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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 24th Jan 2008, 16:41
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Originally Posted by 055166k
Do you think it might be expedient in the short term to do away with the "160 to 4" with a view to stabilised approach further out and at greater height ?
I would say not. It would be a response to a 1 in a million event, that's still unexplained.
Changing the rules suddenly would cause more new problems than it would solve....
There is a long discussion about '160 to 4' elsewhere here on PPRuNe.
I would think any change should be a decision after proper consultation and discussion of all involved, not as a 'solution' to what may be a non-existant problem.
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Old 24th Jan 2008, 16:43
  #22 (permalink)  
 
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I think we shouldn't dismiss the validity of questioning the "160 to 4" procedure. I have very often found that with initial light or variable wind( with a possible tailwind component)or temp. inversion it can be difficult to be stabilized at 500' with engine spool-up only occuring about then. One solution was to have the gear down earlier and hence more drag with the power required above idle,but then so much for noise reduction.
Note the above flown with A320/321's
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Old 24th Jan 2008, 16:58
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Super thread. Exactly what I tried to start a couple of days ago.
Some eight seconds later the thrust reduced on the left engine to a similar level.

It's that "to a similar level" that worries me. If we are looking for ice / wax etc., just what are the chances that there would be any similarity in a fault -- causing a specific reduction -- that was spread in time by 8 seconds?

This sounds horribly like a set of parameters being met that cause such a specific instruction.
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Old 24th Jan 2008, 17:01
  #24 (permalink)  
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Now that's why I asked this earlier:

Is the mechanism for matching thrust demand to engine fuel flow open loop or closed loop. What I mean is, in the event that engine EPR/N1 is not what is being commanded by the thrust levers, does the fuel control unit keep trying to increase fuel flow until it is, or does the fuel metering unit simply set a fixed fuel flow and expect the engine to accelerate to an EPR setting concomitant with it?

If both fuel control units set the same fuel flow, then both engines *should* provide the same thrust. If there is any mis-setting, the result is the same on both engines, although not what has been commanded.
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Old 24th Jan 2008, 18:42
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Is there "ANY" kind of thrust asymmetry protection available in the 777 landing phase??
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Old 24th Jan 2008, 18:55
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I've been maintaining a keen eye on this incident - I'll be very interested to know the final AAIB findings. I personally think the most significant thing to be gained from this latest update is the mention about the state of the engines - i.e. still running above flight idle albeit not at the commanded thrust setting. Whether or not there was a reduced fuel flow (perhaps due to electronic control, physical issue, fuel contamination/waxing, etc...) will be revealed in due course I'm sure.
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Old 24th Jan 2008, 19:40
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Quote from Loose rivets (er, where IS the quote button. Oh well...):

"..just what are the chances that there would be any similarity in a fault -- causing a specific reduction -- that was spread in time by 8 seconds?"

Got it in one. If the aeroplane was in a safe (normal) fuel config for the landing, there is no way that both engines would be fed from the same tank. So the chances of both of them starting to receive bad fuel only 8 seconds apart must be tiny.

Unless it transpires that the engines had been performing badly for some time during the descent (but unnoticed, as no thrust may have been needed), we are probably looking for a single trigger rather than 2 identical ones.

Deliberate interference by powerful EMI must be a slight possibility, and will no doubt be on the list of unlikely causes to be discounted.
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Old 24th Jan 2008, 19:40
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Is there "ANY" kind of thrust asymmetry protection available in the 777 landing phase??
Yes, it is called a "Rudder". It is available in all phases of flight.

The pilot flying, or the autopilot, will apply as much rudder as is necessary to correct up to 115,000 lbs of asymmetric thrust.

Last edited by Lost in Saigon; 24th Jan 2008 at 19:51.
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Old 24th Jan 2008, 22:00
  #29 (permalink)  
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Applying a simplistic view I admit but I can't see how fuel waxing enters the equation.
If waxing was indeed present then when the demand for more thrust/fuel came any wax would inhibit the delivery of fuel and the demands of the 'demanding' unit would not be met. Logically this 'Demanding Unit' would now demand more fuel to achieve required thrust and the thrust levers would be expected to move forward and not back, as happened.
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Old 24th Jan 2008, 22:16
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I don't see any reference to the thrust levers moving back in any of the AAIB reports. Thrust reduction, yes. Thrust lever retardation, no.
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Old 24th Jan 2008, 22:56
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Thrust Assymetry Compensation (TAC)

Is there "ANY" kind of thrust asymmetry protection available in the 777 landing phase?
From the B777 FCOM:

"The TAC continually monitors engine data to determine the thrust level from each engine. If the thrust level on one engine differs by 10 percent or more from the other engine, TAC automatically adds rudder to minimize yaw."

"TAC is available except:
  • when airspeed is below 70 knots on the ground, or
  • when reverse thrust is applied, or
  • when automatically disengaged due to system malfunction or loss of engine thrust data."

Hope that answers the question.
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Old 25th Jan 2008, 00:31
  #32 (permalink)  
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Yes Moggiee, I missed that, it says the engine thrust reduced but didn't mention the lever position.
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Old 25th Jan 2008, 00:35
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"If the thrust level on one engine differs by 10 percent or more from the other engine, TAC automatically adds rudder to minimize yaw."

Interesting... the Boeing Maintenance Manual has a different view...

"The TAC function is active when it is armed and all of these conditions occur:

.....

The thrust difference between the two engines is more than 6000lbs, about 10 percent of the maximum rated thrust....."

Completely different IMHO.

Anyhow... I have the feeling that this is not what the reader wanted to know

Rgds.
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Old 25th Jan 2008, 01:09
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I stand Corrected

In an earlier post (#12) on his thread, I had said:
So did they examine the RR Trent for a similar failing? I'd guess not. Never kick over rocks if you really don't want to know what might crawl out.....
It's been pointed out to me (via a PM) that this is incorrect (i.e. that the FAA had considered at the NPRM stage (of the AD cited in post 12) that the Trent 800 is equally vulnerable to the icing/waxing malady) - which was my surmise from the start.... and of course tends to confirm the validity of the FADEC freeze theory expounded at post #12.....
.
text received:...all below


In a recent post, you rhetorically asked if they considered the RR Trent in the AD, and I think you surmised the answer was negative.

I think this paragraph below is germane, and show that the intent was to include them with the GE, as there was no reason to expect them to respond differently (bold added):




---Quote---
Requests To Specify Lack of Events With Trent 800

Boeing and Rolls-Royce ask that we clarify the summary and paragraph (d) of the NPRM to specify that Trent 800 series engines (the subject of the proposed AD) have not experienced engine surges and internal engine damage due to ice accumulation during extended idle thrust operation in ground fog icing conditions.
We acknowledge that Trent 800 series engines have not experienced the stated events, as described in the "Discussion'' and "Similar Engine Models'' sections of the NPRM. As stated in the preamble of the NPRM, both the Trent 700 series engines and the Trent 800 series engines *have a similar compressor design, and therefore may be subject to the same unsafe condition. *We find that this information need not be added to the AD Summary section, or to any regulatory paragraphs of the AD.
---End Quote---

.
http://www.avionews.com/index.php?co...ante=index.php
.
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Old 25th Jan 2008, 01:53
  #35 (permalink)  
 
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Monkeyman75:

We had new Freezing Fog core ice shedding proceedures since Sept 25 2006 on all RR Trent powered A/C, including the Airbus. Issued in a Boeing Bulletin to all 777 RR Trent operators.

What's different with this AD?
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Old 25th Jan 2008, 04:28
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parabellum Yes Moggiee, I missed that, it says the engine thrust reduced but didn't mention the lever position.

It also doesn't mention the thrust levers being moved manually at any time (although it did in the initial report).

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).

Consequently, it also does not say whether the manual intervention occured within the 11 second delay, or later (maybe they do not know).
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Old 25th Jan 2008, 05:35
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The 'Beeb' is not a reliable source of info!

There have been a huge number of posts removed from here and this is a reminder to those that have seen their handywork disappear why. This is a quick warning to all the wannabe crash investigators that just because the BBC or any other media outlet decide to rewrite and make their own interpretation of the AAIB interim or updated reports, doesn't make them definitive, especially as it is very obvious that they are padding out their pieces with some of the most fatuous and tenuous suppositions that are to be found here. Those of us who do THE job are perfectly aware of how wrong they so often get it and WE are equally aware when someone tries to post their own slant or interpretation on events when they very obviously don't have the knowledge or experience to back it up.

In light of the above, it is quite amazing how many people are still posting their tripe on here when it so obviously contains ignorance based suppositions and proves that a little bit of knowledge can be a dangerous thing. The high number of posts that have been removed from here and placed in the WAG's (Wives And Girlfriends) thread in JB only goes to show what we have to deal with here when trying to keep a serious thread like this reasonably on track.

If you don't quite understand what is meant by "WAG's thread", just think of it as a repository for the less informed and ignorant posts (and their answers). It is a slightly derogatory term for "hangers on" and relates to the media attention placed on the WAG's of professional sportsmen which focusses on bling, handbags, fashion and generally bimboish and lowbrow stuff that we are really not interested in here.

So, may I respectfully suggest that you save us a lot of work and play in the WAG's thread if you are going to reference your posts to ANYTHING other than the AAIB reports as a thread ban may cause offence. Valid, but obviously uninformed, questions may remain provided they contain no suppositions based on dubious sources of information (such as MS fligt sim experience, the words of TLA 'experts' with no real understanding of their meanings or media based fantasy) and other dissemination of related information from technical sources will also be allowed.
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Old 25th Jan 2008, 06:16
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If fuel waxing indeed was an issue, how could this ever be proven ?
Probably there is not enough data on the FDR to reproduce all temeratures of all fuel components. Probably the physics behind waxing is not easy, (formation of wax cristals over time, size and shape of cristals, influence of flow speed, influence of pressure...) so how do you want to reproduce the fuel behaviour in a laboratory, even if you would have a large enough fuel sample from the tanks of the aircraft (don´t know whether there was enough fuel to collect for investigations).
I absolutely doubt that the final AAIB report will tell that fuel waxing was the main reason for the accident, it will always be a much softer statement simply because the final evidence will never be there.

It´s just like the 737 rudder actuator reversal related to temperature difference between hydraulic fluid and actuator valves. Some effect could be reproduced in the laboratory, it is a possible scenario, but the final evidence is missing.
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Old 25th Jan 2008, 07:06
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Chris,
It is difficult to argue with Phils point.
I am sure everyone is looking for the quick answers and given the situation AAIB must be under great pressure.
The data is coming from the data recorders so they can see the throttle movement and the engine reaction.
The 2 reports are actually giving 2 very different scenarios with on saying the engines did not respond and then reponded but rolled back.
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Old 25th Jan 2008, 07:53
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...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?
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