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BA038 (B777) Thread

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BA038 (B777) Thread

Old 11th Apr 2008, 19:49
  #821 (permalink)  
 
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Hi demomonkey,

thanks for your comment. However, I still believe it's technically doable with reasonable effort.

In short we forget how wonderful the MkI eyeball attached to the Mk10 human brain actually is to sense inputs (topography, wind, aeroplane charactersitics) and meld these with prior experience to produce a safe and effective plan.
The words in (my) italics are the key here. Complete engine failure on final approach is evidently judged to be such an unlikely event that it's not included in training or simulator exercises. That was most probably a contributing factor to the fact that the airplane got near the stall and in that process irrevocably lost some dearly needed energy that might have taken it to the runway.

And as others have said, the flight crew is not to blame for it. They were faced with a situation no one had prepared them for and did what they could - saving the lives of all aboard, which in the end is all that matters.
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Old 11th Apr 2008, 20:47
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I would suggest that the loss of energy is due to the double engine failure rather than crew actions?

Jeez, you people....
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Old 12th Apr 2008, 00:12
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Quoting infrequentflyer789:
Also in the report they have said that the leakage was through the spar valves, which means (if I understand the fuel system correctly) that the fuel was from the main wing tanks, not the CWT.

Fuel leakage through the open spar valves, with the boost pumps switched off due to the fire drill, means there must have been a considerable amount fuel in the wing tanks. With the aircraft coming to rest wings level (which can easily be verified from published pictures), wing lower surface at the location of the spar valves has a considerable amount of dihedral downward to the rib separating wing tanks from CWT.

The spar valves are located somewhere outboard of the main landing gear and just inboard of the engine pylons and certainly not at the lowest point in the wing tanks because of spar valve location further away from inboard rib, separating wing tank from CWT, and dihedral sloping upward toward the pylon.

From the last picture taken of the aircraft while still airborne (taken from the left looking up toward the aircraft), one can clearly see the lower wing surfaces. The location of the rib separating the center tank from the right wing tank is clearly visible. The wing lower surface of the right wing tank has a frost layer beween that rib and engine pylon, meaning there is fuel in that tank, at least all the way up to the engine pylon. However, inboard of the rib toward the fuselage, the lower surface of the CWT has no frost which means no fuel is in contact with the lower wing surface in the CWT, hence the CWT is virtually empty.
Frost is also present on the lower surface of the left wing tank but barely visible because the left engine nearly obscures that particular wing surface area. You will notice the absence of frost on the left side of the CWT as well though.


Green-dot

Last edited by Green-dot; 12th Apr 2008 at 00:24.
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Old 12th Apr 2008, 00:37
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Question A Plea?

Ahh...when the "report" is finally published or someone REALLY knows what's going on, would you please publish a separate thread on the subject?

Trying to get some sense out of this one is a bit like herding cats!

G'day
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Old 12th Apr 2008, 08:57
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Software engineering

Sorry to bring these comments up again

Quote:
So in theory it's not that difficult to devise a software that automatically
As someone who has a Software Eng Degree, several years commercial experience with industrial strength, fault tolerant, 'mission-critical' applications (I hate that phrase too) and an ATPL I feel that it would be almost impossible to create such an application.

For a start that the aircraft would need to be fitted with so many sensors that its drag curve would be a vertical line. A database of global obstacles would be so onerous to create that it would be Terabytes of data and commercially un-maintainable.
Are you really aware of the state of the art in flight simulation? Very realistic 3D rendering of landscape and obstacle around airports an beyong. No need for terrabytes of data ...

Obstacles data base are actually used (and thus maintained) to compute landing and take-off restrictions, taking into account all the relevant factors. That kind of software is in use for decades by airlines or specialised service providers. Both data base and software are "accepted" for those computations by the authorities for flight dispatching. (Grey zone, in any way).

Any modern airliner is fitted already by an air data computer. No need for additionnal sensors. Thanks to GPS integration, and differential GPS technology, the position (of the antenna) is known within 10 ft horizontally, 30 ft vertically.

So what? When a runway is selected, a small portion of the obstacle data base should be querried and the data set made readily available for either or both :
- "visual" presentation to the crew (like in a sim)
- emergency trajectory computation by any decent computer. No need for terraflops here ... most of the computations should be done "well before" and the results stored, ready to be retrieved at the time the runway is selected.

As it was stated before, the only important missing information (today) is the "future" wind. The present wind is wel known, the ground wind (somewhat averaged) is known by the tower, but there is no direct link with the onboard computers. And no information "in between" - but good models are available.

I think that the available technology is not put to its maximum use ... Cost is the factor, but also lack of imagination.
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Old 12th Apr 2008, 15:27
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I would suggest that the loss of energy is due to the double engine failure rather than crew actions?
Well, in my world an engine failure means a loss of the energy supply. Which is a good reason to use the airplane's remaining energy in the optimal way.
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Old 12th Apr 2008, 16:14
  #827 (permalink)  
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Biz47,

obviously you have no experience with designing or building SW, let alone SW for what would be a safety-critical application. But you presume to tell a self-identified professional IT person that there is a lack of imagination. There may be and it might well be yours. In a world which does not yet know how to deal with map shifts, building reliable, fault-tolerant, fail-safe systems which do some of those "simple" things you mention, to the required standards of reliability and fault-tolerance, is beyond state of the art.

PBL
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Old 12th Apr 2008, 18:08
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Originally Posted by PBL
obviously you have no experience with designing or building SW, let alone SW for what would be a safety-critical application
I have some ... Not on a big scale, I do admit.
But I have some deep knowledge of flight operations.

I confirm that reliable sofware is already available for terrain visualisation and terrain avoidance :
- Flight simulation software (all levels, PC driven and higher) is amazingly realistic (terrain visualisation and aircraft handling) => allowing approved "Zero flight time" aircraft type rating (no perceived difference between simulator and real aircraft handling).
- Military software allowing flight at low altitude inside valleys ... relying more and more on stored terrain data and less on radar.

And of course, reliable software already exists regarding aircraft handling, allowing auto land in zero-zero, and flight enveloppe protection ...

Please note that airbus flybywire software does not require supercomputers : an Intel 8186 does the job pretty well. 20 years old technology ... much cheaper and much more powerfull hardware is available today - about 1000 times more powerfull ....

Note also that more than 10 years ago, cheap (but certified) general aviation GPS receivers already included some kind of advisory terrain elevation data (grid mora).

And ... today, the most popular general aviation GPS manage to display a nice picture of the runway, and some landscape features as well, when approaching an airport to land.

Where is the lack of imagination? Well nobody did imagine that a modern airliner would meet a dual engine failure on short final, so nobody saw the need for a software to handle that - neither for a pilot training actually. That is lack of imagination.
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Old 12th Apr 2008, 18:21
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Originally Posted by Jumpjim
I would suggest that the loss of energy is due to the double engine failure rather than crew actions?

Jeez, you people....
At the point at which the engine failure became obvious, the plane had X amount of energy - some was potential energy, some kinetic energy. The need was to manage this energy to conserve as long as possible, which means flying the plane in the most efficient way e.g. at best L/D. Unfortunately that's not what the autopilot did - apparently it tried to maintain the glide slope, which will have frittered the energy away.

If you fly too slowly (which is what the autopilot did) you increase the induced drag, too fast and you incrase the form drag. Both will result in the plane not flying as far as it could for the original amount of energy.

See the total drag vs. speed diagram on this URL - where the two cross is the best L/D ratio, and the best speed to fly for the situation G-YMMM was in. Unfortunatly the crew had very, very little time to think about it.

http://pilotsweb.com/principle/liftdrag.htm
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Old 12th Apr 2008, 18:55
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I am a bit pickled, but you are all fantastic, hindsight is a wonderful thing.

I think with the situation they had they did their best in the time available.

Lets praise them instead of nitpicking about what they could have done better.

I wonder whether I and you would have done as well on the day!!!
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Old 12th Apr 2008, 21:42
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Originally Posted by PAXboy
:- of the AAIB who cannot publish an update until they are SURE of what they have found. If they are working on something very serious, then anything less than a full explanation may cause panic in the travelling public. And the airline business is in a bad enough way as it is. If they are thinking of saying, "We have a bunch of ideas - and it could have been any one of them" then that will also cause a bad reaction. They do not 'owe' us anything, until they have something they can say with certainty.
I've just crunched the data on published formal reports by the AAIB back to 2006 (as far back as I could be bothered to go). The average length of time from incident to final report publication is 25.6 months, i.e. a little over two years. This does not and has not stopped them issuing recommendations, where appropriate, before the final report.

I'm quite happy for AAIB to resist the rabid frothing of the news media for sensational information to fill their 24-hour outpourings, and instead concentrate on trying to work out exactly what happened...
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Old 13th Apr 2008, 04:12
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Green-dot, you made a good point for materializing the presence of fuel in the main tanks.

Do you think the following selected area depicts also a frozen surface, which would show in the same time where the fuel extends outboard …?



Now, is the total frozen area large enough to confirm an actual fuel volume of 6500 liters each side ?
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Old 13th Apr 2008, 05:04
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The crew did a remarkable job of using optimum flaps to get close to the runway end. Leaving them in final landing position would have resulted in a bad situation. They should be commended on their skills to save those people.
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Old 13th Apr 2008, 06:27
  #834 (permalink)  

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As this is the rumour network... And I do understand the inverse square law, and all that good stuff. I have to report the "dark" rumors on the street are, that it was electronic interference of some sort.

I have a real struggle to see how it could possibly be, yet the rumor is persistent, and independent.
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Old 13th Apr 2008, 08:05
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RTFM - Spot on!
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Old 13th Apr 2008, 08:49
  #836 (permalink)  
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Biz47,

the difference between our views comes very likely because you are, obviously, amateur and I am expert in the field of embedded safety-critical digital systems.

PBL
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Old 13th Apr 2008, 15:04
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PBL, Demomonkey

I think you are sidetracked by this proposed obstacle-avoidance software. Let's look at the feasibility of the original proposal, which was an autopilot sub-mode that would fly the plane to its best glide speed in case of a serious engine failure.

From a flight safety perspective, it can IMO be treated much like the stick pusher stall prevention device. The functions are in fact quite similar.
The basic difference is that in this case the primary trigger signal comes not from the AoA sensor but from a thrust deficit sensor. The autothrottle already incorporates the function of such a sensor, presumably engineered to flight-critical reliability standards, thus it already exists.

This document from Transport Canada specifies the reliability requirements for stick pushers in transport category aircraft. It says the probability for failure of the device to operate shall not exceed 10^-4. Furthermore, regarding the probability of unwanted operation, a very pertinent issue in this kind of device, no single failure shall cause this and the probability of unwanted operation from all sources shall not exceed 10^-5. IMHO that is easily achievable with well known methods.

In fact, the only additional component not part of standard airplane hardware would be the lift/drag polar curve data. I presume any transport category aircraft-certified computer would have error-correcting memory as a standard feature, so no additional reliability issue there. And the software algorithm is very simple.

Furthermore, this suggested device would only need to be armed at final approach and when at or below the glideslope, greatly reducing its potential to do any harm. Last but not least, it would only ever be triggered when a flight critical "10^-9" event has already happened.

To summarize, if industry can build safe stick pushers, building this device is not a problem.

So please tell me if I've forgotten something crucial. And to repeat: this is about the technical feasibility. The actual usefulness of this suggested device is another discussion.
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Old 13th Apr 2008, 16:12
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Wing tank fuel.

Posted by CONF iture:
Do you think the following selected area depicts also a frozen surface, which would show in the same time where the fuel extends outboard …?
Yes, i believe that is also fuel induced frost.

Now, is the total frozen area large enough to confirm an actual fuel volume of 6500 liters each side ?
I will do some calculations using scale drawings of the fuel tanks.

At first glance, using a B777 front plan view and drawing a horizontal line between the bottom of the wing tanks just outside the left and right pylons with the line disecting the wing tank/CWT rib, the fuel column measured from the bottom skin up reaches two-thirds to the top of the wing tank/CWT rib (i.e. top wing skin). Only one-third of total wing thickness at the rib (from fuel level to upper wing surface) is filled with air.

I could place the drawing here, if I only knew how. Apparently i do not have an attachment option, which according to PPRuNe FAQ i should have.

Should not be too difficult to calculate fuel contents (taking the dihedral of the wing bottom surface into account) if exact dimensions of wing thickness at the ref. rib, chord of the rib between front and rear spar, and distance from rib to just outboard of the pylons are known.

Green-dot
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Old 13th Apr 2008, 18:04
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Interesting line of enquiry, you two, but I would suggest that the resulting figure might be slightly less than the amount on board. It seems likely that the wing bottom surface would have been experiencing positive TATs for at least 10 minutes before the photo was taken.

The aircraft left the Lambourne Hold, according to the AAIB, at FL90. The surface/sea-level temperature was +10C, suggesting something of the order of –8C at FL90. At a typical hold-exit speed of 210kts IAS, I think (in the absence of a Dalton computer) that this would produce a positive TAT. Perhaps someone can tell us? [It is conceivable that the TAT may have become positive prior to the speed-limit point, which would have been 8 minutes earlier. This may have been short-lived, however, and reversed as they slowed down into the hold.]

Time from LAM to touchdown is likely to have been 10 – 15 minutes, during which the skin below the dry part of the fuel tank would have been conducting relative heat towards the ‘wet’ part. This would be eroding the margin of the frost below the ‘wet’ part of the tank, I think. How much would depend partly on the fuel temperature; partly on the TAT.

You should treat whatever figure you arrive with as a MINIMUM estimate. Presence of frost means presence of fuel; conversely, absence of frost does not prove absence of fuel.
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Old 13th Apr 2008, 18:10
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snowfalcon2,

pertinent comments, with which I do not entirely agree, but for reasons unrelated to this thread I probably won't be contributing further.

PBL
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