effects of cavittion causes a significant degradation of performance.

Cavitation however is a symptom of a choking of flow on the inlet of a pump, or the entrainment of air.

An example of cavitation could be the addition of a lighter solvent in the fuel which vaporises at a lower temperature than the bulk of the fuel. The suction pressure of the pump could cause the vaporisation of the additive and so cause vapourisation. The report does not report fuel contamination so this can be ruled out.

Another example of cavitation is when a restriction which is on the discharge side of the pump is suddenly taken away - the suction flow just cannot keep up, the pressure drops and cavitation occurs.

At this point I still go for: failed seals due to low temperature, then regaining flexibility on loss of altitude, then entraining air and causing foaming.

grebllaw123d
18th Feb 2008, 20:28
Mind the following information given in the report:

"The manufacturer assessed both pumps as still being capable
of delivering full fuel flow."

manrow
18th Feb 2008, 20:34
I sense Trolls in here.

I don't care whether the autopilot was left in deliberately or not, - it worked.

I do however know far too many professional pilots who believe they can do better than the automatics, usually wrongly!

booke23
18th Feb 2008, 20:37
I wasn't really getting at fuel waxing/freezing in my previous post......I was more thinking along the lines of water Hygroscopicly suspended in the fuel freezing somewhere in the system during the flight.

However all this is pure speculation at this stage.

Green-dot
18th Feb 2008, 20:38
QUOTE: "BoeingMEL wrote: "............ at least all that garbage about EMI can be buried..... as can fuel exhaustion, windshear, finger trouble, Gordon Brown's heavies..."

To be fair, the report doesn't appear to rule any of that out...

Until the investigation is concluded anything is a possibility!" UNQUOTE

Exactly. Nothing can be ruled out at this time. Reading the report the investigators apparently are confronted with evidence of a normal functioning machine, which for a short period of time did not function normally. In other words, perhaps an intermittent failure of some kind, at this time not yet duplicated.

To quote the report: "The recorded data indicates that there were no anomalies in the major aircraft systems."

Question is then, what could have failed that is not recorded?

Regards,
Green-dot

infrequentflyer789
18th Feb 2008, 21:00
They have just said "Damage to the fuel pumps caused the crash"

I'm certain I read in the report that the pumps would not stop working in this condition.

You're right - report said they were assessed as: "still being capable of delivering full fuel flow".

I was wrong though - I was sure we were going to hear that the cause was the crew pulling the fire handle & engine cutoffs in the wrong order, with the AAIB issuing a safety recommendation to stop it happening again.

[followed by, for the particularly on-the-ball reporter, a retrospective of Kegworth, which will turn out to be "eerily similar" because of the words "fire handle" "engine" and "boeing"... :ugh:]

Bearcat
18th Feb 2008, 21:07
i smell trouble ahead and no one rightly is going to stick their neck out and rightly till the final report is issued.:ouch:

FE Hoppy
18th Feb 2008, 21:08
No Fault Found.

Hmmmm.

The cavitation damage and some FOD in the tanks is not much to go on!!

So there was fuel in the tanks, the metering valve was fully open, and the pump was capable of supplying fuel.

:confused::confused::confused:

tiggerm0th
18th Feb 2008, 21:14
Cavitation??

Mind the following information given in the report:

"The manufacturer assessed both pumps as still being capable
of delivering full fuel flow."


Yes, that's right, and what is your point exactly? A scuffed wall of a car tyre does not alter its stopping distance. A pump that shows evidence of caviatation does not particularily alter its performance, and would still allow full fuel flow. They are both evidence of what has happened.

HOWEVER, a pump that is undergoing caviation at the time however is severely suffering in performance.

Would I buy a centrifugal pump that had suffered cavitation? Yes, no problem, (within limits) the key is not to drive the pump so that cavitation occurs.

Sunfish
18th Feb 2008, 21:18
Data, downloaded from the Electronic Engine Controllers
(EECs) and the QAR, revealed no anomalies with the
control system operation. At the point when the right
engine began to lose thrust the data indicated that the
right engine EEC responded correctly to a reduction
in fuel flow to the right engine, followed by a similar
response from the left EEC when fuel flow to the left
engine diminished. Data also revealed that the fuel
metering valves on both engines correctly moved to the
fully open position to schedule an increase in fuel flow.
Both fuel metering units were tested and examined, and
revealed no pre-existing defects.

This would seem to suggest that the correct signals were getting through to both engines and the engines were trying to respond accordingly. Apparently no other as yet significant defects have been found.

Does this therefore narrow down the possible cause to a momentary absence of liquid Jet A1 under pressure at the burners?

I am totally speculating here, but given that air temperatures were lower than Jet A1 freeze temperature, and even though the tank temperature never got to critical levels according to instrumentation, would it be possible for a deposit (perhaps like cardiac plaque in arteries) of wax or frozen fuel to form somewhere in the fuel pipework?

As it warmed such a plaque could let go, blocking a filter and perhaps giving an engine the mechanical equivalent of a heart attack?

Of course that idea stretches credibility because it had to happen to both engines within seven seconds of each other, there was no EICAS message about clogged filters and it may be physically impossible to put a skin of frozen wax on the inside of a pipe thats carrying -34C jet A1.

No more speculation from me till the next official report.

infrequentflyer789
18th Feb 2008, 21:20
In the case of BA038 the path from the fire handle to the spar valve was damaged, and although the path from the cutoff lever to the valve was ok, it was isolated by the fire handle, which on this occasion was activated before the cutoff lever.


Perhaps worth noting that this was only the case on one wing, on the other side (as I read the report) both paths to the valve were cut by the damage, so no action would have closed it.

Flight Safety
18th Feb 2008, 21:24
Sorry if this sounds oversimplified, but if the tanks had enough fuel, the EECs responded correctly to the request for more power, the fuel metering valves were wide open, and the HP fuel pumps were probably cavitating, then it seems reasonably clear to me that something was probably blocking the fuel flow.

tiggerm0th
18th Feb 2008, 21:29
Common mode failure.

avrflr
18th Feb 2008, 21:39
Several people have said that the A/P did the best possible job under the circumstances. I'm sure the crew did what any good crew would have done in that situation, but.....

If they had disconnected the A/P when the engines failed to respond to the manual throttle movement and flown the aircraft towards the grass, I bet they would have had enough energy for a decent flare. The landing gear might not have liked coming down in the bog, but I'd take that over a no-flare, near the stall plonk-down. Might have saved some poor sod a broken leg.

Of course you don't expect your jet to turn into a glider on short final and it is easy to speculate with the benefit of hindsight. I'm sure somebody will try it in the simulator, let us know how you get on.

Flame on.

borghha
18th Feb 2008, 21:40
Bearcat wrote

from the accident report...

After the aircraft crossed the Ural mountain range in Russia it climbed further to FL380 where the ambient temperature dropped to as low as minus 76°C


What is the enviornmental envelope re min temps for the 777. I know the airbus 320 is -70c


Does anyone know the answer to this question?

sispanys ria
18th Feb 2008, 21:45
What about the cross feed opening and the APU start initiated while the glider was flown by the AP ? What kind of SOP is that ?

L337
18th Feb 2008, 21:52
What about the cross feed opening and the APU start initiated while the glider was flown by the AP ? What kind of SOP is that ?

Well it is not. It is evidence of the crew trying anything to keep flying. They were not exactly in a "SOP" situation. More out of teh box desperate try anything situation.

grebllaw123d
18th Feb 2008, 21:59
I imagine that these actions were initiated by the pilot in the middle (relief pilot or whatever he is called in BA).
Maybe not SOP - but sensible actions in a very stressful and confusing situation!:ok:

Flight Safety
18th Feb 2008, 22:02
Two odd things stand out in the report regarding the fuel.

Initial results confirm that the fuel conforms to Jet A-1 specifications and that there were no signs of contamination or unusual levels of water content.

And...

The specified freezing point for Jet A-1 fuel is -47ºC; analysis of fuel samples taken after the accident showed the fuel onboard the aircraft had an actual freezing point of -57ºC.

Is it normal for Jet A-1 to have a freezing temp of -57C? Is this really pure Jet A-1, or is it mixed with something else?

tiggerm0th
18th Feb 2008, 22:07
If they had disconnected the A/P when the engines failed to respond to the manual throttle movement and flown the aircraft towards the grass, I bet they would have had enough energy for a decent flare.

I somewhat agree. Not only that, if they had put it onto manual, decreased demand on the engines then the blockage/inleakage/cavitation would have subsided then full thrust may have been regained.

However, I don't feel that we can suggest that the chaps on board did anything wrong. There were no fatalities, no serious injuries, and they landed in a distance shorter than I land a Piper Cherokee into.

T.

tiggerm0th
18th Feb 2008, 22:13
Is it normal for Jet A-1 to have a freezing temp of -57C? Is this really pure Jet A-1, or is it mixed with something else?

A good question - any "lights" in the fuel to reduce the melting point would indeed result in a higher liklihood of caviation in the pumps, or could result in waxes in fuel lines to being stripped then passed through once the right temperature (read 'altitude') is reached.

T.

PS Is it normal for fuel not to be tested for water after fuelling?

sispanys ria
18th Feb 2008, 22:23
From the first page of the report. So what are you talking about?!

avrflr,

Glad you said that. I'd duck if I were you ;)I'm talking about these statements from the preliminary report:


• The fuel crossfeed valves were closed in flight according to the flight crew, but the switches were found in the open position and only one valve was open. In the days following the event, the flight crew has added additional details to their report. The crew now believes they opened the valves just prior to impact and the airplane lost power before both valves moved to the open position.
• The auxiliary power unit (APU) began its auto start sequence, even though the buses were still powered. In the days following the event, the flight crew has added additional details to their report. The crew now believes they turned the APU on prior to impact. There was sufficient time before the impact for the APU inlet door to open, but not for the APU fuel pump to turn on or the APU engine to start spooling up.

Is the crew supposed to retract the added comments they made about it ?

NIMBLE
18th Feb 2008, 22:27
UUUUUUUUUUMMMMMMMMMMHHHHHHH???????????Air in the fuel pumps where normally fuel should be???? Me thinks this is starting to go in a certain direction!!!!!!!11:hmm:

andyfrench
18th Feb 2008, 22:31
I read so much rubbish here on pprune and the 777 acident is typical of the misinformed judgemental crap which somehow seems to dominate !! It's a good idea to have these forums but who aside from the newspapers benefits ? hey ho !![

overthewing
18th Feb 2008, 22:35
sispanys ria

I think the 'report' you quoted actually appeared on a US aviation site, and purported to be a leak from the official investigation. The initial report, and the update report on Jan 23rd, didn't mention the pilots' 'recollections'.

So I think we can discount the leak?

avrflr
18th Feb 2008, 22:50
A good question - any "lights" in the fuel to reduce the melting point would indeed result in a higher liklihood of caviation in the pumps, or could result in waxes in fuel lines to being stripped then passed through once the right temperature (read 'altitude') is reached.

T.

PS Is it normal for fuel not to be tested for water after fuelling?
Yes, it is normal. The fuel gets sample tested during the refuel, and for various reasons it is highly unlikely that water can get on to the aircraft at this point. The AAIB reports states that there was not an unusual amount of water in the fuel recovered from the aircraft, so water does not look like a likely culprit.

Also the schedule for water drains and the likely freezing temperature of the fuel was previously explained/predicted by other posters on the forum, so nothing unusual or unexpected there. Even the FOD in the tanks doesn't really explain much. Still a big bloody mystery.

TURIN
18th Feb 2008, 22:51
PS Is it normal for fuel not to be tested for water after fuelling?

Yes.

The fuel at the bowser will be checked by the refueller before and after refuelling.

The fuel in the tanks will only be checked as required by the maint schedule. (Daily Check, First flight of the day, 'x' hours on the ground etc).:ok:

NSEU
18th Feb 2008, 23:02
Is the crew supposed to retract the added comments they made about it ?

In light of the first reports, I took the last report to indicate that, prior to the event (during the normal part of flight down to 700'), the crossfeeds were not activated.
As previously discussed, the 777 SOP for low fuel is to open one crossfeed. The Captain may have suspected low fuel. And two sources are better than one (electrically, too, it seems... cf spar valve comments in the report).

Re the comments about seals....
The fuel is being pumped from the tanks under pressure (by wing tank pumps). Any leaks in seals are going to put fuel to atmosphere, not air to fuel lines. The wing pumps were working normally.

Re the comments about foreign objects being in the tanks being put there by maintenance...
Not diverting attention away from maintenance, but how big are the filters (if there are any) on the refuel bowsers?
Also, I've never been inside the tanks of a 777... Just wondering how big the screens are on the pump inlets (frying pan size?).

I'd love to know what caused this...I've been having similar problems with my car for weeks... and can't figure out whether it's the ECU or fuel flow (coincidentally, I found a plastic foreign object under the pump inlet.... the problem still there after removing it, however) :}

NSEU
18th Feb 2008, 23:19
Re the references to JetA1 in the report....

Would this be because the fuel loaded in Heathrow was Jet A1 and flight crews are told, where there are combinations of fuel types in tanks, to enter the freeze temperature of the fuel type with the highest freeze point into the FMC/CDU?

If they entered -47, then the aircraft would have warned them at -44 (3 degree buffer)....well above the freeze point of the remaining fuel.

I notice the word satisfactorily is used in several places in the report. What margin of error does "satis" allow? :}

avrflr
18th Feb 2008, 23:32
Re the comments about seals....
The fuel is being pumped from the tanks under pressure (by wing tank pumps). Any leaks in seals are going to put fuel to atmosphere, not air to fuel lines. The wing pumps were working normally.
Not sure I follow you there. If there's a blockage upstream of the HP pump the pressure in the line feeding into it is going to go from positive to negative pretty quickly (the pumps cavitated for some reason - not enough pressure being delivered to them is a reasonable inference).

avrflr
18th Feb 2008, 23:37
Re the references to JetA1 in the report....

Would this be because the fuel loaded in Heathrow was Jet A1 and flight crews are told, where there are combinations of fuel types in tanks, to enter the freeze temperature of the fuel type with the highest freeze point into the FMC/CDU?

If they entered -47, then the aircraft would have warned them at -44 (3 degree buffer)....well above the freeze point of the remaining fuel.

I notice the word satisfactorily is used in several places in the report. What margin of error does "satis" allow? :}
Given that the lowest recorded fuel temperature was -34 degrees, it's a moot point.

Hand Solo
18th Feb 2008, 23:47
I don't think you can enter the fuel temp in that way. I believe the Boeings are hard wired to warn you at -37C.

Nov71
18th Feb 2008, 23:56
Four pages of speculation based on an interim report of a basically intact a/c has produced suggested
1. Fuel contamination - culpable
2. FOD blockage of tank outlet - culpable
3. Loss of sealant plasticity due to abnormal low temp (as per Challenger O rings?) which may allow air to be sucked into the fuel flow - not predicted
May explain fuel leak on landing but more likely due to heavy landing
4. Cavitation damage to fuel pump insufficient to impair pump performance, suggesting recent damage - symptom not cause
5. Loss of thrust from both engines at ~200' -SOD's law

Would not partial blockage at tank outlets cause low pressure downstream causing a dose of the 'bends' in the fuel system or draw air in via any sealant crack, thus causing cavitation?

It appears that all pilots, both human & auto, remained fully functional

I await the final report with interest

Fenwicksgirl
19th Feb 2008, 00:17
You can overwrite the fuel freeze point on the FMC.

Huck
19th Feb 2008, 00:20
The first step of any emergency is not - 1. maintain a/p until stickshaker.

You noticed that too, did you.....

NSEU
19th Feb 2008, 00:53
Not sure I follow you there. If there's a blockage upstream of the HP pump the pressure in the line feeding into it is going to go from positive to negative pretty quickly (the pumps cavitated for some reason - not enough pressure being delivered to them is a reasonable inference).

Sorry, I thought you (or someone else) was saying that leaky seals were the root cause of the cavitation problem.... rather than a blockage downstream. Yes, I suppose air might get in through leaky seals with a vacuum in the lines.. but neither air nor vacuum are very good for running engines :)

Elastoboy
19th Feb 2008, 01:24
HP pumps, be they gear (positive displacement / lobe) or centrifical, behave very strangley under many conditions that are just marginally outside of the nominal state. Any one variable can have significant impact to pumping performance. (Cavitation pitting is the clue!!!)
Variables include:
Turbulant vs laminar flow at LP side (Reynolds Number).
S.G. variation at temp deltas (pumps are pre set for nominals NOT all temps).
Clearances and gaps between rotator and stator faces are critical, too big equals deminished performance, too tight equals tip gassing (cavitation).
Contamination (even at < 25 micron level) plays havoc with the flow transition state across the face of the impeller. (Again cavitation).
Cavitation pitting is NOT an instant catastrophic occurance it is a chronic progressive development.
I am sure that many engine HP pumps exhibit similar pitting but I believe that we have a concoction of variables that culminate in an event of the HP pumps creating an air (gas) saturated "fluid" on the HP side that is inadequate to provide the combustion energy required for thrust.
Please, if the AAIB lurk here, look outside of aviation for similar examples of HP hydocarbon pumps behaving very strangly outside of the norm.

Milt
19th Feb 2008, 02:08
Cavitation

You don't have to have air at the HP pumps' inlet to result in cavitation. Cavitation will occur when the pressure is lowerd sufficiently to reach the 'boiling' (vapourisation) temperature of the fuel. That pressure may be
close to a vacuum for fuel already at a very low temperature.

It could well require the services of a Methusela to know just when cavitation damage occurred in the pumps. This could have happened during any periods when there were no booster pumps operating at high altitude with warmish fuel. Cavitation damage occurrs when the pumps lose their continuing lubrication by the fuel itself.

Can any boffins out there give us an idea as to the specific vapourisation pressures of jet fuel over the warm to cold temperature range?

This may be a bit of a useful learn for those who don't already know the dangers of cavitation with jet engine HP pumps.

woodpecker
19th Feb 2008, 03:29
Just a thought.

http://i38.photobucket.com/albums/e146/naptonboy/27l.jpg

CONF iture
19th Feb 2008, 04:29
** a maintenance message indicating excessive water in the center tank was set during taxi on the two previous flight legs, although it cleared itself both times **
** the crew now believes they opened the valves just prior to impact **
** the crew now believes they turned the APU on prior to impact **
** A preliminary review of the EEC data from the right engine shows erratic combustor inlet pressure (P30) **

Troy W (http://www.pilotsofamerica.com/forum/member.php?u=76) … was it only diversion from your friend or was it real leaks or other controlled rumors by THE WSJ ???

… I still expect AAIB would have mentioned any of that … !?


But to be honest I was waiting for a little bit more after 30 days:

-- From FL110 to FL90 in 5 min … surprising they didn’t need any slight thrust increase for that period … How did the engines react ?

-- I know they maintained maybe for a few hours 3000 ft above the planned FL and therefore were probably closer to the optimum, but still they had to hold an extra 5 min, so how to explain they managed to save an impressive 5% on the planned fuel burn ?
Was BA flight planning so much way out, or is there anything else … ?

-- What does mean “The FO took control for the landing at a height of approximately 780 ft”
did he just come back from a pee … or the CPT fell incapacitate … ?

Anyway … “The recorded data indicates that there were no anomalies in the major aircraft systems” … and fuel was not an issue … but still, 100% of the engines refused almost simultaneously to produce the requested thrust ........................................?

Wornout Rubber
19th Feb 2008, 05:09
Can anybody comment as to the best lift/drag speed (best glide speed), with Flaps 30 on a B.777?

The AAIB stated that the speed decayed to 108 Kts.

cwatters
19th Feb 2008, 06:32
I don't recall seeing anything in the report about filter bypass warnings? Is it possible the problem is somehow after the filters but before the pumps that cavitated?

Likewise no mention of the flaps being raised.

soggy_cabbage
19th Feb 2008, 06:53
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.


As my posts appear to dissapear like ice and wax in a fuel line, can anyone who knows what they are talking about explain this quote?

Daysleeper
19th Feb 2008, 07:04
can anyone who knows what they are talking about explain this quote?

Sure what it says is that at a height (distance above the ground) of 720 feet (1 foot = 12 inches) the right engine ( motor that converts thermal energy to mechanical work) thrust (reaction force) reduced (got less). Some 7 seconds (unit of time) later the thrust on the left(opposite of right) engine reduced (got less) to a similar amount. The engines did not shut down (stop) and both engines(left and right) continued to produce thrust at an engine speed above (more than) flight idle(minimum speed in flight) but less than the commanded thrust. (amount of thrust wanted by people at pointy end)
:hmm:

avrflr
19th Feb 2008, 07:04
soggy_cabbage: That quote seems pretty self-explanatory. Which part don't you understand?

Edit: Nice one daysleeper:D

phil gollin
19th Feb 2008, 07:33
Quote:-

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.

unquote

It is interesting to think what could actually cause two separate engines, fuel pumps, filters, etc.... to reduce thrust to "a similar level" but not at the same time.

It is the "similar" which is so intriguing. They could have used so many words and that one could mean anything or NOTHING.

.

NWT
19th Feb 2008, 07:46
At a previous airline I worked for, they had a almost brand new widebody aircraft, suffered a serious engine surge/stall just after rotation. Shortly after engine recovered to required thrust setting, but with abnormal indications (EGT etc). Fuel dumped and aircraft returned. Engine was changed due to internal damage. New engine suffered same problem 2 days later. Extensive investigation revealed plastic pipe blank lodged in fuel feed pipe in pylon area. During high thrust setting, ie take off, fuel flow to engine HP pump was blocked, then almost immediately cleared, then blocked again, causing fuel starvation etc and surging. Initial problem was put down to failure of the original engine, as after the replacement was fitted full power runs were carried out. Offending plastic blank was traced back to manufacturer....

Fil
19th Feb 2008, 08:51
from the accident report...

After the aircraft crossed the Ural mountain range in Russia it climbed further to FL380 where the ambient temperature dropped to as low as minus 76°C


What is the enviornmental envelope re min temps for the 777. I know the airbus 320 is -70c


Does anyone know the answer to this question?

No, but it's not the minus 70 that the single aisle airbus's are limited to. After flying the airbus it was something I looked for and it isn't in any of the manuals (BA) so the 777 can be flown in colder air.

Some posters here have mentioned at what temp the low fuel temp warning would activate. On BA 777's the fuel freeze temp can be changed in the FMC but is set to -40 by default, the actual warning starts at 3 degrees above that value. So by default (but it can be changed) he low fuel temp warningwould sound at minus 37.

davy123
19th Feb 2008, 09:08
Who manufactured the fuel pumps - is it Goodrich who supplies it along with the FADEC?

Teal
19th Feb 2008, 09:47
Offending plastic blank was traced back to manufacturer....Just as an aside, the debris found in various locations in the fuel system:

- a red plastic sealant scraper
- a piece of black plastic tape
- a piece of brown paper
- a piece of yellow plastic.
- a small piece of fabric or paper
- a small circular disc

Surely this can't be normal. Was it all likely introduced during manufacture, or at later stages during maintenance or refueling?

Hand Solo
19th Feb 2008, 09:50
I don't think you'd describe the debris as particularly abnormal. Fuel tanks often have bits of FOD floating around in them. Try to visualise a 6mm diameter plastic disk and think if you are going to be able to spot that in a large, dark fuel tank.

Re-Heat
19th Feb 2008, 10:04
What this seems to be pointing towards is the only situation in which both engines could possibly have suffered the same fate - an as-yet undiscovered maintenance error either at Cardiff, London or Beijing; or more unlikely, a manufacturing defect.

As so much has been ruled out so far, it is even more likely that it is a seemingly minor circumstance that could have caused such an event. What that event is, however, clearly remains a mystery...

RO13ERTS
19th Feb 2008, 10:08
Goodrich supply the FMU, EEC and Actuator suite, not the Pump on the T800.

Teal
19th Feb 2008, 10:27
Try to visualise a 6mm diameter plastic disk....in a large, dark fuel tankSure, but that object was in the water scavenge jet pump.

The first object in the right fuel tank was 30 square cms...how large is the suction inlet screen in comparison?

Perhaps not significant but I would have expected pristine manufacturing and maintenance/fueling standards that would not introduce foreign objects.

Avionista
19th Feb 2008, 11:39
"Eliminate all other factors, and the one which remains must be the truth." - Sherlock Holmes

The AAIB seems to have pretty much eliminated mechanical and/or electrical failures within the engine systems as likely causes of this accident. They are not even suggesting that the cavitation damage within the fuel pump ocurred immediately prior to the crash - it might have happened weeks before the accident. The evidence points to some fuel related factor as it seems to be the only thing common to both engines. Perhaps, there is some aspect of fuel behaviour, under certain conditions, which has not been encountered before and is not yet understood. Cue the fuel chemists!

phil gollin
19th Feb 2008, 11:42
People should note that the problem SEEMED to affect both engines to a similar extent, but at slightly different times.

The latest report states that they have tested the fuel and it conforms to the required specification.

The debris was different in each wing tank and somehow would have had to have the same effect within 7 seconds on each engine (!)

The fuel metering valves were stated to have moved to the correct fully opened position and the filters filters were clean, as for the Fuel Oil Heat Exchangers one was clear and one had slight debris - the high pressure filters were clear.

It SEEMS very unlikely that the fuel was to blame, and the debris could not cause the same effect on the different engines.

-------------

I cannot see that two separate pumps with whatever minor damage could possibly both fail within 7 seconds of each other (whether or not they subsequently showed no fault).

I have no idea about what caused the problem, but would like to know more about any flowmeters that are in the system. Also about what could cause fuel aeration.

However, it gets annoying when people don't think about what the report says before posting. Anything that occurs must be able to affect both engines within a very small timescale.

.

OVERTALK
19th Feb 2008, 11:44
The cavitation damage to the engines' HP fuel pumps would seem to indicate incipient fuel starvation - or at least aeration - for whatever reason. Cavitation damage occurs when the pumps lose their continuing lubrication by the fuel itself. None of the in-tank debris (plastic etc) found would explain one or (particularly not) both engines becoming fuel-starved.
.
The possibilities remain prima facie limited to establishing the reasons why fuel (or trapped water) could be cooled to below freezing point (even if that point is 10 degrees below the specification freezing point for JET A1 of minus 47 deg C). The pointer would seem to be towards either water in the fuel becoming frozen or waxy/illiquid. There's another possibility however.
.

Think about the fact that certain "aerodynamically quiet" areas on an airframe can accumulate de-icing fluids and cause flight-control restrictions (see instances by Googling that term ["aerodynamically quiet"] and recent and historical AD's - particularly for BAe146 etc). In a similar fashion, icing will not form and accumulate uniformly over an airframe. It will first be seen in the corners of windshields, along windscreen wipers, wing/tail leading edges etc. Now think about what can happen in another fluid (i.e. not air, but fuel) that's contaminated with very cold water. What I'm getting at here is that some quiet (or stagnant) areas in an LP fuel system (tanks thru to HP pumps and filters) can support the formation of ice or waxy deposits due to higher (than normal/ambient) super-cooling in certain discrete areas. Result could be (a) large(ish) lump(s) of melting ice that will find its (their) way into a critical passage, filter, pipe or valve and temporarily cause constricted flow-rates. Because both sides of the aircraft mirror image each other, it's conceivable that identical "releases" of this icy lodgment could occur almost simultaneously - and then disappear (i.e. great quantities of water would not be required and wouldn't later be detectable anyway).

However why limit oneself to considering frozen water? Why not just think in terms of fuel itself being trapped and frozen in fluidically quiet areas and becoming supercooled/frozen. I'm guessing that residual fuel in the center tank/tail tank (and their collector tanks) might qualify and could later be induced into the system by both warming and the attitude changes inherent in both descent and configuring (gear and flap extension).

At what temp would the low fuel temp warning activate? On BA 777's the fuel freeze temp can be changed in the FMC but is set to -40 by default, the actual warning starts at 3 degrees above that value. So by default (but it can be changed) the low fuel temp warning would sound at minus 37. However, as with all such system measurements, the validity and usefulness of the sensor data depends upon where the sensor is mounted. Temperature measurement systems are inherently different to pressure measurement. Pressure is exerted equally in all directions. Temperature can vary over a significant range within a large volumetric system, some of whose components/areas are temporarily isolated by dint of being virtually (but not really) empty. If a center tank's output to the wings ceases relatively early in the cruise attitude (because it's "virtually" empty), does that mean it cannot thereafter contribute some previously "baffled" slushy ice-dregs (merely by gravity feed perhaps) once the nose lowers to the descent attitude? It's also a liquid fact that smaller stagnant quantities (such as dregs) will freeze sooner (and also melt sooner) than larger quantities (i.e. in wing tanks) that are being subjected to throughput recirculation (of bypass fuel), engine feed and fuel/oil heat exchanging etc. Wing carried fuel is also heated to some extent by the wing's passage through the air (and by the sun on the upsun side). That's not the case with fuel carried (or baffle-lodged) in the center-section and tail-tanks.

Why would this have happened to BA038 and not other 777's previously? My guess is that there is a trigger temperature and that the period that G-YMMM spent in the super cold pool of their track-miles between the Urals and Eastern Scandinavia ("unusually cold but not exceptional" - UK Met Office) may have allowed such pockets of super-cooled fuel to form into ice. The time of day (sun/no sun) and chosen CRZ hts may have also helped format the process.
.

CLEE
19th Feb 2008, 12:14
This is probably nothing, but since lots of posts are equating a loss of upstream fuel pressure (upstream of the HP fuel pumps in the engine) with a blockage in the fuel line (causing cavitation), it's as well to realise that it's not just a blockage that can result in low fuel pressure from the LP pumps (in the wing tanks).

A few years ago we were completing maintenance on a Fokker 100 and for some reason the LP fuel pump delivery pressure on the pumps in the LH wing were only delivering about 50% of what they should. To troubleshoot, the pumps on the LHS & RHS were swapped - the problem STILL only occurred on the LHS. That eliminated pumps as the problem, and we started to look for a blockage in the lines downstream of the pump as the only explanation. No joy.

During the maintenance input, the LH engine had been changed. This required swapping the IDG (integrated drive generator) from the old to the new engine. During that procedure, human error had misconnected the three phase wiring and earth so that the AC three phase output from the LH engine generator was wrong.

Lesson No 1 - no other system on the aircraft was bothered by this except the LH fuel pumps. Lesson No 2, although the Ground Power Generator Control Units monitor the phase quality of the power coming to the aircraft, ordinary Generator Control Units do not (on that aircraft anyway). So there was no fault announced at the front of the aircraft.

Now in no way am I suggesting that on this 777 the generators magically re-connected themselves in flight. However sudden low pressure from all LH & RH pumps could (in theory) occur if the phase relationship of the power supply to all of the pumps suddenly changed.

Also, if the 777 is anything like the Fokker 100, it might only be the fuel pumps that would have a problem with this. How that might happen in practice I don't know - I would think it so unlikely as to be almost impossible. Also, even if it did happen, I would be surprised if such a loss of pressure starved the engines - but perhaps in combination with other things it is (theoretically) possible.

Lost in Saigon
19th Feb 2008, 12:25
from the accident report...

After the aircraft crossed the Ural mountain range in Russia it climbed further to FL380 where the ambient temperature dropped to as low as minus 76°C


What is the enviornmental envelope re min temps for the 777. I know the airbus 320 is -70c


Minus 80C

http://i2.photobucket.com/albums/y17/msowsun/Aircraft/2e0a91da.jpg

wilyflier
19th Feb 2008, 12:31
Possible , not impossible

One centre tank water scavenge jet pump suffers poor flow due to 6 mm disc
Frozen water accumulates to be discharged at late stage of centre tank scavenge.
Hard ice crystals abrade fuel pump surfaces
Can frothing occur also?
One of the crossfeed valves neither fully open nor closed after crash,(and according to Captain MM would not have been selected open in any case(?))
The enquirys lack of comment indicates it to be serviceable,
But if it had in fact for some reason remained half open throughout the flight it would supply the missing common factor complete with 7 second interval

misd-agin
19th Feb 2008, 12:47
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Best Glide Speed
Can anybody comment as to the best lift/drag speed (best glide speed), with Flaps 30 on a B.777?

The AAIB stated that the speed decayed to 108 Kts.

-----------------------------------------------------------------------

I asked this question to a flight test engineer. He stated for line pilots we could consider approach speed (Vref +5) to be close enough to L/D speed.

Holding speed on my a/c is 1.4 Vsi. Approach speed at Vref + 10 is approx. 1.4 Vsi(typical landing weight). Vref is 1.3 Vso.

Based on his comments, and the relationship of holding speed to Vsi, I'd think L/D is very close to Vref + 5.

We all know that L/D is not at, or even near, 1.0 Vso or 1.0 Vsi.

-----------------------------------------------------------------------

edit -

QRH lists best L/D speeds under 'Dual Engine Failure'. It's 1.5 x Vsi.

If that relationship (1.5x) holds with regards to Vso then L/D would be approimately Vref + 15-20.

davy123
19th Feb 2008, 13:00
Detailed examination of both the left and right engine high
pressure fuel pumps revealed signs of abnormal cavitation
on the pressure-side bearings and the outlet ports.

Would cavitation not have to build up over a period of time in order to be described as 'abnormal'? I appreciate that if solids peppered the bearings and ports then there would be increased pitting, but to such an extent in one flight leg?

What pressure (psi/bar) is the fuel entering/leaving this device?

misd-agin
19th Feb 2008, 13:17
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Best Glide Speed
Can anybody comment as to the best lift/drag speed (best glide speed), with Flaps 30 on a B.777?

The AAIB stated that the speed decayed to 108 Kts.
http://www.pprune.org/forums/images/statusicon/user_offline.gif http://www.pprune.org/forums/images/buttons/report.gif (http://www.pprune.org/forums/report.php?p=3921366) http://www.pprune.org/forums/images/buttons/reply_small.gif (http://www.pprune.org/forums/newreply.php?do=newreply&p=3921366&noquote=1)
-----------------------------------------------------------------------

Anyone have simulator access that can answer the question?

The formula (found on internet...) is TAS x 100/sink rate (FPM) = glide ratio.

IE 250 KTAS x 100/1300' = 19.2

Try a couple of different speeds, and various flap settings (20 being the most interesting due to the reduction in LE slats), and you'll come up with the answer.

Modern airliners are around 18:1. Easy formula is altitude in thousands x 3 = glide distance for a clean aircraft at best L/D speed. Eg, 30,000' x 3 = 90 n.m.

QRH for my a/c has best L/D speeds(clean) listed under "Dual Engine Failure".

Woodenwonder
19th Feb 2008, 13:45
Not an ATPL on 777 - just a one-time instructor.

Whatever the best glide speed might be, a reduction in speed can only build induced drag more and more so the overall L/D must diminish. And this will be non-linear - ever diminishing performance as speed decreases.

Perhaps the autopilot is not the crew's best friend in these circumstances.

PS. I thought landing flap was normally 25 degrees, AAIB say 30 was set.

sevenstrokeroll
19th Feb 2008, 14:37
with so many reports, this AAIB report seems to contradict an earlier report saying the captain reduced the flaps while inflight.

anyone care to make a definitive statement on this?

nice to know the birds didn't do it, right danny?

CONF iture
19th Feb 2008, 15:09
with so many reports, this AAIB report seems to contradict an earlier report saying the captain reduced the flaps while inflight
That flaps reduction was only merely a suggestion ... but like many other things has never been part of any official AAIB report !

sevenstrokeroll
19th Feb 2008, 15:17
its been done to death in movies. duplicate the flight. same weight, same temperatures encountered enroute, same pitch attitude during approach, same fuel type and ammount...just a different plane...and keep it on autopilot to try to duplicate everything.

maybe there is an exact pitch attitude causing a slight change in fuel flow at just a certain amount of fuel remaining.

Mark654321
19th Feb 2008, 15:40
Seems like we have a conundrum and the implausable situation of 2 engines being fuel starved at virtually the same time from some 'fluff' in the tanks (highly unlikely). Concluding with a 40min descent and held in a pattern over Lambourne for 5 minutes I can't see there was any ice either to bung them up. London was cold but not -57C.

I found the AAIB very interesting and thorough for the time allowed to them so far. Thank you.
The weather was mentioned from a METAR at Heathrow I believe at the time but didn't seem to mention anything abnormal when all reports for that day seemed to be anything but normal.
I would like to draw your attention to the METARs for the London area on that particular date here.
http://www.wunderground.com/history/airport/EGKB/2008/1/17/DailyHistory.html?req_city=NA&req_state=NA&req_statename=NA

There are some very interesting Microburst/Windshear activity throughout the day in the London area from SSW with winds in excess of 50kts GUSTING to 75kts out of nowhere!! I also believe that the London Eye was closed at 13:00. Oh no!
Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot/(AUTOPILOT in this case) to reduce engine power if they are unaware of the wind shear. This is where the engines have settled to just above idle. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft's airspeed and increasing its sink rate. The pilot releases AUTOPILOT and demands more power, as passengers and witnesses on the ground 'heard the roar of the engines like a take off'.Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing airspeed further, leaving the aircraft in a low-power, low-speed descent.(Heres where you get your 108kts and on board passengers reporting sudden loss of height). 108kts is not the speed you want on any HEAVY Aircraft at anytime whilst in flight. This can lead to an accident if the aircraft is too low to effect a recovery before ground contact. As the result of the accidents in the 1970s and 1980s, in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have on-board windshear detection systems by 1993. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries. Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years, due to the mandated on-board detection as well as the addition of Doppler weather radar units on the ground.

3 Greens and Happy Landings

Mark in CA
19th Feb 2008, 16:31
Extensive investigation revealed plastic pipe blank lodged in fuel feed pipe in pylon area. During high thrust setting, ie take off, fuel flow to engine HP pump was blocked, then almost immediately cleared, then blocked again, causing fuel starvation etc and surging. Initial problem was put down to failure of the original engine, as after the replacement was fitted full power runs were carried out. Offending plastic blank was traced back to manufacturer....

Exactly the same thing happened to my old VW Bug, only in that case it was a pencil point that had broken off and left floating around inside the gas tank (previous owner must have used a pencil to plug up the gas outlet!). Car would unexpectedly stall, but only while driving on the freeway. Took a while to track it down, but the fix was easy.

nickyjsmith
19th Feb 2008, 16:34
NSEU,

Check your tank vent, if its blocked you can get a vacuum in the tank that causes fuel starvation. Worse on longer journey's.
When you stop,air seeps in while your looking around and the car runs normal again untill a vacuum builds up again.
To test it, stop and take the fuel cap off asap and listen, i've come accross it a few times on different cars.Its a real pig to figure out the first time as it leaves no evidence.

airfoilmod
19th Feb 2008, 16:37
Ice crystals may as well be sand in a pump at that pressure. Tolerating "Water" in Fuel, at any amount obviously opens the door to bad outcomes. Not my Job to point out proximate cause in this one, but a reading of any report gives one a "sense" of where it's going. Cavitation is possible so is Water ice buildup. "Vapour Lock" is a possibility, but so is Delta mmHg in the Low pressure side, a reduction that "pills" ice crystals out of solution to accumulate in parts of the Fuel Journey that cause "Restricted Flow", partial Starvation, and a further reduction, production cycle that exacerbates "Spool Down". I think this report is what I call a "Potentiating release", something intended to set the stage for an "outcome" that is as pleasant as can be to the Players. That two Majors are rewriting Fuel Protocols is a sign this may be true. That BBC already has gotten to the destination I'm implying, is instructive.

airfoilmod
19th Feb 2008, 17:06
So why would both engines protest at the "same time". If the Fuel is homogeneous, separation or "isolation" of supply to each powerplant accomplishes nothing if the "problem" exists in both supplies. What would cause a chronic and consistent "starvation" or "restriction"? Additive that allowed water to be in solution with Fuel, something like Alcohol? Angle of attack, causing a reduction in pressure in both fuel systems at the same time, but one engine, being "hotter", reacts five seconds later? Whatever the outcome of the report, I don't see how anything other than a Fuel scenario could have escaped the scrutiny of this prestigious website.

snowfalcon2
19th Feb 2008, 17:20
Woodenwonder:

Perhaps the autopilot is not the crew's best friend in these circumstances.

I had the same thoughts, as expressed a while ago in this post. (http://www.pprune.org/forums/showthread.php?p=3854688#post3854688)

Just as a remark: The original information we saw indicated that the problem started at 600 ft, and the autopilot was disconnected by the time the aircraft reached 400 ft. The new information says the problem started at 720 ft and the autopilot flew the plane all the way to 175 ft.

The difference is significant in terms of the potential improvement to the glide distance that a "smarter" autopilot might have enabled. As others have said, the aircraft might have retained enough energy to either allow for a flare or, alternatively, make it to the threshold.

Nickyjsmith:

NSEU,

Check your tank vent, if its blocked you can get a vacuum in the tank that causes fuel starvation. Worse on longer journey's.
When you stop,air seeps in while your looking around and the car runs normal again untill a vacuum builds up again.

Tank vent blocking can indeed be quite nasty. I know of it happening in a small plane, with the result that the tank literally was crunched together by the ensuing vacuum. Fortunately the forced landing went well. But based on that, even on a 777 I'd expect such an event to cause visible buckling on the wing skins before fuel flow to the engines would stop.

Just my £0.02 :)

GonzoXL5
19th Feb 2008, 17:47
I work in business systems IT and not in any aviation based profession

Given that they've still been unable to find any indication of a mechanical fault yet this is something significant enough to affect both engines, what do you guys think are the chances of this being a software problem ?

nlarbale
19th Feb 2008, 18:10
From the AAIB's statements on the functioning of the aircraft systems, it appears that all software systems in the Autothrottle and EEC's (and elsewhere) functioned as expected

hetfield
19th Feb 2008, 18:32
Would even 4 engines have done the same.....?

IcePack
19th Feb 2008, 18:34
Mmm! pulling the fire handles at 700 ft. Now would that have the same affect?

Actually I see from other posts that the QAR did not record the last 45 secs. But I guess the FDR would have. So am just wondering when the handles got pulled. After it came to a halt I guess, interesting senario on the post impact fuel spill.
I'll go back under me rock.:confused:

sispanys ria
19th Feb 2008, 18:34
It is really surprising to see that both engines lost power at (almost) the same time, and in addition, they both came to same low RPM (above flight idle). In case of ice obstruction, it's already quite unlikely to expect a simultaneous event, but how can it lead to a similar low thrust setting on both engines ???
It means both ice obstructions did let a similar leak on both sides...:bored:

Correct me if I'm wrong, but up to now, it's only the indicated fuel quantity that was reported, not the actual fuel found in the wreckage...
I just want to remind that Tunintair had a fuel exhaustion accident while indications were showing adequate fuel on board... Is it really impossible to consider low fuel conditions causing the cavitation in high pitch attitude ?

grumpyoldgeek
19th Feb 2008, 18:35
I'm wondering if there's a fuel pressure transducer downstream of the metering valves and if so, was the fuel pressure logged.

snowfalcon2
19th Feb 2008, 18:35
Hi Topslide6,

thanks for the comment. I just like to stress that I'm not blaming the crew in any way here. The situation was apparently so extraordinary that one can't realistically expect more.

grebllaw123d
19th Feb 2008, 18:54
sispanys ria,

You are right - only the indicated fuel quantity is mentioned in the report, but the report also mentions that samples have been taken from the 2 wing tanks.

Later in the report the following statement is given:

A test of the fuel quantity processor unit (FQPU) was satisfactory and its non-volatile memory did not reveal any defects stored prior to the accident.

avrflr
19th Feb 2008, 18:56
Correct me if I'm wrong, but up to now, it's only the indicated fuel quantity that was reported, not the actual fuel found in the wreckage...
I just want to remind that Tunintair had a fuel exhaustion accident while indications were showing adequate fuel on board... Is it really impossible to consider low fuel conditions causing the cavitation in high pitch attitude ?
So you are saying that despite the fact that someone got inside the wing tanks and found a plastic scraper, a piece of tape etc it didn't strike them as odd that there was no fuel in there? You should call the AAIB and point out the mistake they are making.

Re: water/other fuel contamination - there wasn't any. The AAIB report makes fascinating reading. I highly recommend it.

sispanys ria
19th Feb 2008, 19:13
avrflr i'm sorry you cannot post a polite reply. I'm just interrogating myself, and never mentioned fuel exhaustion, but in a low fuel configuration, it might have been possible that the tank LP pumps were not able to provide the adequate fuel to the HP pumps...

As you refer to the small items found in the tanks, I would like to see how you explain that almost simultaneously, two different items would have reduced two fuel flows to two similar values on both engines...

Hand Solo
19th Feb 2008, 19:22
The thing had ten tonnes of fuel on board. There was no low fuel state. None. Ever.

AEROJUANCA
19th Feb 2008, 19:24
Water and ice cristals are normal into the airliners tanks,and comes with the air that vent tanks entering by the NACA inlets near the wing tips,not during refueling.
Also,to much debriss to block both booster pumps per wing tank...the engine fuel feed lines in the 777 are about 3 inches diameter.

avrflr
19th Feb 2008, 19:27
avrflr i'm sorry you cannot post a polite reply. I'm just interrogating myself, and never mentioned fuel exhaustion, but in a low fuel configuration, it might have been possible that the tank LP pumps were not able to provide the adequate fuel to the HP pumps...

As you refer to the small items found in the tanks, I would like to see how you explain that almost simultaneously, two different items would have reduced two fuel flows to two similar values on both engines...
Terribly sorry for being so rude old chap, but ten and a half tons is not a low fuel configuration in my book. Plenty of 777s fly around happily with less than that. I wasn't suggesting that the debris found caused the accident, just pointing out that the contents of the fuel tanks has been given a thorough examination. If there was no fuel in the tanks, or less than the gauges indicated, they would have figured it out, I promise you.

I can't explain how the accident happened, but then neither can the AAIB at the moment and they do this for a living. They are not stupid or corrupt, if the answer was simple, like low fuel or contaminated fuel, they would have told us on day 1.

Pinkman
19th Feb 2008, 19:45
Gerry, you are reading my post literally. Replace 'post - event' with 'after the event' and you will see what I am trying to say: "how can you tell, from the evidence, when you look at the impeller after the event, that cavitation, which is a transient phenomenon, has actually occurred". Apologies for my clumsy british english phrasing:E. And the answer was given above by several people: pitting, damage, caused by huge pressure forces and detonation.

But despite all the slightly sarcastic comments to my earlier post no one has answered my question: If it isnt ice, wax, or water, what other common mode control law reduced one engine seven seconds after the other? I still refuse to believe that there is 100% independant authority and segregation between the two engines at all times and that there is never ever a point or situation at which the two engines in a modern aircraft with digital engine and fuel management controls have the function to compare thrust and balance accordingly. I just cant believe that you guys have to either continually adjust throttles to compensate for small variations in thrust between engines or use rudder trim to keep a constant heading (eg in the cruise)? How fuel inefficient that would be. There must at some point and in some flight modes be a digital comparison between the thrust produced in each engine and my question is still, is there concievably a flight mode or condition such that when one engine unexpectedly loses power (eg if fuel feed blocked by a red plastic scraper) the other engine is electronically limited to avoid... well you tell me. There must be a 777 sparky out there who has the FADEC decision logic.

Otherwise, there's only one explanation.. different pieces of crud left behind in different wings just happened to block respective fuel filter screens just seconds apart. I dont believe that and neither do you...

airfoilmod
19th Feb 2008, 19:50
My reading was that the Fuel was "in Spec". Is that "within Spec" for pure Fuel or does that consider that there is no "Spec" for Water, Alcohol, Glycol, or Canal Mud? I'd like to see collection protocol, transit, and testing methods before I defend the FUEL. (As in, what reality offered in the way of combustible supply to RR1 and RR2.) Just Sayin'.

PBL
19th Feb 2008, 19:52
I must admit, my initial response on hearing about this was that it was probably to do with FADEC or the 777 equivilent. However, on reflection I would have said it's highly unlikely, but still possible. With the certification process these aircraft go through, and how many commercial flights the 777 has done to date, the laws of probability would dictate that it ought to have manifested itself by now.

I have stayed out of this so far, but this supposition must be corrected.

The certification process and 3.6 million flight hours says virtually nothing about manifestation of SW faults. For an example, consider that the certification process and a large percentage of those flight hours failed to discover a significant bug in the flight control SW that manifested itself first in August 2005 out of Perth.

PBL

avrflr
19th Feb 2008, 20:06
My reading was that the Fuel was "in Spec". Is that "within Spec" for pure Fuel or does that consider that there is no "Spec" for Water, Alcohol, Glycol, or Canal Mud? I'd like to see collection protocol, transit, and testing methods before I defend the FUEL. (As in, what reality offered in the way of combustible supply to RR1 and RR2.) Just Sayin'. #124, #125
The spec for the fuel includes that it must contain less than (I think) 30 parts per million of water and that it must be free of solid contaminants. The AAIB report states that there was no significant quantity of water or other contamination in the fuel samples taken. I take this to mean that, being clever chaps, they considered the possibility that fuel contamination caused the accident and they went looking for it. They took samples from various parts of the engines, fuel system and tanks. If there was free, suspended or dissolve water, or solid contamination, or off spec fuel, I for one think they would have found it. I would like to know where this phantom fuel contamination went to hide from the investigators.

Pinkman
19th Feb 2008, 20:15
As an ex-fuel guy, if they've sampled properly, and actually taken a sample down to the worst case temperature and its OK, then its not fuel - period. And hopefully none of you - especially Danny - will remember what I offered to do if I was wrong in asserting it was waxing.

airfoilmod
19th Feb 2008, 20:21
I don't think I'm committing what I deplore. A conclusion of any kind is premature, speculation never is. Having been tested, the Fuel, to me, remains to be exonerated. Speculation, by its very nature, could lead to incrimination, which is silly at this point. My retort had naught to do with clever chaps, but ill advised exoneration of any procuring cause, which the chaps have not done, and someone, I think, thinks they have.

avrflr
19th Feb 2008, 20:27
You must know something about fuel that I don't. If testing won't exonerate it, what will?

I'm all for speculation, but let's have hypotheses that don't require me to believe that the AAIB are morons. Or that jet fuel has acquired some hitherto undiscovered properties that can't be revealed by fuel testing laboratories.

grebllaw123d
19th Feb 2008, 20:44
You say:

"is there concievably a flight mode or condition such that when one engine unexpectedly loses power (eg if fuel feed blocked by a red plastic scraper) the other engine is electronically limited to avoid... well you tell me."

The answer to this is a NO!
Such a feature does not make any sense - imagine what would happen in a takeoff with one engine failing!

Whether the engines on the 777 are digitally connected, I do not know .

I have flown B767 and A340, and these aircraft types have digital connections between their engines.

airfoilmod
19th Feb 2008, 20:47
The Final Report is the one to read. I'm not trying to be coy, the fuel that made its way to the Lab seems to have tested well, within Spec. No argument with that. What's missing is unfortunate. Exact position of Throttle Levers? AOA? Exact temperature of mixture entering LP and HP Pumps? How long Mixture "cavitated"? Where is the liquid (Sic) that transited out the hot end? Indeed, where is the material that hasn't been subjected to "tests". Post incident, there remains a percent accuracy for any test performed having to do with all systems. Without testing ALL the remnants of the tankage, no result is foolproof. These are dedicated professionals, who do excellent forensic work who also happen to be under a huge Microscope. From what's been released, either the throttle settings sequentially slipped to low thrust positions, or Quantity, Quality of Fuel was compromised. Turbofans are not terribly complicated; pump fuel, they provide thrust at very predictable levels. Withold fuel, they reduce output.

misd-agin
19th Feb 2008, 20:49
New software revision had been installed in December. I think it was a FADEC revision.

The accident is a puzzle. Software has not been ruled out by the AAIB(based of FAA letter Jan 29, 2008).

FE Hoppy
19th Feb 2008, 21:16
The problem with trying to attribute the fault to the FADEC (EEC) is that the report states that the metering valves operated correctly in response to the loss of thrust and opened. so the EEC worked. The thrust lever position was correct. This is either fuel or fuel flow or as a friend at work postulated today a large bubble of no oxygen air on the approach.

phil gollin
19th Feb 2008, 21:34
Quote :-

.............. Data, downloaded from the Electronic Engine Controllers
(EECs) and the QAR, revealed no anomalies with the
control system operation. At the point when the right
engine began to lose thrust the data indicated that the
right engine EEC responded correctly to a reduction
in fuel flow to the right engine, followed by a similar
response from the left EEC when fuel flow to the left
engine diminished. Data also revealed that the fuel
metering valves on both engines correctly moved to the
fully open position to schedule an increase in fuel flow.
Both fuel metering units were tested and examined, and
revealed no pre-existing defects. ..........

unquote

A question - IF, repeat IF, the fuel is o.k., the EEC commands were correct, the pumps, filters, etc... worked o.k. - then what is left ?

One thing I don't think is properly mentioned (?) is the ACTUAL operational records of the fuel pumps. Yes, they were tested afterwards and considered capable of working at full capacity, however, I PRESUME that they are variable speed, so would the speed that they were ordered to work at and that they actually worked at be recorded anywhere ?

.

FE Hoppy
19th Feb 2008, 21:37
One thing I don't think is properly mentioned (?) is the ACTUAL operational records of the fuel pumps. Yes, they were tested afterwards and considered capable of working at full capacity, however, I PRESUME that they are variable speed, so would the speed that they were ordered to work at and that they actually worked at be recorded anywhere ?


unless there was a problem with both gearboxes the speed of rotation of the input shaft to the pump is fixed to the speed of N2(3?) and therefore known.

brigordfly
19th Feb 2008, 21:49
Quote from #112:
Now in no way am I suggesting that on this 777 the generators magically re-connected themselves in flight. However sudden low pressure from all LH & RH pumps could (in theory) occur if the phase relationship of the power supply to all of the pumps suddenly changed.

Also, if the 777 is anything like the Fokker 100, it might only be the fuel pumps that would have a problem with this. How that might happen in practice I don't know - I would think it so unlikely as to be almost impossible. Also, even if it did happen, I would be surprised if such a loss of pressure starved the engines - but perhaps in combination with other things it is (theoretically) possible.



Suppose that the APU startup was actually the cause of the problem - Is it possible this was started by accident or automatically started and this messed up the electrical phasing?

TwinAisle
19th Feb 2008, 21:52
Interesting article in the back pages of the Eye this issue. For those who are not familiar with the Eye, it is a satirical magazine, but the last few pages tend to be reserved for more thoughtful writing. Article below, complete:

"A Software Triple Whammy

Air accident investigators have provisionally identified the cause of last month's British Airways Boeing 777 crash at Heathrow as a failure by both engines to provide the necessary thrust of power ahead of landing, causing it to fall short of the runway.

Why both engines shoud fail to respond ro demands from the "Auto throttle" and then from the two pilots is not yet known, but investigators are looking closely at the craft's computer systems. And so they should; aircraft and computer experts warned long ago of a potentially dangerous flaw in the software driving the 400-seater's three primary flight computers (PFCs).

Until the revolutionary 777 (dubbed the "computer with wings"), every passenger jet had a failsafe "triplex" system of PFCs - three computers from three different companies with three different teams writing the software. One computer would fly the aircraft, one would monitor and take over if a problem occurred and the third would be in reserve. A mistake in one was very unlikely to be in the second, and even more unlikely in the third. Thus safety in a craft that was to take autopilot to a new level was all but guaranteed.

So it was - at first - with the 777. Boeing took its proposals for a triplex PFC to the Federal Aviation Administration (FAA) which gave approval. But then things started to unravel. Without putting the software programme out to competitive tender, Boeing appointed GEC Marconi, based in Rochester, Kent, as sole writer. GEC had three different teams of software writers, separated by "chinese walls".

According to Boeing itself, however: "It became apparent that the three separate teams were having to ask Boeing so many questions for clarification that the independence of the three teams was irreparably compromised".

So instead of hiring new teams, which would have delayed the project and may have rendered Boeing liable for penalty payments, the three teams became one. Triplex was forgotten.

The result was 132,000 lines of software code, unprecedented in aviation history, which, it seems, could not be independently checked. The then chairman of the British Computer Society's safety critical systems taskforce, Professor Brian Wichmann, told Computer Weekly magazine (which revealed the potential flaw back in 1995) that "more than 20,000 lines of code are too complex to test" and that 132,000 lines of code were impossible to verify. If a bug or glitch caused a failure during a take off or landing, the plane could crash before the pilot had time to react.

But the system was approved, not by the FAA or the British Civil Aviation Authority (CAA), but by the European Joint Airworthiness Authorities (JAA) based in Holland - by just three JAA specialists.

As the certifying authority in America , the FAA asked for more comprehensive software audits, but then accepted Boeing's argument that the lines of code had already been tested and verified so extensively that any potential for error had been ruled out.

Mary Schiavo, the former director-general at the US Department of Transportation, revealed in her book "Flying Blind, Flying Safe" that the FAA's national software expert later admitted to her: "I'm in a very embarrassing position. To say that the software is safe, I can't tell you that. I can tell you that the software development has followed our procedures". So that's all right, then.

Air accident investigators say that they are focusing on a more detailed analysis of the flight recorder information and examining systems modules and equipment that could influence engine operation.

Clearly there are elements here that don't add up. But an interesting read....

airfoilmod
19th Feb 2008, 21:56
Were both pilots seated properly, restrained correctly? A/P to that height seems unusual, was this addressed? B/A SOP? To now, there isn't enough incontrovertible evidence from FDR, CVR to help the cockpit data along very far. 108 knots can't have been acquired with a functional A/P. Nor could it have been allowed by hand, absent ongoing low thrust. The A/P would not have allowed a Stall; Belly on beats nose in, yeh?. Stall is not a result any pilot would choose purposely, choosing it over belly in. Was the AOA (high) a result of the temporary power surge? Any noise heard on the ground that was unusual? Pop. scraping of metal, changing noise levels?
(Prior to "Landing").

phil gollin
19th Feb 2008, 22:01
Quote:-

unless there was a problem with both gearboxes the speed of rotation of the input shaft to the pump is fixed to the speed of N2(3?) and therefore known. Today 22:34

Thanks. A question - how is the speed controlled ?

Sunfish
19th Feb 2008, 22:13
Stall is not a result any pilot would choose purposely,

Yes it is.

The speculation here is bordering on surreal.

Oldlae
19th Feb 2008, 22:14
Phil,
The speed is controlled by the fuel going to the burners, N1 in my book, this is determined by the demands of the throttles set by the crew. This is basic gas turbine operation.

Oldlae
19th Feb 2008, 22:17
Phil,
The speed is controlled by the fuel going to the burners, N1 in my book, this is determined by the demands of the throttles set by the crew. This is basic gas turbine operation.

Green-dot
19th Feb 2008, 22:25
With reference to thread "AAIB BA38 B777 Initial Report Update 23 January 2008" (now closed):


Post #445, which i wrote, described first hand experience with fuel shutoff valves, the equivalent to the T7 spar valves, which closed uncommanded (no B777 or any Boeing aircraft were involved) during flight which resulted in mishaps.


There were several replies in reaction to my initial post #445 to which i subsequently replied.


I Replied to ChristiaanJ (http://www.pprune.org/forums/member.php?u=105267) (and Chris Scott (http://www.pprune.org/forums/member.php?u=218710)) but my reply, which dealt with testing and certification related to EMI/HIRF has been removed from that thread . . . . .


In response to the now missing reply the following persons responded of which i show abridged versions below:


"Post 451:
CONF iture (http://www.pprune.org/forums/member.php?u=104576) :


I'm still on the fuel but Green-dot is producing an interesting reading ..."


And:

"Post 452:
yamaha (http://www.pprune.org/forums/member.php?u=117013) :

I can't keep quiet any longer. Put EMI to bed. Its nothing more than trying to inject a conspiracy theory into the whole tragic affair.

It reminds me of all the A320 scaremongering 20 years ago. Lightning will bring it down, power cables will bring it down, the computers will be hijacked by virus's.....all humbug.

How many aircraft fly each day and how much EMI is out there 24/7.
Please get real."




To keep the record straight there is now a mis-representation of the actual amount of posts in above mentioned thread. It makes me wonder what caused my reply to be removed.


To "Yamaha" i would like to say that the word "conspracy" will never be part of my vocabulary, neither will scaremongering. I have 34 years of experience working on aircraft and their systems and that experience is based on facts!


With the AAIB Bulletin S1/2008 released yesterday, several issues are no longer a matter of concern. Many other issues are still on the table. EMI/HIRF is perhaps one of those issues.

I will wait for the final report with great interest.


Regards,
Green-dot

Machaca
19th Feb 2008, 23:34
Pumps made by Argo-Tech

excerpts from FAA TYPE CERT. DATA SHEET E00050EN (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgMakeModel.nsf/0/b443906400b2f413862572a4006eb74e/$FILE/E00050EN.pdf) (covers RR Trent 895-17):


NOTE 3. MAXIMUM PERMISSIBLE TEMPERATURES

FUEL
At outlet from HP fuel pump
xxUnrestricted xxxxxxxxxxxxxxxxxxxxxxx140C
xxMax. during transient overshoots on
xxreducing rpm (15 min. limit) xxxxxxxxx165C


NOTE 4. FUEL AND OIL PRESSURE LIMITS

FUEL

MINIMUM FUEL PRESSURE
Between sea level and 45,000 feet, not less than 5 psig plus true fuel vapor pressure, measured at inlet to engine LP fuel pump.

Maximum pressure at the engine inlet (measured at inlet of LP fuel pump)
xxContinuous xxxxxxxxxxxxxxxxxxxxxx60 psig
xxTransient xxxxxxxxxxxxxxxxxxxxxxx70 psig
xxStatic xxxxxxxxxxxxxxxxxxxxxxxxx170 psig



-=MachacA=-

avrflr
19th Feb 2008, 23:36
Were both pilots seated properly, restrained correctly? A/P to that height seems unusual Not to me To now, there isn't enough incontrovertible evidence from FDR, CVR to help the cockpit data along very far. 108 knots can't have been acquired with a functional A/P. Says who? Nor could it have been allowed by hand, absent ongoing low thrust. The A/P would not have allowed a Stall Correct, hence A/P disconnect. Belly on beats nose in, yeh?. Stall is not a result any pilot would choose purposely, choosing it over belly in. Was the AOA (high) a result of the temporary power surge? No, the AOA was high because the A/P was trying to keep the aircraft on glideslope, but since the engines weren't co-operating, it couldn't.

Milt
19th Feb 2008, 23:57
Machaca

Now you are posting specifications for an engine LP Low Pressure whereas most of us believe that the only pumps on the engine are the high pressure gear type pumps.

We would much appreciate knowing the specs for individual High Pressure pumps particularly max output prerssure versus flow rate and minimum inlet pressure to avoid cavitation.

You may be able to throw in the minimum measure of lubricity in fuel for continuous operation and how is it measured.

When we operators talk about LP Low Pressure pumps we mean tank booster or transfer pumps.

MU3001A
20th Feb 2008, 00:27
The first officer took control for the landing at a height of approximately 780 ft, in accordance with the briefed procedure, and shortly
afterwards the autothrottles commanded an increase in thrust from both engines.

A little late for a handover of control don't you think? Is it SOP within BA for copilots to fly such short legs?

glob99
20th Feb 2008, 00:40
The report states,

The recorded data indicates that there were no anomalies
in the major aircraft systems. The autopilot and the
autothrottle systems behaved correctly and the engine
control systems were providing the correct commands
prior to, during, and after, the reduction in thrust.

Explain to me how a software or EMI or EEC problem could cause this accident and nothing bad shows up in the thousands of recorded parameters.:confused:

StudentInDebt
20th Feb 2008, 03:21
MU3001A

BA SOP is for the P1 for the sector (Captain or FO) to hand over control before TOD to the P2. The P2 then flies the aircraft until the P1 becomes visual and takes control, the theory is that at DA the P1 is looking out, sees the runway and takes control for the landing. If operating with the weather above DA this handover of control usually takes place shortly after 1000RA. In this case it appears that the First Officer was the P1 for the sector and the timing of the handover was not unusual, PEK-130 miles before LHR is quite a long leg.

FirstStep
20th Feb 2008, 03:55
Report states that crossfeed valves were closes for entire flight...and 10K fuel balanced for the most part in the two main tanks....
Was center tank fuel uploaded for this flight, and if so, roughly how much?.
As I am unfamiliar with the 777 ( only the 747-200 & -400 ), are the CWT fuel boost pumps "override" pumps, as in the type that will put out more pressure than the mains?. And if so, is there any limitations to having them on below a certain volume?. Could there be any possibility that CWT fuel ( mabye unscavenged fuel ) at highpressure was feeding/cavitating both engines?. Would only the pump cavitate, or would air be induced into the system?.

I know SOP would most likely have had Center tank emptied by now, and pumps switched off, but again I'm looking for a "common feed" so to speak.
Again, I am unfamiliar with the 777, and I know there is a wealth of knowledge spread ( unevenly ) amongst those who peruse this forum.
---------------------------------------------------------------------
As far as the kudos for the "wonderful" job the Autopilot did...
If I were handflying this approach, and lost thrust, would I sacrifice altitude for airspeed ( to prevent a stall ), damm fckng right I would.
And YES, with a loss of thrust, I KNOW I could FLY it to the ground a WHOLE lot better than the A/P. This is airmanship 101 guys. The only question I have is at what point do I realize I had no thrust and then act.

As I have no doubt there are those that will lay into me....lube liberally pre-applied.

Wornout Rubber
20th Feb 2008, 03:56
It seems apparant that the A/P remained engaged down to 175 feet. Although the A/P is a pilot`s best friend in most emergency situations, perhaps it was not the case in this instance.

The speed at 750 feet must have been about 140 Kts.
The speed at 200 feet was 108 Kts.

As the the A/P was engaged, the AFDS was trying to maintain the glideslope, with a probable linear speed decay. The speed loss was approximately 32 kts in 500 feet.

Every kt of speed below VRef 30 would result in a worse Lift/Drag ratio, with the result that the aeroplane could not eek out as much distance as it potentially could if it was flying at it`s best L/D ratio speed (approx VRef). At 108 Kts, the L/D ratio would be significantly reduced.

VNAV PATH
20th Feb 2008, 04:07
:O Hello,

New member in this Forum , and flying 777 outside UK .

My SOP seems quite different than the one described : mine is telling that ( except Cat 2/3 ops ) PF is flying whole sector : no hand over in approach .

Does BA SOP mean the PF has the controls until TOD and take it again at 1000 AGL ? And what reason motivate such a practice ?

Am I correct ?

Thanks !

avrflr
20th Feb 2008, 04:48
Was center tank fuel uploaded for this flight, and if so, roughly how much?About 20800kgs, depending on S.G.

CONF iture
20th Feb 2008, 04:52
Thanks for that clear explanation, I was as surprised as MU3001A, but discovering that specific BA procedure (?), AAIB report makes now more sense.

What still does not make any sense considering that almost simultaneous double eng similar misbehaving:

1- "total fuel on board was indicating 10500 kg" ... but no mention of the actual FOB retrieved from the main tanks ?

2- "the fuel crossfeed valves indicated that they were closed" ...but no mention they have been physically checked closed ?

Also:
- What could justify that amazing 5% reduction over the planned fuel burn ?
- Did the engines actually spool up somewhere in the hold ?

phil gollin
20th Feb 2008, 05:00
Quote :

Phil,
The speed is controlled by the fuel going to the burners, N1 in my book, this is determined by the demands of the throttles set by the crew. This is basic gas turbine operation.

unquote

I meant the speed of operation of the fuel pump.

The fuel valves were open, the throttles (and auto-pilot) demanded an engine speed higher than that acheived, the actual speed of the fuel pumps doesn't seem to have been stated in the special report.

.

HotDog
20th Feb 2008, 05:01
Milt,When we operators talk about LP Low Pressure pumps we mean tank booster or transfer pumps

Not so, the fuel from the aircraft supply system (read tank fuel booster pumps) is fed into a compined LP and HP fuel pump assembly on each engine.

Mr @ Spotty M
20th Feb 2008, 05:22
How can the total fuel on board be measured when an amount is soaking into the grass at the crash site.
Could this be why they AAIB state indicating?

TheInquisitor
20th Feb 2008, 06:05
Somebody mentioned electrical disruption further back in the thread, and I feel it may merit some thought here.

Question - are the tank booster pumps normally in operation in this configuration all fed from the same bus? (not a 777 operator, BTW - I'm assuming here that they're the standard 3-phase type commonly employed).

Reason being, I have come across an odd mode of electrical failure on a previous type, whereby a single phase on a single bus tripping off creates all sorts of oddities - the cause of which are not immediately obvious, since a motor with a dead phase acts as a generator on that phase (still being driven by the 2 good phases) preventing any indications of electrical failure, but sending oddly-shaped wigglyamps to other components.

Might be a worthless tangent, then again, might not be. Just something else to throw into the collective headscratching!

Volume
20th Feb 2008, 06:12
Some new AAIB statement could be found in the Guardian (http://www.guardian.co.uk/world/2008/feb/19/transport.theairlineindustry?gusrc=rss&feed=travel) today:

Kieran Daly, editor of Air Transport Intelligence, said: "The AAIB have effectively ruled out virtually all the potential causes of the accident. They are now following two lines of inquiry. There were signs that not enough fuel was reaching the pumps that finally pump the fuel into the engines and they are looking at the debris in the fuel tanks."


And the Telegraph (http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2008/02/19/nheathrow119.xml) writes

"Detailed examination of… fuel pumps revealed signs of abnormal cavitation (air bubbles) on the pressure-side bearings and the outlet ports. This could be indicative of either a restriction in the fuel supply to the pumps or excessive aeration of the fuel."

NSEU
20th Feb 2008, 07:00
On reflection, I think we can safely ignore the bits and pieces in the tanks as causes of fuel starvation (scraper, paper, etc). Remember that there are 2 pumps in EACH tank. All four tank pump inlets would have to be blocked to cause the problems seen. Each pump is designed to provide enough pressure/flow to supply the onside engine with fuel for all flight regimes (and probably more). The lowest spec I've seen for a 777 wing tank pump is 12psi and 16tonnes/hr

I don't have the wiring schematics handy, but I believe the L/R wing tank pumps are powered by their onside Transfer Busses.

Rgds.
NSEU

PBL
20th Feb 2008, 07:20
TwinAisle cites Private Eye's article, and says "clearly there are elements that don't add up".

There are two major points that "don't add up".

First, the article asserts that FBW systems before the B777 were triple-redundant, designed/written/implemented by three separate teams. That is rubbish.

Second, the article concentrates on the PFC (and maybe on the AIMS). I don't think anybody has suggested there was something wrong with primary flight control.

PBL

sky9
20th Feb 2008, 07:24
Just a thought for the boffins

AAIB report:
Detailed examination of both the left and right engine high
pressure fuel pumps revealed signs of abnormal cavitation
on the pressure-side bearings and the outlet ports. This
could be indicative of either a restriction in the fuel
supply to the pumps or excessive aeration of the fuel. The
manufacturer assessed both pumps as still being capable
of delivering full fuel flow.
If there was a restriction upstream from the HPFP’s would the effect be to cause the pumps to effectively stall and prevent them from working? A cavitating pump does not generally pump much even if it is: “capable of delivering full fuel flow”.
Such a scenario would account for only minimal amount of fuel reaching the engine. Has there been a fleet check of the pumps to ascertain whether this is a one off or a common defect?

There is also a major discrepancy with the planned fuel burn and the actual. Planned burn including taxi fuel would work out at 72,000 kgs actual burn to short finals was only 68,500 based on the AAIB figures. Was this based on FMC fuel indications or gauge indications?
Unless there was a reason for a 5% difference such as an improved route or altitudes it seems a little large to me.

bubbers44
20th Feb 2008, 07:33
Seems like any pilot dealing with decaying power as this was would put speed control requiring to push the nose down below the requirement to not hit the ground before an obstacle. Clearing the fence and hitting a flat surface saved the people in this case so the pilots did all they could under the circumstances even though they had to sacrifice speed to do so. All they could have done was possibly add flaps before hitting the ground to get closer to the runway.

NSEU
20th Feb 2008, 07:41
As I am unfamiliar with the 777 ( only the 747-200 & -400 ), are the CWT fuel boost pumps "override" pumps, as in the type that will put out more pressure than the mains?. And if so, is there any limitations to having them on below a certain volume?. Could there be any possibility that CWT fuel ( mabye unscavenged fuel ) at highpressure was feeding/cavitating both engines?. Would only the pump cavitate, or would air be induced into the system?
I know SOP would most likely have had Center tank emptied by now, and pumps switched off, but again I'm looking for a "common feed" so to speak.
Again, I am unfamiliar with the 777, and I know there is a wealth of knowledge spread ( unevenly ) amongst those who peruse this forum.

The CWT pumps put out more pressure/flow than the wing pumps. Low pressure output at the CWT either due to the pumps being switched off or due to no fuel in the tank will cause the check valves on the CWT pumps to close, preventing air getting into the manifolds. Like the 744, this process will occur well in advance of landing. Remaining fuel is scavenged via jet pumps (and as discussed previously, this process completes well before a normal landing).

I believe someone asked earlier.... There is no gravity feed for the CWT.


As far as the kudos for the "wonderful" job the Autopilot did...
If I were handflying this approach, and lost thrust, would I sacrifice altitude for airspeed ( to prevent a stall ), damm fckng right I would.

...but if pulling back on the stick with the stickshaker ringing in your ears for a few seconds longer meant narrowly missing a large building (and you knew there was a flat surface beyond)? Wouldn't you take the chance that all those built-in margins for stall might work in your favour on that particular day?
Personally, I think man is a gambler... He is taught what is impossible, but when faced with a large immovable object, he goes for the impossible ;)

swiss_swiss
20th Feb 2008, 07:42
phil - you asked about the speed of the HP fuel pumps and how they are controlled - it should be noted that the pumps are not electrical driven but are connected to the the gearbox and as such the gearbox is driven via a shaft which is in turn connected to the N2/N3 compressor spool so the pump speed is not and can not be directly controlled.

regarding the theories about a missing phase from the IDG, this would be sensed and the idg would be disconnected immediatly so that is a non starter

TheInquisitor
20th Feb 2008, 07:57
swiss_swiss,

All things being equal, I agree. But I have personally seen a failure mode where a dead phase on a bus was supplied with enough voltage to keep the genny / bus from tripping off, via the two still live phases powering an electrically driven hyd pump. A generator and a motor are essentially the same thing - you spin a motor with a dead phase, and it will generate on that phase and feed any component connected to that phase on the bus. Frequency lockout wouldn't necessarily pick it up either, if the remaining 2 live phases of the motor could draw enough juice from the bus to keep it spinning at close to its normal running speed.

Make sense, or am I not explaining this very clearly? Just to clarify, this isn't idle speculation, this has actually happened to me (on another type, considerably older than 777)

Sven Sixtoo
20th Feb 2008, 08:08
Phil

If it's anything like the Fuel Control Unit on my aircraft (which also has R-R engines), the HP Pump pushes out considerably more fuel than the engine needs. A variety of spill valves bleed off fuel for various reasons. The most important is a system that ensures a constant pressure drop across the throttle valve. Adjusting that valve then produces a given flow for a given setting regardless of what the pump is up to (within limits obviously). The electronic side of the engine control adjusts the throttle valve, and a monitoring system tells the electronics whether or not the desired result has been achieved.

What seems to have happened here is that the desired result was not achieved, so the electronics continued opening the throttle (that's all they can do in the last analysis) in order to get the desired result - which didn't happen.

swiss_swiss
20th Feb 2008, 08:15
hi inquisitor

your description i can easily follow and can understand how that could be possible. ( i am well versed in motor/generator theory).
mind me asking which type this defect was on and how the system was monitored?

AFAIK (would have to go back and look at my training notes) each individual phase of the 777 is monitored so if 1 is missing then the gen is switched off - would have to confirm this tho.

phil gollin
20th Feb 2008, 08:28
Re. Fuel pumps.

Thank-you "Swiss Swiss" and "Sven Sixtoo".

If I have got it right, the fuel pumps work continuously at maximum rate (when switched on) and the actual fuel flow to the engines is controlled by the control valve AND spill valves which get rid of any excess pressure.

Are the spill valves actuated, or merely like saftey valves, operated mechanically by excess pressure ?


Thanks

.

swiss_swiss
20th Feb 2008, 08:42
they work all the time when the engine is running ( i of course assume u are talking about the engine mounted mechanical driven pumps) and not at maximum rate - maximum rate 100% of the time is not possible because the speed of the pump is determined by the speed of the gearbox and therefore the engine- at any given speed the fuel pumps supply more fuel than is need - this excess fuel is then bled back toward the engine pumps via actuated valves and not by preset pressure relief valves

SILENT_BADGER
20th Feb 2008, 09:13
My experience is on Airbus so I'd be very happy for someone with Boeing fuel system knowledge to correct me on this.

1) Centre tank pumps usually pump at a higher pressure than wing tank pumps so that the centre tank fuel is used up first.

2) When the centre tank is empty sensors in the tank tell the centre tank pumps to stop. By the latter stages of a flight one would expect the centre tank fuel to be used up.

3) It is important that LP tank pumps are immersed in fuel so that they don't introduce air into the system, also the fuel provides a cooling function for the pumps.

4) In some early configurations of the A320 the centre tank pump sensors had a bit of an issue and not switch off the pumps when the tank was empty. This would trigger an ECAM (FUEL CTR TK PUMP LO PR) and we'd switch off the pumps manually. To avoid damaging the pumps there was an SOP not to switch the centre tank pumps on in the first place unless there was a certain amount of fuel in the tank. Its fixed now - well on our fleet anyway.

5) So, you're probably ahead of me, what if the sensors in the 777 center tank failed or were fooled and the pumps operated whilst the tank was dry? Would EICAS alert on something or could they keep going long enough to put enough air into the system to balls things up?

Centre tank obviously feeds both engines albeit via different LP feeds but it is not impossible for two pumps to be running.

Happy to be shot down - I've only given it a few minutes thought.

Cheers,
Silent Badger

tanimbar
20th Feb 2008, 10:05
Hello,
First post; I'm not flight crew, nor engineer, just scientist.
Apologies for trespassing on your professional forum to ask the following but my curiosity has gotten the better of me.

Can fuel on a long, cool flight become stratified?

(Fuel is probably the wrong term considering it is a mass, held within wing tanks, consisting of fuel sensu stricto, other hydrocarbon derivatives, H20, emulsions and various physical particles all of which will behave differently given the same physical environment.)

If the answer is yes then follow-on questions are:
1) Can fuel stratify by density and/or thermal anomaly?

2) Does the ground testing of the fuel take account of possible in-flight stratification?

3) Given that the fuel in both wing tanks was of almost equal weight (bar 30Kg), is it possible for the tank outlets in both wings to encounter a 'bad' layer/stratification at almost the same time?

Regards, Tanimbar

Hand Solo
20th Feb 2008, 10:15
I would think stratification would be unlikely given the aircrafts frequent manouvring and encounters with turbulence. The fuel would have been well and truly mixed up for most of the flight and certainly for the last 30 minutes.

bsieker
20th Feb 2008, 10:25
It is my understanding of the water scavenge jet pumps that they would, while trying to keep the water emulsified in the fuel, would also mix the fuel, preventing stratification.

A minor slip: the imbalance was 300kg, not 30kg, which makes it still less likely that both engines would be fed the same undigestible type of stratum at roughly the same.


Bernd

tiggerm0th
20th Feb 2008, 10:34
you asked about the speed of the HP fuel pumps and how they are controlled - it should be noted that the pumps are not electrical driven but are connected to the the gearbox and as such the gearbox is driven via a shaft which is in turn connected to the N2/N3 compressor spool so the pump speed is not and can not be directly controlled.

MAGNETIC SLIP

What are the sealing arrangements of the HP pumps? Do they have a double mechanical seal or are they magnetically coupled?

If they are magnetically coupled (mag drive) then they can suffer from magnetic slip and lose coupling all together. This would not necessarily however show signs of cavitatation. Magnetic slip can occur quite readily in large pumps when the demand on them is too sudden (on electrically driven ones, the star-delta needs to be set up to apply demand more gradually).

FOAMING

Does anybody know anything about the foaming tendencies of Jet A1, especially at low pressures and temperatures?

akerosid
20th Feb 2008, 10:51
Probably not a huge surprise, but BA has confirmed that 'YMMM is a total loss.

http://www.flightglobal.com/articles/2008/02/20/221689/ba-to-write-off-crashed-777.html

skridlov
20th Feb 2008, 10:55
Hi PPRUNE. Non-pilot,non-anorak, first post. I've been reading the Rumours board intermittently for years so I realise that taking the risk of posting on any technical issue is foolhardy, but here goes. I offer it only for what it is.

Many years ago I was involved in an operation that required pumping lots of water from a river, using a diesel engine (250hp) driving a 4 inch Ajax centrifugal pump. One day I replaced the cage which was fitted to the (non-return) foot-valve - which was intended to prevent large stones being drawn into the suction line. It was badly damaged, so I placed a stainless steel mesh directly across the valve opening (ie "2d" replacement for a "3d" predecessor).

To cut a long story short, we repeatedly encountered delivery failure after about an hour of operation. When checked, we discovered that the pump was partially full of air and consequently was cavitating. I assumed that this was due to a leak on the suction side and repeatedly refitted the suction hose (an 8", heavy, beast) with added sealant and many clamps.

Then it dawned on me to take into account the recent changes to the foot-valve protection. Clearly the restriction to the flow was causing air to gradually come out of solution, hence the eventual cavitation. After replacing the grid with a fabricated box of the same material, the problem vanished.

I have read a lot of the preceding thread. I understand a little of it - in very broad terms anyway. I don't recall if anyone has implied the cause of the "aeration" which has been referred to, except in relation to the possibility of leaky seals on the lp side. But I kept being reminded of my own experience described above. It would only be relevant if there is indeed any dissolved gas/air in jet fuel etc etc, about which I have absolutely no idea.

GearDown&Locked
20th Feb 2008, 11:06
Simple question. How does fuel quality varies when we mix different kinds of fuel in the tanks?

From what I've learned here, normal Jet A1 has a different chemical composition from the fuel this crew used out of China. A chemical reaction between different additives could probably lead to quality degradation regarding its published freezing point.

Walnut
20th Feb 2008, 11:26
Sky9 asked if there has been a random (say 6) check of other BA 777s HP fuel pumps with a time history the same as the crashed a/c. I hope they have. All mechanical parts wear with usage. If they find much the same wear then this is not significant.
What I would consider is the flow path as to why? the engines slowed to just above flt idle, was it commanded and did the throttles close?, or did the pwr just drift away.
I realise that both the a/p & crew asked for more pwr once they became aware but by then it was too late.
I also note the a/c used 5% less fuel than predicted, on a 10hr flt this is 30mins saved. Unless the on route conditions (w/x) where significantly different this is an almost impossible thing to happen.

cats_five
20th Feb 2008, 11:45
Simple question. How does fuel quality varies when we mix different kinds of fuel in the tanks?

From what I've learned here, normal Jet A1 has a different chemical composition from the fuel this crew used out of China. A chemical reaction between different additives could probably lead to quality degradation regarding its published freezing point.

From AAIB Special Bulletin:

"The specified freezing point for Jet A-1 fuel is -47ºC; analysis of fuel samples taken after the accident showed the fuel onboard the aircraft had an actual freezing point of -57ºC."

GearDown&Locked
20th Feb 2008, 11:53
Yes, I've read it on the report also.
I was thinking about how does the hardware (fuel pumps, fuel lines, valves, etc) handle those mixes.

swiss_swiss
20th Feb 2008, 12:23
tigger - the pumps are mechanicly driven via a solid shaft from the gearbox, no magnetic clutch anywhere

FullWings
20th Feb 2008, 12:24
There is also a major discrepancy with the planned fuel burn and the actual. Planned burn including taxi fuel would work out at 72,000 kgs actual burn to short finals was only 68,500 based on the AAIB figures. Was this based on FMC fuel indications or gauge indications?
Unless there was a reason for a 5% difference such as an improved route or altitudes it seems a little large to me.
Actual fuel burn can be less than planned for many reasons that include:

Shorter routings
Better levels
Lower ZFW
Shorter taxi
Lower Cost Index

Over a 12-hour flight, lower actual aircraft mass can have a significant effect, especially if it allows a climb to a more efficient level earlier than planned.

Prangster
20th Feb 2008, 12:58
Having followed the threads good bad and plain ugly and read the AIB reports to date all I can add is curiouser and curiouser said Alice.

RatherBeFlying
20th Feb 2008, 14:23
The AAIB is explicit in saying that flap 30 was selected. No mention is made of decreasing the flap selection.

The usual jet airliner L/D of 18 is for the clean configuration and I suspect that flap 30 yields substantially less. A simulator run would present the raw F30 L/D, but in any case we did have some thrust and the achieved slope and L/D is derivable from the FDR.

There are a number of flapped gliders that use flaps to add drag. The big caution with flapped gliders is not to add too much flap as you can lose considerable altitude reducing flap. Many glider approach accidents have happened when flap was reduced with insufficient altitude.

I would not want to explore in the air how the 777 with thrust restricted to the accident setting reacts to a flap reduction attempt at 600', but it's possible the AAIB will decide to investigate that in a simulator.

In a high drag situation, increasing airspeed increases drag substantially. With flap 30, the best L/D speed would be lower than with lesser flap selections.

With gliders that have powerful spoilers and/or flaps, you can select full spoilers and/or flaps full on and if you are still not coming down steeply enough, add airspeed to steepen the slope.

In this accident, we see a trade of airspeed for glideslope, mostly at the behest of the autopilot. Given the touchdown point and the 108 kt. cited by the AAIB, they were amazingly lucky.

Mariner9
20th Feb 2008, 14:30
"The specified freezing point for Jet A-1 fuel is -47ºC; analysis of fuel samples taken after the accident showed the fuel onboard the aircraft had an actual freezing point of -57ºC."

Guys, you are confusing specifications and actuals. The spec is maximum -47C not exactly -47C. Minus 57 complies with the spec.

Kero is not known for incompatibility problems so mixing differing compositions not currently* thought to be an issue in the industry

*Take it from me, this is currently being investigated :oh:

Danny
20th Feb 2008, 14:35
The AAIB is explicit in saying that flap 30 was selected. No mention is made of decreasing the flap selection.

Which is why each time someone claims or states (mistakenly or presumptuously) that flap 20 was selected, I move the post to the WAG's thread. :rolleyes:

phil gollin
20th Feb 2008, 19:12
Quote (re. fuel pumps) (from Swiss Swiss) :-

they work all the time when the engine is running ( i of course assume u are talking about the engine mounted mechanical driven pumps) and not at maximum rate - maximum rate 100% of the time is not possible because the speed of the pump is determined by the speed of the gearbox and therefore the engine- at any given speed the fuel pumps supply more fuel than is need - this excess fuel is then bled back toward the engine pumps via actuated valves and not by preset pressure relief valves

unquote


Thanks again - very helpful,

So, if the spill valves are actuated to actually relieve the pressure between the fuel pump and the control valves (which were confirmed as being open) are the actuation messages recorded ? If so would their operation have been covered by any of the "satisfactories" in the AAIB report ? The Special Bulletin does say that the EEC worked correctly, but the listing of items tested does not include the spill valves.

Oldlae
20th Feb 2008, 21:25
Scanx3
I am concerned about the quoted fuel figures, they planned to use 72100kgs of fuel and according to the recorded quantity they had used 68500kgs, saving 3600kgs. If this amount was saved by flying higher than planned why not fly at that height everytime. I note that the FQPU was tested but I assume that this unit gathers information from the quantity sensors in the tanks or whatever system the 777 has for fuel quantity, testing of those sensors should be straightforward and I would have thought been mentioned in the report.

Locked door
20th Feb 2008, 22:08
Oldlae,

Ops across the Siberian areas are not as precise as you seem to think they are. The loadsheet (and therefore calculated fuel burn) is provisional based on all male pax and predicted cargo. The final loadsheet is recieved after pushback and contains actual cargo and a Male / Female split therefore the final figures are usually a few tonnes less than the provisional figures. The flightplan fuel burn is derived from the provisional weights and ANTICIPATED atc altitude clearances (remember Siberia is metric non rvsm so there are large differences between available levels). If the crew are able to negotiate more efficient clearances enroute more fuel savings can be made. Also if the crew are able to negotiate better levels enroute the wind may also be more in their favour. All this for 10+ hours flight.

My point is that if all factors go the crews way it is not unusual to land with a few tonnes more fuel than planned. Likewise if all factors go against the crew they will have to use some or all of their contingency fuel. That's what it's there for.

Those of you that think it's significant that BA038 used less fuel than anticipated to get to LHR or that fuel quantity indications were innacurate are barking up the wrong tree.

LD

Oldlae
20th Feb 2008, 22:15
Ref the open spar valves, (LP cocks) I am surprised that the anomaly had not been picked up on the simulator. The FAA requirement to allow the correcting modification, splicing of the wiring, not to be completed until 2010 was probably based on the Boeing cockpit drill being carried out as directed. This appears to be such a simple mod that I question the thought process of however decided not to do the mod asap would not have ever envisaged that the cockpit drill was not being followed to the letter. This lack of communication is always a problem with large companies and I hope that a valuable engineering lesson has been noted. Always check any changes in procedures with the engineers.

Green-dot
20th Feb 2008, 23:14
Quoting the AAIB update bulletin S1/2008:

"The Air Accident Investigation Branch (AAIB) was informed of the accident at 1251 hrs on 17 january 2008 and the investigation began immediately."

Given the specific nature of this accident and the AAIB starting this investigation with an open mind towards any aspects that may have been contributing factors, which means securing the area and start collecting evidence as soon as possible. Is there a protocol to collect or register all personal electronic devices (PEDs) from the persons who "arrived" on this aircraft?

Since i have not read anything about this issue in the bulletin, that could mean several pieces of evidence may be missing if this has not taken place. Or are these PEDs to be traced (which seems harder to achieve) at a later stage if the investigation should have grounds to follow leads which take them in the direction of possible EMI?

Just a thought.

Regards,
Green-dot

SyEng
20th Feb 2008, 23:23
I am not familiar with the 777 fuel system. If anyone is in a position to share schematics or system descriptions I would be most interested. However, with the information gleaned from this and the previous thread plus the AAIB special report, I feel that the following scenario is plausible and deserves further investigation.

This appears to be a common-mode failure. The AAIB appears to have exonerated as far as reasonably possible the engine control systems. Another potential common-mode source seems to be the centre-tank fuel system. According to my reading of NSEU's post #187, the centre tank boost pumps are "override" AKA "priority" pumps i.e. they provide a higher delivery pressure than the wing tank pumps in order to use centre tank fuel first (wing bending relief). This theory rests on that premise.

1) Significant quantities of ice accumulate in the centre tank.

-Posssibly over more than one sector if it does not have chance to thaw during turn-arounds. Maybe supported by CT excessive water warnings reported in, I think, the previous thread, from the American "leak". Maybe exacerbated by scavenge systems contaminated by FOD. Added to during the final descent.

2) Ice melts during descent, hold, approach, yielding a few litres of free water in "empty" centre tank.

3) Config change to flaps 30 at ~1000' leads to attitude change allowing water to migrate to CT boost pump inlets.

Note by design, boost pumps are typically located at lowest point in tank at typical cruise attitude. Increased flap => more nose down for same speed.

4) Fault condition (or crew action) causes CT boost pumps to re-start, or fault condition allowed them to keep running when tank emptied with no indication (or no reported indication).

5) "Override" pump system design causes CT pumps to win over the wing tank pumps and water/fuel mix from CT is consequently supplied to engines in preferance to wing tank fuel.

Note that following the landing, the open LP (spar) valves allowed the contentents of the fuel feed pipes to be deposited onto the sod. Similarly, the CT fuel contents could not be assessed for water contamination as the tanks had been disrupted and subsequently contaminated by the firefighters.

BarbiesBoyfriend
20th Feb 2008, 23:27
AP LEFT IN 'til fell over due (no doubt) to a/c unable to follow GS.

HERO pilot 'took over' and wrote a/c off in high sink rate landing.


:ugh:

wiggy
20th Feb 2008, 23:43
Well if "Hero" pilot hadn't taken over, er "ken" and the aircraft had been left to it's own devices the sink rate could well have have been a darn sight higher, with even more serious consequences..... I think your comment that he "wrote the a/c off" is, putting it politely, ********.

Mad (Flt) Scientist
21st Feb 2008, 00:09
I hope that a valuable engineering lesson has been noted. Always check any changes in procedures with the engineers.

Ah, but WHICH engineers? The operator's engineers are unlikely to be familiar with the details of the certification of the design. Nor, for that matter, are the OEM's support staff, who may be an entirely different part of the organisation to the design staff. Similarly the Airworthiness Authority personnel dealing with operational matters may have little interaction with the Certification teams, and the same applies to those approving STCs, which may not even be approved by the original state of design.

Even if you can track down the right departments (the OEm's design staff, and/or the airworthiness people who actually did the cert approval), there's still no guarantee the people there NOW know why a specific design choice was made.

DraggieDriver
21st Feb 2008, 00:43
1) Significant quantities of ice accumulate in the centre tank.

Syeng, problem with number one there. Centre tank is the warmest tank, so significant quantities of ice are unlikely to accumulate there. The reason fuel temp is measured in the wing tanks is because that is where your coldest fuel will be. Also, centre tank is emptied first, so fuel spends less time in there to cool down. It is very rare to be planning to land with fuel in the centre tank, so you're not going to get buildup of ice over multiple legs.

TheInquisitor
21st Feb 2008, 00:57
swiss_swiss,

ref #191, previous type was C130. Yes, mainly mandraulic and clockwork, so perhaps no direct read-across.

Electrics were monitored for over / under freq (each phase), over & under volt (each phase and average across the 3). If memory serves, any phase drops below 70v or average drops to 90v across all 3, genny trips off. In the specific incident, 1 phase of a genny failed (4 EDGs supplying 4 seperate AC busses). That bus was powering the Aux Hyd pump, which was generating enough back EMF on the dead phase to keep the bus powered and prevent the genny tripping off. The problem only showed itself after the gear was raised and the Aux Hyd pump turned off.

Reason I thought it was worth a mention? It hadn't occurred previously, in some 40 years of ops. Indeed most operator's (myself included) immediate reaction was that such a scenario could not occur, yet once the problem was investigated it became clear that it could. It's an example of something nobody thought of, manifesting itself late in an ac's service life.

I appreciate that modern machinery is way more complex, however, and I imagine more closely monitored to boot - so still, may or may not have relevance! I'm sure 777 operators such as yourself could rule such a scenario in or out of possibility.

Backoffice
21st Feb 2008, 01:41
Now I'm sorry to show my ignorance here, but where is air drawn into the tanks when the fuel is pumped out.
i.e could that be blocked by ice causing too low an air pressure for the pumps to suck the fuel ?

psalm139_9
21st Feb 2008, 01:46
SyEng, inlets for the center tank pumps are not located at the lowest point of the tank. In fact, there is a substantial amount of residual fuel which they can't touch. Once this level is reached, the center pumps are incapable of producing pressure.

Fuel feed then commences from main tanks, while automatic scavenge pumps slowly and automatically transfer the residual center tank fuel to the main wing tanks. Typically, the center tank is literally empty after a flight of this duration.

avrflr
21st Feb 2008, 03:41
The ice theories are the most enticing. I talked to a QF engineer today who was totally convinced it was ice blocking the fuel-oil heat exchangers. So if we accept for a moment that this hypothesis is correct, can someone explain how this fairly large quantity of ice made it's break for freedom and evaded the air accident investigators? Is it possible it melted and ended up on the grass?

The trouble I still have with the ice theories is that, if there is ice in the fuel tanks, it can't end up all in one place. Some would have to be still in the wing tanks waiting to be discovered.

cwatters
21st Feb 2008, 06:36
Problem with ice theories is no water was found in the main wing tanks.

Any significance to that loose union?

sandbank
21st Feb 2008, 07:10
Suppose....just suppose...this accident has nothing whatsoever to do with the fuel or the pumps or the pipes.

Suppose that - for some reason - all that happened is that the autopilot was never disengaged - and stayed in control all the way to the ground.

Would the outcome have been any different?

finncapt
21st Feb 2008, 07:36
I was intrigued by the discrepancy between planned and actual arrival fuel.

Fuel tanks measure volume (albeit it is displayed as mass on the gauges).

Pumps supply volumes.

Engines produce power depending on the mass of fuel.

If the density of the fuel had changed during flight - it may be of sufficiently low density that even with the pumps operating at full volume - the mass of fuel presented to the engine was insufficient to produce the required power.

I am not a chemist, just a retired driver airframes, is it possible that a gas (air) could have been mixed with the fuel to lower the density during flight?

This may explain the apparent fuel discrepancy.

There are many variables that affect fuel usage on long sectors but, with several thousand hours on longhaul (DC10) aircraft, I was suprised to find the aircraft made that much fuel.

I will also say that I think everyone who criticises the crew have the benefit of hindsight and I think I wouldn't have done much different to them.

I have flown several times with the captain when he was a co-pilot and he is one of the better guys!!!

bsieker
21st Feb 2008, 07:47
MiG 15,

again and again and again,

NO!

Any kind of EMI (deliberate or inadvertent) would have shown up as anomalies, and been recorded, and the AAIB report specifically states that

[...] there were no anomalies
in the major aircraft systems. The autopilot and the
autothrottle systems behaved correctly and the engine
control systems were providing the correct commands
prior to, during, and after, the reduction in thrust.

Danny, thank you for moving us to the Jet Blast, where this belongs.

Sven Sixtoo
21st Feb 2008, 07:52
Phil

I get the impression you are uncertain as to the extent to which electrics / electronics play a part in this process.

So, if the spill valves are actuated to actually relieve the pressure between the fuel pump and the control valves (which were confirmed as being open) are the actuation messages recorded ? If so would their operation have been covered by any of the "satisfactories" in the AAIB report ? The Special Bulletin does say that the EEC worked correctly, but the listing of items tested does not include the spill valves.

My understanding is that the EEC is the Engine Electronic Control. Now if the system is anything like the tiny little gas turbine on my aircraft, the EEC has very little to do with the Fuel Control Unit, which is a hydromechanical device that functions entirely automatically and is self-powered, using fuel pressure derived from the shaft-driven HP pump as the motive power for its various servo functions. The EEC input to the FCU is limited to driving an electric actuator which moves the throttle valve. Sensors placed elsewhere in the engine tell the EEC whether the throttle valve movement has produced the desired power, and thus complete the negative feedback control loop. Meanwhile the FCU just gets on with pumping fuel in precise quantities to meet the EEC throttle valve demand, cope with environmental changes and protect the engine from catastrophic failure. Just to give you one example of the sort of thing that goes on inside the FCU, on my aircraft there is a little centrifugal device that, when N1 gets high enough, opens a valve that bleeds fuel from the system. This stops the engine exceeding the designers limiting N1. Of course this gadget never operates unless something else has gone wrong within the system. Another feed of pump output pressure, which is related to N1 (the faster the compressor turns the faster the pump goes) is used to operate the inlet guide vanes. The FCU, with all its internal complexity, is a beautiful bit of engineering and usually extraordinarily reliable. But I would be surprised if there is monitoring by the FDR / QAR or anything in the EEC of the detail of its internal operation.

777drivers / engineers, please correct if my analogy is false.

Sven

cats_five
21st Feb 2008, 07:58
<snip>
There is another common factor namely the environment the aircraft flew through prior to the incident which has not been mentioned in any report.
<snip>

There is a whole section in the latest AAIB report about 'weather'.

Captain Airclues
21st Feb 2008, 08:14
I believe that the figure of 6,900kgs was what the crew had entered into the FMS so as to generate an "insufficient fuel" message. This figure does not include the contingency fuel or any extra fuel (a nice round figure of 79,000 implies that they probably took some extra). If everything went to plan then this contingency fuel, and most of the extra fuel would still be on board at LHR which explains the recorded figure of 10,500kgs at 1000ft.
Perhaps a 777 pilot could confirm that this is correct.

Airclues

NSEU
21st Feb 2008, 08:18
Ref the open spar valves, (LP cocks) I am surprised that the anomaly had not been picked up on the simulator.

What kind of simulator are we talking about here... a flight simulator or a Boeing design program? The flight simulator would have to model damage to specific wiring (I know Level D sims are accurate, but this is ridiculous).

I talked to a QF engineer today who was totally convinced it was ice blocking the fuel-oil heat exchangers.

I'm not familiar with the layout of the Trent plumbing, but surely the fuel-oil heat exchangers are not the coldest part of the fuel feed system? The fuel line goes through the engine strut (which it probably shares with a hot bleed air duct), then through a LP engine pump which would also raise the temperature of the fuel. If it was an internal blockage in the heat exchanger, then wouldn't it have activated the differential pressure switch on the exchanger???

Now I'm sorry to show my ignorance here, but where is air drawn into the tanks when the fuel is pumped out.

From the atmosphere (the tanks have a slightly positive air pressure on them courtesy of NACA scoops)

3) It is important that LP tank pumps are immersed in fuel so that they don't introduce air into the system, also the fuel provides a cooling function for the pumps.

The pumps themselves don't necessarily have to be immersed in fuel. The pumps are actually well above the floors of the tanks. However, the pickups for these pumps are closer to the tank bottoms. The fuel running though the pumps, provides cooling/lubrication.

5) So, you're probably ahead of me, what if the sensors in the 777 center tank failed or were fooled and the pumps operated whilst the tank was dry? Would EICAS alert on something or could they keep going long enough to put enough air into the system to balls things up?

Pressure-wise, CWT Fuel pumps pumping air will be no match for fuel pumps pumping fuel. The wing tanks will simply pump fuel into the manifold causing a backpressure easily strong enough to close the check (one way) valves on the CWT pump outlets. Therefore no air. If BOTH check valves failed on the CWT pumps and they were running.... anybody's guess.

Is the low fuel warning EICAS message generated by the fuel in the tanks...or....only by the FMC?

The EICAS message is generated by fuel in the tanks (The low fuel warning is given by the tank totaliser system). However, the FMC also produces a message if the predicted fuel burn is below the pilot-entered minimum fuel value. There is also a message if the totaliser value varies by a certain amount from the FMC calculated level (but this is after engine start).

If the FMC had been selected as the primary source of fuel quantity on board
during FMC flight planning programming in China and not altered to silence a flashing fuel discrepency message during refuelling...and the
incorrect fuel loaded and signed for...this could be a masked low fuel condition with no warning generated...a rubbish in...rubbish out computer situation....

If the FMC was telling the pilots that the fuel at destination was below minimums, then he would surely double check the flight plan and his FMC programming and resolve the discrepancy. He certainly would NOT fool the FMC into not generating a message even if he could.

On the subject of lack of fuel in tanks.... Didn't the intial report say that one of the fuel tanks was intact? Surely a physical measurement of the remaining fuel would have been made.


Danny! Where are you???? :}

Oldlae
21st Feb 2008, 08:31
Mad (Flt) Scientist,
When a SB is issued by the manufacturer, it must go to someone in the company to decide the priority, in my company that was the Type Engineer. When reading it and it is presumed that they understand the aircraft systems, the person must have wondered why it was issued and the implication that there was a possibility that the spar valves wouldn't be closed when the fire handles are pulled. I would have also thought that the training course for the electrics on the 777 would have shown up the anomaly. Hindsight I know, but having been a TE I know how important it is to realise the implications of SB's etc on Operations.

NSEU
21st Feb 2008, 08:31
My understanding is that the EEC is the Engine Electronic Control. Now if the system is anything like the tiny little gas turbine on my aircraft, the EEC has very little to do with the Fuel Control Unit, which is a hydromechanical device that functions entirely automatically and is self-powered, using fuel pressure derived from the shaft-driven HP pump as the motive power for its various servo functions.

Sounds awfully like you're trying to compare this to a GE engine, Sven?

The Rolls Royce Trent has a fuel pump and a Fuel Metering Unit. The pump is mechanically driven. The FMU is further downstream and simply meters whatever fuel comes out of the pump (according to EEC electronic input).... and delivers this metered fuel to the spray nozzles in the combustor. It would be too confusing to compare the two engine types ;)

Rgds.
NSEU

jafa
21st Feb 2008, 08:37
Ice chips. No doubt about it!

Ah, if BA ain't doing sump checks after fuelling at Shanghai maybe they orta... the fuel there is half water on good days... the icing lights are always coming on. Yes yes yes I know, the truck sample... I like the Chinese.

That 108 knots... um, if we had been hand flying, maybe the old girl would have just sort of naturally wandered on down at Vref or so, then a short transition in ground effect, we would have maybe gone another dozen metres.

I can well see that detail, the speed decay, being overlooked in the mental rushing about caused by the unexpected, inexplicable, sinister and potentially lethal turn of events. It is just that if it had all started say 500 feet higher then with that sort of speed bleed-off we might have had a really high sink rate going by the time we hit the ground.

I mention that only because it is the fashion these days to use the pilot regardless... the chief pilots / fleet managers etc all like it / recommend it / insist on it because they can't trust people to do things they mostly can't do themselves, namely hand-fly an approach on a sunny afternoon. So let's go back to hand flying when hand flying is an option, and keep our motor skills up, and certain piloting-related sub-systems at the back of our brains in the piloting loop.

Just a thought.

bsieker
21st Feb 2008, 08:38
MiG 15

regarding cavitation, there are two phases, the formation of the bubbles, and their collapse.

They form at low pressure, and collapse at high pressure, so it is straightforward that the cavitation damage is on the outlet (high pressure) side, where the bubbles collapse.


Bernd

Oldlae
21st Feb 2008, 08:44
NSEU
I didn't mean by the simulator, I meant that the drill carried out by the pilots undergoing checks etc in the simulator, could have shown up the anomaly when the shut-down procedure was shared between the two.

bsieker
21st Feb 2008, 08:52
I didn't mean by the simulator, I meant that the drill carried out by the pilots undergoing checks etc in the simulator, could have shown up the anomaly when the shut-down procedure was shared between the two.


Only if the simulation included severing of the wiring from the fire handle to the spar fuel valve. Otherwise the valve would have just closed when the fire handle was operated, and no anomaly would have shown.

Is it common in the simulator to simulate such specific damages?

Maybe, just maybe, Boeing should have objected to BA's checklist for unmodifed aircraft, but who knows.


Bernd

snowfalcon2
21st Feb 2008, 09:08
AAIB: "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."

It is interesting that on this topic the latest report essentially repeats the information released earlier and gives no new details about the behaviour of the engines. Was the rpm steady, decreasing or increasing, were there any minor or major surges, did the engines behave identically?

This kind of details may give some clues as to the nature of the fault. For example, I'd expect the signs of a fuel flow impairment to be different than those of a fuel contamination.

My interpretation of this is that the AAIB will use the detail data to simulate any possible fault sources, but wants to do so in peace from any media speculations. And that's OK for me. We'll see the outcome sooner or later. :)

tanimbar
21st Feb 2008, 09:14
Follow-up to my earlier posting (#195, page 10) on the possibility of fuel stratification.
Warning: I'm not crew nor engineer, just scientist.

Hand Solo, you wrote:
"I would think stratification would be unlikely given the aircrafts frequent manouvring and encounters with turbulence. The fuel would have been well and truly mixed up for most of the flight and certainly for the last 30 minutes."

I think you might be mistaken given that the g forces applied to the aircraft during manoeuvers are very low and the resulting force is typically normal to the wing surface (apologies, being very simplistic). The g forces would not be sufficient to disturb a stratification to cause it to break-up, i.e. the fuel becomes fully mixed.

Turbulence could cause a break-up of stratification if violent and with the imposed g forces being applied largely parallel to the wing surface. Moderate turbulence, i.e. the aircraft falls and rises rapidly, and applying g forces normal to the wing surface, might cause disturbance of stratified layers, even cause disruption, but I don't think it would take long for the full stratification to return in calm air. Plus, there are no reports of such events on this flight.

Bsieker, you wrote:
"It is my understanding of the water scavenge jet pumps that they would, while trying to keep the water emulsified in the fuel, would also mix the fuel, preventing stratification."

Thanks for that. Can anyone comment on how these pumps operate and to what 'depth' in the fuel their effect is propogated? Are they designed to fully mix fuel within the total volume of the tanks and so prevent stratification?

Bsieker also wrote:
"A minor slip: the imbalance was 300kg, not 30kg, which makes it still less likely that both engines would be fed the same undigestible type of stratum at roughly the same."

Sorry, a typo. When considering the imbalance, and its possible implications, I had in mind the 300Kg figure. A 5.8% weight imbalance equates to a few centimeters in depth of fuel (for example, 2.9 cm for a fuel depth of 0.5m). Of course, I don't know the dimensions of the fuel tank and so cannot calculate the exact difference in fuel depth between the two wing tanks but, I'd be surprised if the actual difference is greater than 2-3cm. This would possibly suggest that stratification, if it exists, in
both wing tanks would be essentially identical and might be fed into the tank outlet ports at the nearly the same time. Fascinating!

Further, I note now, having missed the text before, that the AAIB Special Bulletin 1/2008 concludes with:
"In addition, comprehensive examination and analysis is to be conducted on the entire aircraft and engine fuel system; including the modelling of fuel flows taking account of the environmental and aerodynamic effects."

Also, someone elsewhere on this thread mentioned that it had been previously thought that the mixing of fuels, provenanced from different sources, was not likely to cause any problems but was now being investigated.

And finally to politics, the elephant in the room: the fuel was sourced from Peking in an Olympic
year.

Regards, Tanimbar

sispanys ria
21st Feb 2008, 09:16
How can one believe that ice obstruction can lead to such a result ?!
First we have 4 LP pumps in the tanks, meaning a quadruple simultaneous obstruction, reducing the fuel flow to a SIMILAR value on both sides (resulting in this just above flight idle thrust). This looks unrealistic to me.
In case of ice obstructions, the fuel restrictions wouldn't have been exactly similar, and thrust values would have been different on each engine.

Martin2116
21st Feb 2008, 09:57
"Further, I note now, having missed the text before, that the AAIB Special Bulletin 1/2008 concludes with:
"In addition, comprehensive examination and analysis is to be conducted on the entire aircraft and engine fuel system; including the modelling of fuel flows taking account of the environmental and aerodynamic effects.""

It seems to me this is the most likely way the AAIB will uncover the cause.

I dont know how much data they have recovered from the flight recorders, but must be enough for some detailed modeling. If the fuel flow is the common mode failure, it was asynchronous by 7 seconds between the engines. Is it not possible the initial response to a demand for thrust was using the fuel in the pipework and any other volumes after the restriction blockage.

So if we know the diameter of the pipework, the speed of the engines, caculated quantity of fuel used in the 3 seconds one engine and 7 seconds the other, cant one calculate the distance back along the fuel lines when the fuel stopped coming? Is there an offcentre pump, tank, point or junction that fits the data?

Just an idea

phil gollin
21st Feb 2008, 12:23
Quote (NSEU) :-

......... The Rolls Royce Trent has a fuel pump and a Fuel Metering Unit. The pump is mechanically driven. The FMU is further downstream and simply meters whatever fuel comes out of the pump (according to EEC electronic input).... and delivers this metered fuel to the spray nozzles in the combustor. ........

unquote

So, the Fuel Metering Unit should have its own data recorded - and hence shouldn't the AAIB Special Bulletin have comented on its staisfactory performance ? The bulletin comments that the correct commands were given by the EEC, but not what data was being recorded by either the FMU or spill valves as to what actions they were performing.

.

TyroPicard
21st Feb 2008, 12:42
phil gollin.. did you miss this bit?
Data also revealed that the fuel metering valves on both engines correctly moved to the fully open position to schedule an increase in fuel flow. Both fuel metering units were tested and examined, and revealed no pre-existing defects.
Regards
TP

phil gollin
21st Feb 2008, 13:07
No, it says the units worked when tested, it did not state that the meausurements recorded by those units (and the spill valves

infrequentflyer789
21st Feb 2008, 13:22
Like I said, it was just a thought!

Just remember, the information recorded and available to the AAIB is recorded electronically. Could an EMP not cause erroneous data to be recorded which appears to show all systems were operating "normally"?

For environment, don't just think of weather! :=



For this to be an interference (or software) fault, with the information the AAIB have released now, it would have to be a fault which:

- reduced engine thrust to below that commanded
- simultaneously faked the data to the FDR etc. to show that everything right through to the fuel valves was responding to commands
- did all this without leaving any other trace of interference in the recorded data

To say that is not credible (for software bug or RFI) is putting it mildly.

Yes it could still have been software failure, or RFI, just as it could still have been aliens. I note the AAIB haven't explicitly ruled out alien involvement either...

SLF3
21st Feb 2008, 14:20
I don't think mixing of different fuel grades or fuel from different sources should be an issue: refineries routinely blend to produce jet fuel in the first place and transfers of different grades between storage tanks are common place.

I'm not convinced by arguments based on fuel freezing - if the TAT was -37 and the fuel freeze point -57 how did it freeze? The freeze point is the appearance of crystals, not the point at which the fuel goes solid, and the margin was 20°C, not the 3°C required.

Stratification of products in storage tanks (caused by density differences) is well known in the liquid gas industry. However, stratification does not occur in ships, and this is attributed to motion. Note the design of the ships minimises sloshing at the liquid surface, it is the induced motion in the bulk fluid that prevents stratification occurring. I would suggest the continuous motion of an aircraft in flight would have a similar effect.

The odd bit in all this is that whatever happened affected both engines at virtually the same time, though the systems are supposedly completely independent. Is there some kind of (hidden) master- slave relationship between the engines buried in the software?

If I understand correctly, there are two pumps per tank - even with a master slave relationship between the engines this seems to rule out obstruction of the pump inlet screens by a foreign object or pump failure.

I think if the investigators knew the answer it would have leaked by now - which makes the whole affair ever more curious.

borghha
21st Feb 2008, 16:57
I'm not convinced by arguments based on fuel freezing - if the TAT was -37 and the fuel freeze point -57 how did it freeze? The freeze point is the appearance of crystals, not the point at which the fuel goes solid, and the margin was 20°C, not the 3°C required.



Indeed, but how about the fuel temperature in the pipework? could it, somewhere down the line, get below the temp measured in the coldest spot of the tank? could part of the fuel, during the descent towards LHR, when idling, begin to freeze and cause an (temporary) obstructon?

airfoilmod
21st Feb 2008, 17:14
Fuel, leaving the Tankage, is under ever increasing pressure from LP pumps toward the HP pumps not to mention, it is moving, both actions would increase its resistance to solidification by introducing friction due to transit, and pressure, which lowers the Freezing Point. At the Low Side of the HP pumps, given a restriction upstream, the Pressure could be drastically reduced, raising the Freezing Point and potentiating fuel solidification or, more likely, transient emulsified water turning to ice which could have blocked critical pathways. This is my current theory, notwithstanding AAIB testing of several samples of collected Fuel.

Again, with a "homogeneous Fuel supply" isolation/separation of Fuel source might have not "protected" either powerplant. The restriction would be timed emphatically on temperatures, demand, and pressure, all of which may have been so close, the isolation would be "overridden" by parameters that could not be prevented by merely "separating" the Fuel sources. The five second lag actually supports this eventuation.

If my theory has weight, it might be better to Fuel the Engines differently.
One from a Wing the other from the Center, given that the location and other factors of the different tanks increase the heterogeneity of the Fuel source.

borghha
21st Feb 2008, 17:32
Thanks airfoilmod, I see your point, but what would be the fuel pressure/freezing point gradient? this might be of some importance on the LP side of any of the pumps. is there any expert out there in fluid dynamics?

Mr @ Spotty M
21st Feb 2008, 17:34
PLEASE PLEASE can we stop talking about BA doing sump checks after refuelling.
I am sure 95% or more do not after refuelling, if l understand correctly the final fuel figure is worked out after final pax & cargo figures are known.
You would then have to wait 2 hours or more for the fuel to settle before the suspended water sinks to the sump drain points.:ugh:

airfoilmod
21st Feb 2008, 17:42
I think we may be going in the same direction. Additionally, it might be more harmful than not to be so focussed on precision in FADEC. I think the system probably worked perfectly so I use it only to suggest that in ETOPS it may actually be helpful to introduce anomalies of benign nature into the control system. If a Fuel source is Homogeneous, sourcing it in different locations in the A/C creates a tolerable "discontinuity" that may introduce a higher level of safety, rather than a lower, albeit predictable, one.

The authority I would suggest, Daniel Bernoulli, dead, but his work lives on.

nickyjsmith
21st Feb 2008, 17:46
What if something was frozen in the system, defrosted and moved to cause an obstruction during the descent ? the aircraft is symmetrical, are the tank layouts ?

Just a thought, as if it was water it would melt and leave no trace.

CONF iture
21st Feb 2008, 17:47
Before anything I’d like to say I read very interesting comments, from different people, with different views …

I’ll be corrected if I say some stupidities …

In these days of high cost energy, the biggest expense for an Airline is … FUEL !
That’s why Airlines are willing to spend big $$$ in order to hire outside companies that teach them how to save fuel and one of the easiest ways to save fuel is by avoiding to travel unnecessary fuel.
In that purpose, flight planning became very accurate (wind – temp – load – perf …), so much that when parking brake is set at the end of the day, actual fuel burn matches very well the planned one. It could be and is, sometimes, higher depending how precisely flight plan was respected and how much delay was on approach, but below … ?
Personally I don’t remember more than a couple hundred kg … but never ever more than half ton … and I’m also talking on more than 8 hours flight.
BA38 improved its fuel burn by more than 3 tons, and I believe BA Flight Planning Department to be somehow experienced.
In my view, to save 3 tons, you need to be much lighter than flight plan …

It could be interesting to compare planned and actual ZFW … if no unusual mismatch appears … then we’ll have to consider … Take Off Weight !?

infrequentflyer789
21st Feb 2008, 17:48
Question for you experts:

Is fuel flow recorded by the FDR? I can't remember.

I believe (and I wouldn't say I'm an expert) that yes, fuel flow is recorded in FDR. Or to be more specific, I think it is a required parameter - the 777 FDR may record other additional parameters as well, I don't know what they would be.

AAIB will also have the QAR in this case, which will probably record even more data.

Also relevant might be, how, and at what point in the line, the fuel flow is measured - I don't know that on the 777. The AIIB state that cavitation damage might indicate excessive aeration of fuel, in which case your fuel flow reading might be really fuel+air - which probably isn't a lot of help.

infrequentflyer789
21st Feb 2008, 18:04
It could be and is, sometimes, higher depending how precisely flight plan was respected and how much delay was on approach, but below … ?


Report says they flew higher than plan, to "accommodate a request from ATC". Also that they were in hold at LHR for only 5mins, which I would have thought was less than expected. How much fuel saving that would account for I don't know, but it would seem likely they would be under rather than over.

RO13ERTS
21st Feb 2008, 18:26
Phil,

The trent 800 FMU is not capable of recording its own positions, however, it will generate a demand discrepancy should the actual vs demand position for the Metering Valve be different. Parameters for the FMU are recorded elsewhere, see post below.

The Spill Valve is built into the FMU, and moves in accordance with the scheduled increases/decreases. As the report says, the FMU's both worked properly on test, therefore, they will have worked properly on the A/C, *if* they were provided with the correct delivery from the pump.

gas path
21st Feb 2008, 18:37
The FMU position is displayed on the EPCS maint. page 1. It is displayed in degrees. It is also recorded on the QAR/FDR.

airfoilmod
21st Feb 2008, 19:09
Pursuant to earlier post, any fluid flowing more rapidly in a pipe, undergoes a lessening of pressure, and a concomitant rise in Freezing point, even boiling point. This is the essence of "vapour lock", a not uncommon problem in liquid under presure supply systems. Once more, if the Fuel is Homogeneous, and temperature/pressure/flow parameters are met, it is easy to understand how any fluid could boil, freeze or restrict flow in a supply system. An HP pump would be the obvious place to suspect, as its delta P is more pronounced than in other locations/parameters. Throw in a maximum demand for Pressure/Flow (FADEC throttle "slam") on a low pressure stable system at low temp. Just Sayin'.

wilyflier
21st Feb 2008, 19:37
Airfoilmod,,,,Please ask your friend Mr. Bernoulli,surely the increased flow and pressure drop go together with a temperature drop and a fall in boiling point

Can a supercoold slushy fluid turn solid: or a superheated one aereate when agitated in extreme circumstances? Ive seen superheated water explode , and we`ve all seen freezing rain.

airfoilmod
21st Feb 2008, 20:04
Of course. Open a cold bottle of soda and watch the liquid instantly be entrained with ice crystals, the temperature hasn't changed (functionally), the low pressure merely raises the freezing point of the beverage. Water can be frozen well above its "boiling point", and be made to boil below its "freezing" temperature. By this I mean there is a concomitant increase in associated boiling point for a liquid freezing out of standard temperature and pressure.

And that is what I'm getting at. Restricted flow from solid particles blocking orifice(s) that starve the engines. I won't quote my former posts regarding how this can happen to two engines at almost the exact same time. It is highly possible. Perhaps even unavoidable. Post #252

NSEU
21st Feb 2008, 20:19
Data also revealed that the fuel metering valves on both engines correctly moved to the fully open position to schedule an increase in fuel flow. Both fuel metering units were tested and examined, and revealed no pre-existing defects.

The FMU position is displayed on the EPCS maint. page 1. It is displayed in degrees. It is also recorded on the QAR/FDR.

The EEC commands the FMU Fuel Metering Valve to a certain position and a feedback resolver on the metering valve sends a signal back to the EEC (closed loop).

I assume the "FMV" position data shown on the EICAS EPCS page is from the feedback (Torque resolver) rather than the EEC command.