Teal

Sure, but that object was in the water scavenge jet pump.

The first object in the right fuel tank was 30 square cms...how large is the suction inlet screen in comparison?

Perhaps not significant but I would have expected pristine manufacturing and maintenance/fueling standards that would not introduce foreign objects.

Avionista

"Eliminate all other factors, and the one which remains must be the truth." - Sherlock Holmes

The AAIB seems to have pretty much eliminated mechanical and/or electrical failures within the engine systems as likely causes of this accident. They are not even suggesting that the cavitation damage within the fuel pump ocurred immediately prior to the crash - it might have happened weeks before the accident. The evidence points to some fuel related factor as it seems to be the only thing common to both engines. Perhaps, there is some aspect of fuel behaviour, under certain conditions, which has not been encountered before and is not yet understood. Cue the fuel chemists!

phil gollin

People should note that the problem SEEMED to affect both engines to a similar extent, but at slightly different times.

The latest report states that they have tested the fuel and it conforms to the required specification.

The debris was different in each wing tank and somehow would have had to have the same effect within 7 seconds on each engine (!)

The fuel metering valves were stated to have moved to the correct fully opened position and the filters filters were clean, as for the Fuel Oil Heat Exchangers one was clear and one had slight debris - the high pressure filters were clear.

It SEEMS very unlikely that the fuel was to blame, and the debris could not cause the same effect on the different engines.

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I cannot see that two separate pumps with whatever minor damage could possibly both fail within 7 seconds of each other (whether or not they subsequently showed no fault).

I have no idea about what caused the problem, but would like to know more about any flowmeters that are in the system. Also about what could cause fuel aeration.

However, it gets annoying when people don't think about what the report says before posting. Anything that occurs must be able to affect both engines within a very small timescale.

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OVERTALK

The cavitation damage to the engines' HP fuel pumps would seem to indicate incipient fuel starvation - or at least aeration - for whatever reason. Cavitation damage occurs when the pumps lose their continuing lubrication by the fuel itself. None of the in-tank debris (plastic etc) found would explain one or (particularly not) both engines becoming fuel-starved.
.
The possibilities remain prima facie limited to establishing the reasons why fuel (or trapped water) could be cooled to below freezing point (even if that point is 10 degrees below the specification freezing point for JET A1 of minus 47 deg C). The pointer would seem to be towards either water in the fuel becoming frozen or waxy/illiquid. There's another possibility however.
.

Think about the fact that certain "aerodynamically quiet" areas on an airframe can accumulate de-icing fluids and cause flight-control restrictions (see instances by Googling that term ["aerodynamically quiet"] and recent and historical AD's - particularly for BAe146 etc). In a similar fashion, icing will not form and accumulate uniformly over an airframe. It will first be seen in the corners of windshields, along windscreen wipers, wing/tail leading edges etc. Now think about what can happen in another fluid (i.e. not air, but fuel) that's contaminated with very cold water. What I'm getting at here is that some quiet (or stagnant) areas in an LP fuel system (tanks thru to HP pumps and filters) can support the formation of ice or waxy deposits due to higher (than normal/ambient) super-cooling in certain discrete areas. Result could be (a) large(ish) lump(s) of melting ice that will find its (their) way into a critical passage, filter, pipe or valve and temporarily cause constricted flow-rates. Because both sides of the aircraft mirror image each other, it's conceivable that identical "releases" of this icy lodgment could occur almost simultaneously - and then disappear (i.e. great quantities of water would not be required and wouldn't later be detectable anyway).

However why limit oneself to considering frozen water? Why not just think in terms of fuel itself being trapped and frozen in fluidically quiet areas and becoming supercooled/frozen. I'm guessing that residual fuel in the center tank/tail tank (and their collector tanks) might qualify and could later be induced into the system by both warming and the attitude changes inherent in both descent and configuring (gear and flap extension).

However, as with all such system measurements, the validity and usefulness of the sensor data depends upon where the sensor is mounted. Temperature measurement systems are inherently different to pressure measurement. Pressure is exerted equally in all directions. Temperature can vary over a significant range within a large volumetric system, some of whose components/areas are temporarily isolated by dint of being virtually (but not really) empty. If a center tank's output to the wings ceases relatively early in the cruise attitude (because it's "virtually" empty), does that mean it cannot thereafter contribute some previously "baffled" slushy ice-dregs (merely by gravity feed perhaps) once the nose lowers to the descent attitude? It's also a liquid fact that smaller stagnant quantities (such as dregs) will freeze sooner (and also melt sooner) than larger quantities (i.e. in wing tanks) that are being subjected to throughput recirculation (of bypass fuel), engine feed and fuel/oil heat exchanging etc. Wing carried fuel is also heated to some extent by the wing's passage through the air (and by the sun on the upsun side). That's not the case with fuel carried (or baffle-lodged) in the center-section and tail-tanks.

Why would this have happened to BA038 and not other 777's previously? My guess is that there is a trigger temperature and that the period that G-YMMM spent in the super cold pool of their track-miles between the Urals and Eastern Scandinavia ("unusually cold but not exceptional" - UK Met Office) may have allowed such pockets of super-cooled fuel to form into ice. The time of day (sun/no sun) and chosen CRZ hts may have also helped format the process.
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CLEE

This is probably nothing, but since lots of posts are equating a loss of upstream fuel pressure (upstream of the HP fuel pumps in the engine) with a blockage in the fuel line (causing cavitation), it's as well to realise that it's not just a blockage that can result in low fuel pressure from the LP pumps (in the wing tanks).

A few years ago we were completing maintenance on a Fokker 100 and for some reason the LP fuel pump delivery pressure on the pumps in the LH wing were only delivering about 50% of what they should. To troubleshoot, the pumps on the LHS & RHS were swapped - the problem STILL only occurred on the LHS. That eliminated pumps as the problem, and we started to look for a blockage in the lines downstream of the pump as the only explanation. No joy.

During the maintenance input, the LH engine had been changed. This required swapping the IDG (integrated drive generator) from the old to the new engine. During that procedure, human error had misconnected the three phase wiring and earth so that the AC three phase output from the LH engine generator was wrong.

Lesson No 1 - no other system on the aircraft was bothered by this except the LH fuel pumps. Lesson No 2, although the Ground Power Generator Control Units monitor the phase quality of the power coming to the aircraft, ordinary Generator Control Units do not (on that aircraft anyway). So there was no fault announced at the front of the aircraft.

Now in no way am I suggesting that on this 777 the generators magically re-connected themselves in flight. However sudden low pressure from all LH & RH pumps could (in theory) occur if the phase relationship of the power supply to all of the pumps suddenly changed.

Also, if the 777 is anything like the Fokker 100, it might only be the fuel pumps that would have a problem with this. How that might happen in practice I don't know - I would think it so unlikely as to be almost impossible. Also, even if it did happen, I would be surprised if such a loss of pressure starved the engines - but perhaps in combination with other things it is (theoretically) possible.

Lost in Saigon

Minus 80C

wilyflier

Possible , not impossible

One centre tank water scavenge jet pump suffers poor flow due to 6 mm disc
Frozen water accumulates to be discharged at late stage of centre tank scavenge.
Hard ice crystals abrade fuel pump surfaces
Can frothing occur also?
One of the crossfeed valves neither fully open nor closed after crash,(and according to Captain MM would not have been selected open in any case(?))
The enquirys lack of comment indicates it to be serviceable,
But if it had in fact for some reason remained half open throughout the flight it would supply the missing common factor complete with 7 second interval

misd-agin

Wornout Rubber
Instead of being 'just another number' I could order a Personal Title and be a bit different

Join Date: Jan 2008
Location: Dubai
Posts: 10


Best Glide Speed
Can anybody comment as to the best lift/drag speed (best glide speed), with Flaps 30 on a B.777?

The AAIB stated that the speed decayed to 108 Kts.

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I asked this question to a flight test engineer. He stated for line pilots we could consider approach speed (Vref +5) to be close enough to L/D speed.

Holding speed on my a/c is 1.4 Vsi. Approach speed at Vref + 10 is approx. 1.4 Vsi(typical landing weight). Vref is 1.3 Vso.

Based on his comments, and the relationship of holding speed to Vsi, I'd think L/D is very close to Vref + 5.

We all know that L/D is not at, or even near, 1.0 Vso or 1.0 Vsi.

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edit -

QRH lists best L/D speeds under 'Dual Engine Failure'. It's 1.5 x Vsi.

If that relationship (1.5x) holds with regards to Vso then L/D would be approimately Vref + 15-20.

davy123

Would cavitation not have to build up over a period of time in order to be described as 'abnormal'? I appreciate that if solids peppered the bearings and ports then there would be increased pitting, but to such an extent in one flight leg?

What pressure (psi/bar) is the fuel entering/leaving this device?

misd-agin

Today, 01:09 #97 Wornout Rubber
Instead of being 'just another number' I could order a Personal Title and be a bit different

Join Date: Jan 2008
Location: Dubai
Posts: 10


Best Glide Speed
Can anybody comment as to the best lift/drag speed (best glide speed), with Flaps 30 on a B.777?

The AAIB stated that the speed decayed to 108 Kts.

-----------------------------------------------------------------------

Anyone have simulator access that can answer the question?

The formula (found on internet...) is TAS x 100/sink rate (FPM) = glide ratio.

IE 250 KTAS x 100/1300' = 19.2

Try a couple of different speeds, and various flap settings (20 being the most interesting due to the reduction in LE slats), and you'll come up with the answer.

Modern airliners are around 18:1. Easy formula is altitude in thousands x 3 = glide distance for a clean aircraft at best L/D speed. Eg, 30,000' x 3 = 90 n.m.

QRH for my a/c has best L/D speeds(clean) listed under "Dual Engine Failure".

Woodenwonder

Not an ATPL on 777 - just a one-time instructor.

Whatever the best glide speed might be, a reduction in speed can only build induced drag more and more so the overall L/D must diminish. And this will be non-linear - ever diminishing performance as speed decreases.

Perhaps the autopilot is not the crew's best friend in these circumstances.

PS. I thought landing flap was normally 25 degrees, AAIB say 30 was set.

sevenstrokeroll

with so many reports, this AAIB report seems to contradict an earlier report saying the captain reduced the flaps while inflight.

anyone care to make a definitive statement on this?

nice to know the birds didn't do it, right danny?

CONF iture

That flaps reduction was only merely a suggestion ... but like many other things has never been part of any official AAIB report !

sevenstrokeroll

its been done to death in movies. duplicate the flight. same weight, same temperatures encountered enroute, same pitch attitude during approach, same fuel type and ammount...just a different plane...and keep it on autopilot to try to duplicate everything.

maybe there is an exact pitch attitude causing a slight change in fuel flow at just a certain amount of fuel remaining.

Mark654321

Seems like we have a conundrum and the implausable situation of 2 engines being fuel starved at virtually the same time from some 'fluff' in the tanks (highly unlikely). Concluding with a 40min descent and held in a pattern over Lambourne for 5 minutes I can't see there was any ice either to bung them up. London was cold but not -57C.

I found the AAIB very interesting and thorough for the time allowed to them so far. Thank you.
The weather was mentioned from a METAR at Heathrow I believe at the time but didn't seem to mention anything abnormal when all reports for that day seemed to be anything but normal.
I would like to draw your attention to the METARs for the London area on that particular date here.
http://www.wunderground.com/history/...q_statename=NA

There are some very interesting Microburst/Windshear activity throughout the day in the London area from SSW with winds in excess of 50kts GUSTING to 75kts out of nowhere!! I also believe that the London Eye was closed at 13:00. Oh no!
Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level. Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot/(AUTOPILOT in this case) to reduce engine power if they are unaware of the wind shear. This is where the engines have settled to just above idle. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft's airspeed and increasing its sink rate. The pilot releases AUTOPILOT and demands more power, as passengers and witnesses on the ground 'heard the roar of the engines like a take off'.Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing airspeed further, leaving the aircraft in a low-power, low-speed descent.(Heres where you get your 108kts and on board passengers reporting sudden loss of height). 108kts is not the speed you want on any HEAVY Aircraft at anytime whilst in flight. This can lead to an accident if the aircraft is too low to effect a recovery before ground contact. As the result of the accidents in the 1970s and 1980s, in 1988 the U.S. Federal Aviation Administration mandated that all commercial aircraft have on-board windshear detection systems by 1993. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries. Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every ten years, due to the mandated on-board detection as well as the addition of Doppler weather radar units on the ground.

3 Greens and Happy Landings

Mark in CA

Extensive investigation revealed plastic pipe blank lodged in fuel feed pipe in pylon area. During high thrust setting, ie take off, fuel flow to engine HP pump was blocked, then almost immediately cleared, then blocked again, causing fuel starvation etc and surging. Initial problem was put down to failure of the original engine, as after the replacement was fitted full power runs were carried out. Offending plastic blank was traced back to manufacturer....

Exactly the same thing happened to my old VW Bug, only in that case it was a pencil point that had broken off and left floating around inside the gas tank (previous owner must have used a pencil to plug up the gas outlet!). Car would unexpectedly stall, but only while driving on the freeway. Took a while to track it down, but the fix was easy.

nickyjsmith

NSEU,

Check your tank vent, if its blocked you can get a vacuum in the tank that causes fuel starvation. Worse on longer journey's.
When you stop,air seeps in while your looking around and the car runs normal again untill a vacuum builds up again.
To test it, stop and take the fuel cap off asap and listen, i've come accross it a few times on different cars.Its a real pig to figure out the first time as it leaves no evidence.

airfoilmod

Ice crystals may as well be sand in a pump at that pressure. Tolerating "Water" in Fuel, at any amount obviously opens the door to bad outcomes. Not my Job to point out proximate cause in this one, but a reading of any report gives one a "sense" of where it's going. Cavitation is possible so is Water ice buildup. "Vapour Lock" is a possibility, but so is Delta mmHg in the Low pressure side, a reduction that "pills" ice crystals out of solution to accumulate in parts of the Fuel Journey that cause "Restricted Flow", partial Starvation, and a further reduction, production cycle that exacerbates "Spool Down". I think this report is what I call a "Potentiating release", something intended to set the stage for an "outcome" that is as pleasant as can be to the Players. That two Majors are rewriting Fuel Protocols is a sign this may be true. That BBC already has gotten to the destination I'm implying, is instructive.

airfoilmod

So why would both engines protest at the "same time". If the Fuel is homogeneous, separation or "isolation" of supply to each powerplant accomplishes nothing if the "problem" exists in both supplies. What would cause a chronic and consistent "starvation" or "restriction"? Additive that allowed water to be in solution with Fuel, something like Alcohol? Angle of attack, causing a reduction in pressure in both fuel systems at the same time, but one engine, being "hotter", reacts five seconds later? Whatever the outcome of the report, I don't see how anything other than a Fuel scenario could have escaped the scrutiny of this prestigious website.

snowfalcon2

Woodenwonder:

I had the same thoughts, as expressed a while ago in this post.

Just as a remark: The original information we saw indicated that the problem started at 600 ft, and the autopilot was disconnected by the time the aircraft reached 400 ft. The new information says the problem started at 720 ft and the autopilot flew the plane all the way to 175 ft.

The difference is significant in terms of the potential improvement to the glide distance that a "smarter" autopilot might have enabled. As others have said, the aircraft might have retained enough energy to either allow for a flare or, alternatively, make it to the threshold.

Nickyjsmith:

Tank vent blocking can indeed be quite nasty. I know of it happening in a small plane, with the result that the tank literally was crunched together by the ensuing vacuum. Fortunately the forced landing went well. But based on that, even on a 777 I'd expect such an event to cause visible buckling on the wing skins before fuel flow to the engines would stop.

Just my £0.02