BA038 (B777) Thread
 

A special bulletin has been released today by the AAIB:

here


And the update of 12th May 2008

Interim report 1 09/08

Interim Report 2 03/09

Interesting 'snippets' from the report


"Detailed examination of both the left and right engine high
pressure fuel pumps revealed signs of abnormal cavitation
on the pressure-side bearings and the outlet ports. This
could be indicative of either a restriction in the fuel
supply to the pumps or excessive aeration of the fuel. The
manufacturer assessed both pumps as still being capable
of delivering full fuel flow."

"Investigations are now underway in an attempt to
replicate the damage seen to the engine high pressure
fuel pumps, and to match this to the data recorded on the
accident flight. In addition, comprehensive examination
and analysis is to be conducted on the entire aircraft and
engine fuel system; including the modelling of fuel flows
taking account of the environmental and aerodynamic
effects."

update -- heathrow crash landing
 

http://www.aaib.dft.gov.uk/cms_resou...8%20G-YMMM.pdf

what does everybody think?
ian shoesmith
bbc news
[email protected]

"spar valves"
 

can somebody give an explanation of what these valves are, and how they play an important role, please?

I think the previous poster who suggested fuel starvation is reading something into the report that isn't there. There's nothing I can see in this report that gives an indication of the cause but it does rule some things out. As to the debris found in the tanks, it's undesirable but not uncommon or necessarily dangerous. Aircraft have been leaving the Boeing plant with bigger bits of FOD than those in the tanks for years.

Shoey - the spar fuel valve is in the fuel tank and the fuel passes through this first on the way to the engine. It then passes through various fuel pumps, heat exchangers and other paraphernalia before it arrives at the engine fuel valve, through which is passes into the engine to be burned. It would appear in this instance that there was a rupture somewhere between the engine and the spar fuel valve, allowing fuel to leak through the open spar fuel valve and out of the system even though the engine fuel valve was closed.

So, with the exception of the HP fuel pump damage, which might be due to insufficient fuel supply or aerated fuel (or something else which escapes me just now) everything was working normally. The FOD in the tanks is probably not sufficiently large to block any of the exit screens/pipes, so what else could cause fuel starvation?

I can almost hear the sound of heads being scratched from here.

Though I must say that every a/c I have worked on. Pulling the fire handles activates the LP (spar valves) and HP (engine fuel valves)

You could have knocked me over sideways when I read that they weren't on the 777!!!???

isn't this secondary to the ground impact and not in the causal chain of the accident?

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.

well there's one good thing...........
 

............ at least all that garbage about EMI can be buried..... as can fuel exhaustion, windshear, finger trouble, Gordon Brown's heavies... (IMHO you understand) Cheers bm:hmm:

Yes it is secondary. The reason the report brought it up is because of the fuel that leaked during the evacuation.

The evacuation checklist aims to shut off things that could lead to fire or other dangers. BA's evacuation checklist has the captain switching off the fuel valves whilst the F/O does the engine fire handles which meant that if the F/O did his bit before the captain then the electric relay controlling the fuel valve would be isolated and the valve didn't close. So the report has issued a sensible recommendation that operators make sure their checklists do things in the right order. It also mentions that Boeing were already in process of changing all 777s so that the fire handles close the fuel valves as well as all the other disconnections so you wouldn't need to do switch the fuel valves separately.

Most other aircraft already have the fire handles/buttons doing all the isolation in one fell swoop already.

Cheers,
SB

The report also mentions a planned descent to FL315 due to "extreme cold"....which did not happen because of an ATC request.

Also mentioned was that later in the flight at FL380 over the Ural mountians the oat was -76. The met office decscribed this as "unusually low compared to the average, but not exceptional"

They seem to be considering the possibility of fuel icing, however the fuel never got below -37.

Puzzling indeed.

What occurs to me when reading this report is just how intensive and detailed an accident investigation has to be.. and should humble some of the 'contributers' here who are repeatedly whinging about the lack of results... additionally, the AAIB is dealing with an a/c for the most part in one piece... just how awsome their task must be when dealing with scattered/burned wreckage begins to become apparant..

I would hazard a guess that they had ruled out fuel icing based on the information in the report. The fuel never got lower than 20C above freeze point and I'm sure the inclusion of the information is to clarify that fuel freezing was highly unlikely to be a factor, just like they included the information about water in the tanks.

A/P kicked off at 175'. Airspeed down to 108Kts by 200' as the a/p tried to maintain the g/s.

Thank god the a/p kicked off.

MungoP

Couldn't agree more. I hope people remember also that this is not a complete investigation report, but one published rapidly due to public interest. The fact that they have already come up with advisory notices without finding a cause shows their attention to detail.
As a SLF I'm grateful they are there looking out for all of us.

So the only manual flying part of the adventure was from < 200' then?

Yes l was surprised about the Auto-pilot, not being a Pilot, l thought that you would disconnect ASAP, would the A/P keep raising the nose if it was not getting any more thrust.
I would like someone to explain if l am wrong please, l know hindsight is a great thing.

Well thank heavens it went in on the undershoot. The combination of ruptured fuel tanks, ruptured O2 cylinders and an ignition source from contact with the runway are just too horrible to contemplate.

Yes, the auto-pilot would try and keep the glide slope at the expense of the speed. (With no thrust)

I think this might end up needing to be clarified a lot :uhoh:, 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.

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.

Fuel pump cavitation is a complex issue, it may take a while to sort that one out.

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 76°C


What is the enviornmental envelope re min temps for the 777. I know the airbus 320 is -70c

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

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?

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?

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.

"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.

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.

Thats rather annoying given that on BBC1 News Tom Symonds was clear that the cause remained unknown. Perhaps Shoey will be along to comment.

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

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

London news update transcript (SKY+) broadcast at 8pm
 

BBC newsroom has been contacted by a pal, and further bulletins should not repeat the fuel pump error (also heard on R4 here).

R

(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.

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.

No. On the contrary, the autothrottle, trying to maintain speed, would increase thrust on the other engine to compensate for the failed one.


Bernd

"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.

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.

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!