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AAIB BA38 B777 Initial Report Update 23 January 2008

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AAIB BA38 B777 Initial Report Update 23 January 2008

Old 25th Jan 2008, 08:53
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Originally Posted by PickyPerkins
Nor whether the levers moved back again after being moved manually (which I understand they should do if the auto thrust/FADEC system is operating normally, as claimed).
We don't know if the autothrottle (Boeing nomenclature) was deactivated or not.

And why should the autothrottle move the levers back when the delivered thrust was insufficient to maintain the desired airspeed? Shouldn't it move them further forward, until the desired airspeed was reached again?

We must also be careful to not throw autothrottle and FADEC together. On the 777 they are completely separate.

The autothrottle mechanically moves the thrust levers, the thrust lever position is signalled to the FADEC (or "EEC"), which then, also evaluating a lot of other parameters (I refer you to some of NSEU's very informative posts in previous threads) regulates fuel flow to achieve the demanded thrust.

The FADEC cannot tell if the thrust levers were moved by the pilots or by the autothrottle system.

Originally Posted by AAIB update to Initial Report
autothrottle and engine control commands were performing as expected prior to, and after, the reduction in thrust.
What exactly is meant by "engine control commands"?

Does the wording imply they did not perform as expected during the thrust reduction?

If what they meant was "FADEC operation", and it resumed normal operation after the thrust reduction, shouldn't it then have responded to the increased demand then?


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Old 25th Jan 2008, 08:57
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The idea that fuel waxing may have been an issue doesn't seem quite right.

EICAS would have brought up the ENG FUEL FILTER L + R and then possibly gone into the by-pass stage.

My point is that the EICAS would have answered the question by now if indeed waxing was an issue.

halas
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Old 25th Jan 2008, 09:06
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Quote:
...engine control commands were performing as expected prior to, and after, the reduction in thrust.

By what definition were they responding as expected AFTER the reduction in thrust?
I read this to Óndicate that the input values to the FADEC were correct for the Thrust Lever position.

"engine control commands" to me is the thrust request. Not the thrust obtained.

I think this rules out any Auto throttle or TLA faults and leaves just the fadec or the engine hardware as culprit.
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Old 25th Jan 2008, 09:07
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EICAS would have brought up the ENG FUEL FILTER L + R and then possibly gone into the by-pass stage.
And I presume sending unfiltered fuel further down the line to the engine itself.
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Old 25th Jan 2008, 09:22
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autothrottle and engine control commands were performing as expected prior to, and after, the reduction in thrust.
I read this statement as implying that the thrust commands (or output values as opposed to FE Hoppy's input values) from the EEC were correct - therefore the problem lies downstream from the EEC.
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Old 25th Jan 2008, 09:32
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I don't see the two AAIB reports as necessarily contradictory. I'd suggest that the first report gave a 'Yes or No' answer to whether the thrust delivered was the same as the thrust commanded - spotlighting this as a key focus of the investigation. The second report offered more detail as to how the failure to produce the required thrust manifested itself. Hopefully the third will offer some insight into the cause. This sequence is just what I'd hope to see as an investigation progresses, and to my mind it's a better approach than allowing free rein to any and all speculation right until the investigation is fully concluded.

Last edited by robdean; 26th Jan 2008 at 10:34. Reason: For clarity
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Old 25th Jan 2008, 09:33
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What would really help this discussion of waxing is a simple schematic of the whole system from thrust lever to the fuel injectors.
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Old 25th Jan 2008, 11:18
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Compressor Stall?

Engine acceleration requires opening of bleed valves/VIGV operation.

I remember a B757 RB211-535C approach incident where an engine flamed out on approach because the BVCU had failed (and hence the IP compressor went into rotating stall).

Just some thoughts.....
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Old 25th Jan 2008, 11:31
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bsieker:
And why should the autothrottle move the levers back when the delivered thrust was insufficient to maintain the desired airspeed? Shouldn't it move them further forward, until the desired airspeed was reached again?
I suspect what the poster was getting at was that the thrust lever servo system should have pulled the levers back in response to the drop-off in thrust.

Which I disagree with if the problem does end up being downstream of the thrust control system, because even a bog-standard cable-based thrust control system (which the 777's system is supposed to emulate) would not pull the levers back in the case of a fuel feed problem, to the best of my (admittedly limited) knowledge.
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Old 25th Jan 2008, 12:46
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If the engines were producing "more than idle thrust" upon impact, the damage ought to have been far more severe than what occured. The port engine appears to be substantially UNDAMAGED, despite inhaling loads of dirt.... The starboard engine, whilst not in great shape, doesn't reflect the sort of damage I'd expect if producing power at impact.
Whatever, the result redefines the term "engine braking", with the aircraft coming to rest after 150m or so...

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Old 25th Jan 2008, 12:55
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Engine braking or Egines broken, Octane?
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Old 25th Jan 2008, 12:55
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Take another look at #1 (ie portside) fan rotor:


Do you see ANY fan blades with more than 50% airfoil length?
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Old 25th Jan 2008, 13:01
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Fuel Waxing

I am under the impression, although lacking the system diagram, that the cold fuel passes through the oil cooler, hence warming the fuel and cooling the oil, to prevent waxy fuel from reaching the engine.
In any event, if the filter bypass opens, allowing waxy fuel to reach the engine, the only result is a slight loss of thrust due to the lowered temperature of flame. The loss however, is in the order of a few percent. Engineers don't like it either because of the potential for interior damage and corrosion to the turbine and exhaust system.
Unless the fuel is so waxy as to actually block the fuel feed, then the incident/accident would have been most unlikely.
If the fuel is found to be the culprit, I will be very, very surprised.
Gremlins normally attack wiggly amps, not pipes or tanks.
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Old 25th Jan 2008, 13:21
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Quote (Chris Scot) :-

Are you being a little unkind to the AAIB? The statement in the initial report that the engines "did not respond" to an autothrottle demand at approximately 600 ft turns out, a week later, not to have been the whole story. But does it make it wrong? ........

unquote


No I am NOT being "unkind". The initial report was incorrect. It wouldn't have been so bad if the AAIB had said in their update something like, "further examination found that contrary to the initial analysis ....." but it didn't it claimed "as previously reported" - that is just silly and wrong.

.

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Old 25th Jan 2008, 13:40
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Do you see ANY fan blades with more than 50% airfoil length?
No but what's your underlying point?
Do you expect fan blades to withstand being dragged through earth/mud for several hundred metres whilst the engine case is being deformed as it is supporting the weight of the wing due to lack of undercarriage?

Last edited by silverelise; 25th Jan 2008 at 13:52.
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Old 25th Jan 2008, 13:43
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Originally Posted by rubik101
I am under the impression, although lacking the system diagram, that the cold fuel passes through the oil cooler, hence warming the fuel and cooling the oil, to prevent waxy fuel from reaching the engine.
Does this help?

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Old 25th Jan 2008, 14:03
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Silverlise, I think his point was that there was enough energy turning the fan blades to get all of them damaged rather than just the ones that were at the bottom of the engine when it met the dirt.

(Edited because a reference to 'flan' blades might have been distracting)
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Old 25th Jan 2008, 15:32
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Fuel waxing -

If the fuel were cold enough to begin to "wax" wouldn't this have been evident in cruise where temperatures are the coldest and fuel flow is relatively high?

I would think that the likelihood of waxing would diminish as the airplane descended in to warmer temperatures.

Further, on the 777, the fuel freeze temperature is entered into the FMS. The EICAS displays fuel temperature in white until the fuel temperature approaches that minimum value at which time the EICAS display the temperature in amber.

There is also a FUEL TEMP LOW EICAS message.

Whatever caused the engines not to respond as expected, it had to be something that affected both sides of the airplane which are virtually independent of each other. Additionally, it must have affected the two independent FADECS rather than any other area of automation since manually advancing the thrust levers would have sent direct signals to the FADECs.

To further compound the mystery, we have to ask why whatever affected this airplane did not affect others. The answers will indeed be interesting.

My kudos to the crew of BA038. I like to think that my crew and I would have done as well in a similar situation.
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Old 25th Jan 2008, 17:30
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@phil gollin - pedant speaking...
As previously reported, whilst the aircraft was stabilised on an ILS approach with the autopilot engaged, the autothrust system commanded an increase in thrust from both engines.
Spot the full stop? The bit the AAIB previously reported is
the autothrust system commanded an increase in thrust from both engines
That is still true.
TP
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Old 25th Jan 2008, 17:33
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sky9

Before any unfiltered fuel entered the engine, an indiction would have been made to the crew through the EICAS as an advisory message.

MATS and QAR would have stored all the data of what the engines were doing before, during and after the turf ingestion. No physical examination will part the information those two hold.

halas
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