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AAIB initial report out on BA B777 crash at LHR

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AAIB initial report out on BA B777 crash at LHR

Old 20th Jan 2008, 15:27
  #101 (permalink)  
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The 777 will fly a three-degree glideslope engines-off clean at 220kts. The art of getting it on the threshold is when to configure. I wouldn't fancy losing (or partially losing) both engines at 600ft though. Fantastic job.

(MLW is either 201t or 208t, depending on the version - MM was 208.6t)
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Old 20th Jan 2008, 15:32
  #102 (permalink)  
 
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You Gimboid (post #100) -

The parameters you gave, 13 miles at 6000' and 250Kts, is an energy state higher than almost every approach we do in real life. Therefore, IMO, it's not quite realistic and is obviously a completely different scenario then the BA 38 incident.

Every airline I've 'dead sticked' in the sim makes it to the runway from 2000' on G/S at 250 kts. If you're FMC is working you can program that into the box or just do the math in your head (727, DC-9, etc).

The newer jets, being much 'slicker', arrive with more than enough airspeed while 727's just barely made it.

Based on my observations, without very strong winds aloft, I think, almost every jet tracking a glideslope @ 250 KTS has a fairly high chance of success. Next sim session I'll try it from 20 DME and 250kts and figure out how high I have to be to have enough energy to reach the field.

Modern airlines glide at about 18:1. In simple pilots talk that's your altitude in thousands x 3 IE, 30,000 becomes 30 x 3 = 90 n.m.
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Old 20th Jan 2008, 15:41
  #103 (permalink)  
 
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Lets say the fuel was contaminated with a large quantity of water.

The water sinks to the bottom of the fuel tank and after 11 hours in the air you would be getting pretty close to powering Rolls Royce's finest on Evian.

Sabotage or HF?

Interesting times ahead for the AAIB.
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Old 20th Jan 2008, 15:54
  #104 (permalink)  
 
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Grounded?

If this was due to a possible electronic/software glitch??????? Why have all 777's not been grounded, pending a more defnitive cause? I would suggest more than a few nervous 777 crews operating at the moment, not to mention pax!
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Old 20th Jan 2008, 15:55
  #105 (permalink)  
 
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The water sinks to the bottom of the fuel tank and after 11 hours in the air you would be getting pretty close to powering Rolls Royce's finest on Evian.
OK I'll bite! Are you insinuating that fuel is somehow drawn from the top of the tanks during flight?
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Old 20th Jan 2008, 15:55
  #106 (permalink)  
 
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Fuel freezing

At last something I can comment on with authority. Yes, Teal, it can happen but it is incredibly rare. I cant remember the last time I heard of it happening. Moggiee is correct - Jet A (US) has a higher freeze point than Jet A-1 (i.e. it freezes more readily). The current spec for the UK is what used to be called DERD 2494 now renamed DefStan 91-91. The spec is - 47 degrees Celsius. The problem with the freezing /waxing theory is that Chinese jet A-1 equivalent (used to be RP-3, now called jet fuel number 3) typically has a lower freeze point (-52C) than Jet A-1.

Thats not to say it could not have happened or that the particular delivery could not have been waxy. But there will have been retention samples taken and freeze point is a test that will have been carried out by an approved lab. BA do take a keen interest in the fuel that goes into their aircraft and I remember being on the recieving end of an assessment visit in Nairobi with painful clarity. It resulted in us buying a whole bunch of new equipment.

The problem is that the normal freeze point for Kerosene is minus twenty or so. Dual purpose kerosine, which can be used in both aircraft and in domestic appliances has both a low freeze point and a high smoke point.

Mistakes can happen.....

P
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Old 20th Jan 2008, 16:27
  #107 (permalink)  
 
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In the sim we did a deadstick autoland from 13 miles out, 6000ft and 250kt. It tracked the 3deg ILS all the way down, we just dropped flaps and gear on the speed schedule. It has a max landing weight in the region of 195 tons, yet the Vref at this weight is less than a 737. It has superbly efficient wings, & is very slippery for a big beast.
No doubt the 777 has efficient aerodynamics, but as I am sure you're all too well aware that as ATC we're rather limited against allowing you to fly at 6000 feet to capture an ILS at 13D, and you'll probably be back at 180kts or thereabouts. In fact, if I have any pilot close to 3000' at 10 miles, they're asking for further descent, so I am not sure this is a relevant example.

Now, I also have a question - from 13D, the 3 degree glide you state your sim tracked all the way down would still have you around 1866' AGL at the threshold (13Nm x 318' per mile descent = 4134 feet of descent). I take it you didn't track the glide at all, just happened to intercept it at around the TDZ, so meaning in this case the 777 DID NOT maintain a 3 degree slope with a double engine failure (which, in this accident, was not what happened anyway). In fact, 6000' in 13 miles is nearer 4.4 degrees (tan-1 461'/6076). The point I am trying to make here is that in the example you gave, using your figures, it was not a 3 degree path that was followed, and in a real world example where you are lower and slower (3000 AGL and 180kts or less, and just about to intercept the glideslope), I doubt the 777 could maintain a power off 3 degree glide.

Finally, correct me if I am wrong (I left engineering without 777 experience), but in every other type I have known, for an autoland, the buses need to be separated, which means 2 independant forms of power. In a twinjet, I would take this to mean a "deadstick autoland" is not possible?

The last point I'd make is that the a 737 at 195 tons would, no doubt have a very high Vref, but I guess Boeing haven't stretched it this far yet?

Last edited by slink; 20th Jan 2008 at 17:05. Reason: Clarify glide question
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Old 20th Jan 2008, 16:32
  #108 (permalink)  
 
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There but for the pace of change...

All I have to say in the matter is "Thank God For good old ILS"

All ILS localiser antenna arrays, need a large protected sterile 'Critical Area' in front of them for the guidance beam to correctly form. Usually this is nothing more than an expanse of grass on which the airport operator can do nothing but regularly mow when operations permit. It is, in this case, the 09R Loc Critical area which presented this 777 with a forgiving, spark supressing, fuel absorbing, cushion.

MLS however, which everyone thought would have replaced ILS years ago, should be able to operate with a much reduced critical area, allowing who knows what to be done with these patches of valuable real estate.

Sometimes the non progress of technology is safer.
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Old 20th Jan 2008, 16:32
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The point here is that, according to the program, the Russian accident investigator maintained that there is more than 1 reason in every accident. In this case, the thing which strikes me was the discovery that the Autopilot on A310's is partially disconnected (some control surfaces) if you "fight" against it for more than 30 seconds-
Well, not sure about the A310, but with every other type I have experienced, the autopilot disconnect is immediate once breakout force is exceeded - ie, give the controls a good heave, and the autopilot will be disconnected. It's something we used to test on the 737s in the hangar with a good old fashioned fishing spring balance!
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Old 20th Jan 2008, 16:37
  #110 (permalink)  
 
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just tried it in the 777 sim...

... and we crashed exactly where they did...

gross weight 200 tons, 143 knots on the glide at 2 miles exactly and shut down the engines. neither the RAT nor the APU has time to start before impact which means only the captains flight instruments are showing. if the f/o was indeed flying, at least one engine was pulling enough power to keep a generator on line and give him flight instruments to look at. sim visual data base has three low buildings at about 1 mile and we cleared those by maybe 10 feet ....don't know if those exist in the real world.
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Old 20th Jan 2008, 16:38
  #111 (permalink)  
 
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Today, 08:59 #76 Wornout Rubber
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Woodpecker,

The reason why 767s get colder fuel temperatures than 777s in the same enviroment, is that fuel temperatures are closely (but not exactly) related to the TAT and not the OAT. The higher speeds on the 777 keep the fuel warmer.


Fuels system engineer working on new jet a/c about 10 yrs ago told me wing thickness was important.

777's typically cruise 2-4000' higher on the same stage length. That temperature difference, below FL370 typically, is warmer for the 767.

You TAT theory can be tested. Fly the same altitude at increasing Mach speeds and note the temperature rise. I'll try it on my next flight. I'm guessing the difference will be very small.
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Old 20th Jan 2008, 16:40
  #112 (permalink)  
 
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Dropp the pilot -

A more accurate scenario would have been to leave the engines at idle. The engines were running, they just 'didn't respond' when increased power was called for.
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Old 20th Jan 2008, 17:37
  #113 (permalink)  
 
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I need more convincing on the cold fuel theory. Even super cold soaked fuel from a long sector has warmed a fair bit once you have been through the stepped descent of the London TMA. I never have a cold soaked fuel problem in the UK, its always elsewhere when you get large straight in descents and the fuel doesnt get a chance to warm.
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Old 20th Jan 2008, 17:49
  #114 (permalink)  
 
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slink.... somne good/interesting points you make, which I shall try to answer. Not really relevant to this incident, so we'll probably sent to the "stupid questions" thread

At "idle" most modern jet airliners plan a 3 degree descent from Cruise altitude. We tend to be above optimum speed, so changing the "idling engines" to "stopped engines" then a 3 degree glide is just about possible, clean (1 in 3 i.e. 3 miles / 1000').

For Engine Out planning, we'd rather go for somewhere 1 in 2, so keeping something in hand for maneouvre... and plan on ~2500' for a 180 degree turn. We then hold the 1:2 approach with some flap ~180K and this needs some speedbrake.... and then ~1500' think about gear, rest of flap and flare. It is eye opening the change at this point i.e. the effect of the gear and extra flap and you do not want to this bit too early

Finally, correct me if I am wrong (I left engineering without 777 experience), but in every other type I have known, for an autoland, the buses need to be separated, which means 2 independant forms of power. In a twinjet, I would take this to mean a "deadstick autoland" is not possible?
You are correct, but only as far as the rules go Airbus AP will quite happily fly engines out, APU on, and really gives you some "capacity". You would normally want do the landing manually, but 1 trainer with too much time on his hands used (in the Sim) to show you could do the 1:2 profile, when the gear went down, also increased RoD and picked up the 3 degree ILS slope (very late!) to an Autoland...

NoD
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Old 20th Jan 2008, 17:55
  #115 (permalink)  
 
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Fuel Config

Just a thought. On the 747-400 at the end of a sector in the normal fuel config, you basically have the four engines each feeding off four independent tanks.

Don't know the tank layout of the 777 but the basic philosophy would still hold true, each engine would be feeding off its own independent tank.

If you are suggesting fuel contamination or water droplets, it would mean that this manifested itself at virtually the same time in each tank?

As an aside, I agree with the suggestions that if it had happened further up the slope the keeping 200 plus kts and intermediate flap and gear up would allow you to basically fly the glide slope or more sensibly, a few dots high, and drop the gear at 500ft.

For this to happen at 600ft, gear down and land flap, there are very few options......sometimes god is a real ****.
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Old 20th Jan 2008, 17:58
  #116 (permalink)  
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I'm puzzled by the concentration on fuel problems as the most likely cause of this accident. I'm a software engineer and not an aeronautical one, but it seems to me that the failure of both the engines to respond to either autothrottle or manual throttle input is just as likely (if not more likely) to be caused by a fault in some common hardware/software subsystem. I started doing a bit of digging around to try to find some details of the architecture of the systems on the 777, and found some interesting references which I've given below. There is at least one large, centralised component that has a part to play in thrust management - the AIMS (Airplane Information Management System). I'm not saying that the AIMS *is* the cause of the problem, just that there *are* common subsystems involved in managing the engines.

BOEING 777 AIRPLANE INFORMATION MANAGEMENT SYSTEM OPERATIONAL EXPERIENCE
http://ieeexplore.ieee.org/iel3/5023...rnumber=635042

The 777 AIMS combines primary flight display, navigation display, EIC.AS display, flight planning, navigation, performance management, guidance, thrust management, central maintenance, airplane and engine monitoring, communications management, digital flight data acquisition, and data conversion gateway functionality into a single integrated system.

[...]

Thrust Management
The thrust management function sends commands to the autothrottle servo motors to move the airplane throttle levers, displays the thrust limit and autothrottle mode, and sends trim commands to the electronic engine controllers.

[...]

AIMS contains more than 600,000 lines of software
Developing the 777 Airplane Information Management System (AIMS): A View from Program Start to One Year of Service
http://ieeexplore.ieee.org/iel4/7/12850/00588382.pdf

The robust partitioning provided by the architecture allows applications to use common resources without any adverse interactions. This is achieved through a combination of memory management and deterministic scheduling of application software execution. Memory is allocated before run time, and only one application partition is given write-access to any given page of memory. Scheduling of processor resources for each application is also done before run time, and is controlled by a set of tables loaded onto each CPM and IOM in the cabinet.
I'm sure the system has been written extremely carefully and tested thoroughly, but there's always the potential for bugs in any piece of software.
 
Old 20th Jan 2008, 18:20
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Some unique shots here

Last edited by JimBall; 20th Jan 2008 at 18:39.
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Old 20th Jan 2008, 18:31
  #118 (permalink)  
 
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Why are some people still going on about fuel icing, contamination etc etc.

It is clear from the report that the engines did NOT fail - they just did not respond to autothrottle / thrust lever commands when the thrust was supposed to come up at 600ft.

I for one will wait for the official report and see what BA tells us in the meantime. I agree that this will probably turn out to be a one-off event - the 777 is a well engineered and very well thought out aeroplane.
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Old 20th Jan 2008, 18:38
  #119 (permalink)  
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Fuel Waxing - the mechanics of it.

I'm not familiar with the chemistry but when the fuel waxes, does it then remain in that waxed stated regardless of an increase in its temperature? i.e. does the wax remain in suspension within the fuel? Logically, as the fuel temperature increases during descent, it will revert back to its non-waxed state - but is the transition linear or does it have to be warmed "more" to change back compared to the amount of cooling originally required?

The reason I ask is that a period of time in the LAM hold requires a thrust setting probably above final approach thrsut and the fuel will only be getting warmer by the time it's on approach.

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Old 20th Jan 2008, 18:50
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My old Airbus is prone to ice on the exterior of the Outer fuel tanks.This happens on even quite temperate days and we don't fly very high so are not exposed to very low temperatures or for 10 hours!The only way to stop it forming is to create an air gap in the tank by feeding fuel from that tank,so that some warming can take place in the descent.Many times I have sat on a very warm ramp an hour after landing and still seen ice on the skin in the tank area.There is no indication of fuel temperature in the cockpit.Cold soaked fuel takes a long time to warm-certainly more than the 30 minutes a 777 would take to get from top of descent to the ground.
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