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BA038 (B777) Thread

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BA038 (B777) Thread

Old 18th Feb 2008, 17:27
  #21 (permalink)  
 
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Originally Posted by Hand Solo View Post
Yes it is secondary, although it does have survivability aspects for the accident. For clarification I wasn't intending to suggest they contributed to the crash.
I think this might end up needing to be clarified a lot , even though the report also specifically says "This was not causal to the accident". Unfortunately it says that in normal type right before the big highlighted box with the safety recommendation in.

Since this is about the only clear conclusion and the only recommendation in the report, I expect it will get a fair bit of press attention and become causal in some peoples minds.

In terms of the rest of it, I'm with the poster who commented on the sound of head scratching. Even the comments on the cavitation damage to the pumps don't sound very confident (as if it might turn out to be a non-issue).

One thing that suprises me slightly is that the report says essentially nothing about flight crew actions (beyond moving the thrust levers) - there's been lots of speculation about whether they changed flap settings etc., none of which is answered in this report. I wonder if they think the crew actions are just not relevant to reveal at this point, or if there is still some uncertainty in that area.
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Old 18th Feb 2008, 17:57
  #22 (permalink)  
 
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I wonder if they think the crew actions are just not relevant to reveal at this point, or if there is still some uncertainty in that area.
One doesn't talk about the crew until you have exhausted all possible contributions by the hardware design and environment that the crew were operating with.
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Old 18th Feb 2008, 18:01
  #23 (permalink)  
 
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Fuel pump cavitation is a complex issue, it may take a while to sort that one out.
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Old 18th Feb 2008, 18:04
  #24 (permalink)  
 
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from the accident report...

After the aircraft crossed the Ural mountain range in Russia it climbed further to FL380 where the ambient temperature dropped to as low as minus 76C


What is the enviornmental envelope re min temps for the 777. I know the airbus 320 is -70c
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Old 18th Feb 2008, 18:14
  #25 (permalink)  
 
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HP fuel pumps on T7

Please can someone help me with the type of HP fuel pumps referred to; centrifical or positive displaced (lobe) type.
Thanks
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Old 18th Feb 2008, 18:30
  #26 (permalink)  
RMC
 
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FS, I agree that establishing the cause of fuel pump cavitation may prove to be complex. The bottom line however is that the only way cavitation happens is if the local fuel pressure drops upstream. The pressure at which this occurs is always between 0 and -30 inches of mercury (-14.7 PSI Gauge).

This pressure gets closer to zero at altitude.It is normally associated with G/A (high fuel flow) situations.

As niether of the above appear to apply then fuel would only cavitate if a restriction in the upstream fuel supply developed. One of the problems with cavitation is that, once the trigger pressure is reached, your fuel supply remains an excited mass of air bubbles until pressures significantly higher than trigger pressure.

As has been said it may be that this evidence of cavitation is eventually found to be unrelated to the incident...who knows?
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Old 18th Feb 2008, 18:52
  #27 (permalink)  
 
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maybe nothing but has no one thought about the fact that TOGA wasnt selected, or was it? seems to me only the thrust levers moved forward. trivial maybe but with TOGA, alot of things are then triggered so maybe a change of tact for the computers etc etc?
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Old 18th Feb 2008, 18:52
  #28 (permalink)  
 
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With regard to the evidence of cavitation in the area of the fuel pumps, as cited in the AAIB interim report:

"This could be indicative of either a restriction in the fuel supply to the pumps or excessive aeration of the fuel."

What could cause excessive aeration of the fuel, particularly when, by all accounts, it had been subjected to cold temperatures for an extended period of time?
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Old 18th Feb 2008, 18:59
  #29 (permalink)  
 
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I just drive - at 175' and 108 kts you're not doing much flying. AOA will be limited by the aircraft, you're in charge of roll, and that's it. Guessing that AOA limit, or stickshaker, kicked it off based on the wording of the report. It states the a/p "disconnected" without any inference of human intervention.

Mr Spotty M - Yes, a/p off and maintaining airspeed is critical. Below L/D speed(basically approach airspeed) gliding distance goes down rapidly, and at stickshaker speeds, or close to that, there will be no airspeed available to flare the a/c to reduce vertical impact loads.
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Old 18th Feb 2008, 19:00
  #30 (permalink)  
 
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HP fuel pumps on T7
Please can someone help me with the type of HP fuel pumps referred to; centrifical or positive displaced (lobe) type.
Thanks
"The gear-type HP pump gets fuel from the:
* LP fuel filter
* Servo return
* FMU bypass.
The HP pump supplies fuel to the FMU and servo fuel to engine
components."

From the 777 AMM for RR Trents.
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Old 18th Feb 2008, 19:03
  #31 (permalink)  
 
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BBC London News

They have just said "Damage to the fuel pumps caused the crash"

I'm certain I read in the report that the pumps would not stop working in this condition.

Last edited by zukini; 18th Feb 2008 at 19:05. Reason: spelling
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Old 18th Feb 2008, 19:11
  #32 (permalink)  
 
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Thats rather annoying given that on BBC1 News Tom Symonds was clear that the cause remained unknown. Perhaps Shoey will be along to comment.
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Old 18th Feb 2008, 19:16
  #33 (permalink)  
 
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The Fire Swith does close the Spar Valve!

Mono, Silent Badger, ...

If all things are still in one piece, the fire handle does close the spar valve on the B777 (see, e. g. the diagram in Section 6.7 of the Flight Manual. I only have seen it for the GE90 variant, but someone posted the equivalent for the Trent in a previous thread).

The report is quite clear on that, just read it carefully. In B777 aicraft not modified according to SB 777-28-0025 there are two separate wiring paths to the spar valve, one from the cutoff lever, and another one from the fire handle. The fire handle, in addition, also isolates the wire path from the cutoff lever, so that the cutoff lever will no longer be able to shut the valve.

Hence the original Boeing checklist for evac and engine fire specified to close the cutoff lever first, and only then activate the fire handle. The BA checklist split the two tasks between the Cpt and F/O, thus creating, as we would say in computer science, a "race condition".

In the case of BA038 the path from the fire handle to the spar valve was damaged, and although the path from the cutoff lever to the valve was ok, it was isolated by the fire handle, which on this occasion was activated before the cutoff lever.

This left the spar valve open, which, in combination with other damage, caused fuel to leak from the aircraft, which, luckily, in this case, did not cause a fire.

With the desirable sequencing (cutoff first, then fire handle) the spar valve would have closed. Althought not causal to this accident, it would have reduced the risk of fire, and therefore merits the safety recommendation.


Bernd
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Old 18th Feb 2008, 19:18
  #34 (permalink)  
 
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I was surprised to see the reference in several places in the report to Jet A-1, my understanding was that Chinese aviation kerosene was called Jet Fuel Number 3 (used to be RP-3). I thought initially that maybe what they were trying to say was that it met at least the spec of Jet A-1 (with which the world is more familiar), but they bang on about Jet A-1 spec of -47deg C and the fact that the actual fuel had a spec of -57 deg C (which is more or less what you would expect from RP-3/JF #3). Unless of course its actually RP-3 marketed as Jet A-1. Doesnt really matter in terms of causation I suppose.

How can you tell if cavitation has occurred post event? Does it actually physically damage the HP rotor/vanes? I thought that once the pressure had been equalised the cavitation stopped and flow resumed?

The debris findings are interesting, particularly the red scraper under the right suction screen. Just supposing one wing fuel suction is suddenly inhibited, notwithstanding the fact that the autothrottle demanded an increase from both engines, in the configuration/mode that the aircraft was in, does the engine management software also limit the differential thrust between the engines. You can see what I am getting at - the seven second difference - could it be that the right engine was starved and the left engine reduced to match, under some limiting algorithm, independant of commands from the autothrottle.

I'm an oilhead so tell me if I am talking cr*p. It wouldnt be the first time.

Pinkman
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Old 18th Feb 2008, 19:22
  #35 (permalink)  
 
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London news update transcript (SKY+) broadcast at 8pm

"It was damage to fuel pumps that caused a BA plane to crash land at Heathrow last month. 136 passengers and the crew on board were safely evacuated"
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Old 18th Feb 2008, 19:23
  #36 (permalink)  
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BBC newsroom has been contacted by a pal, and further bulletins should not repeat the fuel pump error (also heard on R4 here).

R
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Old 18th Feb 2008, 19:35
  #37 (permalink)  
 
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Originally Posted by Pinkman
How can you tell if cavitation has occurred post event? Does it actually physically damage the HP rotor/vanes? I thought that once the pressure had been equalised the cavitation stopped and flow resumed?
(To correct a possible misunderstanding first: this is about the high-pressure fuel pump, not the HP rotor of the engine proper.)

Cavitation is the formation and subsequent collapse of small cavities (fuel vapor, dissolved air, ...) in a liquid due to low pressure, e. g. on the suction side of a pump. The collaps can be so violent that it damages the material. It is one of the limiting factors in the design of ship propellers.

does the engine management software also limit the differential thrust between the engines.
The aircraft is designed to handle TOGA power on one engine and zero thrust (shut down/windmilling) on the other. The rudder will compensate. Also, the engine management of each engine is independent of the other.

Would be quite interesting during an engine failure at takeoff or go-around, when the computer would limit the other engine, too, depriving the aircraft of full power when it most needs it.

You can see what I am getting at - the seven second difference - could it be that the right engine was starved and the left engine reduced to match, under some limiting algorithm, independant of commands from the autothrottle.
No. On the contrary, the autothrottle, trying to maintain speed, would increase thrust on the other engine to compensate for the failed one.


Bernd
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Old 18th Feb 2008, 19:37
  #38 (permalink)  
 
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"What could cause excessive aeration of the fuel"

AFAIUI, it can be caused by the fuel flow being restricted, especially to a fast running pump.
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Old 18th Feb 2008, 19:40
  #39 (permalink)  
 
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Here's my theory then:
(bearing in mind I have no knowledge of the 777 fuel system - but that won't stop the D*ily M*il from quoting it):


Some seals/joints were deformed during the extreme low temperatures earlier on in the flight. The seals however had lost flexibility and still maintaining a seal, or at least weeping only slightly. As the atmospheric pressure was low at altitude, the weeping would be from the fuel system to the outside.

As the aeroplane descended, the seals warmed up, regaining flexibility then flicked back, but this caused an inleakage of air into the fuel lines. The inleakage of air caused the pump cavitation observed.

By this point also the water that would have been thawing in the tank at this point - perhaps by putting the landing gear down it caused more condensation to drop from the walls of the fuel tanks.

Anyhow, going back to the air. As the air was drawn into the inlet side of the high pressure fuel pump, this induced sudden foaming exacerbated by the presence of the water in the fuel lines. This choked the fuel pumps and disrupted fuel supply.

Tiggermoth

If this had been the case, then the way to have recovered from the situation would have been to stop demand on the fuel system, allowed the fuel to fill the suction line, then slowly put an increased demand on the pumps while throttling back the discharge side. As the autopilot just put a greater demand on the fuel system, it just made it worse.
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Old 18th Feb 2008, 19:41
  #40 (permalink)  
 
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BoeingMEL wrote: "............ at least all that garbage about EMI can be buried..... as can fuel exhaustion, windshear, finger trouble, Gordon Brown's heavies..."

To be fair, the report doesn't appear to rule any of that out...

Until the investigation is concluded anything is a possibility!
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