ajaaron
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Question On The Accident Of BA B777, G-YMMM

Sun Nov 02, 2008 12:01 am

I have read the interim report regarding the incident on Boeing 777-236ER, G-YMMM in Jan 2008, http://www.aaib.gov.uk/publications/...ports/boeing_777_236er__g_ymmm.cfm

and I know it sounds obvious, so I VERY humbly offer this question:

If ice accretion caused restricted fuel flows to the engines, how could it be that both engines suffered from restricted fuel flows at exactly the same time, i.e. just 7 seconds apart, when each engine was being supplied independently from the other by its own main tank in its respective wing?

Statistically is it possible that that ice accretion would have occurred (and sufficiently so), and precisely at the same time on both independent fuel supplies to the engines?

Does it sound a likely occurence?
 
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Starlionblue
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 02, 2008 1:26 am



Quoting Ajaaron (Thread starter):
Statistically is it possible that that ice accretion would have occurred (and sufficiently so), and precisely at the same time on both independent fuel supplies to the engines?

I'm going to nitpick a bit here. You can't really say "statistically possible". If there is any possibility, it can happen, however unlikely. Statistics and probability describe possibilities and results. Statistics and probabilities do not rule out improbable events. They just describe how unlikely such events are.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
Starglider
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 02, 2008 3:12 am



Quoting Ajaaron (Thread starter):
Does it sound a likely occurence?

Not really. After reading all the reports issued by the AAIB regarding this accident, chances of ice accretion being the cause are very remote but not impossible.

However, 5 litres of water spread out over 3 tanks in 100.000 litres (79.000 kg) of fuel just does not add up as a likely cause. The water content in the fuel was below average for a B777. Furthermore, business jets with much less wing volume fly at 40.000 ft in the winter and never encounter such a problem. Cold soaking the fuel on a business jet is reached much sooner than on a jet the size of a B777. In this particular case of BA038, the fuel temperature never went below -34 C, and stayed well above fuel waxing or freezing temperatures.

Any accumulated ice in the center tank (no more than 1.6 litres according to the AAIB) when departing from Beijing, most likely would have melted and have been consumed during the climb. Even if the ambient temperature was -6 C on the ground and fuel temperature in the left main tank was -2 C after refuelling, the fuel in the center tank must have been warmer (was +5 C when refuelling) during taxiing and climbing to initial cruising altitude with most of the center tank enclosed between the airconditioning system below it, the cargo compartments in front and behind it and the cabin above it. The report states that the left main tank fuel temperature remained at -2 C until cruising altitude, center tank fuel most likely was much warmer. Firstly because the fuel entered an empty center tank during refuelling (contrary to the fuel in the wing tanks, where the added, + 5 C fuel mixed with the cold soaked fuel remaining from the previous flight). Secondly because the fuel in the center tank remained relatively warm for a longer period of time until well into the cruise because it is enclosed as explained above.

The engines are fed from the center tank until it is almost empty (800 kg) before switching to the main (wing) tanks which contained no more than 3.4 litres of water in the fuel spread evenly over the two main tanks according to the AAIB.

Ice being the cause is a hypothesis, nothing more.

Personally I believe there are more factors in play here (not ice) which acted simultaneously at a very inconvenient moment.


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tdscanuck
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 02, 2008 5:58 am



Quoting Ajaaron (Thread starter):
If ice accretion caused restricted fuel flows to the engines, how could it be that both engines suffered from restricted fuel flows at exactly the same time, i.e. just 7 seconds apart, when each engine was being supplied independently from the other by its own main tank in its respective wing?

Because both engines throttled (or tried to) at the same time.

Based on the AAIB's initial report, ice was slowly building inside both systems while on descent. So, although the accumulation rate would probably vary subtly from side to side, the exchangers were both becoming restricted by ice formation. Since the fuel on both sides was essentially the same, the amount of ice formation should be roughly the same. The restriction wasn't enough to cause a problem at flight idle, but when they commanded a thrust increase on both sides it was enough to starve the engines.

Quoting Starglider (Reply 2):
After reading all the reports issued by the AAIB regarding this accident, chances of ice accretion being the cause are very remote but not impossible.

Of course it's remote...otherwise it would have happened before. The ice formation isn't time critical...the triggering event was throttling up, which happened simultaneously on both sides.

Tom.
 
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 02, 2008 1:49 pm

Quoting Tdscanuck (Reply 3):
Of course it's remote...otherwise it would have happened before. The ice formation isn't time critical...the triggering event was throttling up, which happened simultaneously on both sides.

That remains a hypothesis that has yet to be proven. Many tests described in the AAIB report end with "further tests are required." Most of the water in the main tanks (max. 1.7 litres per tank) was entrained in the fuel so soon after refuelling. When frozen as entrained ice crystals a large portion must have been consumed before it could be a factor in ice accretion. This means that a considerable less amount of ice could have accreted. The report for instance only found 0.25 ltr and 0.1 ltr of free water in the left and right main tank respectively (probably checked hours if not days after the accident). This quantity was considered to be relatively low for a B777. If the FOHEs were clogged by ice I would assume the water contamination in the FOHEs or downstream fuel lines would have been significantly higher. No mention of it in the AAIB report other than a few tiny water droplets in the fuel filters and housings which might have been free water which naturally settles in these areas.

All BA B777s were taking part in a sumping program which revealed that none of the 43 aircraft showed evidence of significant quantities of free water. G-YMMM was actually sumped under ideal conditions during the last 2 stop-overs at Heathrow only days apart.

The report refers to recorded data not revealing any anomalies in the aircraft systems. Question remains what is and what is not recorded and if data is recorded, could a parameter have been been corrupted by some "byzantine fault" scenario? Furthermore, I have trouble with the AAIB statement regarding the spar valves. Especially that if a spar valve were to fail, an enunciated warning would have been presented on the flight deck. When reading the maintenance documentation on this subsystem I come to the conclusion that there could be a scenario where this may not be the case. But for that to happen several holes in the Swiss cheese would have had to line up. From what I have read in the report the spar valve control system has been thoroughly examined and tested. Question remains, were these bench tests or tests of the actual spar valve system in the accident aircraft?

The report mentions and i quote: "Extensive testing [of the spar valve control system] to induce an uncommanded movement, that remained unrecorded, could not identify any such failure modes." My perception after reading this is that there are failure modes which can close the spar valves by uncommanded movement. But this would have been recorded on the FDR, provided the recorded parameters were not corrupted.

If due to some electrical or electronic spike the spar valve control system and FDR recordings were corrupted, that would open up other scenarios for the cause to this accident.

I agree, such a scenario is remote but no more remote than two donks rolling back due to ice with the minute quantities of ice found in the systems of this particular aircraft.



Just my   
Starglider

[Edited 2008-11-02 05:58:29]
 
OldAeroGuy
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 02, 2008 4:09 pm



Quoting Starglider (Reply 4):
Furthermore, I have trouble with the AAIB statement regarding the spar valves. Especially that if a spar valve were to fail, an enunciated warning would have been presented on the flight deck. When reading the maintenance documentation on this subsystem I come to the conclusion that there could be a scenario where this may not be the case.

I think that the simultaneous build up of ice crystals in both FOHE's is more probable than a simultaneous failure (ie closure) of both spar valves. This is especially true since it has not been possible to detect or reproduce the postulated spar valve closures.

After all, the fuel for both engines was an identical mixture. Water entrained in the wing fuel would have been crystalized for nearly the entire flight as wing tank temperatures were less than 0 deg C. Since fuel flows for both engines were the same through out the flight, it's quite probable that a fuel flow restriction due to ice on one engine would be mirrored by an identical restriction on the other engine.

The application of Occam's razor in this situation would seem to be relevant.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
Starglider
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 02, 2008 9:13 pm



Quoting OldAeroGuy (Reply 5):
The application of Occam's razor in this situation would seem to be relevant.

True.

Bust just to put everything in perspective, both spar valve control relais are located adjacent to one another on the same panel, millimeters apart.


Starglider.
 
OldAeroGuy
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 02, 2008 11:29 pm



Quoting Starglider (Reply 6):
both spar valve control relais are located adjacent to one another on the same panel, millimeters apart.

And the failure mode was?

I doubt that the AAIB has jumped to an unsupportable conclusion.
Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
 
tdscanuck
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 02, 2008 11:55 pm



Quoting Starglider (Reply 6):

Bust just to put everything in perspective, both spar valve control relais are located adjacent to one another on the same panel, millimeters apart.

Yes, but the power and sense lines are on totally different sides of the aircraft and are on separate circuits. In order for this to be a cause, our unidentified fault would have had to effect both relays in the same way (i.e. hit both circuits in the same way), and both position sensors would have had to fault the same way (they're on opposite sides of the wing), and both data channels to the FDR (which is a digital system independant from the actuation system) would have had to be corrupted in the same way, and the EICAS would have had to miss both actuators doing it.

Tom.
 
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RE: Question On The Accident Of BA B777, G-YMMM

Mon Nov 03, 2008 6:53 am



Quoting Tdscanuck (Reply 8):
our unidentified fault would have had to effect both relays in the same way (i.e. hit both circuits in the same way),

But (hypothetically) if an unidentified fault were to influence the polarity of both relais (open to close), the spar valve actuators would follow this command and there would be no warning on the flight deck because the logic is such that an alert would only be generated if the spar valve actuator would not follow commanded position, e.g. in case of a jammed actuator.

Regarding fault message on EICAS (or according to above scenario the absence of one) see AMM/SDS 28-22-00 "Fault Indications"


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tdscanuck
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RE: Question On The Accident Of BA B777, G-YMMM

Tue Nov 04, 2008 3:03 am



Quoting Starglider (Reply 9):
Regarding fault message on EICAS (or according to above scenario the absence of one) see AMM/SDS 28-22-00 "Fault Indications"

Good point...seems like an odd design to me. How would you know if your relay had frozen?

Tom.
 
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RE: Question On The Accident Of BA B777, G-YMMM

Tue Nov 04, 2008 7:25 pm



Quoting Tdscanuck (Reply 10):
How would you know if your relay had frozen?

On the B777 in this, and I stress very remote condition, you won't know until you are confronted with the consequences. Only if the crew would have pre-selected the fuel synoptic or the fuel management maintenance page the spar valve position is visible to them (provided their attention is actually drawn to their position).

There are no spar valve in-transit lights.

Although the fuel synoptic is occasionally selected during a flight, chances are that it is not for most of the flight phases and usually not during approach unless there is a specific reason to do so. The maintenance pages are usually the domain of the maintenance department, again only selected by the flight crew if there is a reason to do so.


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planewasted
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RE: Question On The Accident Of BA B777, G-YMMM

Thu Nov 06, 2008 12:47 am

Why use relays in such a modern aircraft as the 777? A semiconductor based solution is cheaper, lighter, smaller, more reliable and generates less heat.
 
tdscanuck
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RE: Question On The Accident Of BA B777, G-YMMM

Thu Nov 06, 2008 2:06 am



Quoting PlaneWasted (Reply 12):
Why use relays in such a modern aircraft as the 777? A semiconductor based solution is cheaper, lighter, smaller, more reliable and generates less heat.

The 777 went into service in the mid-90's, which means it's technology got defined in the late 80's/early 90's. Solid-state was around then, but much less established than it is now. I also have no idea of the certification status of solid-state relays at that time.

Tom.
 
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RE: Question On The Accident Of BA B777, G-YMMM

Thu Nov 06, 2008 10:27 pm



Quoting OldAeroGuy (Reply 7):
And the failure mode was?



Quoting Tdscanuck (Reply 8):
position sensors would have had to fault the same way (they're on opposite sides of the wing), and both data channels to the FDR (which is a digital system independant from the actuation system) would have had to be corrupted in the same way

In theory:
It crossed my mind that on the B777 spar valve control circuitry, when selecting the fuel control switches from cutoff to run, each fuel control switch positions several multi-functional relays in the respective (left / right) power management panel (PMP) from cutoff to run. One of these relays in each PMP provides an AIMS position signal (run or cutoff) by which the aircraft information management system (AIMS) is provided with system status and likely provides the FDR with parameter signals regarding fuel control switch (thus indirectly spar valve) position.

From one of the relays in each of the PMPs a run signal goes to the respective left / right spar valve control relays which are not located in the PMPs. The PMPs are part of the electrical load management system (ELMS). The PMPs are located on the left and right side of the aircraft. Spar valve control relays are placed at a separate (central) location. With the spar valve control relays in run (open) position, the signal commands the spar valve actuators to go from closed to open position. When the actuators reach commanded position, limit switches cut power to the actuators.

My perception is that with the fuel control switches on the flight deck and the relays in the PMPs in the run position, if (hypothetically) the spar valve control relays further downstream in the circuit were to change polarity due to some unidentified fault/influence, this could perhaps not affect the relays upstream in the PMPs. If that would be the case, the position signal to AIMS would still be a run signal with the spar valve control relays in cutoff (closed) position.

Suppose our unidentified fault was of temporary nature. The spar valves take approximately 15 seconds to transit from open to closed position and another 15 seconds back to open position. Hence, if the fault lasted anything short of 30 seconds, the spar valves would have only partially closed but would have restricted fuel flow considerably before opening again as the spar valve control relays returned to normal (run/open) condition . . . . . .

Just a theory of course.


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Jetlagged
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RE: Question On The Accident Of BA B777, G-YMMM

Fri Nov 07, 2008 3:47 pm



Quoting Starglider (Reply 14):
Suppose our unidentified fault was of temporary nature. The spar valves take approximately 15 seconds to transit from open to closed position and another 15 seconds back to open position. Hence, if the fault lasted anything short of 30 seconds, the spar valves would have only partially closed but would have restricted fuel flow considerably before opening again as the spar valve control relays returned to normal (run/open) condition . . . . . .

You are assuming that as soon as the spar valve moves towards closed a significant restriction is created. If not, the window for the fault to have such an effect on fuel flow is much shorter than 30 seconds. The failure scenario requires a significant and fairly constant restriction over a period of time. A varying restriction would effectively throttle the fuel flow and so vary engine power. Power didn't vary, it simply didn't respond to acceleration demand. The power levers remained at maximum throughout.

This speculation also ignores what phenomenon would move the relays to CLOSE then return them to OPEN with just the right timing. I suspect the AAIB have been through all the possible valve failure scenarios.

The AAIB ice accretion scenario is the most likely that has been identified, but as yet has not been reproduced in testing AFAIK.

Quoting PlaneWasted (Reply 12):
Why use relays in such a modern aircraft as the 777? A semiconductor based solution is cheaper, lighter, smaller, more reliable and generates less heat.

The reason for using relays is often to control a high powered motor from low voltage logic. Can solid state relays switch 115V AC?
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
tdscanuck
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RE: Question On The Accident Of BA B777, G-YMMM

Fri Nov 07, 2008 9:28 pm



Quoting Jetlagged (Reply 15):
A varying restriction would effectively throttle the fuel flow and so vary engine power.

Not necessarily...the spar valves feed low-pressure fuel from the boost pumps to the engine pump. You only get a throttling effect at the engine if the spar valve restricts the flow below what the fuel metering valve on the engine wants to deliver. Most of the time, the spar valve isn't the major restriction. The spar valve is sized for considerably more flow than it ever sees in cruise/descent.

Quoting Jetlagged (Reply 15):

The reason for using relays is often to control a high powered motor from low voltage logic. Can solid state relays switch 115V AC?

You can get them to do 440VAC now, but I'm not sure if the weight is adequate for aircraft use.

Tom.
 
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RE: Question On The Accident Of BA B777, G-YMMM

Fri Nov 07, 2008 10:21 pm



Quoting Jetlagged (Reply 15):

The reason for using relays is often to control a high powered motor from low voltage logic. Can solid state relays switch 115V AC?

115VAC is no problem ... I have several relays at home (for an interface project between a pinball machine and a traffic light) that switch 115VAC at 15A apiece. The huge extra capacity means no need for cooling the units when switching the relatively small wattage bulb in the traffic light. They switch on anything from 3 to 20 vdc input.

They are rated, actually, to 230VAC, but at half the current from 115VAC. This makes them compatible in Europe too.

Considering these are off-the-shelf items, I'm sure in the aerospace world if they needed a 400+ volt solid state relay, it would be easily acquirable as a custom made item, if not already available.

The only considering you have to take into effect is a normal relay doesn't create much heat; you just energize the coil, and the contacts snap!, and the current flows.

Solid state relays on the other hand generate heat proportionate to the voltage and current you intend to pass through them. If you don't dissipate the heat, the unit will melt down and fail.

- litz
 
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Jetlagged
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RE: Question On The Accident Of BA B777, G-YMMM

Sat Nov 08, 2008 5:54 am



Quoting Tdscanuck (Reply 16):
Not necessarily...the spar valves feed low-pressure fuel from the boost pumps to the engine pump. You only get a throttling effect at the engine if the spar valve restricts the flow below what the fuel metering valve on the engine wants to deliver. Most of the time, the spar valve isn't the major restriction. The spar valve is sized for considerably more flow than it ever sees in cruise/descent.

I realise that, but if the spar valves were closed enough to prevent acceleration then already they are throttling the fuel flow. If they continue to close further before re-opening then the restriction will increase, further reducing fuel flow and so RPM. But that didn't happen in this case as far as is known.

Even with these slow moving valves, the restriction will only be effective when the valves are nearly closed, therefore for a very much shorter time than the 30 second valve close/open cycle.
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Starglider
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RE: Question On The Accident Of BA B777, G-YMMM

Sat Nov 08, 2008 10:09 am



Quoting Jetlagged (Reply 15):
The failure scenario requires a significant and fairly constant restriction over a period of time. A varying restriction would effectively throttle the fuel flow and so vary engine power. Power didn't vary, it simply didn't respond to acceleration demand. The power levers remained at maximum throughout.

This speculation also ignores what phenomenon would move the relays to CLOSE then return them to OPEN with just the right timing.

I emphesize it is a theory, in this scenario it is indeed a question of timing but if the valves were to close while the engines increased their demand for fuel, engine behaviour could very well appear to be not responding to acceleration demand. I put it in the same unlikely category as two engines rolling back within seven seconds due to the current hypothesis of ice accretion.

Furthermore, what happens to the fluid dynamics once the fuel has passed a more than 75 to 80% closed spar valve with an engine accelerating but then deprived of the required fuel flow? The fuel flow (what is left of it) most likely would become turbulent in the near empty fuel lines which might have upset the balance, making the fuel flow unstable in the engine feed system. The mass of an accelerating engine, suddenly deprived of demanded fuel, would not decelerate immediately, resulting in cavitation.

Once in this scenario, the instability reaches the FMVs and burners it might have taken too much time for the engines to recover and accelerate. Before they could, they hit the ground.

Another issue is FADEC. As long as a FADEC equipped engine gets fuel, it will tenaciously cling to life. This article in the link below might be interesting regarding that aspect. It deals with another type of turbofan engine but the principle is similar:

http://www.codeonemagazine.com/archi...rticles/jan_90/flameout/index.html

With regards to the phenomenon, without a trace of evidence and being of a temporary nature, that would be like trying to find a needle in a haystack. It could have been aircraft system related such as poor bonding or moisture in the electrical circuits. An electronic spike eminating from one of the other aircraft systems during the approach phase. Or it could have been a source alien to the aircraft and don't even rule out a combination of several distubances at the same time, lining up the holes in the Swiss cheese where, if only one of the factors did not contribute, nothing would have happened to cause this accident and there would have been nothing to report in the news

Quoting Jetlagged (Reply 15):
The reason for using relays is often to control a high powered motor from low voltage logic. Can solid state relays switch 115V AC?

The spar valve control system a 28V DC system.


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tdscanuck
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RE: Question On The Accident Of BA B777, G-YMMM

Sat Nov 08, 2008 4:34 pm



Quoting Starglider (Reply 19):
The fuel flow (what is left of it) most likely would become turbulent in the near empty fuel lines which might have upset the balance, making the fuel flow unstable in the engine feed system. The mass of an accelerating engine, suddenly deprived of demanded fuel, would not decelerate immediately, resulting in cavitation.

I don't think this could actually happen...you'd certainly get turbulence between the spar valve and the engine fuel pump, but the pressure rise across the engine fuel pump is a few thousand psi...that's going to squash any cavitation bubbles and instabilities long before they get to the fuel nozzles.

It would get you cavitation at the engine fuel pump inlet, which was observed on the BA airplane as I recall, but that just tells you there was a restriction...it's consistent with either a spar valve or fuel exchanger restriction.

Quoting Starglider (Reply 19):
I put it in the same unlikely category as two engines rolling back within seven seconds due to the current hypothesis of ice accretion.

I wouldn't put them at the same level of unlikeliness...we know there was water in the fuel and we know that flight was unusually cold...icing is a reasonable result and explains all subsequent events.

In the case of the hypothetical spar valve event we've got no idea what condition would cause it.

Tom.
 
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RE: Question On The Accident Of BA B777, G-YMMM

Sat Nov 08, 2008 6:59 pm



Quoting Jetlagged (Reply 15):
The reason for using relays is often to control a high powered motor from low voltage logic. Can solid state relays switch 115V AC?

Yepp. But maybe the 777 was to early for solid state.
I read that the A350 will have only solid state circuit breakers.
 
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Jetlagged
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RE: Question On The Accident Of BA B777, G-YMMM

Sat Nov 08, 2008 8:59 pm



Quoting Starglider (Reply 19):
I emphesize it is a theory, in this scenario it is indeed a question of timing but if the valves were to close while the engines increased their demand for fuel, engine behaviour could very well appear to be not responding to acceleration demand. I put it in the same unlikely category as two engines rolling back within seven seconds due to the current hypothesis of ice accretion.

Your theoretical scenario relies on a transient restriction being able to reproduce the cavitation. The ice accretion theory allows for a constant restriction to take place. According to the data time history in the AAIB interim report EPR and EPR command began to diverge about one minute before the crash. The maximum time for a complete cycling of the spar valves is 30 seconds, full open thru closed back to full open. If a 75% closure of the valves is required to reproduce the effect, that only provides a maximum event duration of 7.5 seconds, assuming the valves fully closed before they open beyond 75% again.

http://www.aaib.dft.gov.uk/cms_resources/G-YMMM%20Interim%20Report.pdf

I regard the AAIB theory (ice accretion triggered by fuel flow demand) as being much more likely than the unexplained simultaneous failure of two relays coinciding with increased thrust demand. With respect, the AAIB have studied the evidence, and the entire fuel system, first hand.

Quoting Starglider (Reply 19):
Furthermore, what happens to the fluid dynamics once the fuel has passed a more than 75 to 80% closed spar valve with an engine accelerating but then deprived of the required fuel flow? The fuel flow (what is left of it) most likely would become turbulent in the near empty fuel lines which might have upset the balance, making the fuel flow unstable in the engine feed system. The mass of an accelerating engine, suddenly deprived of demanded fuel, would not decelerate immediately, resulting in cavitation.

However the fuel line is blocked, the resulting cavitation is the same so I don't see how this supports your theory.

Quoting Starglider (Reply 19):
Another issue is FADEC. As long as a FADEC equipped engine gets fuel, it will tenaciously cling to life. This article in the link below might be interesting regarding that aspect. It deals with another type of turbofan engine but the principle is similar:

Even FADEC can't do much about insufficient fuel pressure.
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RE: Question On The Accident Of BA B777, G-YMMM

Sat Nov 08, 2008 10:30 pm



Quoting Tdscanuck (Reply 20):
we know there was water in the fuel and we know that flight was unusually cold...icing is a reasonable result and explains all subsequent events.

Recorded minimum fuel temperature was -34C (-49F). Even if minimum TAT was -45C and minimum SAT was -73C, the fuel (at -34C) did not reach low enough temperatures to cause the fuel to wax.

Fuel temperatures on the accident flight were low but not unique with other flights experiencing lower temperatures which was revealed as a result of data mining 13,000 flights performed by B777 with RR engines.

Quantities of water in the fuel (40ppm dissolved, 30ppm entrained or free) estimated to have been present on this flight are considered a very low level for a B777 according to the AAIB.

Data mining revealed that fuel flows during approach on the accident flight reached more than 12,000 lbs per hour, although not unique, this was at the edge of the family for data analysed.

The theory according to the AAIB is that perhaps the temperatures (low but not the lowest) and high fuel flow during approach (at the high end of the family) could have released accreted ice at stagnation points somewhere in the engine feed fuel lines and blocked the LP pump inlets or FOHEs.

My remark on this theory is: 12,000 lbs/hr fuel flow may be at the high end during approach but many similar flights under similar conditions (perhaps with relatively higher water content in the fuel compared to the low water content of the accident flight) must have made go-arounds which require higher fuel flows but apparently do not release accreted ice and do not encounter this problem?

Those go-arounds, also occurring on a regular basis but without similar effects have not been covered in the AAIB reports sofar. Without any evidence or clear explanation to this accident in 13 years of B777 operation, this is why I place this "ice" scenario in the same category and as unlikely as the possibility of the uncommanded closing of the spar valves.

Quoting Tdscanuck (Reply 20):
It would get you cavitation at the engine fuel pump inlet, which was observed on the BA airplane as I recall, but that just tells you there was a restriction...it's consistent with either a spar valve or fuel exchanger restriction.

Cavitation was found on the HP pump pressure outlet ports on both engines caused by restricted fuel flow, leading to low inlet pressure at the HP pump.



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tdscanuck
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RE: Question On The Accident Of BA B777, G-YMMM

Sat Nov 08, 2008 11:53 pm



Quoting Starglider (Reply 23):
Quoting Tdscanuck (Reply 20):
we know there was water in the fuel and we know that flight was unusually cold...icing is a reasonable result and explains all subsequent events.

Recorded minimum fuel temperature was -34C (-49F). Even if minimum TAT was -45C and minimum SAT was -73C, the fuel (at -34C) did not reach low enough temperatures to cause the fuel to wax.

I didn't say the fuel was unusually cold, I said the flight was unusually cold. A key thing to keep in mind is that the fuel temperature probe is only in one location and there is very limited fuel circulation on a 777 (as compared to some other designs), so the fact that the fuel probe only measured -34C doesn't tell you much about the temperature anywhere else, most especially at the heat exchanger.

Quoting Starglider (Reply 23):
Quoting Tdscanuck (Reply 20):
It would get you cavitation at the engine fuel pump inlet, which was observed on the BA airplane as I recall, but that just tells you there was a restriction...it's consistent with either a spar valve or fuel exchanger restriction.

Cavitation was found on the HP pump pressure outlet ports on both engines caused by restricted fuel flow, leading to low inlet pressure at the HP pump.

You can't get cavitation on the outlet of the HP pump...that would require something downstream sucking fuel and there's no such entity. Cavitation requires you drop the pressure below the fuel vapour pressure...the actual pressure on the outlet side of the HP pump is in the thousands of psi. The HP pump is pushing fuel against the combustor pressure and the outlet port is the highest pressure point in the entire fuel system. Low inlet pressure will cause cavitation on the impeller and inside the pump, not on the outlet.

Tom.
 
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 09, 2008 1:17 am

Quoting Tdscanuck (Reply 24):
You can't get cavitation on the outlet of the HP pump...that would require something downstream sucking fuel and there's no such entity. Cavitation requires you drop the pressure below the fuel vapour pressure...the actual pressure on the outlet side of the HP pump is in the thousands of psi.

See page 18 of the AAIB Interim report, top of first column:

http://www.aaib.gov.uk/cms_resources/G-YMMM%20Interim%20Report.pdf

Apparently the AAIB got it wrong? If I read the AAIB report correctly, the cavitation marks on the pressure outlet ports must have been caused by bubbles as they collapse at the pressure side of the HP pumps.


Starglider

[Edited 2008-11-08 17:18:46]

[Edited 2008-11-08 17:19:13]
 
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 09, 2008 5:05 am



Quoting Starglider (Reply 25):
Apparently the AAIB got it wrong? If I read the AAIB report correctly, the cavitation marks on the pressure outlet ports must have been caused by bubbles as they collapse at the pressure side of the HP pumps.

I doubt they got it wrong, they were able to replicate the cavitation marks during testing.

According to the AAIB report the restriction would have to be 95% of the area to produce cavitation at the HP pump. The roll back due to loss of fuel pressure lasted for about a minute before the crash. Rather than being a transient movement the spar valves would have had to be very nearly closed for the last minute of the flight. Page 8 of the interim report shows that the AAIB paid close attention to the spar valves in their investigation.
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 09, 2008 5:15 am



Quoting Starglider (Reply 25):
Apparently the AAIB got it wrong?

I have to assume not...they're a lot better at this than I am. However, I'm totally baffled by the physics that could make this happen. Prior to my aeronautical days, I worked with huge (multi-ton) extremely high pressure (22000 psi) pumps...we cavitated them all the time and you'd get damage on the moving pump parts where the work was being done, but you couldn't get it on the outlets because the bubbles had collapsed long before they got there.

Quoting Starglider (Reply 25):
If I read the AAIB report correctly, the cavitation marks on the pressure outlet ports must have been caused by bubbles as they collapse at the pressure side of the HP pumps.

This is certainly what it sounds like, but I don't see how it can physically occur. Cavitation is a low pressure phenomenon...the outlet side of an HP fuel pump is a very high pressure place. You get local low pressure areas on the working sides of the pump rotors (impellers/gears/etc.) but, in order for fluid to move to the outlet at all, it has to be at higher pressure than the fluid outside the outlet, and that's at very high pressure in an HP fuel pump. I'm mystified.

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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 09, 2008 12:10 pm



Quoting Tdscanuck (Reply 24):
A key thing to keep in mind is that the fuel temperature probe is only in one location and there is very limited fuel circulation on a 777 (as compared to some other designs), so the fact that the fuel probe only measured -34C doesn't tell you much about the temperature anywhere else, most especially at the heat exchanger.

On page 11 of the AAIB interim report it is explained how the OEM previously undertook tests with racks of thermocouples and determined that the coldest spot is in the inboard section of the LH main tank. That is where the temperature probe is located. One could wonder, why not a second probe in the RH main tank? Apparently the OEM determined that the RH main tank will generally be slightly warmer due to the presence of two hydraulic oil-to-fuel heat exchangers in the RH main tank and only one such heat exchanger in the LH main tank. I agree it does not say much about the temperature near the engine FOHEs but I would not be surprised if the OEM has tested near that location during design and development of the aircraft as well.

Quoting Jetlagged (Reply 26):
According to the AAIB report the restriction would have to be 95% of the area to produce cavitation at the HP pump. The roll back due to loss of fuel pressure lasted for about a minute before the crash. Rather than being a transient movement the spar valves would have had to be very nearly closed for the last minute of the flight. Page 8 of the interim report shows that the AAIB paid close attention to the spar valves in their investigation.

Exactly my point. The AAIB paid close attention to the spar valves and as I have posted previously, my opinion differs with regards to their conclusion that warnings are being presented and uncommanded movement will be recorded under all circumstances. But I humbly accept the fact that the AAIB does magnificent work finding the true causes of aircraft accidents. They have the expertise and the subject aircraft to investigate and substantiate their conclusions. But I will anxiously wait for their final report on this particular accident.


How coincidental that the 95% restriction published in the AAIB interim report is identical to the tests performed in the article from January 1990 I have posted in reply 19 of this thread. To put these tests in perspective, one of the many reasons these tests were conducted was the fact that on the subject aircraft type, the fuel shutoff valve closed uncommanded (quoted as some weird failure in the article but which actually had occurred) on several occasions, sometimes rolling back the engine, shutting down the engine during taxiing, or resulting in a crash because the engine flamed out in flight. I know it as a fact because I have had firsthand experience with this phenomenon on this aircraft type and the corrective actions taken back in those days.

Quoting Tdscanuck (Reply 27):
I'm mystified.

So am I.


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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 09, 2008 7:05 pm



Quoting Starglider (Reply 28):
How coincidental that the 95% restriction published in the AAIB interim report is identical to the tests performed in the article from January 1990 I have posted in reply 19 of this thread. To put these tests in perspective, one of the many reasons these tests were conducted was the fact that on the subject aircraft type, the fuel shutoff valve closed uncommanded (quoted as some weird failure in the article but which actually had occurred) on several occasions, sometimes rolling back the engine, shutting down the engine during taxiing, or resulting in a crash because the engine flamed out in flight. I know it as a fact because I have had firsthand experience with this phenomenon on this aircraft type and the corrective actions taken back in those days.

In the test you refer to the engine flamed out after a series of auto-restarts. On the BA 777 the engine rolled back to a steady condition above flight idle.

In your original post you referred to a time period of less than 30 seconds (the time for the spar valve to motor closed then open). The restriction lasted much longer than that. To support your spar valve theory both spar valves would have had to be stuck in an intermediate, nearly closed position without creating a warning. That would either require an uncommanded series of open/close commands, or the relay contacts to be in an intermediate position. Yet after the crash the valves were certainly open, because they had to be manually closed to stop fuel spilling out.

A lot depends on how the FDR records spar valve position.
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 09, 2008 8:19 pm

I've seen cavitation damage on the output sides of high pressure hydraulic pumps where nobody thought cavitation would be possible. Cavitation bubbles were making it past the impeller and collapsing in microseconds from the pressure on the high side, but the speed of the fluid moving to collapse the bubbles was causing extremely high pressure, short lived shock waves at the pump outlet. The waves didn't seem to travel far because the tiny bit of elasticity of the lines was enough to dampen them after a few inches, but they were beating the hell out of the impeller.
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RE: Question On The Accident Of BA B777, G-YMMM

Sun Nov 09, 2008 11:25 pm



Quoting Jetlagged (Reply 29):
In the test you refer to the engine flamed out after a series of auto-restarts. On the BA 777 the engine rolled back to a steady condition above flight idle.

I don't intend to claim that the test in the article is identical to a posible failure in the BA777. It does show that unexplained incidents/accidents back then were investigated to try and find answers to puzzling issues. And these tests were done with boost pumps OFF and air refuel door open (which reduces tank pressure to ease air refuelling), reducing fuel pressure to a minimum.

The F110 turbofan flamed out after strugling to keep running for a short while. First of all the F110 is a lighter low bypass turbofan when compared to a Trent 895. The Trent 895 has a much higher mass and is a high pypass turbofan with a much larger fan disc. Perhaps the mass of the larger/heavier engine could have been enough to keep it running for a longer period of time in which it would rolled back for a while before eventually flaming out. The moment from the start of the fault to the moment the aircraft hit terra firma is not that long. Who knows what the result may be, doing such a test with a Trent 895 and with boost pumps ON.

By the way, the solution to prevent uncommanded closing of the fuel shutoff valves on the subject aircraft type tested was the addition of an EMI filter to the shutoff valve connector. Later on improvements were made to the contact pins in the connector as corrosion to the pins could possibly create a path for an electrical current between pins causing the valve relay to move from open to close.

Quoting Jetlagged (Reply 29):
In your original post you referred to a time period of less than 30 seconds (the time for the spar valve to motor closed then open). The restriction lasted much longer than that. To support your spar valve theory both spar valves would have had to be stuck in an intermediate, nearly closed position without creating a warning. That would either require an uncommanded series of open/close commands, or the relay contacts to be in an intermediate position. Yet after the crash the valves were certainly open, because they had to be manually closed to stop fuel spilling out.

As long as the spar valve actuators follow control relays commands there would be no warning. And yes, perhaps there were several intermittent faults which kept the spar valves oscillating at a near closed position. Being intermittent, the fault may have dissappeared just as randomly as it had appeared or (depending on the source of the anomaly) the impact with the ground made it dissappear. Soon after, the flight crew pulled the fire handles. The rest regarding the open spar valves and the reason why has been published in the AAIB reports. It was a matter of incorrect sequencing by pulling the fire handles before selecting fuel control switches to cutoff on an aircraft in a pre SB configuration. In a post SB configuration the valves would have closed regardless of sequence.

Quoting Jetlagged (Reply 29):
A lot depends on how the FDR records spar valve position.

Yes, indeed but if the FDR depends solely on the AIMS position signal as mentioned in reply 14, then in this scenario the recorded OPEN signal would be false.

Since the AAIB is convinced that uncommanded closing of the spar valves could not go undetected, this suggests there must be other sensors as well that monitor and record valve position. I don't know if this is the case.


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RE: Question On The Accident Of BA B777, G-YMMM

Mon Nov 10, 2008 11:59 am

It is conceivable, but it requires repeated unexplained intermittent faults on both spar valve control relays. It seems even more remote a possibility than the ice accretion scenario postulated by the AAIB.

My point about the spar valves needing to be closed manually was that they were found open after impact, not about any crew error. If the valves had been nearly closed during the last minute of flight they were open again on ground.
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RE: Question On The Accident Of BA B777, G-YMMM

Mon Nov 10, 2008 3:15 pm

Maybe I'm not getting it here.. I've read the thread and haven't seen anyone explain in a credible fashion why ice isn't likely to be the source of the problem. It's all fine to speculate on the cause if someone can give me a *good* reason why ice couldn't be to blame. Until then all this chatter of relays and whatnot is:

a) Without any supporting evidence
b) Directly contradictory to the findings of professionals
c) Irresponsible

I really had thought we were better than that around these parts.
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RE: Question On The Accident Of BA B777, G-YMMM

Mon Nov 10, 2008 8:01 pm



Quoting Jetlagged (Reply 32):
My point about the spar valves needing to be closed manually was that they were found open after impact, not about any crew error.

That was my point as well. Sequence was mentioned, not to blame the crew, just to explain a pre and post SB configuration and why (in a pre SB config.) they could remain open.

Quoting Osiris30 (Reply 33):
It's all fine to speculate on the cause if someone can give me a *good* reason why ice couldn't be to blame.

Because of the relative small amount of water/ice present and, if it were enough to cause such problems during an approach, why did it not occur before during go-arounds under similar conditions?

If you have read all the posts seriously you would have noticed that I differ in opinion regarding the AAIB statement that, under all circumstances, there would be a warning on the flight deck if the spar valves were to close uncommanded and I have referred to the OEM documentation as to why I differ in opinion. If you are in the know about the system and how it is described in that documentation you'd know why.

I have had experience with fuel shutoff valves closing uncommanded in the past (different aircraft type but also a 28V DC system) which means conditions exist for such a phenomenon to occur. I will not go into details here but if anyone is interested, just PM me.

Quoting Osiris30 (Reply 33):
c) Irresponsible

You are entitled to an opinion. Just like anyone else on this forum.


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RE: Question On The Accident Of BA B777, G-YMMM

Mon Nov 10, 2008 8:44 pm



Quoting Starglider (Reply 34):
Because of the relative small amount of water/ice present and, if it were enough to cause such problems during an approach, why did it not occur before during go-arounds under similar conditions?

So because you have never won the lottery means it is impossible (assuming you play of course)? Not exactly the most stellar line of logic I've heard of when discussing causes of aircraft crashes. Usually the first time a problem occurs marks an interesting time in history where it's the FIRST OCCURENCE.

The first Comet crashes due to metal fatigue for example, were the first time that happened. The first time we had a fan disc let go in a DC-10 #3 engine was the first time we saw all the hydraulic lines severed like that.. I could go on and literally type volumes here, but I hope you get the point, namely: Just because it hasn't happened before is a piss poor reason to discount it.

Quoting Starglider (Reply 34):
If you have read all the posts seriously you would have noticed that I differ in opinion regarding the AAIB statement that, under all circumstances, there would be a warning on the flight deck if the spar valves were to close uncommanded and I have referred to the OEM documentation as to why I differ in opinion.

That's great, but that wasn't what I was asking for clarification on. I was asking why ice isn't a perfectly reasonable and likely explanation, not why you think something else might have been the case. Two different questions. Your answer of; "Because of the relative ..." I have already dealt with above.

Quoting Starglider (Reply 34):
If you are in the know about the system and how it is described in that documentation you'd know why.

I'm not disputing the point you put forward could have happened, or even that it won't happen one day. I'm disputing the fact that you are choosing to disregard the findings (albeit preliminary) of experts in the field, because it hasn't happened before, and then advancing an alternate theory with no factual basis IN THIS CASE.

Quoting Starglider (Reply 34):
I have had experience with fuel shutoff valves closing uncommanded in the past

I don't doubt you for a minute.

Quoting Starglider (Reply 34):
(different aircraft type but also a 28V DC system)

Milspec is everywhere. Hardly unique and I would wager most planes that are more than 10 years old in design are all milspec when it comes to DC voltage (unless we go way back and then things get really weird  Wink )

Quoting Starglider (Reply 34):
which means conditions exist for such a phenomenon to occur.

So because it has happened before on (I'm assuming) an alternate type somehow makes it more likely to have happened here than ice based problems? Using your logic, if it was a likely cause of an accident it would have happened many times before, but hasn't, therefore is the incorrect item to be blamed for the failure.

Quoting Starglider (Reply 34):
You are entitled to an opinion. Just like anyone else on this forum.

This isn't just *my* opinion. Everytime we have any sort of crash around here, we usually try to remind ourselves (i.e. the A.net community) that speculation on the source of a crash is irresponsible. In this case to come out and say the AAIB is wrong because something else could have caused the crash and that what they caused the crash never happened before, is just irresponsible.

I *will* give you ample credit for freely admiting that it is jsut a theory, so perhaps I was a little harsh with the irresponsible comment as it relates to your advancing of a possible alternate cause, but your out of hand dismissal of their findings, with no contradictory evidence is, frankly, irresponsible.
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RE: Question On The Accident Of BA B777, G-YMMM

Tue Nov 11, 2008 12:37 am



Quoting Osiris30 (Reply 35):
In this case to come out and say the AAIB is wrong because something else could have caused the crash and that what they caused the crash never happened before, is just irresponsible.

You are taking my words out of context here. I said I differ in opinion on one issue regarding flight deck warnings in relation to spar valve position and from there on I asked myself "what if . . . . ." If questions pop up, they are there for a reason and I start trying to find answers!

I do not discard the ice hypothesis, but up to this moment, that is what it is and I just happen to be a critical reader of the interim report. Until tests PROVE the cause to be ice there is no harm thinking outside the box and this accident has fascinated me because there is no clear explanation for it sofar. I have noticed not only on A.net but on another forum as well, that if you start to ask questions not in line with the established train of thought some come bashing in out of nowhere and start the finger pointing.

Quoting Osiris30 (Reply 35):
So because it has happened before on (I'm assuming) an alternate type somehow makes it more likely to have happened here than ice based problems? Using your logic, if it was a likely cause of an accident it would have happened many times before, but hasn't, therefore is the incorrect item to be blamed for the failure.

Apparently you are the first person that is sure to KNOW what did or did not cause this accident. I'm not blaming anything for the failure. I just happen to have another theory.

I'll tell you this, after doing some research in the aircraft systems, only weeks after the accident I simply asked (on another forum) which cabin layout G-YMMM was in at the time of the crash, a 3 or 4 class configuration. Not hardly a top secret issue now is it? My question was removed within minutes. The same happened to several other related questions. The answers to these questions may have a certain rellevance to my train of thought and to remove them without a clear explanation (non ever was given) by the mods at that particular forum mystified me. Eventually I got my answer regarding the cabin layout through other means. So, you bet I will look critically at this accident.

The only things that count are the FACTS so i will anxiously await the official final report which will be a while before being published from what I have read in the reports issued sofar.

Furthermore, I could be wrong but I thought that forums like these exist to share thoughts and opinions. Yet you accuse me of irresponsibility for a difference in opinion regarding only one aspect in the report and for not accepting that aspect at face value. Then your perception is that I blame another system to be the cause of this accident, that is simply not true and you are again putting my words out of context.


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RE: Question On The Accident Of BA B777, G-YMMM

Tue Nov 11, 2008 1:04 am



Quoting Osiris30 (Reply 33):
I've read the thread and haven't seen anyone explain in a credible fashion why ice isn't likely to be the source of the problem.

Part of the issue is that *none* of the causes of this accident are likely...any that were would have been seen long ago. Since all the options are unlikely to start with, you can get led down the garden path when comparing relative probabilities when all the numbers are tiny to start with.

Quoting Starglider (Reply 34):
Because of the relative small amount of water/ice present and, if it were enough to cause such problems during an approach, why did it not occur before during go-arounds under similar conditions?

Whatever happened in this case had to be be a very unique combination of factors...otherwise we'd have seen it before. That means that we don't really know how similar "similar conditions" need to be for the comparison to be valid. It's entirely possible (probably likely) that this precise set of conditions never happened before.

For just one example, if you have no water in the fuel the icing problem is impossible. If you have a lot of water in the fuel, it may also be impossible because water has a higher heat capacity than fuel and you may not be able to pull enough energy out in the heat exchanger to cause ice formation before the water has passed through...so there may be a very fine balance between water percentage, fuel flow, and temperature to even get ice formation in the first place. Then you have to have it line up exactly with a demand for more thrust...it's possible that we've been 60 seconds, or even 1 second, away from this type of event happening many times in the past but nobody ever knew it.

Quoting Starglider (Reply 36):
Until tests PROVE the cause to be ice there is no harm thinking outside the box and this accident has fascinated me because there is no clear explanation for it sofar.

No test is going to prove that ice was the cause...any evidence that could have done that has long since gone. The closest we'll ever get is proving that ice could have been the cause, and that it's more likely than anything else we can think up.

By the way, I wholeheartedly agree with you that there's no harm in thinking outside the box and I, for one, don't find the speculation on a.net "irresponsible." The speculation, and ensuing firefight/clarification/discussion is always educational, even if it doesn't get us one iota closer to knowing the cause of the crash.

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RE: Question On The Accident Of BA B777, G-YMMM

Tue Nov 11, 2008 1:55 am



Quoting Starglider (Reply 36):

You are taking my words out of context here.

Apologies, you're right. I did mix two posts and that was wrong of me, so I officially (  Wink ) withdraw criticism c above.

Quoting Starglider (Reply 36):
I have noticed not only on A.net but on another forum as well, that if you start to ask questions not in line with the established train of thought some come bashing in out of nowhere and start the finger pointing.

My issue has never been with your alternate line of reasoning. My issue is with the fact that you a) *appeared* to completely disregard the ice theory (thank you for clarifying) and b) are failing to apply the same litmus test to your own 'cause' as you are applying to the official cause.

Quoting Starglider (Reply 36):
Apparently you are the first person that is sure to KNOW what did or did not cause this accident. I'm not blaming anything for the failure. I just happen to have another theory.

I don't think I said one way or the other *at all* what *did* cause the crash. Rather I pointed out you aren't applying the same test to your argument as you are applying to the AAIB's argument.

Quoting Starglider (Reply 36):
I'll tell you this, after doing some research in the aircraft systems, only weeks after the accident I simply asked (on another forum) which cabin layout G-YMMM was in at the time of the crash, a 3 or 4 class configuration. Not hardly a top secret issue now is it? My question was removed within minutes. The same happened to several other related questions. The answers to these questions may have a certain rellevance to my train of thought and to remove them without a clear explanation (non ever was given) by the mods at that particular forum mystified me. Eventually I got my answer regarding the cabin layout through other means. So, you bet I will look critically at this accident.

I really don't see how your posts being removed from some other forum have an relevance on what I took issue with. Hell I'm pretty sure they don't even have an relevance on the accuracy of your hypothesis. It's likely just a bunch of forum Nazis somewhere  Smile

Quoting Starglider (Reply 36):
The only things that count are the FACTS so i will anxiously await the official final report which will be a while before being published from what I have read in the reports issued sofar.

Agreed.

Quoting Starglider (Reply 36):
Furthermore, I could be wrong but I thought that forums like these exist to share thoughts and opinions. Yet you accuse me of irresponsibility for a difference in opinion regarding only one aspect in the report and for not accepting that aspect at face value.

See above for what I *perceived* in your posts and took issue with. It wasn't that you have a different thought, but rather you ... oh nevermind I said it once above already  Wink
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RE: Question On The Accident Of BA B777, G-YMMM

Tue Nov 11, 2008 9:41 pm



Quoting Starglider (Reply 36):
I'll tell you this, after doing some research in the aircraft systems, only weeks after the accident I simply asked (on another forum) which cabin layout G-YMMM was in at the time of the crash, a 3 or 4 class configuration. Not hardly a top secret issue now is it? My question was removed within minutes. The same happened to several other related questions. The answers to these questions may have a certain rellevance to my train of thought and to remove them without a clear explanation (non ever was given) by the mods at that particular forum mystified me. Eventually I got my answer regarding the cabin layout through other means. So, you bet I will look critically at this accident.

I don't think that having posts removed about this accident on another forum is any indication of anything sinister about it so why does that cause you to look more closely at the accident investigation? Forum mods tend to be ultra careful with subjects of this kind. Maybe your questions were so obscure as to look suspicious.

Alternative theories are to be welcomed, but I doubt the AAIB has a closed mind on the cause. They would not and could not be doing their job properly if they did. The report is still only interim and they themselves say the probable cause (ice) is an unlikely occurrence. But it is still a single cause triggered logically, whereas your theory requires a number of coincidences to occur rendering it much less probable.
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RE: Question On The Accident Of BA B777, G-YMMM

Wed Nov 12, 2008 12:06 am



Quoting Osiris30 (Reply 38):
Apologies, you're right. I did mix two posts and that was wrong of me, so I officially ( ) withdraw criticism c above.

Apologies accepted.  checkmark 

Quoting Osiris30 (Reply 38):
I really don't see how your posts being removed from some other forum have an relevance on what I took issue with. Hell I'm pretty sure they don't even have an relevance on the accuracy of your hypothesis. It's likely just a bunch of forum Nazis somewhere



Quoting Jetlagged (Reply 39):
I don't think that having posts removed about this accident on another forum is any indication of anything sinister about it so why does that cause you to look more closely at the accident investigation? Forum mods tend to be ultra careful with subjects of this kind. Maybe your questions were so obscure as to look suspicious.

It was a plain and simple request for information about the cabin layout (3 or 4 class config.). Other topics removed which I posted, related to this accident, which others replied to, were related to the continuous updating of documents such as RTCA DO-160 and EUROCAE ED-14 dealing with "backdoor coupling" and the ever increasing number of electronic devices taken onto aircraft (serviceable and/or damaged) generating electric fields / spurious emissions. Discussion was based on facts readily available simply by googling for it but for some reason they were removed. Nothing sinister about the posts at all. Again I have no clue why those questions and discussions were removed. The result only created a biassed thread for anyone trying to read its history.

Quoting Jetlagged (Reply 39):
Alternative theories are to be welcomed, but I doubt the AAIB has a closed mind on the cause. They would not and could not be doing their job properly if they did. The report is still only interim and they themselves say the probable cause (ice) is an unlikely occurrence. But it is still a single cause triggered logically, whereas your theory requires a number of coincidences to occur rendering it much less probable.

As mentioned earlier, I think the AAIB has the reputation of doing thorough and sound investigations. Even the AAIB doubts that the relatively low amount of water/ice believed to be present could have been capable of blocking the fuel flow. Of course ice is a logic and attractive candidate to try and explain the cause but I am convinced the AAIB remains open minded to other scenarios. However, accidents/malfunctions usually occur due to several contributing factors. Often such factors individually may do no harm and lay dormant until an additional factor is introduced and things go bad.

Starglider
 
Starglider
Posts: 657
Joined: Sun Oct 01, 2006 12:19 am

RE: Question On The Accident Of BA B777, G-YMMM

Wed Dec 10, 2008 9:12 pm

Another engine rollback is under investigation:

http://www.flightglobal.com/articles...w-like-trent-800-engine-issue.html


Starglider

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