BA038 (B777) Thread [Archive] - Page 3

bill_s

29th Feb 2008, 01:31

Some basics might help. An electromechanical relay is a coil of wire around a magnetic (iron) core, and a magnetic armature that moves one or more electrical contacts when the coil is energized.

A typical 28 volt DC relay needs at least 18 to 20 volts DC applied to its coil in order to operate. There is no way any radio frequency signal short of a lightning hit will operate or release the relay directly.

Here's the fine print part: relays often have a diode wired across their coils for the purpose of suppressing the high voltage pulse that the coil generates when you suddenly de-energize it. Depending on the speed of this diode, and the topology of related circuits, the diode could turn RF energy induced into the wiring to the coil into enough DC to operate the relay, or drop it out if it is already energized.

The problem is that you would have to induce some 20 volts of stray RF directly into the relay coil wiring, for a significant period of time, for this scenario to work. This RF would have to penetrate both the acft metal hull and any shielding on the wires. I have no idea how much RF power would have to be delivered to the outside of the hull, but my guess is it would be upwards of tens of kilowatts. This massive amount of RF power would probably disrupt other electronics in the acft long before it acted directly on any relay.

(DC = direct current, RF = radio frequency energy)

Halfnut

29th Feb 2008, 02:06

http://www.alliedpilots.org/Public/PublicRelations/Hotline/hotline.asp

This is APA Communications Director Gregg Overman with the APA Information Hotline for Thursday, February 28.

777 SAFETY ALERT: This afternoon American Airlines Flight 229, a Boeing 777 bound from MIA to LAX, had the left engine hang up on approach to LAX at approximately 2,000 feet. The auto throttles were on and the left engine hung at approach idle as the right engine accelerated normally. It is believed that the left engine would not respond to throttle inputs for 10-15 seconds before finally responding and accelerating to the commanded thrust. The right engine performed normally. Maintenance has downloaded the DFDR data and Maintenance action to be taken (ATBT) will be to sump the fuel tanks, pull the engine fuel filters and check for contamination, BITE check the EEC and check the MAT for any existing faults. We will continue to update you as we get more information.

ve3id

29th Feb 2008, 02:27

I think you are barking up the wrong tree here! I just did a quick calculation using 28 Volts across a 1k ohm load (the coil at RF) and used 140dB for space loss beyond the e-field.

I came up with 78 TeraWatts!

If someone was using that kind of RF power, you would know. All the lights in and around the airport would dim!

GlueBall

29th Feb 2008, 03:50

Halfnut you don't specify engine model; is AA using Rollers?

hellsbrink

29th Feb 2008, 04:08

I've seen RR engines on some AA 777's, Glueball

Graybeard

29th Feb 2008, 04:25

Will the authorities ground the 777 fleet now? They did it to the worldwide DC-10 fleet in 1979, wrongly as it turned out.

GB

Milt

29th Feb 2008, 04:51

Another Acceleration Hangup on a B777.

This may be a blessing in disguise!!

Remarkable and let's hope 'they' can identify the cause. Presumably the fuel system and FADEC is standard to whatever engine leaving only the engine driven High Pressure fuel pump/s as a variation.

What doesn't seem to have been covered so far on this thread are the ACUs - Acceleration Control Units. Are these part of the FADEC and hence computer controlled and are there variable compressor guide vanes and compressor bleeds in the engines also being controlled by the ACUs?

Remember the RR Avons and the gubbonry necessary to have them accelerate cleanly what with guide vanes and bleed valves inter related. Was it 18 or 28 seconds from idle to max following a throttle slam? Once had one of these in an F-86 Sabre maladjusted under investigation hicoughing around a circuit with some of the surges putting out the fires. Saved from deadsticking with easy HOT relights.

For the uninitiated a HOT Relight is defined as one that can often reliably be achieved within 10 seconds of a flame out.

NSEU

29th Feb 2008, 06:00

The pressure relief valve can also open to relieve air or fuel pressure if there is too much pressure during refueling."

I can confirm Jetdoc's(?) statement that the (2) surge tank pressure relief valves have both positive and negative pressure relief. There is a description in the AMM on how to check the valve using positive and negative pressures. So the vacuum in tank theory seems to have lost its momentum :p

On being sanctimonious...
My company does not allow me to take information from company computers without permission.
All the Boeing manuals on my company computers have very prominent copyright statements.
No doubt similar restrictions are placed on those posting pictures here.

Also...
Some of the technology used on the 777 is classified material. Boeing recently got into trouble for selling airplanes to a foreign country.

What copyright de-restrictions are placed on Americans are not necessarily the same as those placed on other nationalities.

There is a difference between being sanctimonious and worrying about fellow PPRuNers losing their jobs and being punished financially and/or being imprisoned.

Rgds.
NSEU

NSEU

29th Feb 2008, 06:50

"Gustep and all of the other "Vacuum/Blocked vent" theorists, there is a "low fuel pressure" warning that would no doubt have been triggered (and recorded) should this have occurred."

Note however...
The trigger point for fuel pump low pressure switches can be as low as 4psi... The output of the boost pumps is, nominally, 12psi... so the pumps can, in theory operate down to almost 1/3rd the normal pressure before a warning is triggered.

Also, flow rate can vary depending on the type of 777 boost pump ...even though the output psi is the same. One 777 airline AMM shows the data for 2 pumps. Flow rate for one is 16,000kg/hour. Another is 19,320 kg/hr.

I'd be interested to know what sort of pressures are required to open the check valves on the boost pumps.

Rgds.
NSEU

28L

29th Feb 2008, 13:27

Ref the AA 777 incident yesterday, I understand the problem was probably with the autothrottle rather than the engine. Entirely different to the BA 777 incident.
Hope this stops the thread diverging.....I'm learning a lot about cavitation and bubbles here!

sky9

29th Feb 2008, 14:34

Just a follow up of the AA :

http://www.flightglobal.com/articles/2008/02/29/221923/american-investigates-as-777-engine-fails-to-respond-to-throttle.html

woodpecker

29th Feb 2008, 16:14

I understand the problem was probably with the autothrottle rather than the engine
Rather a strange statement, let me explain.

The auto throttle drives the throttles>>> the throttle position (angle) is sent to the Fadec (EEC)>>>the Fadec sends demands to the fuel metering valves to give the appropriate power.

For the problem to have been due to the autothrottle system it would have needed a failure that, when power was demanded, only moved one of the throttles forward (which could have simply have been rectified by moving the lagging throttle forward).

ianwood

29th Feb 2008, 16:34

28L wrote:
Ref the AA 777 incident yesterday, I understand the problem was probably with the autothrottle rather than the engine. Entirely different to the BA 777 incident.
Hope this stops the thread diverging.....I'm learning a lot about cavitation and bubbles here!

28L, is your source for this information something different than the flightglobal.com article? I ask only because, IMHO the article reads very similarly to those that came out immediately after the BA038 incident with reference specifically to the autothrottle.

Incidentally, American 777s do use the Trent 800.

Far-flung possibility (feel free to shoot it down mercilessly): If I remember correctly, there was a recent FADEC software update on the BA 777 (Dec 2007 if I remember correctly). Now, I know all systems performed "as expected" according to the AAIB including the FADEC. However, it is possible that this update introduced an anomaly that would go undetected in testing, was not logged in system events, and would only occur under specific circumstances.

Does anyone know if the FADEC software on the AA 777s has been recently updated as well? Might be worth looking into.

Whether mechanical or software, I do know that these types of intermittent issues are especially tricky to deal with in that they tend to correct themselves before anyone can take a close enough look to find the problem. Even the most elaborate or event logging across numerous systems cannot capture every detail.

I am SLF with nothing more than a PPL but have been following this thread in great detail and appreciate all the expert input that has come up so far. BA038 is proving to be a very complex incident. It reminds me of TWA800 in that respect. TWA800 taught us a lot when it was finally resolved. Hopefully, BA038 will do the same.

Halfnut

29th Feb 2008, 17:20

29feb08/1022
To: All Pilots
Subject: B777 Lax Event

Initial Review Of Dfdr Data By Rolls Royce Indicates A Very
Different Event Than What British Airways Experienced. There
Were A Number Of Markers In The Ba Event That Are Not Present
In Our Dfdr Data. I Know This Is A Very Concerning Event And I
Will Keep You Informed As Details Become Available.

For The 777 Fleet Team,
Ca Jim Dees

overthewing

29th Feb 2008, 17:48

Is there any way of knowing how many times a single engine has shut down on the same type, at the same stage of flight? If there's a systemic problem that affects engines infrequently, statistics suggest that sooner or later both engines will be affected at the same time. That would make the BA38 incident just 'sooner' rather than later.

I'm sure the AAIB are combing the figures for clues, but I'd be interested to know how many incidents of single engine throttle-up 'issues' at any stage of the flight, on this type, professionals are aware of?

PJ2

29th Feb 2008, 18:56

If organizations keep all their FOQA/FDA data, then there is the possibility, given sufficient software capability, to "retrospectively" analyse the data for such an event.

mocoman

1st Mar 2008, 01:53

Can anyone cast any creadence upon a rumour that I have heard:

Airbus do engine software updates sequentially following a trial period; Boeing do parallel updates on all fixtures.

:hmm:

sky9

1st Mar 2008, 07:10

pax2908

1st Mar 2008, 07:23

Re. software updates. Please allow a general question: what is actually certified? Not really the a/c itself (after a s/w update, it is no longer the same a/c). How does the regulating authority handle this issue?

avoman

1st Mar 2008, 10:11

software updates are treated in a similar manner to hardware items. Only software from the manufacturer, or approved by the manufacturer from an approved source can be used on an aircraft. It must be installed in the approved manner only and installation certified by an appropriately licenced and approved engineer. No deviation is acceptable, unless that is also approved by the manufacturer and regulatory authority.

Static in the Attic

1st Mar 2008, 14:33

..."then what?"
... well Ed, I guess you point the nose at the threshold rather than 10nm CF & yell 'Mayday'. Surely better than radar vectors around the houses... don't you think?

sky9

1st Mar 2008, 14:46

"Suppose we do a power response test and the engines don't respond. Then what?"

Subject to height I would suggest that you shut one engine down leave for 30 secs then attempt a relight. (OK start the APU as well to give you start pressure).

RomeoTangoFoxtrotMike

1st Mar 2008, 17:58

If my memory serves me correctly didn't an engineer once replace the oil drain plugs incorrectly on all 3 engines on a tristar.
HP Rotor Drive covers on a 737-400, see AAIB Report here (http://www.aaib.gov.uk/cms_resources/3-1996%20G-OBMM.pdf).

Jumbo Driver

1st Mar 2008, 19:58

Should software updates be done to both engines on a twin at the same time? Whether this is or is not the cause of the problem it surely is prudent not to change the operating criteria of both engines at the same time.
If my memory serves me correctly didn't an engineer once replace the oil drain plugs incorrectly on all 3 engines on a tristar.

Yes, sky9, I think the event you have in mind involved the omission of all the O-ring seals on the magnetic chip-detector plugs on an Eastern Air Lines Lockheed L-1011 on May 5 1983.

This incident (amongst others) is mentioned on page 2 of this (http://www.flightsafety.org/amb/amb_sept_oct99.pdf) interesting Flight Safety Foundation article entitled Simultaneous Engine Maintenance Increases Operating Risks.

There was also a very similar incident involving a BAe 146 (RAF 32 Sqn) in 1997, which is described in the preceding bulletin here (http://www.flightsafety.org/amb/amb_jul_aug99.pdf).

CAP 718: Human Factors in Aircraft Maintenance and Inspection (http://www.caa.co.uk/docs/33/CAP718.PDF) is both relevant and interesting reading.

JD
:)

Woodenwonder

1st Mar 2008, 21:20

Quote:"Suppose we do a power response test and the engines don't respond. Then what?"

Re-configure for best glide, and FLY THE AIRPLANE! (Sorry, not wanting to shout, but can't get the italics I wanted.)

wilyflier

1st Mar 2008, 23:15

See thread (Ameican investigates as 777 engine fails to respond)
...American quotes engine hung up for 15 secs before responding
...Olaf Husted post #24 refers to possible compressor icing after long descent with engine de icers ON , but inneffective at idle
...Orester,post #23 quotes high Alpha

UNCTUOUS

1st Mar 2008, 23:29

from this link (http://www.washingtonpost.com/wp-dyn/content/article/2008/02/29/AR2008022903544.html) (Washington Post article)
.
"The parts for commercial airliners such as the Boeing 727 and 737 were once manufactured almost exclusively in the United States. But the parts on today's big jets, such as Boeing's 777 and its planned 787, are made in such countries as China, Japan, Brazil, Italy, France and Australia, in addition to the United States. Boeing, Pratt & Whitney, GE and other plane manufacturers buy parts made overseas largely because they are cheaper.
But the bargain-hunting has come at a price, according to a new report by the Transportation Department's inspector general.
.
"Neither manufacturers nor FAA inspectors have provided effective oversight of suppliers; this has allowed substandard parts to enter the aviation supply chain," reads the report, dated Feb. 26. The agency released the report yesterday after it was made public by the Project on Government Oversight, a nonprofit organization that focuses on government accountability.
The report cited four engine failures in 2003 -- three on the ground, one in flight -- that were traced to "unapproved design changes made by a . . . supplier" of speed sensors on engine fuel pumps."

777fly

1st Mar 2008, 23:43

I am not sure that there are any software implications in this incident. The AAIB report makes it clear that the engine software and systems responded correctly but there was a problem with the fuel supply, not the demand. This is supported by the evidence of HP pump cavitation.

There is actually one instance in which Boeing suggest that a fuel supply problem can cause engine thrust deterioration or flame out. This is when an engine has to revert to suction feed at altitude, following dual wing tank pump failure. (No mention is made of the low altitude situation.) The background to this advisory is that dissolved air in the fuel is released at altitude and can accumulate in the suction feed pipe. The implication being that low atmospheric pressure in the tank, plus air in the suction line will disrupt or cut off the fuel flow under suction conditions.

Note from the fuel synoptic posted by Jet11 that the suction pipe is connected directly to the engine fuel supply manifold, isolated only by a pressure bypass valve. Also that the tank pump nominal output pressure is 14psi, but can be much lower. For argument's sake, I will assume that the suction bypass valve opens when there is a pressure differential around 5 psi and it closed under normal conditions.

In Bejjing, BA038 takes on fuel with an unusually high dissolved air content. This is due to a combination of production/storage/transportation/pumping and weather variables. The a/c flies high, long and cold. The air is released from the fuel and the suction pipe is completely filled with (relatively) dense air. This air remains in the pipe during the descent and once below 6000ft is inceasingly pressurised by denser ambient air and rising temperatures. At 1000ft atmospheric pressure reaches 14psi, equalling the fuel manifold pressure. Fuel hydrostatic static pressure and adiabetic heating add another 6psi, causing the suction bypass valve to open against tank pump pressure.This releases pressurised air into the fuel supply manifold and disrupts the fuel supply. This happens in both tanks, but not simultaneously. The opening of the valve could be assisted by a momentary drop in fuel manifold pressure during the initial power demand.

Just another theory. My assumptions about the bypass valve are probably wrong. Also, why is this not a regular occurence? Maybe the fuel and meteorological conditions were unique to BA38. Maybe we have always been operating close to this scenario.........

Jetdoc

2nd Mar 2008, 03:19

The output of the boost pumps will be added to the atmospheric pressure. Its always implied that we are talking about values above atmospheric pressure because it acts across the whole system- input and output.

777fly

2nd Mar 2008, 06:17

Jetdoc: Your observation does not neccessarily invalid my theory. There is no way of knowing what pressures would be reached in the suction pipe ( would Boeing have checked it under all conditions?) and it could maybe reach higher values than I suggested. Do you know what diff pressure opens the suction bypass?

PBL

2nd Mar 2008, 07:07

Jumbo Driver, sky 9,

you may find the report on the oil-seal incident you mention in our Compendium of Computer-Related Incidents with Commercial Aircraft (http://www.rvs.uni-bielefeld.de/publications/compendium/) under the heading "Eastern Airlines L1011 Common Mode.... 1983"

pax 2809 asks what is actually certified? Not really the a/c itself (after a s/w update, it is no longer the same a/c)

Actually, yes, the aircraft itself is what carries the airworthiness certificate. Yes, it is not in a strict sense "the same aircraft" after a SW update, and this is what worries SW safety people. However, it is the same aircraft as far as the certificate goes, unless something awful is installed and discovered (in which case the certificate will be quickly revoked). SW updates are offered by the manufacturer of the kit at regular intervals and each is "controlled" by controlling the process by which it is developed and offered and what it must show to be approved. There is an incremental process: for example, I don't think one has to perform MC/DC testing on each update. MC/DC testing is an extremely high resource swallower and according to some who have tried to quantify its effects, does not appear to improve the quality of the code.

Attempting to control quality by process does not work to guarantee SW quality; there is no significant correlation that anybody has been able to nail down. However, it "stands to reason" (whatever that phrase might mean) that controlling the process by which code is developed is likely to lead to better quality SW than without.

ArcticLow says
Precising the AAIB: the human-electro-mechanical chains of command from the left hand on the throttle levers all the way to (and including) the valves that control fuel flow all worked correctly. and presages his further discussion on this premiss.

I doubt anyone at the the AAIB would agree to such a statement without qualification. People who do forensics of this sort know that your conclusions are only as good as the data you have, and the data you have is both selected (in terms of parameters) and sampled (recorded only at discrete time points). And that goes for *all* data. Suppose a computer commands a valve to open. You may record the command on the data recorder. Is the position of the valve sensed? You may record that also if it is. Is the sensor working correctly? You are unlikely to record that, but maybe there are duplicated sensors. But was there a common-mode failure of the sensors allowing erroneous readings to be recorded? These are not theoretical questions: all of the phenomena mentioned have arisen in recent incidents. There is one phenomenon that is even worse: in which one and the same data signal is interpreted in two different ways by two different devices (say, one way by the data recorder and another way by the intended receiver). Such phenomena are known as Byzantine failures, and a series of such almost led to the airworthiness certificate for one common aircraft type being withdrawn in the early 2000's, which would have been a commercial disaster for worldwide aviation.

So there are a lot of ways in which reality can slip through the cracks in the data. I think you have to take the AAIB wording literally: "The recorded data indicates that there were no anomalies in the major aircraft systems." The first five words of that sentence are essential. They go on to say that the autopilot and autothrottle "behaved correctly" and that the EEC and associated systems "were providing the correct commands". That leaves large parts of the continuous physical-electric-electronic causal system for producing thrust that are not yet addressed by what has been written. I have confidence that they will be, if nothing else is found in the meantime.

PBL

Jetdoc

2nd Mar 2008, 08:11

The bypass valves are simply check valves or non return valves. They are held closed by fuel pressure from the tank boost pumps. If the tank boost pumps are off and the crossfeed valves are closed so that pressure is supplied by the other side of the aircraft, the engine driven pumps can suction feed by pulling the bypass valves open.

FranklyMidear

2nd Mar 2008, 10:41

From shawk, Post 521: " Unusual acoustical standing waves in piping can restrict fuel flow"Interesting thought: The 777 uses a 'new' acoustic based fuel volume measurement technique. Presumably a pure ultrasonic tone is far more likely to set up a standing wave than the white noise from a vibration source, unless of course the designers had already thought of the potential problem and use frequency agility based on prime numbers?

I guess such a standing wave would only occur at particular resonant path lengths dependent on air and/or fuel volume shapes. I'm not sure if short wavelength standing waves would restrict fuel flow, but they would cause cavitation, as happens in ultrasonic cleaning.

Green-dot

2nd Mar 2008, 11:27

Quoting Bill_s, ref. post #527:

The problem is that you would have to induce some 20 volts of stray RF directly into the relay coil wiring, for a significant period of time, for this scenario to work. This RF would have to penetrate both the acft metal hull and any shielding on the wires. I have no idea how much RF power would have to be delivered to the outside of the hull, but my guess is it would be upwards of tens of kilowatts. This massive amount of RF power would probably disrupt other electronics in the acft long before it acted directly on any relay.

(DC = direct current, RF = radio frequency energy)

and quoting ve3id, ref. post #529:

I think you are barking up the wrong tree here! I just did a quick calculation using 28 Volts across a 1k ohm load (the coil at RF) and used 140dB for space loss beyond the e-field.

I came up with 78 TeraWatts!

If someone was using that kind of RF power, you would know. All the lights in and around the airport would dim!

Bill_s and ve3id:
Reading your posts it seems you are assuming the (possible) source originated from outside the aircraft.

The scenario could be considerably different if the (possible) source originated from inside the aircraft.

To my knowledge certain transmitting PEDs can generate electric fields in the order of 20 Volts (or more) in relatively close proximity to the transmitting PED.

Next question is if something like this had occurred, could it have affected the relays, subject to this discussion? For those "in the know" about where these relays are located and how they "function in the total scheme of things" (Ref. AMM), it would be something to at least take into consideration.

Regards,
Green-dot

skridlov

2nd Mar 2008, 11:34

From a recent post:
"This is because air dissolved in the liquid will tend to come out of solution at low pressures, and contribute a partial pressure of air to the contents of any macroscopic cavitation bubble. When that bubble is convected into a region of higher pressure and the vapor condenses, this leaves a small air bubble that only redissolves very slowly, if at all."

I previously posted a description of what may be an analogous event which I encountered when I had a small mining operation. It was evidently disregarded as unworthy of comment:). It may be that I do not understand the above, but it seems to imply a similar phenomenon.

To amplify my original post slightly I'd just add that the gradual accumulation of air in the pump casing had little or no noticeable effect until it suddenly reached a critical threshold, at which point the pump quite abruptly ceased to deliver. This ocurred several times, after repeated re-priming, before the cause of the problem (increased restriction of flow in the suction line) dawned on me.

Flagon

2nd Mar 2008, 12:47

Don't have time to read the whole thread, so apologies, but does anyone know how the engines responded at 1000'AGL when they would have been at approach power IAW BA SOPs?

dxzh

2nd Mar 2008, 13:03

Fuel "excessive aeration" theory might be explained, in part, by a significant difference in more recent Boeings (post TWA800). Recent Boeings use centre tank fuel scavenge jet pumps with motive power provided by main tank fuel from the main tank booster pumps. Moreover in 777s such jet pumps remain on, once main tanks reduced to half full, for remainder of flight; whereas in 737s (other than more recent NGs) they are powered on for just 20 minutes (and link only to main tank No1) before a shut-off valve removes motive power.

Accordingly, here is another slightly different, ridiculously remote Swiss cheese line-up:

1. Jet A-1 in main tanks was saturated with air, either when uplifted and/or as result of prior centre fuel scavenge tank jet pump operation on fuel remaining from previous sectors. (Kerosene has much greater propensity to become saturated with air and retain it even at altitude compared to say water.)

2. Centre fuel scavenge tank jet pumps operated to entrain and dissolve air into fuel in each main tank thereby supersaturating fuel in each main tank during last hour of flight (by then last of centre tank fuel has been scavenged). Increasing pressure on descent helped increase the level of saturation.

3. LP main tank booster pumps were unaffected by supersaturated fuel (i.e. no cavitation) and no LOW PRESS alert as a result.

4. Cavitation induced in HP pump as flow increased by order of magnitude on finals.

5. Relatively cold fuel made cavitation much more pronounced (see http://naca.central.cranfield.ac.uk/reports/arc/cp/1128.pdf) and, once started, cavitation worsened preventing increase in flow rate as more and more air was released into system. Some flow was maintained, just insufficient.

6. Suction feed as alternate to HP pumps was ineffective and/or became ineffective to increase fuel flow to level demanded. Perhaps due to:

(a) inherent weakness of suction feed (for inadequacy of suction feed in certain flight conditions, see article on UAL flight 767 which in climb at altitude suffered rollback when boost pumps turned off and suction feed alone turned out to be insufficient - http://findarticles.com/p/articles/mi_m0UBT/is_17_15/ai_73578123/pg_4).

(b) secondary effect of release of air at HP pumps as a result of cavitation impeding operation of suction feed bypass.

(c) reduced flow at suction feed inlet from main tanks due to (i) FOD blockage on right hand side at least and/or (ii) perhaps (if sufficient newly melted water, derived from ice formed in surge tanks or centre tank, could have entered and frozen in main tanks) ice particles.

Yellow Shark

2nd Mar 2008, 13:59

From Message #561: The 777 uses a 'new' acoustic based fuel volume measurement technique. Presumably a pure ultrasonic tone is far more likely to set up a standing wave than the white noise from a vibration source, unless of course the designers had already thought of the potential problem and use frequency agility based on prime numbers?

I guess such a standing wave would only occur at particular resonant path lengths dependent on air and/or fuel volume shapes.

Just to clarify this, the ultrasonic fuel measurement for each tank is calculated from sensors contained in individual hollow probes (a total of 20 in each wing tank and 12 in the centre tank). As such, the ultrasonics themselves would not be capable of creating any standing wave.

3Greens

2nd Mar 2008, 14:14

Flagon, BA sop's don not say approach power has to be set by 1000'. Think you are mistaking that for the 500' "gate".

777fly

2nd Mar 2008, 14:37

Thanks Jetdoc. So if the suction bypass valves are simply check valves, at any time the pressure of air trapped in the suction pipe exceeds the fuel manifold pressure, they should open and allow the pressurised air into the engine fuel supply.......?

Jetdoc

2nd Mar 2008, 17:15

777fly

The suction pipe is really not all that long. Just imagine the pipe with a screen on the end of it similar to the boost pump pickups. Its attached directly to the engine feed manifold. The check valve is in the pipeline. The pick up point is somewhat low in the tank about 3 or 4 inches above the tank bottom. Its location is the 4th manhole cover outboard from the wing root and probably less than 10 feet inboard of the engine pylon. One would hope that it is still covered by fuel and not full of air even at that point in the flight.

Rightbase

2nd Mar 2008, 19:26

or:

Why I fly Warriors, in nice warm weather.

Have you noticed how dirty a snowman gets as it melts?

(The situation on the approach to LHR was worse than worst case design spec, so look at worst case design spec. Any numbers are very approximate, but spurious precision has been retained so that their origin is recognisable)

900Kg of 'unusable' fuel in the centre tank - recovered to the outboard (high) end of the main tanks by the fuel scavenge system when main tank pumps are working. That's 2000 lbs or 250 UK gallons.

Up to 138 (US?) gallons of water trigger the water-in-fuel warning - 115 UK gallons. So the centre tank dregs to be scavenged could be half water, half fuel. It gets scavenged 'water first' in the cruise.

The water (mostly) comes in with ambient air replacing fuel used. At altitude the air is cold and even if saturated has relatively low water content. In cloud, the air is saturated and carries suspended droplets as well. Climbing uses fuel at the greatest rate. Climbing on the centre tank through cloud brings water into the centre tank at the greatest rate - but it is a relatively short phase of a long haul flight.

The water droplets, either carried in as cloud or condensing with adiabatic expansion of the air in the tanks, collect in the fuel, and slowly settle at the water scavenge pump inlet (at cruise attitude) and get 'burned off' when the scavenge pump discharges them adjacent to the inlet of the pump supplying the engine. Water only accumulates at the water scavenge inlet if it arrives faster than the scavenge pump sucks it away. Nothing can go wrong ...

But what if it is not water? What if the local condensation and ingested cloud is ice?

Ice granules will not coalesce to form droplets, so the layer of ice granules at the bottom of the centre tank will only be scavenged near the scavenge pump inlet. Instead of flowing to the lowest point as liquid water would, the granules will roll down a local embankment of granules, under the gravitational influence of their small density difference. So whilst the scavenge pump will keep the local area clear, ice crystals will settle like snow everywhere else. The scavenge pump will only start to clean the whole tank when the tank temperature rises above freezing, and the crystals melt into droplets and globules of water that run down to the scavenge point. If the whole take-off, climb and cruise has been in sub-zero temperatures, the centre tank water scavenge pump will be off before this happens. Centre tank clearance of water will start when (and if) the centre tank temperature rises above freezing in the descent, when it will be cleared to the main tank by the fuel scavenge pump, and burned off from the main tank by the main tank water scavenge.

If the centre tank does not collect enough heat to provide the latent heat needed to melt the ice slush before engine shutdown, the slush will melt on the ramp and collect in the sump, potentially causing a water warning at next start-up. This will clear when the water scavenge pump burns it off. If sub-zero ramp and take-off temperatures freeze this water, it almost certainly stays frozen until the next descent. There is no scavenge whilst burning fuel from the centre tank because the inlet is frozen solid. There is then a double dose of solid and powdered ice to be cleared by the fuel scavenge pump when it melts during the next descent.

Provided there is a big enough safety gap between bottom level of useable fuel and the top level of water, and assuming the scavenge pumps can't pump water quickly enough to bring the aircraft down, nothing can go wrong ....

But the snowman effect spoils the party. As a snowman thaws, the atmospheric dirt and dust collected by the falling snow (and, admittedly, dirt collected by the youngsters who built the snowman) is caught and concentrated by its receding and shrinking surface. This is not just because the dirt cannot evaporate. Even a melting snowman seems able to cling on to his dark coat whilst shedding melt water. The dirt is efficiently concentrated on the surface. So after several journeys in freezing or near freezing ground temperatures, or maybe a couple of such journeys through a particularly dirty industrial atmosphere, it is easy to visualise the dregs of the centre tank water and slush, with all its accumulated micro-muck, being slopped into the fuel scavenge area by a change of trim or even by the unstable progression of the melt process.

So water with concentrated micro-dirt surges into both fuel scavenge areas of the centre tank, to be promptly fed to the main tanks, draining 'fairly promptly' to the water scavenge points on the main tanks, to be then fed to the engines - affecting both engines within a few seconds of one another.

The dirt nucleates cavitation, killing throughput of the LP engine pump.

Fire away ...

HarryMann

2nd Mar 2008, 22:11

Cripes...! :D

or maybe a couple of such journeys through a particularly dirty industrial atmosphere,

Nowhere near Beijing then :hmm:

Leodis737

2nd Mar 2008, 22:56

So when does this happen (Rightbase)?

5LY in post 326 had an answer:

The center tank pumps are manually turned off when the tank reaches 2000 pounds (900Kg.). The scavange system then operates automatically to draw out the remaining fuel when the total fuel remaining reaches 29,000 pounds (do your own conversion).

Doing the conversion gives 13,500kg. The aircraft landed with roughly 10,500kg on board (AAIB). So rough calc suggests the scavenge from the Center to BOTH Main tanks started about half an hour before landing (72,000kg/12 hours = 6,000kg/hr of flight). At what point did it reach the possible contamination - depends on rate of flow of scavenge system... - but the contamination reached both main tanks at roughly the same time... thrust reduction 7 seconds apart ... (pure speculation of course!).

HarryMann

2nd Mar 2008, 23:42

For both engine LP pumps to fail to deliver sufficient flow (and failing in such a way as to prevent suction feed) within a few seconds of each other due to prior events (wear/damage) or instantaneous fault is as unlikely as to be unimaginable.

For the fluid diet of the pumps and engines to be the cause, fed from different tanks, and again with that vital proviso - within a few seconds of each other - the common centre tank and its associated functions at that stage of flight must be involved ?

This seems to be the guts of what we are hearing now...

Sorry, just thinking out loud to focus my feeble ability to keep up with some of the earlier posts, but the Dirty Snowman has put nucleation and cavitation into contextual focus for me... Any more ..ations needed to complete this picture?

SeldomFixit

2nd Mar 2008, 23:43

Leodis - don't lose any sleep over purely speculating on this thread. Subtract 2 AIB report posts from the total and the rest equals the number of other speculators sharing the beach with you :ugh:

Rightbase

3rd Mar 2008, 00:23

melting slush in the otherwise empty centre tank collapses and slips down into the fuel scavenge intake areas - both at about the same time.

The fuel scavenge pumps (powered by pressure from the main tank pumps) then start to transfer the muck to the main tanks. With one wing slightly higher than the other (was the cross wind from the South on that day?) this purged muck slides down one main tank faster than the other, so the right engine gets it first, followed a few seconds later by the left.

Only an idea, but as a jigsaw puzzle piece it looks to me like a better than average fit.

But what do I know? I only fly singles with gravity feed.

Rightbase

3rd Mar 2008, 00:49

Just to stop others making money out of my fabulous (good word?) idea:

Suggestions - published so that they are freely available, not patented:

Refuel the tank to be used for climb out with warmed fuel, so that the water build up can be purged before it freezes. This will also prevent legacy build up from previous flights. It might even thaw legacy frozen muck so that maintenance can drain it off before engine start.

Lag the tank so that it stays warm.

Warm tanks in flight so that the water purge works in the cruise, instead of leaving it to the warmer descent phase.

Disable the fuel purge from at least one side before you are committed to needing power to make the field.

Rightbase

3rd Mar 2008, 00:57

HarryMann - some forensic found-ation would be nice!

NSEU

3rd Mar 2008, 01:02

"The fuel scavenge pumps (powered by pressure from the main tank pumps) then start to transfer the muck to the main tanks."

Note that muck in the CT has to pass through an inlet screen before it is scavenged, then through a narrow opening at the jet pump, then, flowing from the outlet of the jet pump, has to jump over baffles, pass through wing boost pump inlet screens, then has to pass through one or two engine filters (depending on where the blockage is), one of which has an impending bypass alert..... and almost simutaneously affect two engines.

Not saying it couldn't happen, but.... :p

So rough calc suggests the scavenge from the Center to BOTH Main tanks started about half an hour before landing (72,000kg/12 hours = 6,000kg/hr of flight).

The CT fuel scavenge rate is 400Kg/per hour minimum. By your calculations the centre tank would have 700Kg remaining on landing :confused:

ve3id

3rd Mar 2008, 01:55

The fact that there is a measurement of 20 Volts per metre does not mean that the voltage will be developed across a coil. The problem is that the relay coil has a very low impedance compared to air, even looking like a short circuit. There is no way a PED would create a field strong enough to drive the current.

Sorry, it is out of the question.

If a PED had interfered with the high-impedance parts of a circuit that controlled a relay, then I could see it, but by the fact that we are talking about relays acting on their own I assume that has been ruled out

777fly

3rd Mar 2008, 04:09

Rightbase: "Climbing in cloud brings water into the centre tank"
How does that happen? During the climb the air in the centre tank will be venting overboard as it expands due to reducing external atmospheric pressure. How does cloud vapour overcome that, to get into the tank? Even if it could, the volume of tank occupied by the 4000kg or so of fuel used in the climb could not contain enough 'cloud' to hold a signficant amount of water. Also it is very rare to spend more than a few minutes in cloud during the climb, if at all.

Green-dot

3rd Mar 2008, 05:56

Rightbase

3rd Mar 2008, 06:56

This is air pollution muck - NSEU - and its sink rate is very low within its water carrier.

Thanks for the 400Kg/hr figure. I was wondering how 'leisurely' the scavenge was.

If a dam of ice released a cascade of clean water into the water scavenge area, would 400kg/hr of water be enough to slow the engine? If so, the case against the dirty snowman collapses, and 'my' finger points to ice in the fuel.

Rightbase

3rd Mar 2008, 07:16

Yes, 777fly, my outsider's view is showing its weaknesses.

The increase in volume of 4000Kg (about 320 - correction - 5000 litres) can only be supplied by adiabatic expansion if there is enough air in the tank to start with. If there is enough then there should be no contamination in the climb.

Edit3 - hope I'm on better form today ...
Tank capacity 100K litres, fuel upload approx 25k litres. Air to be expanded 75k litres. Expansion needed to replace fuel spent in the climb 5k litres. Estimate about 100mB (adiabatic) pressure change or 3000ft. There is an order of magnitude more air than needed to replace the fuel burned.
(OK - you professionals knew all this - I'm conceding as graciously as I can!)

Are the occasional 'water-in-fuel' warnings spurious? If not, 100+ gallons of water (and associated pollutants depending on where it came from) gets in some how.

NSEU may have exonerated the dirty snowman with his 400kg/hour scavenge rate for clean water. Could that cause the problem?

Jetdoc

3rd Mar 2008, 08:50

I think we should all remember that the scavenge system does not feed the remains of the center tank directly into the main tank boost pump pickups but rather it stirs up the fuel around the pickup.

ionagh

3rd Mar 2008, 09:23

If a PED had interfered with the high-impedance parts of a circuit that controlled a relay, then I could see it, but by the fact that we are talking about relays acting on their own I assume that has been ruled out

Could that possibly mean high-impedance parts or connections in the wiring close to the relays somewhere between the relays and ELMS?

If there were high-impedance points in the connections then the relay would probably fail to operate at all. After all it will require something in the region of 100mA to operate. ie around 3 watts.

Just toying with ball-park figures, but imagine a relay with a 28 volt coil. Number of turns required would result in around 1mH inductance and a self-resonant frequency around 1MHz.
What sort of field strength at 1 MHz would be needed to induce 3 watts in the coil??

P (dBm) = E (dBuV/m) - 20log F + Antenna gain - 77,2
Putting the relay coil and associated wiring as a 0 gain antenna and Power as 34 dBm the required field is 111 dBuV/m, or less than 1 V/m.

Do medium wave transmitters still exist?

Rightbase

3rd Mar 2008, 10:44

According to the diagram provided in an earlier post (sorry - can't find it at the moment) the centre tank fuel scavenge outlet is at the outboard (high) end of the main tank. To get to the engines any water scavenged from the centre tank at this stage has to find its way back down to the main tank water scavenge pump which moves it to near the boost pump inlet.

(see post 461 Page 24 )

Robin400

3rd Mar 2008, 15:17

After all the theory and in depth physics, I wonder what the flight crew were doing as the speed decayed from 175 to 108 kts before the autpilot disconnected.:eek:

wilyflier

3rd Mar 2008, 15:51

Reducing speed at 1knot per second?

pls8xx

3rd Mar 2008, 18:43

Pump output can be greatly diminished by cavitation and would give the results seen in this case.

The HP gear pumps show evidence of cavitation. Two of the principle determinants for the point at which cavitation occurs are pump speed and fluid viscosity.

A mixture of hydrocarbons such as jet fuel can have a temperature dependent change of viscosity while in the liquid state.

Cold high viscosity fuel along with an increase in pump speed at thrust demand may have lead to the cavitation.

Fuel that was not frozen but chilled to a high viscosity would have flowed through the large surface area filters without producing an alarm. And the boost pumps may have handled it without a problem too.

But how and why did it happen on this flight and not so many other flights?

I look back to a post on the previous thread (pg 20, post 394, by Glueball)
http://www.pprune.org/forums/showthread.php?t=310013&page=20

"The temperature probes in the 747-400, 777, and MD-11 are located where the bulk of the fuel is coldest. However, some fuel may be colder than the fuel measured by the probes, such as the fuel that is in contact with the lower wing skin. This creates a temperature gradient in the fuel tank from the wing skin to the location of the probe.
As fuel travels to the boost pump inlets, the bottom, cold layer flows through small flapper valves located on solid tank ribs next to the bottom wing skin. These valves are used to control fuel slosh. Thus, the cold fuel tends to flow toward the boost pump inlets."

Speculation ...

This flight started with fuel from a location that had cold weather. The initial FL resulted in a cold soak. The original flight plan was prudent for the conditions expected. The crew elected to honor a request from ATC to climb to FL348, ambient temperature approximately -65ºC. This was followed by a climb to FL380, -76C.

It seems to me that there was an extreme gradient between the coldest side of the tank and the fuel probe. While the the bulk of the fuel remained above the min. temp, I think that along the tank side, the fuel was chilled to the point that it congealed on the tank surface. Maybe it was still not frozen but perhaps so high in viscosity that it no longer moved with fuel currents to be remixed.

Once the tank surface warmed at arrival this high viscosity fuel would be released to flow toward the boost pump. Since the temp probe is not located at the pump inlet, it may have never seen this cold spot in the fuel.

Holes in the cheese ...

To reproduce this you have to cold soak the fuel. Expose the tank to extreme cold to get the required gradient, tank to probe, otherwise the crew will see the problem and correct for it. On arrival, a little more fuel in the tanks and the currents might not direct the cold spot to the pump or less fuel and the sloshing of flight maneuvers might remix the fuel. And the timing has to be just right. Early release of the cold spot gives a greater time for the remix of the fuel. A late release and the plane is on the ground before the cold spot gets to the pumps.

On this flight it all lined up. And it's going to be hell to prove. I bet it's a long time before we see a final report.

Green-dot

3rd Mar 2008, 22:55

After digging into some background information i ran into an article in a "Code One Magazine" which may be of interest. Although a totally different type of aircraft, the article deals with a ground test to simulate a certain failure in the fuel system of a F-16. The similarity with the subject T7 is that its engine is also electronically controlled and the result of a simulation, engine rollback, comes remarkably close to what was experienced on G-YMMM. I know, totally different aircraft types but engine response seems not so different. Please read on . . . .

In this simulation the writer of the article, a senior experimental test pilot, describes the following:

Quote:
"Recently, I saw how the engine would quit in other than the normal airstart-test type of atmosphere. Although these tests were accomplished on the ground, I saw several things that I hadn't seen before, things I'd like to pass along. You can never know too much about the airplane.

I am sure that you are aware that there have been some as yet unexplained flameouts with the F110 engine. What we attempted to do was to try to induce as many potential malfunctions in the airplane as possible in order to shed some light on some of the past accidents/incidents. The airplane was tied down in the normal manner and then fully instrumented to record all the parameters of interest.
One large impression that I had was how tenaciously the engine will cling to life if it has fuel. On one test, we had artificially closed the master fuel shutoff valve to only five percent of capacity, that is, ninety-five percent closed. (There is no way that you can do this in your airplane without some really weird failure, or a plumbing change like we had for the purposes of this test.) The boost pumps were off and the refueling door was open so the system was depressurized. The engine was in idle and running just fine. The test point called for me to snap-accel the engine to ninety-five percent. The engine only briefly touched ninety-five percent, immediately rolled back to ninety-two percent, and hesitated there for a few seconds. It then rolled back to about eighty-seven percent for a few seconds. Subsequently it flamed out, but had an automatic restart accomplished in time to catch the rpm at eighty percent. It stayed there for another few seconds, and then flamed out again. The engine then auto-transferred to secondary engine control, or SEC, and got another auto restart at seventy-two to seventy-three percent. It maintained this condition for a little while and then flamed out again, with an auto restart in SEC at about sixty-five percent where it stayed for a while and then slowly continued to decay toward zero rpm. From this, and other similar runs, I feel that if the engine is operating properly, you have little fear of its quitting as long as the aircraft is providing fuel." Unquote

The article was written in 1990.

Here is the link to the complete article:

http://www.codeonemagazine.com/archives/1990/articles/jan_90/flameout/index.html

In the simulation above it was elected to close the main shutoff valve for 95 percent, only 5 percent capacity, and the engine managed to retain RPM above idle several times for "a little while", even with boost pumps off and the fuel system depressurized (air refuel door open).

Focussing on the T7 dual engine roll back again, just suppose both spar valves had temporarily closed for, say, 80 percent (only 20 percent capacity remaining) with boost pumps on. Could that have resulted in engine roll backs to a stabilized thrust level above flight idle and cause cavitation at the engine pumps as the engines initially responded and then rolled back because reduced fuel flow did not meet engine demand?

I assume similar simulations such as mentioned above would have to be performed to find answers or are there other means (computer simulations?) to observe engine behaviour under such conditions?

Regards,
Green-dot

stilton

3rd Mar 2008, 23:03

Despite the lack of conclusive evidence in this case does anyone know if BA, or any other 777 operator has changed any procedures in the wake of this accident, and what that change might consist of ?

Smilin_Ed

4th Mar 2008, 03:09

Has anyone heard anything more on the 777 rollback incident at LAX?

Dream Buster

4th Mar 2008, 07:14

Would people expect the BA crew to have their blood / fat tested after such an incident?

I have just started a thread on this subject which has been 'kindly' moved to Ground & Other Ops Forums - Questions! Thanks guys.

cats_five

4th Mar 2008, 07:21

Blood yes - alcohol & drugs. Fat? :confused:

wilyflier

4th Mar 2008, 12:09

I may have been concentrating too much on the waxing and ice effects of very low outside temps during cruise .
...We are advised the uplifted fuel (stored and delivered at subzero temp) exceeded specs by a long chalk . being non waxing at temps down to -57 instead of only the normally expected -47ish
...The Chinese gave us a blend specially suited to their expected ISA minus 20 or worse. Its like our road fuel we get a more volatile blend in the winter.
...But how was this blend achieved? Was it just a generally lighter distillate? or was it by adding a much more volatile "additive"? With motor fuel it might be "quickstart" or alcohol.
.. The uplift was mostly in the CTW tank with the Wing tanks topped off.Fuel management in flight used a large proportion of the uplift at first., the remainder mixing with and somewhat diluting the original wing tank fuel.
...Finally descending into warmer UK temps at LHR, did this volatile cause gassing, cavitation, and loss of fuel pump efficiency when an extra demand reduced manifold pressures on the suction side of the pumps?

777fly

4th Mar 2008, 13:06

wilyflier:

Quote: " The uplift was mostly in the CTW tank with the wing tanks topped off".
This could not be the case. The aircraft would have arrived with, at a guess, 15000kgs (total) in the wings, assuming that there was no tankering. During refuelling the wing tanks have to be completely full before any fuel goes in to the centre tank. As the total wing tank capacity is about 58,000kgs, about 21500 kgs would have been put into each wing tank. The aircraft left with 79000kgs onboard, so about 21000kgs must have gone into the centre tank. In other words roughly the same amount of fuel was uplifted into each tank.

wilyflier

4th Mar 2008, 14:20

Sorry about the sloppy refuelling Captain777
...My main thrust was higher volatility fuel and "vapour -lock" above normal propensity.

Dream Buster

4th Mar 2008, 14:48

Cats Five - Fat is the long term storage place in your body for all the nasties that enter it either deliberately or by accident.

27 airline pilots were checked by UCL in 2006 and each and every one of them had abnormal amounts of toxic chemicals in them, which in turn produce mind bending effects.(sic)

Why not check for it as it could be relavent evidence and more importantly, it shouldn't be there.

DB :eek:

skiesfull

4th Mar 2008, 18:54

Fat test??! This topic is rapidly turning from speculation to pure fantasy.

777fly

4th Mar 2008, 19:05

wilyflier: Yours is as good a speculation as any other.

111boy

4th Mar 2008, 20:22

come on...a fat test is obvious, who would trust a skinny pilot ?

airfoilmod

4th Mar 2008, 20:37

One of my suggested possibilities 300 posts (or so) ago. I also suggested the possibility of ethanol contamination as well, (explaining an unusually low fuel freeze point also: -57). This would increase "fuel" susceptibility to disastrous vaporisation at the HP pumps and consequent cavitation/damage to pump lobes. Although the AAIB tested samples, and they tested "OK", it isn't possible to have tested "all" fuel. Forgive the poor Physics memory, Wily. Something else that could have found its way into the tanks by mistake is Glycol, or DeIce fluid.

skiesfull

4th Mar 2008, 21:12

"Dear Willie,
Sorry I f****d-up the landing - it's because I'm a fat b@st@rd."

Smilin_Ed

5th Mar 2008, 01:06

There has been much discussion as to whether faulty software or Electromagnetic Interference (EMI) played a part in this accident. Before we can ascribe fault to either, we need to know just what the software does. The fuel control has multiple inputs but, presumably, only two outputs, fuel metering valve and inlet guide vane positioning, both of which have been reported by the AAIB to have functioned properly.

What else does the fuel control ultimately do? If the fuel control does more, can someone help us out here? If not, to blame the software or EMI looks to me like barking up the wrong tree. Again, apparently appropriate electrical signals reached the engines and individual parts of the engines responded accordingly but the engines just didn't produce the demanded power.

If there are no other fuel control outputs, it would seem that, since the high-pressure fuel pumps have been reported by the AAIB to work normally and there was enough fuel flow to keep the engines running at just above flight idle, it boils down to some characteristic of the fuel which reduced its ability to burn properly OR some flow restriction, presumably upstream of the high-pressure pumps.

Of course, there is a third possibility, the tanks simply started to run dry. :eek:

wilyflier

5th Mar 2008, 08:31

Skies fool,
At least I`ve done more good ones than bad ones
Hillyflier

skiesfull

5th Mar 2008, 10:55

Me too, Hilly -I just couldn't resist it. Dreambuster's "blame it on the pilots for fat-induced decision-making flaws" belongs to the 'Aliens' post!

Hydroman400

5th Mar 2008, 12:16

Yes....indeed, I think someone has been on the blow :8. " While at 600ft, all I could think of was a fat juicy big mac....yyyyyummmmmmmmm, then all of a sudden we hit the ground....but it's not my fault, the burgers altered my mind...."

Squawk_ident

5th Mar 2008, 15:16

On the 29/02 on the french "rcoco.com" forum, one of the contributor joined to his post a report about the fuel scavenge system on the 777-300ER/777-200LR aircraft. I don't think that this (undated) report has been published here.
I believe it might be of some interest for most of you. And thank you for this very interesting discussion.
quote:

SUBJECT: 777-300ER/777-200LR Failure to Scavenge Fuel

/A/ Service Related Problem 777-SRP-28-0118
/B/ Fleet Team Digest Article 777-FTD-28-07002

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

SUMMARY:
Note: This message contains important information relevant to flight operations and airplane dispatch, please distribute accordingly.

Several 777-300ER operators have reported intermittent occurrences of airplanes landing with as much as 2200 lbs/1000kgs/300 gallons of fuel in the center tank. Boeing theorizes that this is an indication that the fuel scavenge system has malfunctioned. A failure such as this of the fuel scavenge system reduces the range of the airplane and could potentially lead to fuel exhaustion in the event additional failures occur which require use of all planned reserve fuel. To address this concern, Boeing recommends that 777-300ER and 777-200LR operators review their fuel reserve policy to ensure adequate reserves exist for each mission.

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

DESCRIPTION:

The scavenge system is designed to transfer fuel from low areas of the center wing tank to the main tanks after the override pumps are shut off. Scavenging this additional fuel from the center tank increases the fuel available for engine use. The 777-300ER and 777-200LR airplanes have incorporated scavenge system design changes intended to increase the amount of fuel scavenged and reduce the amount of trapped unusable fuel in the center tank to approximately 3 gallons. These changes included relocating the fuel scavenge inlet further inboard, while the water scavenge inlet location remained unchanged. Additionally, the fuel scavenge outlet and float valve were moved further outboard to allow fuel scavenge to be initiated earlier in flight.

Several 777 -300ER operators have reported intermittent occurrences of airplanes landing with as much as 2200 lbs/1000kgs/300 gallons of fuel in the center tank. Boeing theorizes that this is an indication that the fuel scavenge system has malfunctioned. These instances have only occurred on long routes originating from colder climates and have led to the conclusion that an excessive amount of water is entering the fuel scavenge system and is freezing during scavenge operations. Because the water scavenge inlet was not co-located with the fuel scavenge inlets it is more likely for water to be ingested in the scavenge system. Additionally, as the outlet float valve location is further outboard in the main tank than previous, the scavenged center tank fuel has more exposure to the cold soaked main fuel tank prior to reaching the scavenge discharge. Indications are that the water in the scavenge system is freezing prior to discharging in the main tank. Frozen water (or ic!
e) in the scavenge system could result in a low rate of scavenge or no fuel scavenge.

Failure of the fuel scavenge system could result in airplanes landing with as much as 2200 lbs (1000 kgs) of fuel in the center tank. During mission planning and dispatch, this fuel in the center tank was considered usable fuel. However, failure of the fuel scavenge system in flight renders this 2200 lbs (1000 kgs) of fuel as unusable. There is no indication to the flight crew that the scavenge system has failed and the fuel is unusable. Failure of the fuel scavenge system essentially reduces the range of the airplane and could potentially lead to fuel exhaustion in the event additional failures occur which require the use of all planned fuel reserves.

Boeing review has determined that the failure to completely scavenge the center tank is the result of system configuration changes unique to the 777-300ER and 777-200LR airplanes. This issue has been placed in our Service Related Problem (SRP) process for resolution and is the subject of the REF /B/ Fleet Team Digest article.

DESIRED ACTION
===============
Boeing recognizes each operator establishes its own fuel reserve policy. Some operators choose to add additional conservatism to existing regulatory fuel reserve requirements. In addition, we note that not all routes and/or operators have shown a susceptibility to this condition. This may be because of environmental conditions, individual airline water sumping policies, or different operator fuel system procedures.

Boeing suggests 777-300ER and 777-200LR operators review their operation for exposure to trapped center tank fuel and their maintenance policy related to water sumping.

We recommend operators establish a policy to monitor center tank fuel quantity upon arrival of each flight. If trapped center tank fuel above 400 lbs (200 kgs) is discovered, we recommend a further review of fuel reserve and maintenance policies as noted above.

If operators chose to address this issue by uploading additional fuel, Boeing recommends operators notify their flight crews that additional fuel has been loaded to mitigate the potential for up to 2200 lbs (1000 kgs)of unusable fuel following failure of the scavenge system.

For operators who have seen the trapped center tank fuel condition and chosen to adjust their fuel reserve policy, we recognize it may be possible for this condition to be resolved on future flights due to a change in environmental conditions or maintenance practices.. If this situation arises, we believe it appropriate to adjust fuel reserve policies to original levels provided they continue the monitoring policy on a flight by flight basis for trapped center tank fuel.

Although these failure to scavenge occurrences have only been reported on the 777-300ER, any Boeing recommendations should also be applied to the 777-200LR as it has an identical center tank fuel scavenge system.

If further information is needed regarding the subject, please contact your local Boeing Field Service Representative. If your local Field Service Representative is unavailable, you may contact the appropriate Airline Support Manager or call the BCA Operations Center at (206) 544-75

unquote

original link:
http://www.rcoco.com/viewtopic.php?t=30693&postdays=0&postorder=asc&start=600

Regards.

tanimbar

5th Mar 2008, 18:08

Squawk_ident's post is interesting, especially where it says:
"Indications are that the water in the scavenge system is freezing prior to discharging in the main tank. Frozen water (or ice) in the scavenge system could result in a low rate of scavenge or no fuel scavenge. "

But the report does not comment on what effect such frozen water would have on systems 'down stream' of the scavenge system.

Seems to be an odd omission but I assume Boeing concluded that it was not worth comment.

regards

Smilin_Ed

5th Mar 2008, 18:36

If there were fuel trapped in the center tank and if the fuel quantity system were working properly, the crew should have noticed it. AFAIK there has been no report that the crew noted anything less than sufficient fuel in the feed tanks to make a safe landing. If the feed tanks did run dry, that points to a fuel quantity indication problem. :sad:

grebllaw123d

5th Mar 2008, 18:43

I qoute from post #608:

"On the 29/02 on the french "rcoco.com" forum, one of the contributor joined to his post a report about the fuel scavenge system on the 777-300ER/777-200LR aircraft. I don't think that this (undated) report has been published here."

According the AAIB reports, BA38 was a 777-236ER.

It appears that this SB is valid only for the -200LR and not for the -200ER

SpeedyG

5th Mar 2008, 18:46

May seem irrelevant, but can we rule out the fact that the temperature was so cold the fuel was gelling and perhaps that led to the scavanging effect.

After impact etc this would possibly not be detected??

seekayess

5th Mar 2008, 19:11

The Manager, OzJet, Safety & Security has posted the following:

FW: B777 crash
The following comment is taken from the African Pilot newsletter. It might seem bizarre but with no answers as yet I suppose they have to keep digging. Not sure ECM could affect the FADEC, but then, this has never happened before and any accident has a cause, or 2, or 3!!!

Did Gordon Brown's Electronic Counter Measures suite cause the B777 to malfunction and crash at Heathrow?
The mystery of the January 2008 B777 crash at London’s Heathrow deepens, with investigators reporting no evidence of mechanical failure. Investigators also ruled out the possibility that fuel froze during the flight or was contaminated, or that a bird strike was to blame. So what caused the engines to fail to produce enough thrust on the final approach? The engines were still operating; though at idle, when the crash crews arrived only minutes after the event. The thrust levers were at full or maybe at the ‘go round’ position. So what might have happened? Given the aircraft had fuel, that was not contaminated and the engines were operating...?

I have had reports that at the time the aircraft passed over a perimeter road, by an extraordinary coincidence, it passed low and directly overhead British Prime Minister Gordon Brown's car detail, which was delivering Brown and his entourage to Heathrow. This cavalcade apparently travels with a significant and powerful electronic counter measures (ECM) package to jam (fry) roadside IEDs, deflect and avoid incoming missiles and so on. Did the PM's ECM package interfere with the engine management controls (called FADEC) on the B777? FADEC (Full Authority Digital Engine Control) has been in operation for decades - first operated on the B767 and the B777. It has had millions of miles of trouble free motoring and is apparently extremely robust in an operational sense.

FADEC power plant operation provides for independent engine operations. Therefore any FADEC problem would not normally lead to a double engine failure. Unless of course, the problem existed in the parameters and/or the engine control information provided to the FADEC due to a malfunction, or possibly ice, or maybe after being subjected to extremely high intensity electromagnetic radiation from a nearby source. Given that ice has been ruled out - and there was a double malfunction, could an ECM response have contributed?

So far all reports about this crash have covered the outstanding airmanship of the crew to limp the stricken aircraft at a very low altitude and low speed over the perimeter fence and crash land with no loss of life, or serious injuries, AND the complete bafflement as to what caused this aircraft to crash - given the fabulous operational record to date its type. Perhaps there will be no conclusion entered? If Brown's security measures contributed, will that fact ever be fully investigated or reported on?

My knowledge not being up to scratch on the subject, thought I would run it past you folks!

Green-dot

5th Mar 2008, 20:24

Quoting Smilin Ed:

Again, apparently appropriate electrical signals reached the engines and individual parts of the engines responded accordingly but the engines just didn't produce the demanded power.

According to the AAIB report, engine fuel control systems functioned normally. However, if fuel flow (from airplane fuel system) to the engines is somehow restricted and cannot meet engine (control) demand, the engines will only accelerate to- and stabilize at the restricted fuel flow value, regardless of normally functioning fuel controls on the engines.

Green-dot

Chris Scott

5th Mar 2008, 20:42

Quote from ArcticLow [Mar02/19:06]:
....At 1000ft radio, the F/O takes control, states "Man Land 280 baro", but leaves the automatics in. At this stage, everything looks OK and they're just waiting to lose the last few knots of extra speed and so the power still hasn't come up fully. They've already discussed this and agreed that idle power is OK at 1000 but they'll go around if still not fully stabilised by 500 radio, as per SOP. At just under 750ft, the speed reaches the bug, the levers move forward to maintain target speed, but nothing happens. The levers go to climb power, still nothing. The F/O applies TOGA power, nothing. The rest is in the AAIB report. I don't know if this is what happened, but it's starting to look like a high probability hypothesis, on the available evidence. Thank God it wasn't a scarebus...

For someone who claims not to be a pilot, you are remarkably well briefed. Perhaps by one of the Boeing Airways pilots on a long overnight flight?
He has certainly sold you the whole package -
1) If a Boeing has an incident/accident, it cannot possibly be caused by any fundamental problem with the aircraft. :rolleyes: Possibly rogue fuel? :ok:
2) If an Airbus has an incident/accident, it's because the designer was simply bonkers. := :{

Sound familiar? So, since you have begged the question, can I stay off-topic to speculate what [U]MIGHT have happened if the Trent engines on an A330, for example, had behaved like the ones on G-YMMM did, at the same point on the same approach?

Unlike the B777, the A330 autopilot would be programmed to Go-Around mode as the pilot selected TOGA thrust. Because the aeroplane would be unable to comply, the AP would drop out almost immediately, and the FD demands on the PFD (ADI) would be of no help. The PF would switch it off (or ask the PNF to do it).

"Fly the aeroplane." The FBW would be doing just that. It would maintain 1-G flight in pitch, and zero-rate in roll; holding a steady descent path and keeping the wings level (if they were when the AP disengaged). Because of the shortage of thrust, the speed would be decaying, as per the B777.

Workload permitting, the PF might select the FPV display on his/her PFD. This, with the FD switched off, would display the "Bird", a symbol showing the drift and descent trajectory. This might be useful to check a steady profile is being maintained initially, even though the nose is being raised steadily as the speed falls. Because of this, the "picture" of the world outside the cockpit windows is starting to look very wrong.

Meanwhile, the aeroplane is still pretty much on the glide-slope. The FBW continues to run the stabiliser-trim in the "nose-up" sense. A decision must be made. The PNF's attempts to revive the engines have failed; we are going to undershoot the runway. What is the best (minimum-drag) speed on this aeroplane with landing flaps and gear extended? Haven't the foggiest idea, but we must be already below it! Time to trade height to stop the IAS going any lower. Slowing down equals MORE DRAG. We'll have to maintain this speed as long as we can. Tweek the sidestick forward a bit, then release. Down we go, is that enough?

We've got to get over the perimeter road – which has already disappeared somewhere under the nose – and into the field. The speed is still decaying slowly, unfortunately, but this aeroplane cannot be stalled. As the IAS drops to Alpha-prot, about 13% above stalling speed, the stabiliser stops trimming back; the stick must now be pulled harder and harder to stop the nose dropping. Can we keep a tiny bit of speed in hand to avoid hitting the ground too hard?

The rad-alt calls "ONE HUNDRED". Pull a bit harder. "FIFTY". Stick hard back - we've got to miss that road. We're close to Alpha-max (5% above stalling speed). Now, the aeroplane will descend of its own accord to avoid stalling. "TWENTY", BANG, we are down...

DC2 slf

5th Mar 2008, 21:44

Smilin Ed said:

"There has been much discussion as to whether faulty software or Electromagnetic Interference (EMI) played a part in this accident."

EMI, like "pilot error" is something which leaves little or no evidence behind, and so is a convenient explanation for otherwise inexplicable events.

Modern airliners emit a lot of radio frequency radiation for many purposes, and their instruments and controls are made to tolerate quite high levels of EMI.

infrequentflyer789

5th Mar 2008, 21:59

If there were fuel trapped in the center tank and if the fuel quantity system were working properly, the crew should have noticed it.

Also the AAIB should have noticed, and their report says:The total fuel on board was indicating 10,500kg, which was distributed almost equally between the left and right main fuel tanks

They also reported significant fuel spill at the crash site, so we know it didn't run completely dry (regardless of indicated quantity). Also, the engines continued to get some fuel (report implies they rolled back to somewhere above flight idle and stayed there).

The quoted report on fuel scavenge failure is very interesting though, not least because it matches some of the previous speculation here on fuel scavenge + ice.

Milt

5th Mar 2008, 22:08

Engine Compressor Guide Vanes?

Could I recommend we should look at the part that the engine guide vanes play in the scheme of things. They have received precious little attention so far no doubt because of the concentration in trying to determine a single source for a two engine event.

Presumably the engines compressor guide vanes angular control is by the FADECs which are in turn are driven by dedicated computerised software. Wonder where the FADEC software is physically located.

Can someone describe the Trent guide vane system? How many stages of vanes and are they all interconnected? Over what RPM range do they do their thing? Are they hydraulic, fuel pressure or bleed air driven? Are they anti-iced and what is likely to be the effects of malfunctioning or non functioning during engine acceleration?

Chris Scott

5th Mar 2008, 22:16

Quote from grebllaw123d:
According the AAIB reports, BA38 was a 777-236ER.

Hi, Are you are under the impression that the Boeing Service Bulletin only applies specifically to B777-300ER and B777-200LR?

Think you will find that Boeing dash-numbers ending in "0" or "00" are a kind of generic description. The second and third [U]numerical digits merely indicate the airline.

Since I was a lad, all Boeing airplanes bought new by BOAC and BA have had dash-numbers ending in "36", e.g., B707-436, et cetera.

Chris

Editing PS: Ah, now I see what you mean, I think. Is it the "ER" designation you are querying? If so, I don't know. Sorry.

Oldlae

5th Mar 2008, 22:30

Chris, read post #608 the SB refers to 777-300ER and 777-200LR not 777-200ER.

infrequentflyer789

5th Mar 2008, 22:38

After digging into some background information i ran into an article in a "Code One Magazine" which may be of interest.

Interesting. This passage caught my eye:

"then you will start to hear strange noises as the combustion becomes unstable in the engine. You will hear a low rumble as parts of the ejector nozzles become too lean to burn."

Various news reports have eye witnesses reporting odd engine noises ("roaring" and "rumbling")...

Chris Scott

5th Mar 2008, 22:50

Oldlae,

Yeah - Guess that's the point grebllaw123d was making. So, I suppose we can assume that the 200ER has not been fitted with the fuel scavenge system design changes that the Service Bulletin is addressing.

Smilin_Ed

6th Mar 2008, 01:05

The AAIB report says that the compressor inlet guide vanes responded properly to the demand for more power.

SeldomFixit

6th Mar 2008, 03:45

....which loaded the compressors more, without the required F/F increase....

spannersatKL

6th Mar 2008, 05:20

Wilyflyer
See the AAIB report that shows that whilst the fire handles were pulled the LP valves did not close....the crew shut down procedures has now been modified...

wilyflier

6th Mar 2008, 09:08

SpannersatKL
..Yes,but the engines should have been unable to run if the HP cocks were closed by the fire drill , thus still a little niggle about not much fuel should have been leaking if the engines really continued to run on the ground ,(which I doubt)
..But a bigger niggle as to was there evidence was to show the CWTs were really as dry as they should have been by then; in order to disprove any ice block misbehaviour of the CWT scavenge systems?

lomapaseo

6th Mar 2008, 13:11

SeldomFixit

....which loaded the compressors more, without the required F/F increase....

:confused:

even I can't read between the lines when there are no lines posted as a reference to your post:(

airfoilmod

6th Mar 2008, 16:04

With all due respect, "what's Old is New again." Noises on approach, human factors, stall avoidance, etc. 108 knots with an AOA at that value, is Scary. That Aircraft is not to be stalled, period. (Notwithstanding nonsense replies from bug smasher pilots.) Fuel exhaustion is still on the table, but so is information starvation. This thread is on life support.

FAStoat

12th Mar 2008, 12:29

Oh by the way,am I correct in thinking David Learmont said in one of the initial reports that the Crew experienced a slight Power Surge problem when they came off the Lam hold????This has not been mentioned in any of the threads I have read,or is a figment of my imagination?

F14

12th Mar 2008, 16:34

err, wasn't GB flying a BA777 too??

Think u'll find it was good old Vmca on approach, long flight tired crew. Engine No2 failed (check the pics) Crew went heads in, didn't spot speed decay till below Vmca, ie, cannot go around!! must trade height for speed.
The lads did a good job because every body walked away, however couldn't be a great landing because unable to use the aircraft again.

Basically there are no "new" aircraft accidents, just a swiss cheese that when the holes in training, hardware, automatics and Humans line up we get an incident...

Smilin_Ed

12th Mar 2008, 19:18

Everyone keeps forgetting that the original report by the AAIB says that the fuel metering valves opened in response to the Autothrottle and the crew's movement of the power levers. This means that the engines should have accelerated but instead, they hung up at just above idle. This clearly indicates a lack of sufficient fuel at the fuel metering valves. The real question is why?
[/COLOR]

SilsoeSid

12th Mar 2008, 21:29

DingerX Feel free to look at the maps and tell me where you would expect the motorcade to be "under the approach path.

How about anywhere between Hounslow and the crash site!

http://i52.photobucket.com/albums/g11/silsoesid/Hounslow.jpg

Or
Within 2 miles of the crash site....anywhere in this picture!

http://i52.photobucket.com/albums/g11/silsoesid/27L-1.jpg

Or
Anywhere east of 27L threshold with the aircraft below 10,560'!

http://i52.photobucket.com/albums/g11/silsoesid/east.jpg

Or if you want a road map, how about an autoroute route from Downing Street to Heathrow.
Notice, if you will, how the A4 dips nicely south towards the approach path!

http://i52.photobucket.com/albums/g11/silsoesid/autoroute.jpg

X-37

13th Mar 2008, 15:12

Why are so many posts hostile to the idea of electronic interference?
The AAIB have not ruled this out in fact they don't seem to know what caused the accident.
It could be electronic, it could be that contaminated fuel got to both engines within 7 seconds ( or rather fuel didn't get to..).
Incidentally Both lifts in the Control Tower failed at exactly the moment of the incident. This may not be relevant, on the other hand it may well be. Was the accident caused by fuel,was it a burst of RF, was it Gordon Brown, was it onboard cellphones/laptops????
None of us here know the cause.
The AAIB are not saying, so all bets are on until they tell us what happened.

grumpyoldgeek

13th Mar 2008, 15:19

Why are so many posts hostile to the idea of electronic interference?

Because many of us have worked in electronics for years. We intuitively understand antenna theory, the inverse square law and the incredible difficulty of transmitting significant amounts of electrical energy through air.

tubby linton

13th Mar 2008, 17:34

This tool is interesting.The last time I looked I think the day of the accident had been sanitised.
http://lhr.webtrak-lochard.com/template/index.html

airsound

13th Mar 2008, 18:36

This tool is interesting
It certainly is, tubby. Thanks for the heads up. Shame about 17 Jan though.

airsound

Green-dot

13th Mar 2008, 20:42

Because many of us have worked in electronics for years. We intuitively understand antenna theory, the inverse square law and the incredible difficulty of transmitting significant amounts of electrical energy through air.

With respect to all with a profession in aviation, regardless of background, intuition does not always win it from the facts.

I personally had to replace main fuel shutoff valves in engine feed systems (back in the 1980s) because intuition and inverse square laws did not prevent them from closing in an unscheduled manner then. The valves were redesigned and the wiring was provided with an improved electrical connector. And guess what, EMI filter modules were added between the connector and fuel shutoff valve.

I won't go into specific details here, not about what i wrote above nor specific T7 details, not that it matters much because i have noticed that a thread posted by me regarding a GENERAL explanation of the EMI phenomenon yesterday has been removed without the decency of the "puppet-masters" on this forum to explain to me as to why!!! And it has not been the first time with regards to BA038. That makes me wonder.

Regards,
Green-dot :oh:

Del Prado

13th Mar 2008, 21:15

Incidentally Both lifts in the Control Tower failed at exactly the moment of the incident

I believe that was an on going problem throughout the day and NOT coincident with the accident.

grumpyoldgeek

13th Mar 2008, 21:48

I personally had to replace main fuel shutoff valves in engine feed systems (back in the 1980s) because intuition and inverse square laws did not prevent them from closing in an unscheduled manner then. The valves were redesigned and the wiring was provided with an improved electrical connector. And guess what, EMI filter modules were added between the connector and fuel shutoff valve.

Means nothing unless you know that the fault was caused by *radiated* EMI rather than conducted.

Oldlae

13th Mar 2008, 22:30

The AAIB bulletin stated that the fuel control system was working, the open spar valves were blamed on a faulty shut-down procedure, so it is unlikely that all the RF and the PM's ECM affected the elecrics. So it is obvious innit, all the RF and ECM made the fuel boil thus sending fuel vapour to the engines, causing cavitation and lack of thrust.:)

spannerhead

14th Mar 2008, 20:13

I did hear that recently an ADIRU fault on a bmi A320 was positively fixed by changing an oven in the forward galley. If the cause was some kind of interference then it could have come from the most unlikely sources.

airfoilmod

15th Mar 2008, 00:19

Re: Gordon Brown? If his motorcade/Aircraft/Jaguar had anything to do with the incident, (HIGHLY unlikely), it was inadvertent, sloppy, witless. This isn't to say that if the PM was not present or even tangentially involved, that something like what is being discussed couldn't have happened any other way. There seem to be two camps; Brown's retinue downed the flight, or nothing like it happened or could have happened. In the middle is enough room to drive a twelve tonne lorry. Jammer, MW, hostile elements; there is room for all manner of conspiracy, why stop at Brown or Vapor Lock? Curiouser and curiouser. The only changes I've noticed is a minor change in fueling protocol by two yank Legacies, if there were something that could affect the Fleet, it would be common knowledge. Right, AAIB?

boguing

15th Mar 2008, 23:57

Don't fly 'planes or design them. Good at Physics though.

One cause, two possibilities, one result.

Cause. The fuel got too cold and separated to constituent parts. One or more of which didn't like pumps.

Possibilities. Either there was a software warning that was ignored; or there was not a software warning and fuel temperature was therefore not known to the crew.

Result. Engines didn't do the expected.

If I'm wrong I'll buy a personal title. If I'm right I'll buy a personal title.

pls8xx

16th Mar 2008, 00:39

boguing (http://www.pprune.org/forums/member.php?u=130979),
Am I to understand you would be in agreement with my more detailed hypothesis in post #589, page 30?

boguing

16th Mar 2008, 11:21

pls88x

Certainly on the temperature.

Gradient, possibly a factor earlier in the flight although I'd expect that the smallest amount of turbulence would mix the fuel in a part full tank later in the trip. So even if the gradient 'hid' the true temp, the true figure would have been measured after a slosh or two.

My point is that the fuel temp was not spotted, either by the software or the crew.

4Greens

16th Mar 2008, 11:44

Real World.

Nobody knows what happened.

So, why haven't all aircraft with this fitout and engine been grounded?

wilyflier

16th Mar 2008, 12:13

Boguing.
....I dont think mixing is guaranteed by smooth flight sloshing .In a seperate discipline we are warned not to rely on it to mix oil and petrol for 2 strokes.
.
...Now Mr Physics, tell us how much the boiling (gassing) point of hydrocarbon liquids goes down if its freezing(waxing) point has been Orientally cunningly reduced from -47 to -57 ?....Would it be as low as say +10 or +11 centigrade ?
...The same discipline warns us of the Vapour lock problems with usage of winter road fuel above around 7000ft.

Does the in flight fuel temp indicate prominently all the time ?(Seperate from TAT leading edge temp) or is there just a warning show at a value that has been preset according to the crews understanding of the specification of the uplifted fuel?

M.Mouse

16th Mar 2008, 12:58

Does the in flight fuel temp indicate prominently all the time?

On the upper EICAS screen in the lower right hand corner the fuel temperature is displayed below the fuel quantity indication. TAT is displayed in the top right corner. Display is always on.

Fuel freeze point is entered manually on the PERF INIT page of the FMC. In BA it defaults to -40°C but is overwritten as appropriate. This figure is used by the aeroplane systems to generate the FUEL TEMP LOW warning should the fuel temperature reach withing 3°C of the entered value.

chris weston

16th Mar 2008, 18:17

wilyflier:

I have the greatest of respect for the quality of the postings in this thread, let's see if this one is vaguely up to scratch.

It's the Vapour Pressure of the mixture that's changing with sinoadditives and it's that that drives any change in Boiling Point, a pedantic point but one that's crucial in this "vapour lock = outa gas" hypothesis.

Stick out neck time.

Fuel stratification and subsequent cavitation did get my vote - it fits the evidence beautifully , but ....... we ain't heard a thing from AAIB.

The hypothesis is eminently testable and (i) I would be amazed if they hadn't had a go at it already and (ii) if the numbers stacked up in a repeatable way and the model "worked" (iii) we'd have heard by now.

I know, call me naive. Fire away.

CW

airfoilmod

16th Mar 2008, 19:28

Chris, this is a popular theory, though it brings up unfortunate implications. (See posts #261 and #602). Fuel Exhaustion is "ostensibly" ruled out, Fuel Starvation has been obliquely addressed, Engine Response is on the Record, Fuel contamination has gotten little buzz. (Notwithstanding the public disclosure of Fuel "testing" absent conclusion as to causation or even postulate). The Fuel "tests" were left unaddressed, and the public does what it always does, "assume" that means the Fuel is cleared. Not so. Everything points to Fuel issues, storage, transit, pumping, temperature, Engine response, etc. even quality. My fear is that the longer the investigative authority takes to update, the more it will overemphasize the ultimate outcome of the investigation. (I hope "Sinoadditve" does not become a new word in the Dictionary, for many reasons.)

chris weston

16th Mar 2008, 20:23

airfoilmod

I've read every post on this thread but had forgotten 261 and 602. I blame being born in the 40s plus Scotch .......:ok:

I absolutely agree with you, fuel fits best. My caveats obviously obscured that central point.

But but it's very straightforward to do mass spec et al, n times on fuel samples so........... it's back to either my naivity that no news means they've found zip or ........we're looking at something more sinister over possible (temporary?) information suppression.

Yes I know AAIB needs no political clearance etc ad nauseum, if however we don't get a damaging fuel report until after the Olympics we may want to revise that view.

From naive to cynic in one bound!:D

........... and my apologies for "Sinoadditives", I liked it at the time.

CW

Southernboy

16th Mar 2008, 21:26

I agree with 4 Greens. It's an ETOPS aircraft too. Has it been restricted?

M.Mouse

16th Mar 2008, 21:38

Has it been restricted?

On what basis?

Concorde had an, arguable, design fault which could have led to another catastrophe. When the reason for the BAW38 accident is known a decision can be made.

If the worldwide fleet is grounded or restricted now what reasons would be given? If no cause of the accident is found then would those 600+ aircraft remain grounded?

lomapaseo

17th Mar 2008, 03:03

Groundings are temporary restrictions against the original issue type certificate while it is being brought into compliance with the original specification.

From what I have read there is no varriance identified against the type certificate. For all we know this could be purely an operational problem and nothing wrong with the design.

If you support grounding, then just how far do you go? All recently certified aircraft from all manufacturers? Or do you feel there is supportable evidence to limit it to one manufacturer?, one aircraft type?, one model year?, one or two line numbers? or one operator's fleet?

When all is said and done what corrective action will place this large piece of capital ownership back into service in what time frame?

Certainly any of you can play a ouija board and decide to ground yourselves from flying anytime you choose, but I prefer to abide by a process of a rational approach.

wilsr

17th Mar 2008, 04:52

There *is* what appears to be a new interim bulletin on the AIB site - but my Adobe reader hangs when 95% of the .pdf has downloaded and freezes my comfuser so I don't know if it's "new" or one of the "old" bulletins. Anyone read it?

Doha_lad

17th Mar 2008, 05:09

wilsr

The only report I can see which refers to this accident is the 'old' report S1/2008 issued in Feb.

DL

alph2z

17th Mar 2008, 06:14

Approx how much time is required to cause the cavitation damage within the pumps that is experienced here.

I would assume that it would take a long long time; much more than a few hours.
.

Smilin_Ed

17th Mar 2008, 09:41

Regardless of how long it took to produce the observed damage, the report says the pumps are still serviceable.

wcorcoran

17th Mar 2008, 15:19

The cavitation issue may never have been known if this accident never happened. Although it was not causative, we all must agree that it was unexpected. Perhaps several hundred lives were just saved twenty years from now as a result of this incident. The problem with cavitation is kind of like the difference between a styrofoam cup and a glass. The cavitation can cause the liquid (in this case fuel) to become aerated.

The other issue with cavitation relates to debris. The cavities (no matter how small) can not form unless something is lost or disintegrated. While the fuel flow was not impeded, the combination of the aeration and potential debris from the cavitation may have had some impact.

Oldlae

17th Mar 2008, 16:23

RR will know the cavitation history from examination of the Trent engines previously returned to them for repair/inspection/overhaul over the years.

Secondly, it is now 2 months since G-MMMY undershot, the AAIB are obliged to issue a report after 30 days which they have done, but are they obliged to issue a report every 30 days?

PBL

17th Mar 2008, 21:14

If you support grounding, then just how far do you go?

I would like to remind everybody that, according to a senior avionics manager at Honeywell in a paper published in a major conference, a certain aircraft type came within days of having its airworthiness certificate withdrawn in the early 2000's because of irresoluble problems with data transfer on critical data buses (known as "Byzantine faults").

That answers the question "how far do you go?": maybe all the way, depending.

PBL

shortfinals

18th Mar 2008, 13:54

Boeing doesn't know the answer, apparently, but it knows what it's looking for:

http://www.flightglobal.com/articles/2008/03/14/222219/fuel-system-tests-continue-following-777-crash-landing.html

SF

cats_five

18th Mar 2008, 14:22

From the URL above:

"Boeing's chief pilot flight operations safety division Capt Dave Carbaugh says the fact that the fuel pumps, according to the UK Air Accidents Investigation Branch (AAIB), (http://www.aaib.dft.gov.uk/latest_news/special_bulletin_s1_2008.cfm?view=print&)had been cavitating not long before the impact"

Is my mind going? I only remember a statement in the latest AAIB report that there was damage caused by cavitation, not anything about when it might have occured.

shortfinals

18th Mar 2008, 15:36

Cats, be assured your mind's not going. Your link to the AAIB's statement proves that. But had you wondered whether Capt Carbaugh might know something you don't?

TeachMe

18th Mar 2008, 16:06

Question 1: If we cracked open a 100 average fuel pumps on a 777, how many would show signs of cavitating?

Question 2: How could one tell that the cavitation was recent, and if recent how recent (aka this flight vs flight before)?

Bobbsy

18th Mar 2008, 17:10

My (layman's) understanding of the AAIB investigation process is that the manufacturer (and airline for that matter) will be closely involved at all stages. It seems likely, therefore, that Boeing and their chief pilot would have inside information about the direction the investigation is going.

What surprises me is that he "went public".

Bobbsy

pax2908

18th Mar 2008, 17:12

To Question 2: perhaps the output pressure would show fluctuations which (depending on the sample rate) could indicate that the pump is cavitating?

SuperT

18th Mar 2008, 17:36

Just remember that the cavitation is an indication that the pump had been operating with low inlet pressure, the pump itself would only have its performance mildly affected- ie not enough to cause the event

chris weston

18th Mar 2008, 22:05

SuperT

I'm not sure we can imply that a problem with cavitation is only a function of low inlet pressure on a pump - with or without nucleation within the liquid phase, although I agree, nucleation will help considerably.

Presumably you're imply waxing or freezing of the fuel thereby restricting flow rates to the donkeys, lowering fuel pump inlet pressures and causing the cavitation. My own non specialist view is that the waving/freezing hypothesis died the death aeons ago on this most worthy of threads in the light of the published AAIB data showing fuel fpt to be < - 50 C - a suspiciously low figure.

The following, somewhat familiar by now hypothesis, alleges no failure of the pumps.

I'm rusty on this but cavitation can easily produce a mixture of true gas phase mixture fuel components (which may or may not contain air as well) from the mixture alone by mechanical induction in a liquid phase mixture if component(s) in the fuel mixture have a low enough Mr or levels of intermolecular bonding and the kinetic energy input is high enough.

I'm guessing, but I reckon any gas phase component's t½ at say +80C will be long enough for it to be a nuisance.

And and ……if the fuel has stratified in the tanks as well, with the low Mrs with their lower densities at the top of the tanks, they'll be fed to the donkeys at higher concentrations just when the FADECs go with throttle up so to speak, at the end of the flight.

As an aside, the pumps are defined as serviceable by the AAIB interim report. Now I take that to mean that they're within spec and I presume spec refers to volumetric efficiency against a liquid phase. If the poor things are trying to pump lots of eminently compressible of bubbles we will have considerably reduced flow rates and outa-gas symptoms.

Just what were the additives? What was their Mr?

It's already been said and it's still true, AAIB have done their job entirely to spec and we have to wait for the full report, but hey this is PPRuNe after all! :ok:

Green-dot

19th Mar 2008, 06:56

What Boeing is doing
Boeing doesn't know the answer, apparently, but it knows what it's looking for:

http://www.flightglobal.com/articles...h-landing.html (http://www.flightglobal.com/articles/2008/03/14/222219/fuel-system-tests-continue-following-777-crash-landing.html)

SF

Reading the article, i assume the tests will not be limited to fuel properties alone, as the article suggests, but fuel system tests including failure simulations of fuel system components upstream of the engine pumps? I would not be surprised they will do tests equivalent to what i have posted in post #590. In this case for 2 donks instead of one.

Regards,
Green-dot

GemDeveloper

19th Mar 2008, 07:08

Just to build a little on the last few posts:

The investigation reports that the fuel pumps are serviceable. I take that to mean that the flow/pressure characteristics, when the pumps are bench tested on fuel, are within specification.

But when the cavitation damage occurred (and, depending on the causes, the design of the flow paths, and the metallurgy, that might have occurred relatively quickly), we can speculate that the output from the pump(s), would have been some dual phase mixture. If such a mixture was being passed by the pumps at the time the FADEC requested a higher fuel flow in response to the increased power demand, it is possible that the fuel flow rate within such a mixture would be insufficient.

Zorst

19th Mar 2008, 08:07

My (layman's) understanding of the AAIB investigation process is that the manufacturer (and airline for that matter) will be closely involved at all stages

You can bet your bottom dollar that the manufacturers (RR and Boeing) are throwing much bigger resources at this than the AIB ever can.

smuff2000

19th Mar 2008, 08:24

Being involved in automotive design for many years may I answer a question from a day or so ago, cavitation damage can appear very rapidly, even to the point of destroying a pump in a matter of minutes in extreme cases.

oxo

19th Mar 2008, 12:48

But when the cavitation damage occurred (and, depending on the causes, the design of the flow paths, and the metallurgy, that might have occurred relatively quickly), we can speculate that the output from the pump(s), would have been some dual phase mixture. If such a mixture was being passed by the pumps at the time the FADEC requested a higher fuel flow in response to the increased power demand, it is possible that the fuel flow rate within such a mixture would be insufficient.

Sure, but remember that the pumps don't just have cavitation for no reason. There must have been an upstream flow restriction, and it is that flow restriction which caused the reduced flow out of the pump, not the cavitation at all.

Smilin_Ed

19th Mar 2008, 13:10

smuff2000 wrote: "Being involved in automotive design for many years may I answer a question from a day or so ago, cavitation damage can appear very rapidly, even to the point of destroying a pump in a matter of minutes in extreme cases."

And I would expect that, since the pressures and flow rates generated by these pumps are many times that ever found in any automotive applications, the damage rate would be even faster.

Milt

19th Mar 2008, 21:24

Chris weston

We've been waiting for someone like you to enhance our knowledge of the 'phenomonen' of cavitation of our high pressure fuel pumps. But then you confuse us by introducing Mr and Mrs without explaining what that happens to be. Help!!

I know a little more than most about cavitation having been triggered into researching the subject after having an irretrievable flame out on a Canberra's left Avon engine followed by the loss of the remaining right engine on short final. Fortunately we just made it to the under run and rolled out on the runway, much to the relief of the nav who had been tempted to leave via his ejection seat. But that incident was caused by the failure of all four variable stroke piston pumps which rapidly disintegrated when they started to pump water instead of fuel. There were some signs of earlier cavitation damage discovered.

Experience in those days soon revealed that short periods of operation of the HP pumps at high altitude soon had HP pumps cavitating and then we were learning that jet fuel starts to boil/gassify/produce vapour as the suction side of HP pumps reaches some low value not far above a vacuum. Presumably there will be a temperature variable.

I asked in a previous post whether anyone could produce some feel or appreciation for the likely pressure below which one could expect cold fuel, with some water content, to start vapourising which would consequently mark the onset of HP pump inlet cavitation. Such a number will set a bench mark as to the extent of fuel flow restriction necessary to substantially reduce output flow.

I would expect that fuel cavitating at the pump intake would instantly return to total liquid at the considerably higher pressures at its outlet with its flow then moderated by the FADEC demand.

Can you elaborate and give us an enhanced comprehension as to the operation of the conventional HP pumps particularly under marginal conditions.

Apart from opening up further instructive speculation this will prompt pilots to a better appreciation of the train of events which result when they move those thrust levers and the booster pump switches.

4Greens

20th Mar 2008, 07:28

A thought:

What is the situation with EROPS/ETOPS certification?

It would seem that there should be some reassessment in view of the uncertainty.

hetfield

20th Mar 2008, 07:54

If both donks dead, forget about ETOPS.

M.Mouse

20th Mar 2008, 12:35

What is the situation with EROPS/ETOPS certification?

It would seem that there should be some reassessment in view of the uncertainty.

ETOPS certification does not depend on guesswork. If and when the causes of the accident are known then it may or may not be looked at depending on the relevance of the accident causes.

chris weston

20th Mar 2008, 23:03

Milt,

I must needs disappoint and apologise to you if you feel I have mislead you and others, engineer I ain't and I can't advise on something as safety critical as the operation of conventional HP pumps in likely cavitation situations. I can speculate that the 777 had a fuel mixture problem probably revolving around additives.

I have great respect for those people such as yourself for whom cavitation is a not merely an armchair and academic exercise.

I'm simply a pure-ish chemist who has the privilege to work with some competent Chem Eng people who, amongst other things, pump a lot of hydrocarbons around.

We agree that any gas phase hydrocarbons et al will go back to liquid phase more or less instantly and that their half life (t ½) will be short.

Mr is the Mass of a Molecule on the C scale and I am further to blame for not rendering it as such in my text. I typed the r s as the subscripts they should be at the time but on cutting and pasting, the site rendered them as "Mr" and in the plural as "Mrs" - I forgot the understandably limited word processing on the site, Word it ain't. I also typed +8 (superscript) O Centigrade which became +80C ………. a singularly startling LHR Jan temperature. My fault, not Danny's, I know the site does this, mea culpa.

CW

UNCTUOUS

21st Mar 2008, 09:28

Chris Weston said
I can speculate that the 777 had a fuel mixture problem probably revolving around additives.
(Does this include an icy slush from the nether regions of the center-section tank mixed in with the fuel-feed (and emanating from the situation described below)?:
.
The scenario below describes Boeing's awareness of a problem but poor analysis of it and absolutely no extrapolation to a situation where the inflight temperatures were largely irrelevant - simply because the water in the center-section tank had been frozen (beijing overnight temps being below 7 degrees Celsius), therefore not able to be sumped (i.e. drained) and accumulating over a lengthy period. Furthermore Boeing's engineers failed to foresee or entertain any scenario wherein thrust could be affected. Boeing simply addressed the undesirability of the icy accumulations making inroads into the planned reserve fuel.

SUBJECT: 777-300ER/777-200LR Failure to Scavenge Fuel

/A/ Service Related Problem 777-SRP-28-0118
/B/ Fleet Team Digest Article 777-FTD-28-07002

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

SUMMARY:
Note: This message contains important information relevant to flight operations and airplane dispatch, please distribute accordingly.

Several 777-300ER operators have reported intermittent occurrences of airplanes landing with as much as 2200 lbs/1000kgs/300 gallons of fuel in the center tank. Boeing theorizes that this is an indication that the fuel scavenge system has malfunctioned. A failure such as this of the fuel scavenge system reduces the range of the airplane and could potentially lead to fuel exhaustion in the event additional failures occur which require use of all planned reserve fuel. To address this concern, Boeing recommends that 777-300ER and 777-200LR operators review their fuel reserve policy to ensure adequate reserves exist for each mission.

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

DESCRIPTION:

The scavenge system is designed to transfer fuel from low areas of the center wing tank to the main tanks after the override pumps are shut off. Scavenging this additional fuel from the center tank increases the fuel available for engine use. The 777-300ER and 777-200LR airplanes have incorporated scavenge system design changes intended to increase the amount of fuel scavenged and reduce the amount of trapped unusable fuel in the center tank to approximately 3 gallons. These changes included relocating the fuel scavenge inlet further inboard, while the water scavenge inlet location remained unchanged. Additionally, the fuel scavenge outlet and float valve were moved further outboard to allow fuel scavenge to be initiated earlier in flight.

Several 777 -300ER operators have reported intermittent occurrences of airplanes landing with as much as 2200 lbs/1000kgs/300 gallons of fuel in the center tank. Boeing theorizes that this is an indication that the fuel scavenge system has malfunctioned. These instances have only occurred on long routes originating from colder climates and have led to the conclusion that an excessive amount of water is entering the fuel scavenge system and is freezing during scavenge operations (and for sumping?). Because the water scavenge inlet was not co-located with the fuel scavenge inlets it is more likely for water to be ingested into the scavenge system. Additionally, as the outlet float valve location is further outboard in the main tank than previous, the scavenged center tank fuel has more exposure to the cold soaked main fuel tank prior to reaching the scavenge discharge. Indications are that the water in the scavenge system is freezing prior to discharging in the main tank. Frozen water (or ice) in the scavenge system could result in a low rate of scavenge or no fuel scavenge. (But ice in the form of melting slush?)

Failure of the fuel scavenge system could result in airplanes landing with as much as 2200 lbs (1000 kgs) of fuel in the center tank. During mission planning and dispatch, this fuel in the center tank was considered usable fuel. However, failure of the fuel scavenge system in flight renders this 2200 lbs (1000 kgs) of fuel as unusable. There is no indication to the flight crew that the scavenge system has failed nor that the fuel is unusable. Failure of the fuel scavenge system essentially reduces the range of the airplane and could potentially lead to fuel exhaustion in the event additional failures occur which require the use of all planned fuel reserves.

Boeing review has determined that the failure to completely scavenge the center tank is the result of system configuration changes unique to the 777-300ER and 777-200LR airplanes. This issue has been placed in our Service Related Problem (SRP) process for resolution and is the subject of the REF /B/ Fleet Team Digest article.

DESIRED ACTION
===============
Boeing recognizes each operator establishes its own fuel reserve policy. Some operators choose to add additional conservatism to existing regulatory fuel reserve requirements. In addition, we note that not all routes and/or operators have shown a susceptibility to this condition. This may be because of environmental conditions, individual airline water sumping policies, or different operator fuel system procedures.

Boeing suggests 777-300ER and 777-200LR operators review their operation for exposure to trapped center tank fuel and their maintenance policy related to water sumping.

We recommend operators establish a policy to monitor center tank fuel quantity upon arrival of each flight. If trapped center tank fuel above 400 lbs (200 kgs) is discovered, we recommend a further review of fuel reserve and maintenance policies as noted above.

If operators chose to address this issue by uploading additional fuel, Boeing recommends operators notify their flight crews that additional fuel has been loaded to mitigate the potential for up to 2200 lbs (1000 kgs)of unusable fuel following failure of the scavenge system.

For operators who have seen the trapped center tank fuel condition and chosen to adjust their fuel reserve policy, we recognize it may be possible for this condition to be resolved on future flights due to a change in environmental conditions or maintenance practices.. If this situation arises, we believe it appropriate to adjust fuel reserve policies to original levels provided they continue the monitoring policy on a flight by flight basis for trapped center tank fuel.

Although these failure to scavenge occurrences have only been reported on the 777-300ER, any Boeing recommendations should also be applied to the 777-200LR as it has an identical center tank fuel scavenge system.

If further information is needed regarding the subject, please contact your local Boeing Field Service Representative. If your local Field Service Representative is unavailable, you may contact the appropriate Airline Support Manager or call the BCA Operations Center at (206) 544-75

Spaz Modic

21st Mar 2008, 09:45

:eek: QUOTE]Boeing doesn't know the answer, apparently, but it knows what it's looking for:[/QUOTE]

Yeah - It's called GAS, as in AVTUR ;)

Green-dot

21st Mar 2008, 11:40

because the water in the center-section tank had been frozen (beijing overnight temps being below 7 degrees Celsius), therefore not able to be sumped (i.e. drained) and accumulating over a lengthy period.

The temperature in Beijing was -7 deg. Celcius according to AAIB Bulletin S1/2008, page 3. I assume the poster intended to mention it as -7 deg. C.

On page 5 of the same bulletin the AAIB states:

"The aircraft's fuel tanks were last checked for water in the fuel on the 15 january 2008 at Heathrow; this was prior to its refuelling for the outboard sector to Beijing."

For the record i just checked: temperatures at Heathrow on 15 january were between +6 and +11 degrees Celcius, depending on the time of day. Warm enough to drain water. For sure if the airplane had been at Heathrow for a considerable time. I don't know the aircraft's flight history prior to the outbound sector to Beijing so alot depends on the aircraft's turnaround time between flights. But if it had an overnight stay, temperatures were well above freezing at Heathrow the day before (the whole week before, for that matter). Any water drained in such conditions is all the water accumulated in the tanks prior to refuelling.

Fuel samples taken from the aircraft after the crash have sofar ruled out contamination or unusual levels of water content, neither did sump samples taken from the main wing tanks reveal "significant quantities of water" according to AAIB Bulletin S1/2008, page 4.

Apparently water did not accumulate over a lengthy period.

Green-dot

sky9

21st Mar 2008, 14:54

Just 3 questions to the experts:
1 If there is sign of cavitation in the pumps, I presume that it involves wear on the impellers. What happens to that wear or debris; does it travel downstream or is there a filter to capture it before it enters the cans?
2 When the propeller on my boat cavitates the RPM increases but the boat goes nowhere; the prop effectively stalls. Does the same happen to the fuel pumps?
3 If temperature of fuel is an issue why hasn't anyone restricted the altitude or fuel temp of the 777 as a "safety first" prudent measure or are they looking for another incident to verify their research?

bsieker

21st Mar 2008, 17:03

1 If there is sign of cavitation in the pumps, I presume that it involves wear on the impellers. What happens to that wear or debris; does it travel downstream or is there a filter to capture it before it enters the cans?

Cavitation does not necessarily cause abrasion, it may in mild cases only "dent" the surface.

Having said that, there is a high-pressure filter specifically to prevent debris from the HP pump to enter the engine and damage it permanently. It also has no bypass: if the HP pump fails, the engine would not work anyway.

2 When the propeller on my boat cavitates the RPM increases but the boat goes nowhere; the prop effectively stalls. Does the same happen to the fuel pumps?

The HP pump is a gear-type pump, not impeller-type. If it cavitates, it will still work, although slightly less efficient, because the liquid with bubbles in it is now compressible, unlike the completely liquid fuel it normally pumps.

3 If temperature of fuel is an issue why hasn't anyone restricted the altitude or fuel temp of the 777 as a "safety first" prudent measure or are they looking for another incident to verify their research?

Fuel temperature is almost certainly not an issue here. There is no need for a general altitude restriction, because fuel temperature is monitored. Warning is given well above critical temperature. Countermeasures to keep the fuel from cooling further exist, such as increasing mach number (cruising speed) or descending to lower flight levels if and when necessary. Both measures increase fuel consumption.

There are fuel temperature limits, but just how far above the known freezing point would you consider "prudently safe"? Is 13 degrees C enough?

In this incident fuel temperature was still very far in the "safe" region. Freezing point according to spec would have been -47C or lower, actual freezing point was -57C, lowest recorded fuel temperature was -34C.

Bernd

MU3001A

21st Mar 2008, 18:10

I agree that it's unlikely the actual temperature of the fuel on flight BA38 ever got below what are currently understood to be safe limits, but the fact that there is only one fuel temp sensor in one location of one wing of the B777 does introduce at least the potential for a single point failure.

bubbers44

21st Mar 2008, 18:27

Why were a lot of flights that day on similar routes descending to FL250 and this flight recorded a low of -34C fuel temp? ISA-20 was the temp aloft that day according to a previous post. Once installed is the fuel temp sensor ever calibrated or checked?

slip and turn

21st Mar 2008, 19:41

So just one fuel temp sensor, and what proportion of the remaining fuel does it measure? Is the wing tip the coldest position? What are the fluid dynamics of hydrocarbons in a tank at critically low temperatures? Does it become a little like a soft boiled egg i.e. would the temperature be homogenous throughout the tank or would Apollo 13 type tank stirrers be required to make it so?

Tree

22nd Mar 2008, 00:35

I am interested re the tested fuel f/p of -57.
That seems unusual for traditional jet fuel.
I have experienced unusual engine exhaust conditions using jet fuel in a third world country. Turns out they were short of jet fuel so they blended in a substantial amount of avgas.

foxtrot charlie

22nd Mar 2008, 01:02

At the risk of being shot down, my own view is that this was a 'proof of concept'. no evidence just a feeling. sometimes that's all that's needed though.

Pinkman

22nd Mar 2008, 21:04

I am interested re the tested fuel f/p of -57. That seems unusual for traditional jet fuel.

Tree, thats because it isnt 'traditional' Jet fuel like Jet A (f/p -40C), Jet A1 (f/p -47C). The Chinese call it Jet fuel number 3, it used to be called RP-3 and it meets or exceeds the spec of Jet A-1 for every parameter as far as I am aware. I am not sure what the historical reason is but its always been that way as long as I've been around fuels. Interestingly many of the chinese and soviet higher freeze point fuels also have lower flash points. But I dont think thats relevant in the current case.

wilyflier

23rd Mar 2008, 01:32

moderator ,did you block my message yesterday?
This thread leads to what I was saying about fuel being ok for not waxing at extremely low temps when tested,, but light enough to boil at low above zero temps when subject to pump suction

HotDog

23rd Mar 2008, 03:32

wilyflier, I'm not the moderator but will save him from answering you. Your post was not blocked but deleted together with my reply to your ridiculous theories. Give it a rest mate, the AAIB are working on it. There is nought you can suggest that hasn't been looked at by experts in the field which most probably have no test tube freaks amongst them.:ugh:

Flight Safety

23rd Mar 2008, 15:20

Is there any possible way the fuel could have been Russian TS-1, which has a freeze point of -57C, min low flash point of 28C, and has properties and test methods similar to Jet A-1?

Tree

23rd Mar 2008, 17:57

Pinkman and Flight Safety. Thanks, I can find technical references to TS-1 with f/p of -50 (see below). The only one that fits close to a f/p of -57 is obsolete JP-3 (further below). Excuse the format, the graphs do not copy properly.

Selected Specification Properties of Jet Fuels
Fuel Jet A Jet A-1 TS-1 Jet B
Specification ASTM D 1655 DEF STAN 91-91 GOST 10227 CGSB-3.22
Acidity, mg KOH/g 0.10 0.015 0.7 (mg KOH/100ml) 0.10
Aromatics, % vol, max 25 25.0 22 (% mass) 25.0
Sulfur, mass% 0.30 0.30 0.25 0.40
Sulfur, mercaptan, mass% 0.003 0.003 0.005 0.003
Distillation, °C:
Initial boiling point — Report 150 Report
10% recovered, max 205 205 165 Report
50% recovered, max Report Report 195 min 125; max 190
90% recovered, max Report Report 230 Report
End point 300 300 250 270
Vapor pressure, kPa, max — — — 21
Flash point, °C, min 38 38 28 —
Density, 15°C, kg/m3 775–840 775–840 min 774@20°C 750–801
-------------------------------------------------------------
..............................Jet A...A-1..TS-1...................Jet B
Freezing Point, °C, max –40 –47.0 –50 (Chilling point) –51
-------------------------------------------------------------
Viscosity, –20°C, mm2/sec, max 8 8.0 8.0 @ –40°C —
Net Heat of combustion, MJ/kg, min 42.8 42.8 42.9 42.8
Smoke point, mm, min 18 19.0 25 20
Naphthalenes, vol%, max 3.0 3.00 — 3.0
Copper corrosion, 2 hr @ 100°C, max rating No. 1 No. 1 Pass (3 hr @ 100°C) No. 1
Thermal stability
Filter pressure drop, mm Hg, max 25 25 — 25
Visual tube rating, max <3 <3 — <3
Static test 4 hr @ 150°C, mg/100 ml, max — — 18 —
Existent gum, mg/100 ml, max 7 7 5

Freeze Point Flash Point
Fuel Introduced Type RVP, psi °C max °C min Comments
JP-1 1944 kerosine –60 obsolete
JP-2 1945 wide-cut 2 –60 obsolete
JP-3 1947 wide-cut 5–7 –60 obsolete
JP-4 1951 wide-cut 2–3 –72 U.S. Air Force fuel
JP-5 1952 kerosine –46 U.S. Navy fuel
JP-6 1956 kerosine -54 XB-70 program, obsolete
JPTS 1956 kerosine –53 Higher thermal stability
JP-7 1960 kerosine –43 Lower volatility, higher thermal stability
JP-8 1979 kerosine –47 U.S. Air Force fuel
JP8+100 1998 kerosine –47 U.S. Air Force fuel containing an additive
that provides improved thermal stability
JP stands for Jet Propulsion.

Tree

23rd Mar 2008, 18:00

Pinkman.
Interestingly many of the chinese and soviet higher freeze point fuels also have lower flash points. But I dont think thats relevant in the current case.

I believe that is a wide-cut fuel type which has been phased out elsewhere.

Tree

23rd Mar 2008, 19:11

Flight Safety: Is there any possible way the fuel could been Russian TS-1, which has a freeze point of -57C, min low flash point of 28C, and has properties and test methods similar to Jet A-1?

Russian fuel grades
GOST Standard 10227-86 lists four grades of fuel, TS-1, T-1, T-2 and RT. Each has a category of quality with TS-1 having both a higher and first category, of which only the first category appears to be in use. An update of Table 1 of this specification, listing TS-1, RT and two Military fuels, T-8V and T-6, requires an antioxidant and a lubricity improver in RT, T-8V and T-6. Another amendment noted is a change in the Crystallization Point (similar to our freezing point) in TS-1 and RT to -50 °C with -60 °C and -55 °C respectively being produced by user demand.

Flight Safety

23rd Mar 2008, 19:21

This quote is from this link on the Shell website, aviation fuels for civil aircraft.

http://www.shell.com/home/content/aviation-en/productservice/aviationfuels/detail/worldwideciviljet_10081004.html

Former Soviet Union and East European Jet Fuels

Soviet kerosine type jet fuels are covered by a wide range of specification grades reflecting different crude sources and processing treatments used. The grade designation is T-1 to T-8, TS-1 or RT. The grades are covered either by a State Standard (GOST) number, or a Technical Condition (TU) number. The limiting property values, detailed fuel composition and test methods differ quite considerably in some cases from the Western equivalents.

The principle grade available in Russia (and members of the CIS) is TS-1.

The main differences in characteristics are that Soviet fuels have a low freeze point (equivalent to about -57 degrees C by Western test methods) but also a low flash point (a minimum of 28 degrees C compared with 38 degrees C for Western fuel). RT fuel (written as PT in Russian script) is the superior grade (a hydrotreated product) but is not produced widely. TS-1 (regular grade) is considered to be on a par with Western Jet A-1 and is approved by most aircraft manufacturers.

Eastern European countries have their own national standards with their own nomenclature. Many are very similar to the Russian standards but others reflect the requirements of visiting international airlines and are similar to Western Jet A-1 in properties and test methods.

Tree

23rd Mar 2008, 19:58

From Air BP:

Jet Fuel No 3 (formerly known as Grade RP-3) – is similar to Jet A-1 and is the
grade supplied at all civil airports in mainland China.

From Exxon Mobil:

Footnote to TS-1
(2) Temperature for start of crystallisation. TS-1 fuels with freezing point not above -50°C intended for use in all climatic zones except zone 11 (GOST16350-80).In zone 11 TS-1 fuel with freezing point above -50°C may be used when ground temperature is below - 30°C for 24 hours before take-off. TS-1 fuel with freezing point not above -60°C intended for use in zone 11 shall be produced as required by the consumers. (3) In case of dispute, the heat of combustion shall be determined by GOST 21

Fuel System Icing Inhibitors (FSII)

Water dissolved in fuel can come out of solution at low temperatures in the form of very fine droplets. Although the amounts are small, the droplets formed can freeze at altitude and cause filter plugging. Fuel system icing inhibitors have been developed to protect the system from this problem. The most widely used additive is diethylene glycol monomethyl ether (DiEGME). The use of FSII is required in UK and US military jet kerosine and although optional in many civilian specifications is very seldom used.

Tree

23rd Mar 2008, 20:04

Flight Safety;

Thanks. I have been researching several different sites (but not Shell yet) and fuel companies. There seems to be a variation in specifications between sites/companies. I also note that China imports approximately 30% of their jet fuel.

infrequentflyer789

25th Mar 2008, 01:33

(Does this include an icy slush from the nether regions of the center-section tank mixed in with the fuel-feed (and emanating from the situation described below)?:

No.

a) you've got the wrong aircraft (your boeing quote is about 777-200LR and 777-300ER, G-YMMM was 777-200ER)

b) we already know BA38 didn't land with a large amount of unused fuel in CT (which is clearly not the same scenario as the reports you are quoting)

wilyflier

25th Mar 2008, 09:49

Tree,
You seem to have a closer understanding than I do.
...Ive seen fire prorection tables that seem to show that aviation kerosenes with lower freeze points have lower flash points ; and elsewhere that tied to a fall in flash point is a fall in boiling point , at a roughly equivalent rate.
...Im getting at this -57c freeze point fuel being able to start bubbling with demand suction at an oat of 8-11 as it wamed up on final approach.
...Can you determine what the boiling point at MSL might be , and what would the inlet suction pressure have to fall to , to cause a further drop in boiling point (and gassing aeriation).?

Railgun

28th Mar 2008, 16:31

Does anyone know if the airframe has been declared a hull loss?

CaptJ

28th Mar 2008, 20:20

Yes, and paid out on.

see previously entries in this thread.

Skipness One Echo

28th Mar 2008, 20:52

Unusual that the airframe still carries full British Airways colours and titles.

boaclhryul

28th Mar 2008, 21:16

Unusual that the airframe still carries full British Airways colours and titles.

All the white paint and tape was saved for the T5 opening, in case that's declared a hall loss :)

Michael

airfoilmod

29th Mar 2008, 20:30

This is basically what I see.

1. Given the examination of EEC, FADEC, and appurtenant equipment,
Lack of quantity or quality of Fuel is expected to have caused this
incident.
2. A second Trent Balk at throttle was found to be not pertinent to BA38.

3. Cavitation, though found, has not been identified as Causative.

4. Everything AAIB have supplied has resulted in continuing operation of the Fleet.

5. Two major carriers have modified their fueling procedures (AA, UA).

From this a reasonable set of conclusions can be advanced.

1. Nothing AAIB have released has caused a system wide recommendation.

2. Nothing AAIB have released has caused an Airline recommendation.

3. Several reasonable theories remain which may infer other than the
resultant lack of action.

4. A responsible body as AAIB, not finding any cause to recommend
changes in equipment or procedures, has left open the door to very few
alternate albeit certainly not unobserved causative scenarios.

5. These few alternates are perforce limited to very unique and incident related possibilities. (Not generally assumed to happen again).

6. Microwave sensitive equipment could be adversely affected by tramp signal, returning to 100% post incident functionality, without record of anomaly.

7. Destruction of the Center Tank in the Landing phase may rule out an implication of tank issues.

8. There is probably good reason to withold investigative communication from the public.

pasoundman

29th Mar 2008, 20:39

airfoilmod
This is basically what I see.

1. Given the examination of EEC, FADEC, and appurtenant equipment,
Lack of quantity or quality of Fuel is expected to have caused this
incident.

With TEN TONNES of fuel in the tanks how do you reckon fuel QUANTITY might be an issue ?

The fuel has also AFAIK been tested for quality and found to be just fine.

airfoilmod

29th Mar 2008, 20:53

With all due respect, if a powerplant is not supplied with fuel, it will not run. No matter the location of ample fuel elsewhere. Take a breath, and make the attempt to see the possibilities in what you read. Starvation or exhaustion are not parsed by inanimate objects. Starvation and exhaustion are two iterations of fuel "availability".

Green-dot

30th Mar 2008, 09:39

Quoting Airfoilmod:

4. A responsible body as AAIB, not finding any cause to recommend changes in equipment or procedures, has left open the door to very few alternate albeit certainly not unobserved causative scenarios.

5. These few alternates are perforce limited to very unique and incident related possibilities. (Not generally assumed to happen again).

6. Microwave sensitive equipment could be adversely affected by tramp signal, returning to 100% post incident functionality, without record of anomaly.

7. Destruction of the Center Tank in the Landing phase may rule out an implication of tank issues.

8. There is probably good reason to withold investigative communication from the public.

Perhaps this is why several posts have been removed from this thread (some of mine amongst others)? Maybe some theories approximate the true cause of this incident. If so, this thread is biassed but my theory still stands:

1. A unique set of circumstances causing disruption of a subsystem.

2. Disruption resulting in a reverse logic electrical failure downstream of ELMS. Why downstream of ELMS? Because relays within ELMS provided correct signals to the engine controls according to the AAIB.

3. Spar valve control may have been affected.

4. Provided the fuel synoptic page was not selected at the time of the system disruption (I don't think so in the approach phase), no visible or aural alerts to the flight crew were presented because this may be inherent to the subsystem design (not going into details here because it is proprietary information).

5. Disruption was of a temporary nature and reset itself.

6. Impossible to prove without testing, which is a task for the AAIB, Boeing and RR to do so and find the facts.

Regards,
Green-dot

airfoilmod

30th Mar 2008, 15:03

Rightbase

30th Mar 2008, 16:01

Airfoilmod:

5. Two major carriers have modified their fueling procedures (AA, UA).

I'm curious - do you have a link to the details by any chance?

airfoilmod

30th Mar 2008, 16:23

Affirmative.

lomapaseo

30th Mar 2008, 18:47

I would agree that because of the way the investigation has unfolded, the incident was caused by factors narrowly applicable to BA38. Having released information that isn't considered to threaten continued and consistent operation of the Fleet or the Airport, I would assume AAIB is satisfied; if not, their inquiry obviously doesn't have disquieting implications.

Interesting assumption. Do you see any parallels with the response following similar one-off situations of TWA800 and Concorde? Or were these uniquely different combinations and of no relationship to such assumptions?

airfoilmod

30th Mar 2008, 19:14

What a leap. My post was meant to emphasize the lack of an extrapolative conclusion by AAIB; If there was such a finding, of course broader implications would have been entertained. If AAIB are keeping things close,
that's their prerogative, and no safety questions applicable to the Fleet or
crashsite are appropriate. Am I missing something? If a Jag hits a tree,
do we garage all Jags and cut down big trees? For goodness sake don't assume my metaphor implies Pilot error.

(TWA800, CONCORDE, TENERIFE- all fascinating, but off thread, I think)

SpeedyG

30th Mar 2008, 21:14

What was the official fuel remaining in the aircraft at time of impact? Does the AAIB actually know this? All I could see was "a significant amount of fuel leaked' Now question was was there sufficient fuel to flow without cavitation in the pumps??

Rightbase

30th Mar 2008, 22:33

airfoilmod

Affirmative.

Care to share? Google not delivering - perhaps I'm not touching the right nerve.

CONF iture

30th Mar 2008, 22:39

Also interesred ...

Rightbase

31st Mar 2008, 08:04

snanceki: re Airfoilmod

There is probably good reason to withold investigative communication from the public.There are several I can think of, but the obvious reason is they must be free to speculate about all the possibilities in order to eliminate those they can. Publishing their speculations would lead to the inevitable 'selective emphasis' that sells newspapers by disturbing the public. L-38 has a timely example of this.

re your questions:

If they don’t understand the causal reasons how can anybody assume
1. Causal factor(s) was unique to BA038
2. Causal factor(s) won’t happen again.
Should we assume they don’t understand the causal factors?Now you are doing it.
The 777 has built up a good safety record. The process leading to this failure has only been seen on this flight. Selective emphasis describes this as unique then suggests complacency within the investigation by asking for evidence of uniqueness.

The investigators will obviously look at what was different about this flight. They will also look into all the history - of this aircraft type and of all past accidents - to see whether anything already known about might have contributed here. As the investigation proceeds factual evidence and careful testing will eliminate some possibilities, and may reveal new factors to be considered and eliminated in turn.

This process has to be undertaken by specialists who understand aircraft, who know how accidents happen, and who are as dispassionate as it is possible to be in sometimes harrowing circumstances.

Air travel is as safe as human endeavour can make it - and that is very safe.

And that doesn't sell newspapers.

Sallyann1234

31st Mar 2008, 11:31

@Airfoilmod
I find your point 6 rather cryptic (my fault I'm sure).
What is a 'tramp signal'? Do you mean an interfering EM pulse?
How could such a pulse cause fuel control signals to be changed, without the real-time recordings of those same signals also showing the change?

snanceki

31st Mar 2008, 13:32

Now you are doing it.
The 777 has built up a good safety record. The process leading to this failure has only been seen on this flight. Selective emphasis describes this as unique then suggests complacency within the investigation by asking for evidence of uniqueness.

The investigators will obviously look at what was different about this flight. They will also look into all the history - of this aircraft type and of all past accidents - to see whether anything already known about might have contributed here. As the investigation proceeds factual evidence and careful testing will eliminate some possibilities, and may reveal new factors to be considered and eliminated in turn.

This process has to be undertaken by specialists who understand aircraft, who know how accidents happen, and who are as dispassionate as it is possible to be in sometimes harrowing circumstances.

Air travel is as safe as human endeavour can make it - and that is very safe.

And that doesn't sell newspapers.

Maybe I'm being difficult but I don't understand your answer. Maybe you are just reiterating what I said. Maybe not.

I understand the investigation process. I am an engineer myself and although I don't work in Aviation the fundamental process is common to any machine / structural investigation.
Please remember I was replying to a previous post.

All I asked was how is it possible to assume that the causal factors surrounding BA038 are UNIQUE to that aircraft and flight? i.e. No implications to other of the type or operator.

If the cause is unknown anything is possible and this may therefore also affect other aircraft of the same type or operator once the cause is known.

Engines do not just Roll Back; Something caused the event to happen. Most of the tabled possibilities are indeed EXTREMELY unlikely but the very fact they have cause one event means that without change a reoccurrence could happen.

If the cause is already known then that opens up other (also unlikely) possibilities.

najavens

31st Mar 2008, 14:32

najavens

31st Mar 2008, 14:52

RF is certainly a possibility and one that rarely leaves any tangible evidence for investigators to see. Most internal RF will be preventable due to electrical systems and wiring being shielded to prevent leakage affecting a neighbouring system, but external RF is a problem and can affect A/C in a perplexing manor of ways. I believe that it is rare for a fuel system to be compromised by RF as there are mechanical systems as back-up, and I believe there are 2 FCU’s on the aircraft, one for each engine. For both to be affected by RF would be unusual. I dare to speculate that it may just be a fuel flow problem due to contamination of the fuel, most of which was dumped onto the runway post landing! Time will tell.

chris weston

31st Mar 2008, 15:25

najvens,

With you all the way sir, my thoughts entirely - but that's only to be expected from a test tube freak such as myself! :ok:

CW

FullWings

31st Mar 2008, 17:18

I have been informed by a collegue in Flight Safety, who has contacts with the AAIB, that they are currently running with the idea that there was fuel starvation to both engines. It seems that only 10% of the fuel required was making it to the engines causing them to wind down. The problem however is identiying the part of the fuel system at fault.
I think that came out pretty early on...

from the AAIB initial report, Feb. 2008:

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.
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.
Basically, there doesn't seem to be any new information; indeed, even the speculation seems to be repeating itself now...

Sunfish

31st Mar 2008, 17:44

At the risk of getting labelled a spotter I would observe that with any aircraft incident:

1. The mechanical engineer thinks the cause is mechanical failure.

2. The Electrical engineer thinks the problem is electrical.

3. The radio ham thinks it's RF interference.

4. The computer programmer thinks the cause is software problems.

5. The chemist thinks it's the fuel.

.........and so on, as this thread conclusively demonstrates.

To put it another way, everyone examines things through the lens of their own experience.

The trouble is that while I'm sure the speculation is well meaning in most cases, very little of that experience has anything much to do with commercial aviation.

I'd rather wait for the experienced folk at AAIB and Boeing to tell all of us what they believe the probable cause is, because as an engineer who first was responsible for an oil terminal and airport jet fuel handling facility, then spent six years with an airline engineering division, followed by an aerospace/defence outfit, and flies little tiny aircraft, I'm buggered if I know what happened.

Rightbase

31st Mar 2008, 20:39

Snanceki said:
Maybe I'm being difficult but I don't understand your answer. Maybe you are just reiterating what I said. Maybe not.

Possibly because I am guilty of not answering it. An apology and explanation are in order.

Sorry.

Explanation:
I just hate the way some news media capitalise on natural human curiosity by sensationalising matters. They do so, in a subtle way, by using sensationalising words. A hand-made objet d'art can be unique, never to be repeated. Using the Unique word to describe an event is imprecise to say the least. I would grudgingly go along with 'hitherto unique'

If you are objecting to the word Unique, then I am agreeing and reiterating that point, prompted by rather than answering your rhetorical question.

Like you, I can't believe anybody is saying 'We know what caused it, and it can't happen again, so there's no need to publish it.' They wouldn't dare. My take on the silence so far is the investigators - a collective name for members of the AAIB, Boeing, RR, possibly other regulatory bodies and probably several component suppliers - still don't know what happened, so I presume they are still speculating (testing hypotheses). And none of this speculation has resulted in a convincing causal sequence they can publish - yet.

I can see three possible outcomes. The worst outcome is we don't know what happened. The best is we know what happened and we can stop it happening (at least in that way) on future flights. In the middle is we know what happened and the only way we know of stopping it happening again is to not fly.

As somebody said a long time ago, the final report is the one to read.

Sorry again. Please let me plead oversensitivity, caused by excessive exposure to spin and hyperbole.

L-38

1st Apr 2008, 06:46

I thank my fellow airman of discussion for their endeavored reply's ("RF" theory for BA B-777 #720), .

In long ago past service as a B-742 FE, I recall that every time the lead ground fueller (when in the cockpit) would key his radio phone (to communicate with his fueling partner at the wing), that my automatic pressurization outflow valves would drive closed.

snanceki

1st Apr 2008, 07:15

Thanks Rightbase.
Appears we are in the same camp.

However! waiting for the final report would put Forums like this "out of business".
Postulating what might have happened (on inadequate and potentially incorrect information) is surely what Forums like this are all about.
Although trained as an engineering investigator (and therefore trained to have an open mind) I never cease to be amazed by the power of collective thinking and the unusual and sometimes bizarre suggesting that are made. Some are easy to dismiss, others propositions less so and some are definitely feasible.
The main benefit from my perspective is increased understanding of how systems with which an individual is not adequately familiar become clearer.

Whatever the outcome of this investigation the cause is going to be UNUSUAL/UNPRECEDENTED but not UNIQUE.
If its happened once, it can occur again, unless suitable changes take place

Although I appreciate that the AAIB wish to dot the I's and cross the T's before publishing any further report I'm surprised that more "informed" data hasn't hasn't leaked out.
Now in one sense this is exactly as it should be, but with so many interested parties involved and an intact aircraft I find it surprising that (virtually) no recommendations have come from the AAIB impacting other aircraft of the type / operator. This suggests to me that they currently have no recommendation to make.

So either the team has:
1. No idea what caused the accident in which case the extremely improbably theories need to be considered more seriously or
2. The findings are worrying with significant impact to other aircraft / operations (although likelyhood of a repeat is low based on experience to date) and any action must be thoroughly researched and backed up due to implied costs etc. of taking action. ETOPS?

I wonder which is nearer to being correct? I favour option 2.

Either way the situation is IMHO unprecedented!

A no cause identified outcome is IMHO not viable since the aircraft is largely intact.

Interesting.

Chris Scott

1st Apr 2008, 11:07

Quote from snanceki:
A no cause identified outcome is IMHO not viable since the aircraft is largely intact.
[Unquote]

It’s a wonderful situation, isn’t it? Pity all those electrons have rearranged, though. I like your analysis. It is a blessed irony that the very presence of the hardware must be a mixed blessing for the investigators. Don’t we have it easy…

Kolossi

1st Apr 2008, 12:55

Sunfish:

To put it another way, everyone examines things through the lens of their own experience

I recall a similar post previously regarding the speculation around BA38. Perhaps it was you? I resisted the temptation to comment on that occasion, but as it's been raised again, here goes ... :)

What you say is right, but would you prefer it if the Mechanical Engineer started spouting about fuel chemistry?

Sunfish:
The trouble is that while I'm sure the speculation is well meaning in most cases, very little of that experience has anything much to do with commercial aviation.

You see, unless the poster has experience of commercial aviation, you seem to infer that their contribution is of less value - ie you are asking them to stick to their field!

As I see it, it's entirely sensible that those with experience of other fields make a contribution, even if they don't have experience of commercial aviation. A link pin is a link pin, chemistry is chemistry, software has bugs :hmm: - aviation is another field, but it doesn't change the laws of the universe. If those with commercial aviation experience - who I'm sure are for the most part intellegent enough to understand the suggestion from other fields being put forward - can "through the lens of their own experience" spot a flaw in a posted suggestion, they should raise it to rebuf the argument and the poster should accept that. Unless they have a counter argument of course, that's how a debate works isn't it ? :)

Whilst there have been some truly terrible posts on this and other threads despite the moderators considerable efforts, I also see a lot of people being shot down when they make a suggestion, with the responder (who often has a valid point, but agressively made) seeming to have implied that the poster is saying "I categorically state this is what is happened" when in all but a few cases they haven't.

If only commercial aviator's opinions are of value, then make the forum closed or read-only. For my part I think the mix of interested engineers from many disciplines is a real strength of PPrune, albeit that there is a problem due to it being used by Journos - who don't understand engineering uncertainty - as gospel fact for a sensationalist headline.

I'd rather wait for the experienced folk at AAIB and Boeing to tell all of us what they believe the probable cause is

That's been posted many times too. I read every one of the first 1000 or so posts on this subject, but drifted away from the thread when useful additions dried up (interesting diagrams of the fuel system were still being added until quite recently). As you see I pop back from time to time. But this is the thread for people wanting to speculate, postulate and question, so if you just want to wait for the AAIB report, surely it's best to "move along, nothing to see here ..." :confused:

because as an engineer who first was responsible for an oil terminal and airport jet fuel handling facility, then spent six years with an airline engineering division, followed by an aerospace/defence outfit, and flies little tiny aircraft, I'm buggered if I know what happened.

Yup, as an CEng MIET in Electronic Engineering and a software professional, I'm similarly buggered (though that's probably as expected because as you say I don't have professional experience of commerical aviation) :hmm:, and am keen to see the AAIB report.

But let's not stop some of the current issues with the use of PPrune kill one of it's greatest strengths, namely the excellent experience of interested individuals across multiple engineering fields.

Just need to find a way to make them float to the top amongst the youngsters, innexperienced but wannabe engineers, and Journos trying to provoke a headline.:ok:

lomapaseo

1st Apr 2008, 14:14

Snanceki
So either the team has:
1. No idea what caused the accident in which case the extremely improbably theories need to be considered more seriously or
2. The findings are worrying with significant impact to other aircraft / operations (although likelyhood of a repeat is low based on experience to date) and any action must be thoroughly researched and backed up due to implied costs etc. of taking action. ETOPS?

I wonder which is nearer to being correct? I favour option 2.

I also favor option 2. Similar to what we knew about TWA800 at the same time. I hate mysteries in investigating multiple probabilistic combinations when the actual chain is missing links which can not be verified easily. Yet the aviation comunity including the users awaits a definitive corrective action to make the outcome all go away. If the wrong recommendation is chosen it not only is costly but it also adds new unforseen risks of its own. Therfore one can not rush quickly into expressing a conclusion.

I usually am satisfied to put a close watch on the data stream information that the fleet has to offer relative to suspected contributing causes and see what theory is best supported by monitoring the continued airworthiness of the suspected product line..

An no I don't yet have in mind a definition of the suspected product line, be it Rolls, Boeing or the whole damn fleet. I am only drawn to the parallels following TWA800

411A

1st Apr 2008, 14:45

The problem however is identiying the part of the fuel system at fault.

The fuel system apparently did not fail, as such, but rather fuel icing appears to be the cause.
This specific flight was one (perhaps the only one) that did not descend into warmer air along the route, due to very cold ambient temperatures aloft.

Not necessarily the FD crews fault, it could well have been that the actual fuel tank(s) fuel temperatures were incorrectly displayed.

airfoilmod

1st Apr 2008, 17:36

You don't attribute your quotes, so I need to distinguish the perception that you made re: a "post" of mine.

It is plausible that AAIB do NOT have evidence of causation that would implicate other aircraft, because if a warning to the Fleet were necessary, it hasn't happened; being the responsible chaps they are, I assume they will not endanger the Fleet by being "coy" with important announcements.
Asking for clarification is appropriate, but if one so obviously misunderstands a Post, one should withold personal conclusion and further confusion. By and Large the comment here is excellent, it comes from a number of interesting sources and the tenor and respect is exquisite. I am curious as any, and refutation of possible conclusions is interesting also, but of course, much remains to be known. In the meantime, I will read your posts and others with great relish.
(411A your surmise is compelling, When others were descending, BA was Climbing)

pls8xx

1st Apr 2008, 18:10

I'm already on record for a theory. I'll go a bit more.

The permanent fix is to change the gear ratio for a reduction in pump speed and an increase in pump size to maintain the flow capacity.

In the interim, if the fuel is already cold soaked, don't continue a long exposure to an extremely low OAT. Do what every other prudent pilot did that day; get down.

snanceki

1st Apr 2008, 19:16

@airfoilmod
My quote was attributed to Rightbase Post 719.
Since it was only a couple of posts earlier I thought the connection was self evident. Obviously not.
Stuart

Pinkman

1st Apr 2008, 19:22

I'm with Sunfish.

However, as a chemist & oil co. fuel guy, its obvious it was the fuel..

Sunfish

1st Apr 2008, 19:40

Kolossi, you make the point that everyone should contribute from their field, within their realm of competence, and I agree with you. However many of the posts on this thread completely discount the design expertise of the aircraft manufacturer and the skill and experience of the pilots who operate the aircraft and the engineers that maintain them. That to me is unwise.

By way of example, while I now have to admit that Occams razor suggests that fuel system design or the fuel itself is the culprit, the "water in the fuel" crowd do not appear to know that fuel quality is monitored daily and that the petroleum industry has been paranoid about jet fuel quality, especially water contamination, since at least 1980. Before the fuel reached the aircraft, it will have passed at a minimum three filter/coalescer units designed to remove any water or particulate contamination.

Similarly Boeing would be well aware of the risk and catered for it in its design, so how someone can blandly say on page one or page two of a thread "it's water contamination" is beyond me.

The same goes for RF interference. Do the proponents of this theory think Boeing has never heard of it, nor allowed for it's in it's detailed designs? Same for pump cavitation theorists, do you think the engine designers are idiots?

That's why people get irritated by some posters - because their helpful suggestions imply that the designers, pilots or maintainers of the aircraft were idiots.

airfoilmod

1st Apr 2008, 20:12

My experience in discussion is that people who are "irritated" by information may have an agenda regarding the outcome or the "reputation" of those who may own a piece of the potential fault. I will admit that some content here does tip a certain naivete regarding Aerospace and Aviation in general, but isn't that to be expected in a Forum such as this? I strive to look for the useful in things I encounter, and encourage people in general to have a good time; this isn't rocket science, and I have found a sub-group of true professionals here. This is a fascinating investigation, no one was killed, and as has been pointed out, the Hull is intact and may end up at Embry-Riddle some day. Cheers and happy typing. Bill

snanceki

1st Apr 2008, 20:27

@ Sunfish Post 739
Similarly Boeing would be well aware of the risk and catered for it in its design

But I have to add that sometimes even FMECA Failure Mode and Effects Analysis (of which I am sure you are aware) and best design practice can miss certain possibilities.

Just as an example take the PH co-cylindical tailfin actuator - unpredicted behaviour due to dirt ingress on 737. Enough said.

Maybe a SINGLE temp thermistor in just one tank is not appropriate.
Maybe current min. fuel temp in combination with the way it gets measured, is not appropriate.

Although Occam indicates the likelyhood of the simplest potential cause it does not at this stage exclude the other "way out" possibilities such as EMI. Be tolerant. You can always claim you were right when others are finally proved wrong.

@ Airfoilmod

Cranfield might be more appropriate! It's certainly nearer.

airfoilmod

1st Apr 2008, 20:34

Or the unlubricated and uninspected stripped Jack screw in the Alaskan MD80 elevator. Like the man said, Human Endeavor, weak link, etc. As an engineer, you focus on How? The Public wants Why? Why is about fate and gross mistakes and should be elsewhere. Where is Cranfield? And as has been pointed out, the Fuel Temp. sender is in the Port (warm) wing.

Green-dot

1st Apr 2008, 20:36

Quoting najavens:

I have been informed by a collegue in Flight Safety, who has contacts with the AAIB, that they are currently running with the idea that there was fuel starvation to both engines. It seems that only 10% of the fuel required was making it to the engines causing them to wind down. The problem however is identiying the part of the fuel system at fault.

I believe that it is rare for a fuel system to be compromised by RF as there are mechanical systems as back-up, and I believe there are 2 FCU’s on the aircraft, one for each engine. For both to be affected by RF would be unusual.

1. Identifying the parts:
The parts most suspect IMHO, are of electro-mechanical composition and the mechanical part(s) of this hybrid design are the first components one finds in the engine fuel feed system, upstream from the engines toward the fuel tanks, the spar valves. They only have 2 positions: open or closed. These mechanical parts in the engine fuel feed lines are nothing but slaves to what the electrical parts, the control relays, of this "symbiosis" instruct them to do.

The electrical parts of this hybrid design are buried deep inside the aircraft's hull, nowhere near their mechanical "slaves." If for whatever reason these electrical parts were to be disrupted in their normal functioning and moved their mechanically slaved "other half" from open to closed position, starvation of fuel to both engines would be guaranteed.

Just a theory until proven by facts of course but if they were affected, i won't dare speculate as to what the source of the disruption could have been. Maintenance issues? Items carried on board, alien to the aircraft's systems? Were such items located in the passenger cabin or perhaps in a cargo hold? The list of possibilities could be long . . . . . and the possible causal source a needle in a hay-stack.

2. Mechanical back-up:
With reference to mechanical systems as a back-up in the fuel system, i would not be so sure about that. See my first comment above, "identifying the parts."

3. Both engines:
Too much emphasis is put on RF affecting both engines, this has already been discounted by the AAIB report which confirmed FCUs and EECs functioned as advertised. What ever the source of the disruption, if it could have potentially affected parts of the left and right engine fuel feed systems, especially when in close proximity to eachother in a confined space, the electrical parts mentioned in my first comment above would be very likely candidates in the chain of events resulting in the generally perceived theory of a common source failure (they meet close proximity and confined space criteria, making them perhaps vulnerable to a single, yet to be defined, causal source).

Green-dot

snowfalcon2

1st Apr 2008, 20:52

Good posts. Aviation safety development is based on openness and frankness. While adhering to that leads to extraneous "naive worthless contributions" and other extra "noise", my feeling is that that is still preferable to suppression of possibly valuable contributions, for whatever reason. In other words, moderation could preferably be limited to obvious repetitions and totally clueless contributions. But contributions by people from a different field should IMHO be treated with some respect. There just might be nuggets in them. :ok:

Just my $0.02.

Rightbase

1st Apr 2008, 22:25

Sunfish - have you seen the post by UNCTUOUS (P35 post681) where in response to several reports from users, Boeing theorise that water might be accumulating in the centre tanks of -200LR and -300ER aircraft?

Whilst the system for dealing with water is different in the -200ER, that doesn't mean that water can't get into the -200ER tank.

Leodis737

1st Apr 2008, 22:56

And the timing fits. Scavenge from centre tank to wing tanks starts when 29,000 pounds of fuel (total) remaining - i.e. roughly within half an hour of landing on the grass at LHR. (see post 572).

Sunfish

1st Apr 2008, 23:27

Wait for the report.

Milt

2nd Apr 2008, 04:10

Cargo/Container Restraint.

Does anyone know the design horizontal g restraint on baggage and freight containers for the B-777?

The peak decelleration may have come close to max. Did any break loose?

bsieker

2nd Apr 2008, 08:41

And the timing fits. Scavenge from centre tank to wing tanks starts when 29,000 pounds of fuel (total) remaining - i.e. roughly within half an hour of landing on the grass at LHR. (see post 572).

No.

With the main tank pumps ON, a scavenge system operates automatically to transfer any remaining center tank fuel to the main tanks. Fuel transfer begins when either main tank quantity is less than 29,000 pounds. (my emphasis.)

So not 29,000 total, but 29,000 in either wing tank. Wing tank capacity is 64,100 pounds, each.

Bernd

Shaka Zulu

2nd Apr 2008, 08:48

Post incident I know religiously check the Maint Page Fuel QTY pages for evidence of water in the tanks.
About a month ago it showed water centre tank on a GE 200ER, called up Maintrol and they had to come out to check it (ai drip stick the centre tank).
No water apparent in the sample though.

tanimbar

2nd Apr 2008, 10:18

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

I've been wondering what would the AAIB do if they could not demonstrate to a high probability the cause of the accident.

If, for example, the AAIB were convinced, but could not prove, that the fuel quality had been lowered by cold-soaking for an extended period of time and that this had lead, in some unknown way, to the accident, what would they do?

Presumably the AAIB would operate on the precautionary principle.

Might they:
1) Recommend/demand/regulate the avoidance of extended flight-time in very cold air masses;
2) Demand that if aircraft had experienced extended flight-time in very cold air masses then they must loiter for some time in warmer air before descending for landing?

If the AAIB was thinking of 1 or 2 then, I assume, they would have to convince, and then act in unison with, equivalent international agencies - that would take a lot of time and effort. And just imagine how all the other interested parties would react.

Of course, in this scenario the AAIB have until the coming northern hemisphere winter to sort this out.

Has such a situation (i.e. no proof but suspicion of likely cause) arisen before and how did the industry react?

Regards, Tanimbar

wilyflier

2nd Apr 2008, 12:23

It is suggested that Ba038 was given an unusually long soak at -75degrees.(Around ISA -20 ....more unusual in that it is also suggested that most aircraft on this route took precautions and reduced altitude)
...How many hours would this have been? and did it continue to top of descent? Would all engine internal temperatures be running as much as 20 degrees lower than at the same altitude with a normal OAT of ISA.?
Did it at any time in a long Flight-Idle descent pass through an area of high humidity and low temp (cloud)?
.....We do know that a long taxi at Idle in fog can produce "core-icing", in spite of having engine deicing switched on, which has to be countered by a clearing procedure Thus allowing continued operation without damage occurring.
....Does mild core icing ever occur in flight?preventing acceleration?
....Could an amount of core ice accumulate ,and not be readily shed, on such a descent commencing with a core temp already 20 degrees lower than usual? with normal engine deicing operating at less than 100%?

NOT very likely.If it had happened thus, there would probably be data records,or signs of internal damage. Or the engines would not have run down again after they had succeded in initial acceleration.
...I still think its fuel,plus a bit of wind shear.

lomapaseo

2nd Apr 2008, 18:32

Has such a situation (i.e. no proof but suspicion of likely cause) arisen before and how did the industry react?

Regards, Tanimbar

Think TWA800...you should know the rest.

and the AAIB/NTSB can't demand or regulate, their job is to investigate and recommend. An no they won't wait for the winter months. As soon as they learn anything that can/should be fixed, even if not in the causal chain they will issue a recommendation for the industry to act on.

Leodis737

2nd Apr 2008, 19:07

jake brake

2nd Apr 2008, 20:26

Green-dot.
Solenoids are sometimes activated by stray electromagnetic fields
as you suggest. A Shorts 3-30 fell on its "knees" when an errant field
pulled the gear pins, while pax were unloading. Perhaps it happened more
often.

bsieker

2nd Apr 2008, 21:24

bsieker, thanks for your correction on when fuel scavenge starts. With the correction: 13,500kg+13,500kg+900kg of fuel (total) remaining when scavenge started; 10,500kg remaining on approach to LHR (AAIB); difference 17,400kg; fuel being used at a rate of maybe 5,700kg/hr; so fuel scavenge would have started about 3 hours before it hit the grass, i.e. in cruise. Would scavenge have finished, though?

Yes.

The figures look reasonable; at the minimum fuel scavenge rate of 400kg/h, it would take 2 hours, 15 minutes to scavenge all of the 900kg from the center tank. It would have finished some 45 minutes before the end of the flight.

Likely before TOD, but I cannot know for certain.

Bernd

Leodis737

2nd Apr 2008, 21:57

alright then - switching horses, are the light patches on the underside of the wing behind the engines in this photo unusual or not?

http://forum.skyscraperpage.com/showpost.php?p=3300323&postcount=47

it's a long time since I stood at the end of a runway in winter...

(sorry if this point has already been settled)

Rightbase

2nd Apr 2008, 22:15

bsieker:
Likely before TOD, but I cannot know for certain.

Assuming the scavenge system was working properly, and not iced up.

infrequentflyer789

2nd Apr 2008, 23:08

Assuming the scavenge system was working properly, and not iced up.

If scavenge failed then the fuel sensors did too, as the AAIB state that the fuel was (indicated to be) in the wing tanks on approach.

Green-dot

2nd Apr 2008, 23:47

To jake brake:

Solenoids are sometimes activated by stray electromagnetic fields
as you suggest. A Shorts 3-30 fell on its "knees" when an errant field
pulled the gear pins, while pax were unloading. Perhaps it happened more
often.

Inadvertent in-flight liftdumper deployment on a Fokker 100 comes to mind from several years ago. The cause was determined to be the combination of EMI from a faulty Flight Control Computer on a wheel speed signal, a skid control box power-up spike on landing gear DOWN selection, and liftdumper arming before landing gear extension.

Ref. sources below:

http://www.federalregistersearch.com/1998/5/18/98-13062-filed.asp

http://www.federalregistersearch.com/1999/4/16/99-9512-filed.asp

Green-dot

Rightbase

2nd Apr 2008, 23:58

If scavenge failed then the fuel sensors did tooThe subject of the observation was the timing rather than the failure of fuel scavenge. Ice could possibly delay the onset of scavenge until some time during the descent, then melt away and be scavenged as water.

and:
Quote from Boeing relating to 777-300ER (not exactly the same scavenge arrangement, but might or might not have similar lack of instrumentation - any lurking 777 pilots able to comment or confirm?)

There is no indication to the flight crew that the scavenge system has failed nor that the fuel (remaining in the CWT) is unusable.(My bold formatting, and my italic clarification)

infrequentflyer789

3rd Apr 2008, 01:30

the spar valves. They only have 2 positions: open or closed. These mechanical parts in the engine fuel feed lines are nothing but slaves to what the electrical parts, the control relays, of this "symbiosis" instruct them to do.

The electrical parts of this hybrid design are buried deep inside the aircraft's hull, nowhere near their mechanical "slaves." If for whatever reason these electrical parts were to be disrupted in their normal functioning and moved their mechanically slaved "other half" from open to closed position, starvation of fuel to both engines would be guaranteed.

Interesting theory...

Problem: I would expect spar valve position would be recorded (although I don't know details on 777 FDR or QAR), and that a difference between recorded and commanded position would raise a warning (which would also be recorded). AAIB reported no recorded anomalies.

Counter: I think you've previously theorised that spar valve could be detected / recorded as only open/shut and therefore "partly shut" could go unrecorded ?

Problem: Spar valves were found to be open.

Counter: [Again, I think you've theorised this] could be a transient effect that reset/cleared before impact.

Problem: The issue didn't clear before impact, because we know the engines stayed at below commanded thrust until impact (AAIB).

Thoughts: impact cleared the problem - bounced the valves open again, or the electrical disruption ceased at impact. Hmm.

Or, there is a return flow of fuel around the HP pump, and that feedback makes me wonder if this system could possibly get kicked into an abnormal flow state which then self-sustains. ie. your transient spar valve incident causes reduced flow and cavitation at the HP pumps, affects the return flow, which then continues to cause reduced flow even after the spar valve is open again.

My gut feeling is it can't happen like that, on the other hand, it's a long time since I did any fluid dynamics, so maybe...

infrequentflyer789

3rd Apr 2008, 01:52

Ice could possibly delay the onset of scavenge until some time during the descent, then melt away and be scavenged as water.

But if scavenge was still in progress (and maybe sending a big slug of water through to the wings) then CT wouldn't be reading empty.

Quote from Boeing relating to 777-300ER (not exactly the same scavenge arrangement, but might or might not have similar lack of instrumentation - any lurking 777 pilots able to comment or confirm?)

I've read that before also, and I understand it as no explicit indication / warning of scavenge system failure, not that you couldn't infer it from CT fuel level readings, and I am pretty sure that all 777s have fuel level sensors in every tank. Happy for any 777 pilot to tell us we're (maybe both) wrong though.

Checkers

3rd Apr 2008, 02:53

Ice could possibly delay the onset of scavenge until some time during the descent, then melt away and be scavenged as water.

I like that line of thinking, but fuel temperatures often stay below freezing until well after landing. It seems like there would be, at most, a "snowstorm" of small ice crystals in the liquid fuel.

Sunfish

3rd Apr 2008, 05:56

We have been around all these theories in the previous 30 something pages. If,for example, there was a "snow storm of ice crystals" then why wasn't there a "fuel filter" EICAS warning (or whatever the correct terminology is)?

Let's wait for the report.

Volume

3rd Apr 2008, 07:22

are the light patches on the underside of the wing behind the engines in this photo unusual or not?

This is perfectly normal icing of the wing in the area where the cold fuel is stored. Fuel temperature during landing is very often well below 0°C, and most probably was during this flight. You can see this icing even on the upper surface of the wing, if there is still a larger amount of fuel present. I have several such photographs taken from the passenger windows.

This is why I seriously doubt that melting ice was somehow involved in the accident. All water in the tank was still in the solid state during impact, which of course could have been at any size of particle from fine "ice dust" to solid chunks.

Green-dot

3rd Apr 2008, 23:21

Quoting infrequentflyer789:

Interesting theory...

Problem: I would expect spar valve position would be recorded (although I don't know details on 777 FDR or QAR), and that a difference between recorded and commanded position would raise a warning (which would also be recorded). AAIB reported no recorded anomalies.

Counter: I think you've previously theorised that spar valve could be detected / recorded as only open/shut and therefore "partly shut" could go unrecorded ?

That is indeed the question, spar valves in transit from open- but not reaching the closed position may perhaps not be recorded. Warning to the crew in case of unscheduled closing of the spar valves (with the fuel switches still in "Run" position) is unlikely. I refer to the AMM and/or WDM for further information as to why this may be the case. Also, if the fuel synoptics page was not displayed (or not observed) at the time, the crew would have no visual indication of actual spar valve position.

Problem: Spar valves were found to be open.

Counter: [Again, I think you've theorised this] could be a transient effect that reset/cleared before impact.

Correct, also see my comments below.

Problem: The issue didn't clear before impact, because we know the engines stayed at below commanded thrust until impact (AAIB).

Thoughts: impact cleared the problem - bounced the valves open again, or the electrical disruption ceased at impact. Hmm.

- I would not know if the disruption (if present) did or did not clear before impact. If, as i have theorized, spar valve control was disturbed by reverse logic: the valves take 15 seconds to fully close and another 15 seconds to fully open again. Theoretically the valves could have been in transit for over 25 seconds (from open - to not fully closed - and back to open) limiting fuel flow to the engines for this period of time and perhaps causing cavitation to the pumps when the engines demanded more fuel but did not receive it.

- The AAIB Special Bulletin 1/2008 from page 5 onward explains why the spar valves were found open. Aircraft wiring was in pre SB 777-28-0025 configuration. The SB was introduced to ensure the spar valve closes when the fire handle is pulled regardless of sequencing. I do, however, still have a question regarding the AAIB findings and subsequent explanation related to the spar valve control relays found in open position. Were they found in the open position because of incorrect sequencing as reported or perhaps because of possible reverse logic still in effect at the time of- or just before impact? With reverse logic this would imply:
- Fuel switch in "Run": control relay in Cutoff position;
- Fuel switch to "Cutoff": control relay in Run position.

In other words, could selecting fuel switches to "Cutoff" at the time of the fire drill have inadvertently re-opened the spar valves (already closing or closed) due to a reverse logic condition still in effect before the fire switches were activated, thus assuming in this case that the sequence was correctly performed, contrary to the report? In the pre SB configuration the second wire path goes directly to the spar valve actuator and de-energizes the control relay circuit, latching the relays in last selected position if i am correct, i could be wrong. Depending on when exactly the fire switches were activated, the impact and damage to the landing gear may have severed the wiring to the valve actuators, preventing the close signals from reaching the valves?

Or, there is a return flow of fuel around the HP pump, and that feedback makes me wonder if this system could possibly get kicked into an abnormal flow state which then self-sustains. ie. your transient spar valve incident causes reduced flow and cavitation at the HP pumps, affects the return flow, which then continues to cause reduced flow even after the spar valve is open again.

My gut feeling is it can't happen like that, on the other hand, it's a long time since I did any fluid dynamics, so maybe...

To my knowledge there is a return flow path of excessive fuel returning from the fuel control unit to the HP pump. As fuel flow reduces due to the suspected flow restriction, return flow to the HP pump reduces as more fuel is demanded by the now "fuel hungry" FCU.

See also my post #590 on page 30 of this thread which describes engine behaviour when fuel flow to the engine is deliberately restricted. The test with a 95 percent closed fuel shutoff valve was done because the aircraft type involved had encountered unscheduled closing of the valve due to reverse logic on several occasions, resulting in incidents and accidents.

Green-dot