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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, 13:12
  #81 (permalink)  
 
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Therefore and nevertheless now another hypothesis concerning systems: If i go about the RR Trents on the T7, there is a possibility of no malfunction but still having the effect described. The engine anti-ice is mostly switched to "AUTO". I have witnessed its reluctance to go to "ON" more than once. Lets assume the inlet PT probe was iced up, apparently a ice warning was out that day and prolonged holding in the FL80 to 120 is a ice prone region. As the EEC uses EPR as parameter for thrust setting, in conjunction with the thrust lever angle, a demand either by the auto throttle or the pilots might have been ignored by the EEC, simply because with a PT inlet iced up and PT outlet not, the difference would fake "differential pressure", thus thrust, to the EEC. It would say "I am already giving you what you're asking for", not increasing FF and the result would be the low thrust setting persisting with a functioning and happy EEC.
Far fetched?
Are the T2P2 probes only heated when ice is detected? I think not. Does the FADEC only use EPR or does it use N1 also?
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Old 20th Jan 2008, 13:15
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Pitch control

As an A330 pilot I am most interested in the aspect of 'pitch control' during the seconds after the double engine loss.

I do not want to start another A vs B discussion but want to ask a question about the 777 flybywire.

1. I do not think that pulling your aircraft (any aircraft) into a stall at the last moment, will postpone your impact. The impact will be earlier instead.

2. Loss of all engines during approach is not trained in the simulator, because it is regarded as negative training; there is no way you can prevent a crash landing.

3. The confrontation with loss of all engines during approach in real life, will make your brain and thus body go into survival mode; only the very most essential tasks will be executed. (Making a PA call is certainly not one of them)

4. While in survival mode, I can imagine the brain is having difficulties making smooth control inputs. (As with making a smooth and firm brake application during an emergency stop with your car.) I think on the Airbus the FlybyWire system would give you a "superhuman" postponement of the impact in this situation, just by pulling the stick fully backwards all the time. (This is why you have to do exactly this after a terrain warning)

While waiting patiently on the investigation what caused the engine problems, my question is: Is it possible to stall a B777 with manual inputs?

Last edited by astonmartin; 20th Jan 2008 at 13:29. Reason: typo
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Old 20th Jan 2008, 13:24
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Hoppy, you're right, the inlet PT is heated anyway. However the Potomac incident showed us that icing in the inlet region can lead to misreadings of the inlet pressure. That intrigued me then aswell, leading to my raising the issue today.
The Trent takes only EPR in the nomal mode, N1 when reverted (soft or hard) to the alternate mode. On the GE it takes N1 for normal.
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Old 20th Jan 2008, 13:24
  #84 (permalink)  

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The AAIB initial report did not say that there was a flame out, only that the thrust levers failed to respond to a demand for increased power. A couple of newspapers made the same statement today.

As I read the AAIB statement the engines were still running at some at preset indeterminate (low?) power setting. This raises the question as to when the APU started up automatically and what caused it to do so, and did the RAT extend and for what reason. If they had both activated during the approach rather than on touchdown it might just identify the cause of the "hung" engines. I remember some years ago on the 767 someone had malfunctioning N1guages pulled the CB to reset them and the RAT fell out as it sensed a double engine failure.

AAIB report:-
"At approximately 600 ft and 2 miles from touch down, the Autothrottle demanded an increase in thrust from the two engines but the engines did not respond. Following further demands for increased thrust from the Autothrottle, and subsequently the flight crew moving the throttle levers, the engines similarly failed to respond."
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Old 20th Jan 2008, 13:30
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Don't know if the APU did start or not. APU inlet door is open, APU wont start until it's open, maybe loss of busses AFTER 'landing' caused APU to attempt auto start...
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Old 20th Jan 2008, 13:49
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BBC reports of Sunday Mirror interview with SFO

BBC News has the following quote from a Sunday Mirror interview with the SFO:

"Suddenly there was nothing from any of the engines, and the plane started to glide," he said.

"I didn't think we'd clear the fence at first. As we landed I was bracing myself for an enormous thud. But instead of one thud, there was a series of thuds as it bounced along the grass."

He added: "Eventually it shuddered to a halt. While I was trying to stop the plane. I struggled to try and keep it in a straight line."

He said that after the crash "there was no sound at all. No sound from the engines, no sound from behind.

"I turned around and composed myself and heard a lot of commotion behind me. I realised that staff were trying to carry out the drills to evacuate all the passengers."
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Old 20th Jan 2008, 14:07
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Wingswinger:
Does anyone else think that this might call into question the whole concept of the CDA?
The intent of Continuous Descent Approaches (CDA) was initially to reduce noise and has morphed to include reduction in fuel usage (and therefore emissions). Trials by SAS and others in Australia have show savings of between 500Kg and 1000Kg of fuel are possible per approach for some CDA variants (obviously dependent on aircraft type and the normal non-CDA procedure with which it is compared).

Unfortunately, the definition of CDA is vague in the extreme. Some CDA like those at LHR are effectively interceptions of the glide slope at well above normal heights after a normal descent. Others involve continuous descent from last en-route fix at flight idle (or more) to intercept the glide slope from above. AFAIK once established on the glide slope the aircraft will fly the descent according to operators SOPs or the ANSP requirements for that airport. In all cases that I am aware of the aircraft would thus be stabilized in descent as they are now with sufficient power to fly a normal 3 or 2 1/2 degree glide slope at least at the same distance from touchdown as they are now.

Therefore I do not see that CDA per-se would have any bearing on this although it is possible that the BA or NATS procedures for flying them at LHR might have some impact. However, as these procedures have been flown successfully by B777s for some time - including apparently the aircraft landing ahead - I doubt that there is anything in these procedures that had a direct causative relationship to the accident.
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Old 20th Jan 2008, 14:13
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my two penn'orth after silently reading every post

I make no apology for posting what some of you may brand "speculation" on a website named "Rumour Network". Surely every rumour is speculative.

I have read and considered every single post in both threads, and frankly I feel I have suffered just as much wasted bandwidth (and irritation) from the self-congratulatory pomposity of the "in-crowd" criticising the garbage of the ignorant as from the garbage of the ignorant itself.

Particularly inappropriate, in my opinion, was the endless praise for flight and cabin crew. Until we know what happened it is no more appropriate to broadcast such assumptions as truth than it is to broadcast technical assumptions about failure modes as truth. Those blindly praising colleagues on no evidence, and even castigating others who fail to do likewise, are really just praising themselves, no ?

I agree that once we learned of the simultaneous double "failure to respond" it did begin to look likely that the flight crew did well from that point on, but it is not yet totally certain that the problem was not exacerbated by crew actions or omissions. Only the report can tell us how much praise or blame is due, and until then anything else is correctly categorised as what you all so hate - "speculation".

Enough of that - here is my very own bit of "speculation" :

Did anyone else notice post 59, and think it might be significant ?

Here it is :
"And the airtemp over UK at the time was some of the coldest I've ever experienced with OAT at higher levels down to -70 degrees celsius."

This sounds as though this was pretty abnormal. How abnormal ? Could the fuel have got below freezing point ? Is there only one fuel temperature sensor ? Could it (or even both) have failed ?

Alternatively, we have heard doubts expressed about the quality of Chinese fuel. Could it have been out-of-spec, so that it froze at, say -40degC instead of the required -47degC ? Presumably the sensor would have no way of knowing this, and if it works on a 3deg margin, it would have issued no warning.

Do these two things, taken together (or even the first point on its own), suggest fuel waxing as a cause?

Previous posters have judged it unlikely that fuel waxing or other contamination could cause two engines fed from different tanks to suffer flow restriction more or less simultaneously, and ruled it out of consideration on the basis of that simultaneity.

But perhaps they didn't get into difficulty simultaneously. If they had been close to idle or at fairly low fuel flow for several minutes, they could have got into difficulty that many minutes apart. Only the call for power which revealed the problem was simultaneous on both. The actual fuel flow restriction need not have commenced simultaneously at all.

So you see, I am hypothesising about possible causes. I am inviting a discussion of evidence for and against. Perhaps what I suggest can be demolished by the more knowledgeable at a stroke, in which case thats fine, the picture becomes a little clearer.

I understand that for too many here the rule is

"What I post is calm analysis, what you post is inappropriate speculation, what he posts is ignorant rubbish"

...and yes, there is much rubbish and (inevitably) much duplication (not everyone can read every word of both threads)...... but in my opinion the policy of allowing open posting gives us more than we lose - and the truly useless postings would waste less bandwidth if others could refrain from adding to the dross by complaining about them. If you watch you will see that the worst do get periodically culled.

So what do you think ?

Freakily cold conditions? Poor fuel freezing too easily? Fuel waxing not simultaneous, but simultaneously revealed ?
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Old 20th Jan 2008, 14:17
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With a fuel issue, albeit 1) lack of 2) contaminated or 3) cold soaked, I can't see how they would fail almost simultaneously. I would have thought there to be at least a couple of minutes between them. Each engine doesnt burn the exact same amount at the exact same time so I would have expected a lag.
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Old 20th Jan 2008, 14:26
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as fg32 said, if fuel waxing occured and affected the engines ability to recieve fuel, it's likely that the situation developed at different times in each engine during descent, but it was only readily apparent when a large thrust increase was demanded, which the engines where then unable to comply with due to restricted fuel flow

seems well within the realm of the possible
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Old 20th Jan 2008, 14:35
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You might all be wrong.

It is not at all unplausible that ATC at LHR has "maintain 160 to 4" which means that at 4 You slam the throttles back to idle to reduce to, what, maybe 138 for the landing. The autothrottle is as stupid as the train (it only follows the track) it only demands more power when the speed drops below placard speed. If by some mysterious reason a spurious signal has told the engines to spool down to ground idle instead of flight idle it takes a considerable time to accelerate them up to full power, and they will most likely not do that in syncron. And when the speed drops below minmum for flaps full, 5 seconds is like an eon, if You try to maintain glidepath You will get too high angle of attack and then loose even more speed, so, unless there is something untold, They did a good job flying the airplane onto the ground rather letting it fall out of the air.
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Old 20th Jan 2008, 14:41
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Originally Posted by astonmartin
[...]
my question is: Is it possible to stall a B777 with manual inputs?
Yes.

The Flight Manual states in the section about pitch envelope protection:

Stall protection reduces the likelihood of inadvertently exceeding the stall angle of attack by providing enhanced crew awareness of the approach to a stall or to a stalled condition.

[...]

The pilot must apply continuous aft column force to maintain airspeed below the minimum maneuvering speed. Use of the alternate pitch trim levers does not reduce the column forces.
Mernaing with sufficient force on the column the handling pilot can stall the machine, the protection system only provides enhanced crew awareness against inadvertent stall by pilot inputs.

In addition, at the Introduction to the Flight Control System Overview (FM, Section 6.9) it states that

The pilot always has ultimate control authority; the flight control computers cannot override a pilot command.
I'm not judging the different systems, only noting that this is different from Airbus FBW pitch protection, which will not allow stalling the aircraft by pilot input.


Bernd

Last edited by bsieker; 20th Jan 2008 at 14:56. Reason: Added Paragraph, Typo
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Old 20th Jan 2008, 14:47
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Any Back-up Fuel Control?

Would the crew have had the ability to assume any manual or secondary control over the engines? Is there any option to disengage the FADEC, or use it in a degraded mode? I realize they had very little time, but am just trying to understand whether this would even have been possible. Hawk
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Old 20th Jan 2008, 14:47
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I've been flying in that cold air mass (-69C at worst) and was faced with low fuel temperatures (-40C at one stage) but noted that the fuel was considerably above this temperature by the time we arrived in the London area. Given the aircraft in question also spent some time in the LAM hold below FL150 I'd be rather surprised if fuel waxing was present.
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Old 20th Jan 2008, 15:04
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Waxing seems plausible. Do we know how similar engines and fuel systems accommodate waxing?

For a moment, let's assume that waxing was the problem. - What needs to be done now, to prevent it happening again on the next flight from China?
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Old 20th Jan 2008, 15:08
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Originally Posted by Hand Solo
I've been flying in that cold air mass (-69C at worst) and was faced with low fuel temperatures (-40C at one stage) but noted that the fuel was considerably above this temperature by the time we arrived in the London area. Given the aircraft in question also spent some time in the LAM hold below FL150 I'd be rather surprised if fuel waxing was present.
Once cold soaked, fuel can stay cold for a VERY long time. It takes a long time to cool down and a long time to warm up (Ive seen the underside of the wing on my aeroplane covered in ice for some considerable time after landing in the Arizona desert).

I'm not speculating here - I'll wait for the report. However, low quality fuel or the use of Jet A as opposed to Jet A1 has caused me some "inconvenience" in the past.
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Old 20th Jan 2008, 15:14
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I've seen the ice too, but there's a big difference between underwing icing (fuel below 0C) and fuel waxing (fuel a loooong way below 0c). I flew largely the same route and profile as the accident aircraft and made a point of noting the fuel temperature once we landed, which was around the -25C mark. If fuel waxing occurs at this high a temperature then I'll stand corrected but it was always my understanding you'd need to be well on your way to -40C before waxing was an issue. For info top of descent on this route to touchdown is a typically 40 mins in increasingly warm air.
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Old 20th Jan 2008, 15:14
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astonmartin
Don't know about the 330, but a double engine failure in the A320 (i.e. AC Bus 1+2 fail as well) leads to Alternate Law - so bang goes your stall protection. Is the 330 different?
But if one/both engines are running but fail to respond, Normal Law protections would still be there.
TP
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Old 20th Jan 2008, 15:14
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I have total confidence in the 777.

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.

It is a superb piece of kit, I've had a very happy flying career to date on it. Believe me when I tell you this is going to be a (dramatic!) glitch which will most likely never be seen again.
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Old 20th Jan 2008, 15:23
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On the move again

As I watch, she's trundling along at a surprisingly good lick on an array of huge trailers
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