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BA038 (B777) Thread

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BA038 (B777) Thread

Old 20th Feb 2008, 22:08
  #201 (permalink)  
 
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Oldlae,

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

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

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

LD
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Old 20th Feb 2008, 22:15
  #202 (permalink)  
 
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Ref the open spar valves, (LP cocks) I am surprised that the anomaly had not been picked up on the simulator. The FAA requirement to allow the correcting modification, splicing of the wiring, not to be completed until 2010 was probably based on the Boeing cockpit drill being carried out as directed. This appears to be such a simple mod that I question the thought process of however decided not to do the mod asap would not have ever envisaged that the cockpit drill was not being followed to the letter. This lack of communication is always a problem with large companies and I hope that a valuable engineering lesson has been noted. Always check any changes in procedures with the engineers.
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Old 20th Feb 2008, 23:14
  #203 (permalink)  
 
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Quoting the AAIB update bulletin S1/2008:

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

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

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

Just a thought.

Regards,
Green-dot

Last edited by Green-dot; 21st Feb 2008 at 19:20. Reason: Edited for clarity.
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Old 20th Feb 2008, 23:23
  #204 (permalink)  
 
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Theory

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

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

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

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

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

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

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

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

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

Note that following the landing, the open LP (spar) valves allowed the contentents of the fuel feed pipes to be deposited onto the sod. Similarly, the CT fuel contents could not be assessed for water contamination as the tanks had been disrupted and subsequently contaminated by the firefighters.
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Old 20th Feb 2008, 23:27
  #205 (permalink)  
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AP LEFT IN 'til fell over due (no doubt) to a/c unable to follow GS.

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


 
Old 20th Feb 2008, 23:43
  #206 (permalink)  
 
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Barbiesboyfriend

Well if "Hero" pilot hadn't taken over, er "ken" and the aircraft had been left to it's own devices the sink rate could well have have been a darn sight higher, with even more serious consequences..... I think your comment that he "wrote the a/c off" is, putting it politely, ********.
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Old 21st Feb 2008, 00:09
  #207 (permalink)  
 
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Originally Posted by oldlae
I hope that a valuable engineering lesson has been noted. Always check any changes in procedures with the engineers.
Ah, but WHICH engineers? The operator's engineers are unlikely to be familiar with the details of the certification of the design. Nor, for that matter, are the OEM's support staff, who may be an entirely different part of the organisation to the design staff. Similarly the Airworthiness Authority personnel dealing with operational matters may have little interaction with the Certification teams, and the same applies to those approving STCs, which may not even be approved by the original state of design.

Even if you can track down the right departments (the OEm's design staff, and/or the airworthiness people who actually did the cert approval), there's still no guarantee the people there NOW know why a specific design choice was made.
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Old 21st Feb 2008, 00:43
  #208 (permalink)  
 
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1) Significant quantities of ice accumulate in the centre tank.
Syeng, problem with number one there. Centre tank is the warmest tank, so significant quantities of ice are unlikely to accumulate there. The reason fuel temp is measured in the wing tanks is because that is where your coldest fuel will be. Also, centre tank is emptied first, so fuel spends less time in there to cool down. It is very rare to be planning to land with fuel in the centre tank, so you're not going to get buildup of ice over multiple legs.
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Old 21st Feb 2008, 00:57
  #209 (permalink)  
 
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swiss_swiss,

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

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

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

I appreciate that modern machinery is way more complex, however, and I imagine more closely monitored to boot - so still, may or may not have relevance! I'm sure 777 operators such as yourself could rule such a scenario in or out of possibility.
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Old 21st Feb 2008, 01:41
  #210 (permalink)  
 
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Now I'm sorry to show my ignorance here, but where is air drawn into the tanks when the fuel is pumped out.
i.e could that be blocked by ice causing too low an air pressure for the pumps to suck the fuel ?
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Old 21st Feb 2008, 01:46
  #211 (permalink)  
 
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SyEng, inlets for the center tank pumps are not located at the lowest point of the tank. In fact, there is a substantial amount of residual fuel which they can't touch. Once this level is reached, the center pumps are incapable of producing pressure.

Fuel feed then commences from main tanks, while automatic scavenge pumps slowly and automatically transfer the residual center tank fuel to the main wing tanks. Typically, the center tank is literally empty after a flight of this duration.
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Old 21st Feb 2008, 03:41
  #212 (permalink)  
 
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The ice theories are the most enticing. I talked to a QF engineer today who was totally convinced it was ice blocking the fuel-oil heat exchangers. So if we accept for a moment that this hypothesis is correct, can someone explain how this fairly large quantity of ice made it's break for freedom and evaded the air accident investigators? Is it possible it melted and ended up on the grass?

The trouble I still have with the ice theories is that, if there is ice in the fuel tanks, it can't end up all in one place. Some would have to be still in the wing tanks waiting to be discovered.
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Old 21st Feb 2008, 06:36
  #213 (permalink)  
 
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Problem with ice theories is no water was found in the main wing tanks.

Any significance to that loose union?
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Old 21st Feb 2008, 07:10
  #214 (permalink)  
 
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A Ridiculous Suggestion.....

Suppose....just suppose...this accident has nothing whatsoever to do with the fuel or the pumps or the pipes.

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

Would the outcome have been any different?
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Old 21st Feb 2008, 07:36
  #215 (permalink)  
 
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Volume and mass

I was intrigued by the discrepancy between planned and actual arrival fuel.

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

Pumps supply volumes.

Engines produce power depending on the mass of fuel.

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

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

This may explain the apparent fuel discrepancy.

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

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

I have flown several times with the captain when he was a co-pilot and he is one of the better guys!!!
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Old 21st Feb 2008, 07:47
  #216 (permalink)  
 
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MiG 15,

again and again and again,

NO!

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

Originally Posted by AAIB Special Bulletin
[...] there were no anomalies
in the major aircraft systems. The autopilot and the
autothrottle systems behaved correctly and the engine
control systems were providing the correct commands
prior to, during, and after, the reduction in thrust.
Danny, thank you for moving us to the Jet Blast, where this belongs.
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Old 21st Feb 2008, 07:52
  #217 (permalink)  
 
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Hydromechanical Fuel Control vs Electronic Engine Control

Phil

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

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

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

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

Sven
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Old 21st Feb 2008, 07:58
  #218 (permalink)  
 
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Originally Posted by Mig15 View Post
<snip>
There is another common factor namely the environment the aircraft flew through prior to the incident which has not been mentioned in any report.
<snip>
There is a whole section in the latest AAIB report about 'weather'.
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Old 21st Feb 2008, 08:14
  #219 (permalink)  
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I believe that the figure of 6,900kgs was what the crew had entered into the FMS so as to generate an "insufficient fuel" message. This figure does not include the contingency fuel or any extra fuel (a nice round figure of 79,000 implies that they probably took some extra). If everything went to plan then this contingency fuel, and most of the extra fuel would still be on board at LHR which explains the recorded figure of 10,500kgs at 1000ft.
Perhaps a 777 pilot could confirm that this is correct.

Airclues
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Old 21st Feb 2008, 08:18
  #220 (permalink)  
 
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Ref the open spar valves, (LP cocks) I am surprised that the anomaly had not been picked up on the simulator.
What kind of simulator are we talking about here... a flight simulator or a Boeing design program? The flight simulator would have to model damage to specific wiring (I know Level D sims are accurate, but this is ridiculous).

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

Now I'm sorry to show my ignorance here, but where is air drawn into the tanks when the fuel is pumped out.
From the atmosphere (the tanks have a slightly positive air pressure on them courtesy of NACA scoops)

3) It is important that LP tank pumps are immersed in fuel so that they don't introduce air into the system, also the fuel provides a cooling function for the pumps.
The pumps themselves don't necessarily have to be immersed in fuel. The pumps are actually well above the floors of the tanks. However, the pickups for these pumps are closer to the tank bottoms. The fuel running though the pumps, provides cooling/lubrication.

5) So, you're probably ahead of me, what if the sensors in the 777 center tank failed or were fooled and the pumps operated whilst the tank was dry? Would EICAS alert on something or could they keep going long enough to put enough air into the system to balls things up?
Pressure-wise, CWT Fuel pumps pumping air will be no match for fuel pumps pumping fuel. The wing tanks will simply pump fuel into the manifold causing a backpressure easily strong enough to close the check (one way) valves on the CWT pump outlets. Therefore no air. If BOTH check valves failed on the CWT pumps and they were running.... anybody's guess.

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

If the FMC had been selected as the primary source of fuel quantity on board
during FMC flight planning programming in China and not altered to silence a flashing fuel discrepency message during refuelling...and the
incorrect fuel loaded and signed for...this could be a masked low fuel condition with no warning generated...a rubbish in...rubbish out computer situation....
If the FMC was telling the pilots that the fuel at destination was below minimums, then he would surely double check the flight plan and his FMC programming and resolve the discrepancy. He certainly would NOT fool the FMC into not generating a message even if he could.

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


Danny! Where are you????
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