Fuel System Schematics
 

Report states that crossfeed valves were closes for entire flight...and 10K fuel balanced for the most part in the two main tanks....
Was center tank fuel uploaded for this flight, and if so, roughly how much?.
As I am unfamiliar with the 777 ( only the 747-200 & -400 ), are the CWT fuel boost pumps "override" pumps, as in the type that will put out more pressure than the mains?. And if so, is there any limitations to having them on below a certain volume?. Could there be any possibility that CWT fuel ( mabye unscavenged fuel ) at highpressure was feeding/cavitating both engines?. Would only the pump cavitate, or would air be induced into the system?.

I know SOP would most likely have had Center tank emptied by now, and pumps switched off, but again I'm looking for a "common feed" so to speak.
Again, I am unfamiliar with the 777, and I know there is a wealth of knowledge spread ( unevenly ) amongst those who peruse this forum.
---------------------------------------------------------------------
As far as the kudos for the "wonderful" job the Autopilot did...
If I were handflying this approach, and lost thrust, would I sacrifice altitude for airspeed ( to prevent a stall ), damm fckng right I would.
And YES, with a loss of thrust, I KNOW I could FLY it to the ground a WHOLE lot better than the A/P. This is airmanship 101 guys. The only question I have is at what point do I realize I had no thrust and then act.

As I have no doubt there are those that will lay into me....lube liberally pre-applied.

A/P disconnect
 

It seems apparant that the A/P remained engaged down to 175 feet. Although the A/P is a pilot`s best friend in most emergency situations, perhaps it was not the case in this instance.

The speed at 750 feet must have been about 140 Kts.
The speed at 200 feet was 108 Kts.

As the the A/P was engaged, the AFDS was trying to maintain the glideslope, with a probable linear speed decay. The speed loss was approximately 32 kts in 500 feet.

Every kt of speed below VRef 30 would result in a worse Lift/Drag ratio, with the result that the aeroplane could not eek out as much distance as it potentially could if it was flying at it`s best L/D ratio speed (approx VRef). At 108 Kts, the L/D ratio would be significantly reduced.

:O Hello,

New member in this Forum , and flying 777 outside UK .

My SOP seems quite different than the one described : mine is telling that ( except Cat 2/3 ops ) PF is flying whole sector : no hand over in approach .

Does BA SOP mean the PF has the controls until TOD and take it again at 1000 AGL ? And what reason motivate such a practice ?

Am I correct ?

Thanks !

About 20800kgs, depending on S.G.

Thanks for that clear explanation, I was as surprised as MU3001A, but discovering that specific BA procedure (?), AAIB report makes now more sense.

What still does not make any sense considering that almost simultaneous double eng similar misbehaving:

1- "total fuel on board was indicating 10500 kg" ... but no mention of the actual FOB retrieved from the main tanks ?

2- "the fuel crossfeed valves indicated that they were closed" ...but no mention they have been physically checked closed ?

Also:
- What could justify that amazing 5% reduction over the planned fuel burn ?
- Did the engines actually spool up somewhere in the hold ?

Quote :

Phil,
The speed is controlled by the fuel going to the burners, N1 in my book, this is determined by the demands of the throttles set by the crew. This is basic gas turbine operation.

unquote

I meant the speed of operation of the fuel pump.

The fuel valves were open, the throttles (and auto-pilot) demanded an engine speed higher than that acheived, the actual speed of the fuel pumps doesn't seem to have been stated in the special report.

.

Milt,Not so, the fuel from the aircraft supply system (read tank fuel booster pumps) is fed into a compined LP and HP fuel pump assembly on each engine.

How can the total fuel on board be measured when an amount is soaking into the grass at the crash site.
Could this be why they AAIB state indicating?

Somebody mentioned electrical disruption further back in the thread, and I feel it may merit some thought here.

Question - are the tank booster pumps normally in operation in this configuration all fed from the same bus? (not a 777 operator, BTW - I'm assuming here that they're the standard 3-phase type commonly employed).

Reason being, I have come across an odd mode of electrical failure on a previous type, whereby a single phase on a single bus tripping off creates all sorts of oddities - the cause of which are not immediately obvious, since a motor with a dead phase acts as a generator on that phase (still being driven by the 2 good phases) preventing any indications of electrical failure, but sending oddly-shaped wigglyamps to other components.

Might be a worthless tangent, then again, might not be. Just something else to throw into the collective headscratching!

Some new AAIB statement could be found in the Guardian today:

And the Telegraph writes

On reflection, I think we can safely ignore the bits and pieces in the tanks as causes of fuel starvation (scraper, paper, etc). Remember that there are 2 pumps in EACH tank. All four tank pump inlets would have to be blocked to cause the problems seen. Each pump is designed to provide enough pressure/flow to supply the onside engine with fuel for all flight regimes (and probably more). The lowest spec I've seen for a 777 wing tank pump is 12psi and 16tonnes/hr

I don't have the wiring schematics handy, but I believe the L/R wing tank pumps are powered by their onside Transfer Busses.

Rgds.
NSEU

TwinAisle cites Private Eye's article, and says "clearly there are elements that don't add up".

There are two major points that "don't add up".

First, the article asserts that FBW systems before the B777 were triple-redundant, designed/written/implemented by three separate teams. That is rubbish.

Second, the article concentrates on the PFC (and maybe on the AIMS). I don't think anybody has suggested there was something wrong with primary flight control.

PBL

Just a thought for the boffins

AAIB report:
If there was a restriction upstream from the HPFP’s would the effect be to cause the pumps to effectively stall and prevent them from working? A cavitating pump does not generally pump much even if it is: Such a scenario would account for only minimal amount of fuel reaching the engine. Has there been a fleet check of the pumps to ascertain whether this is a one off or a common defect?

There is also a major discrepancy with the planned fuel burn and the actual. Planned burn including taxi fuel would work out at 72,000 kgs actual burn to short finals was only 68,500 based on the AAIB figures. Was this based on FMC fuel indications or gauge indications?
Unless there was a reason for a 5% difference such as an improved route or altitudes it seems a little large to me.

Seems like any pilot dealing with decaying power as this was would put speed control requiring to push the nose down below the requirement to not hit the ground before an obstacle. Clearing the fence and hitting a flat surface saved the people in this case so the pilots did all they could under the circumstances even though they had to sacrifice speed to do so. All they could have done was possibly add flaps before hitting the ground to get closer to the runway.

The CWT pumps put out more pressure/flow than the wing pumps. Low pressure output at the CWT either due to the pumps being switched off or due to no fuel in the tank will cause the check valves on the CWT pumps to close, preventing air getting into the manifolds. Like the 744, this process will occur well in advance of landing. Remaining fuel is scavenged via jet pumps (and as discussed previously, this process completes well before a normal landing).

I believe someone asked earlier.... There is no gravity feed for the CWT.


...but if pulling back on the stick with the stickshaker ringing in your ears for a few seconds longer meant narrowly missing a large building (and you knew there was a flat surface beyond)? Wouldn't you take the chance that all those built-in margins for stall might work in your favour on that particular day?
Personally, I think man is a gambler... He is taught what is impossible, but when faced with a large immovable object, he goes for the impossible ;)

phil - you asked about the speed of the HP fuel pumps and how they are controlled - it should be noted that the pumps are not electrical driven but are connected to the the gearbox and as such the gearbox is driven via a shaft which is in turn connected to the N2/N3 compressor spool so the pump speed is not and can not be directly controlled.

regarding the theories about a missing phase from the IDG, this would be sensed and the idg would be disconnected immediatly so that is a non starter

swiss_swiss,

All things being equal, I agree. But I have personally seen a failure mode where a dead phase on a bus was supplied with enough voltage to keep the genny / bus from tripping off, via the two still live phases powering an electrically driven hyd pump. A generator and a motor are essentially the same thing - you spin a motor with a dead phase, and it will generate on that phase and feed any component connected to that phase on the bus. Frequency lockout wouldn't necessarily pick it up either, if the remaining 2 live phases of the motor could draw enough juice from the bus to keep it spinning at close to its normal running speed.

Make sense, or am I not explaining this very clearly? Just to clarify, this isn't idle speculation, this has actually happened to me (on another type, considerably older than 777)

HP Pump speed
 

Phil

If it's anything like the Fuel Control Unit on my aircraft (which also has R-R engines), the HP Pump pushes out considerably more fuel than the engine needs. A variety of spill valves bleed off fuel for various reasons. The most important is a system that ensures a constant pressure drop across the throttle valve. Adjusting that valve then produces a given flow for a given setting regardless of what the pump is up to (within limits obviously). The electronic side of the engine control adjusts the throttle valve, and a monitoring system tells the electronics whether or not the desired result has been achieved.

What seems to have happened here is that the desired result was not achieved, so the electronics continued opening the throttle (that's all they can do in the last analysis) in order to get the desired result - which didn't happen.

hi inquisitor

your description i can easily follow and can understand how that could be possible. ( i am well versed in motor/generator theory).
mind me asking which type this defect was on and how the system was monitored?

AFAIK (would have to go back and look at my training notes) each individual phase of the 777 is monitored so if 1 is missing then the gen is switched off - would have to confirm this tho.

Re. Fuel pumps.

Thank-you "Swiss Swiss" and "Sven Sixtoo".

If I have got it right, the fuel pumps work continuously at maximum rate (when switched on) and the actual fuel flow to the engines is controlled by the control valve AND spill valves which get rid of any excess pressure.

Are the spill valves actuated, or merely like saftey valves, operated mechanically by excess pressure ?


Thanks

.

they work all the time when the engine is running ( i of course assume u are talking about the engine mounted mechanical driven pumps) and not at maximum rate - maximum rate 100% of the time is not possible because the speed of the pump is determined by the speed of the gearbox and therefore the engine- at any given speed the fuel pumps supply more fuel than is need - this excess fuel is then bled back toward the engine pumps via actuated valves and not by preset pressure relief valves

Did the centre tank LP pumps put air into the system?
 

My experience is on Airbus so I'd be very happy for someone with Boeing fuel system knowledge to correct me on this.

1) Centre tank pumps usually pump at a higher pressure than wing tank pumps so that the centre tank fuel is used up first.

2) When the centre tank is empty sensors in the tank tell the centre tank pumps to stop. By the latter stages of a flight one would expect the centre tank fuel to be used up.

3) It is important that LP tank pumps are immersed in fuel so that they don't introduce air into the system, also the fuel provides a cooling function for the pumps.

4) In some early configurations of the A320 the centre tank pump sensors had a bit of an issue and not switch off the pumps when the tank was empty. This would trigger an ECAM (FUEL CTR TK PUMP LO PR) and we'd switch off the pumps manually. To avoid damaging the pumps there was an SOP not to switch the centre tank pumps on in the first place unless there was a certain amount of fuel in the tank. Its fixed now - well on our fleet anyway.

5) So, you're probably ahead of me, what if the sensors in the 777 center tank failed or were fooled and the pumps operated whilst the tank was dry? Would EICAS alert on something or could they keep going long enough to put enough air into the system to balls things up?

Centre tank obviously feeds both engines albeit via different LP feeds but it is not impossible for two pumps to be running.

Happy to be shot down - I've only given it a few minutes thought.

Cheers,
Silent Badger

Can fuel become stratified?
 

Hello,
First post; I'm not flight crew, nor engineer, just scientist.
Apologies for trespassing on your professional forum to ask the following but my curiosity has gotten the better of me.

Can fuel on a long, cool flight become stratified?

(Fuel is probably the wrong term considering it is a mass, held within wing tanks, consisting of fuel sensu stricto, other hydrocarbon derivatives, H20, emulsions and various physical particles all of which will behave differently given the same physical environment.)

If the answer is yes then follow-on questions are:
1) Can fuel stratify by density and/or thermal anomaly?

2) Does the ground testing of the fuel take account of possible in-flight stratification?

3) Given that the fuel in both wing tanks was of almost equal weight (bar 30Kg), is it possible for the tank outlets in both wings to encounter a 'bad' layer/stratification at almost the same time?

Regards, Tanimbar

I would think stratification would be unlikely given the aircrafts frequent manouvring and encounters with turbulence. The fuel would have been well and truly mixed up for most of the flight and certainly for the last 30 minutes.

It is my understanding of the water scavenge jet pumps that they would, while trying to keep the water emulsified in the fuel, would also mix the fuel, preventing stratification.

A minor slip: the imbalance was 300kg, not 30kg, which makes it still less likely that both engines would be fed the same undigestible type of stratum at roughly the same.


Bernd

MAGNETIC SLIP

What are the sealing arrangements of the HP pumps? Do they have a double mechanical seal or are they magnetically coupled?

If they are magnetically coupled (mag drive) then they can suffer from magnetic slip and lose coupling all together. This would not necessarily however show signs of cavitatation. Magnetic slip can occur quite readily in large pumps when the demand on them is too sudden (on electrically driven ones, the star-delta needs to be set up to apply demand more gradually).

FOAMING

Does anybody know anything about the foaming tendencies of Jet A1, especially at low pressures and temperatures?

Probably not a huge surprise, but BA has confirmed that 'YMMM is a total loss.

http://www.flightglobal.com/articles...ashed-777.html

causes of aeration
 

Hi PPRUNE. Non-pilot,non-anorak, first post. I've been reading the Rumours board intermittently for years so I realise that taking the risk of posting on any technical issue is foolhardy, but here goes. I offer it only for what it is.

Many years ago I was involved in an operation that required pumping lots of water from a river, using a diesel engine (250hp) driving a 4 inch Ajax centrifugal pump. One day I replaced the cage which was fitted to the (non-return) foot-valve - which was intended to prevent large stones being drawn into the suction line. It was badly damaged, so I placed a stainless steel mesh directly across the valve opening (ie "2d" replacement for a "3d" predecessor).

To cut a long story short, we repeatedly encountered delivery failure after about an hour of operation. When checked, we discovered that the pump was partially full of air and consequently was cavitating. I assumed that this was due to a leak on the suction side and repeatedly refitted the suction hose (an 8", heavy, beast) with added sealant and many clamps.

Then it dawned on me to take into account the recent changes to the foot-valve protection. Clearly the restriction to the flow was causing air to gradually come out of solution, hence the eventual cavitation. After replacing the grid with a fabricated box of the same material, the problem vanished.

I have read a lot of the preceding thread. I understand a little of it - in very broad terms anyway. I don't recall if anyone has implied the cause of the "aeration" which has been referred to, except in relation to the possibility of leaky seals on the lp side. But I kept being reminded of my own experience described above. It would only be relevant if there is indeed any dissolved gas/air in jet fuel etc etc, about which I have absolutely no idea.

Simple question. How does fuel quality varies when we mix different kinds of fuel in the tanks?

From what I've learned here, normal Jet A1 has a different chemical composition from the fuel this crew used out of China. A chemical reaction between different additives could probably lead to quality degradation regarding its published freezing point.

Sky9 asked if there has been a random (say 6) check of other BA 777s HP fuel pumps with a time history the same as the crashed a/c. I hope they have. All mechanical parts wear with usage. If they find much the same wear then this is not significant.
What I would consider is the flow path as to why? the engines slowed to just above flt idle, was it commanded and did the throttles close?, or did the pwr just drift away.
I realise that both the a/p & crew asked for more pwr once they became aware but by then it was too late.
I also note the a/c used 5% less fuel than predicted, on a 10hr flt this is 30mins saved. Unless the on route conditions (w/x) where significantly different this is an almost impossible thing to happen.

From AAIB Special Bulletin:

"The specified freezing point for Jet A-1 fuel is -47ºC; analysis of fuel samples taken after the accident showed the fuel onboard the aircraft had an actual freezing point of -57ºC."

Yes, I've read it on the report also.
I was thinking about how does the hardware (fuel pumps, fuel lines, valves, etc) handle those mixes.

tigger - the pumps are mechanicly driven via a solid shaft from the gearbox, no magnetic clutch anywhere

Actual fuel burn can be less than planned for many reasons that include:

Shorter routings
Better levels
Lower ZFW
Shorter taxi
Lower Cost Index

Over a 12-hour flight, lower actual aircraft mass can have a significant effect, especially if it allows a climb to a more efficient level earlier than planned.

That 777 incident
 

Having followed the threads good bad and plain ugly and read the AIB reports to date all I can add is curiouser and curiouser said Alice.

The AAIB is explicit in saying that flap 30 was selected. No mention is made of decreasing the flap selection.

The usual jet airliner L/D of 18 is for the clean configuration and I suspect that flap 30 yields substantially less. A simulator run would present the raw F30 L/D, but in any case we did have some thrust and the achieved slope and L/D is derivable from the FDR.

There are a number of flapped gliders that use flaps to add drag. The big caution with flapped gliders is not to add too much flap as you can lose considerable altitude reducing flap. Many glider approach accidents have happened when flap was reduced with insufficient altitude.

I would not want to explore in the air how the 777 with thrust restricted to the accident setting reacts to a flap reduction attempt at 600', but it's possible the AAIB will decide to investigate that in a simulator.

In a high drag situation, increasing airspeed increases drag substantially. With flap 30, the best L/D speed would be lower than with lesser flap selections.

With gliders that have powerful spoilers and/or flaps, you can select full spoilers and/or flaps full on and if you are still not coming down steeply enough, add airspeed to steepen the slope.

In this accident, we see a trade of airspeed for glideslope, mostly at the behest of the autopilot. Given the touchdown point and the 108 kt. cited by the AAIB, they were amazingly lucky.

Guys, you are confusing specifications and actuals. The spec is maximum -47C not exactly -47C. Minus 57 complies with the spec.

Kero is not known for incompatibility problems so mixing differing compositions not currently* thought to be an issue in the industry

*Take it from me, this is currently being investigated :oh:

Which is why each time someone claims or states (mistakenly or presumptuously) that flap 20 was selected, I move the post to the WAG's thread. :rolleyes:

Quote (re. fuel pumps) (from Swiss Swiss) :-

they work all the time when the engine is running ( i of course assume u are talking about the engine mounted mechanical driven pumps) and not at maximum rate - maximum rate 100% of the time is not possible because the speed of the pump is determined by the speed of the gearbox and therefore the engine- at any given speed the fuel pumps supply more fuel than is need - this excess fuel is then bled back toward the engine pumps via actuated valves and not by preset pressure relief valves

unquote


Thanks again - very helpful,

So, if the spill valves are actuated to actually relieve the pressure between the fuel pump and the control valves (which were confirmed as being open) are the actuation messages recorded ? If so would their operation have been covered by any of the "satisfactories" in the AAIB report ? The Special Bulletin does say that the EEC worked correctly, but the listing of items tested does not include the spill valves.

Scanx3
I am concerned about the quoted fuel figures, they planned to use 72100kgs of fuel and according to the recorded quantity they had used 68500kgs, saving 3600kgs. If this amount was saved by flying higher than planned why not fly at that height everytime. I note that the FQPU was tested but I assume that this unit gathers information from the quantity sensors in the tanks or whatever system the 777 has for fuel quantity, testing of those sensors should be straightforward and I would have thought been mentioned in the report.