BA 777 @ LHR

[Dmmoore]

Second, considering the AAIB, NTSB, Boeing and GE have not released any significant operator letters on the subject we can assume for the moment that what ever the cause, the AAIB, NTSB, Boeing and GE think the cause is......

Minor point, the accident aircraft had Rolls-Royce Trents.

I knew that
Thanks for the correction.

[Hypersonic]

Thanks Don for your continued patience!!

I had not forgotten what you had said previously but your reiteration has made your position clearer, at least to me.

In my usual style I do not disagree with your contention...you have my respect... but I always seek to examine what doesn't fit etc.

Now I have learned something new from this latest exchange namely that the wing tanks have two pickups per side (one pickup per boost pump) and that they are positioned inboard and for/aft ...as you would expect to allow for (approach angle) attitude change and dihedral on the wings.

However, you did not respond to.....Quote Hypersonic "I am also interested in how air is prevented from entering the pickup should one of the pickups become uncovered (above fuel level) due to aircraft attitude and the relatively "flat" nature of the tanks.

What prevents air getting in to the engine supply (LP) pipe work especially since you stated that HP pump suction can maintain supply should the boost pump fail. Presumably there must be a one way valve so that fuel delivered by the correctly operating boost pump closes the output somehow from the pump which is above the fuel level.

So maybe the design does not cater for one pickup being above fuel level since this condition is supposed to be excluded by other procedures / equipment.
Maybe this is why the AAIB stated that they were going to do flow etc on the fuel system?

A further issue is with respect to the failure to issue a TSB or whatever you call them. You state that the correct procedure for tank level calibration is in the relevant paperwork. However you propose that somehow it must have been incorrectly applied on this specific aircraft, or the system failed in some unique way.

If so, lets say due to BA training then why no bulletin aimed at BA or other airlines to ENSURE that procedure is being followed. IF procedure has been delivered incorrectly once by somebody, it can occur again.

Finally, is the pprune fuel level estimate process using the size of the frost marking on the underwing, a total load of bull? Seemed quite reasonable to me on the surface and lower altitude warming would impact results in such a way to underestimate the amount of fuel remaining,

Still don't see a bullet proof contention.

[Peminu]

Myself also don't want to finish your patience Don, but I will really appreciate your opinion on the cavitation damage: how often are the pumps checked and what is the possibility of the damage being made on a previous flight?

Forgive me if I insist, but I am thinking that a partially damaged pump will have a smallest NHSP (head succion power) than a normal one, and if there is a low fuel condition, and supposing that the pumps are nor checked frequently, this can add a hole so the swiss cheese.

The other question has to be: is there a way to know that the cavitation damage was produced on the last flight?

Thanks in advance.

[Gabriel]

However, you did not respond to.....Quote Hypersonic "I am also interested in how air is prevented from entering the pickup should one of the pickups become uncovered (above fuel level) due to aircraft attitude and the relatively "flat" nature of the tanks.

In the sketchy diagram I posted some slots above I ommited a check valve immediatly after each low pressure pump. Remember that this is what I understood about the fuel system from the explanations given in internet forums. So a confirmation from Don would be in order.

[Gabriel]

Myself also don't want to finish your patience Don, but I will really appreciate your opinion on the cavitation damage: how often are the pumps checked and what is the possibility of the damage being made on a previous flight?

Forgive me if I insist, but I am thinking that a partially damaged pump will have a smallest NHSP (head succion power) than a normal one, and if there is a low fuel condition, and supposing that the pumps are nor checked frequently, this can add a hole so the swiss cheese.Thanks in advance.

I don't know if this answers your question but:
- The pumps were found capable of delivering rated flow.
- Initial cavitation damage (small holes on the surface) doesn't affect pump performance.

[Hypersonic]

However, you did not respond to.....Quote Hypersonic "I am also interested in how air is prevented from entering the pickup should one of the pickups become uncovered (above fuel level) due to aircraft attitude and the relatively "flat" nature of the tanks.

In the sketchy diagram I posted some slots above I ommited a check valve immediatly after each low pressure pump. Remember that this is what I understood about the fuel system from the explanations given in internet forums. So a confirmation from Don would be in order.

So how does the check valve work (suppose to).

No doubt it is supposed to stop reverse flow of fuel OK but how does it stop air being sucked up by the HP.
What is the HP pump suction pressure vs the boost pump output pressure.

How does the system stop air getting into the supply if one pickup is uncovered?
Does the pump auto stop if it is pumping air? i.e. pichup above fuel level. If not is this not a problem in its own right?

[Dmmoore]

Thanks Don for your continued patience!!

However, you did not respond to.....Quote Hypersonic "I am also interested in how air is prevented from entering the pickup should one of the pickups become uncovered (above fuel level) due to aircraft attitude and the relatively "flat" nature of the tanks.

What prevents air getting in to the engine supply (LP) pipe work especially since you stated that HP pump suction can maintain supply should the boost pump fail. Presumably there must be a one way valve so that fuel delivered by the correctly operating boost pump closes the output somehow from the pump which is above the fuel level.

As Gabriel stated, there are check valves located after each boost pump. When one pump is sucking air, the pressure from the other pump closes the non fluid pumping boost pumps check valve. In the event the pumps are inop, fuel is drawn through the check valves from the suction of the main engine pump.

A further issue is with respect to the failure to issue a TSB or whatever you call them. You state that the correct procedure for tank level calibration is in the relevant paperwork. However you propose that somehow it must have been incorrectly applied on this specific aircraft, or the system failed in some unique way.

If so, lets say due to BA training then why no bulletin aimed at BA or other airlines to ENSURE that procedure is being followed. IF procedure has been delivered incorrectly once by somebody, it can occur again.

Because the AAIB, NTSB, Boeing and RR do not think the event has any connection to any other aircraft. I proposed the mis-calibrated fuel quantity system as
one explanation for fuel starvation. There are others I'm sure, I don't know what they are.

Finally, is the pprune fuel level estimate process using the size of the frost marking on the underwing, a total load of bull? Seemed quite reasonable to me on the surface and lower altitude warming would impact results in such a way to underestimate the amount of fuel remaining,

Still don't see a bullet proof contention.

The frost markings "IF" visible would be accurate enough however they would dissipate quickly after landing. Maybe lasting an hour, not much longer.

And no, there is no billet proof answer except that fuel stopped reaching the engines within a very short time of each other.

[Dmmoore]

Myself also don't want to finish your patience Don, but I will really appreciate your opinion on the cavitation damage: how often are the pumps checked and what is the possibility of the damage being made on a previous flight?

Forgive me if I insist, but I am thinking that a partially damaged pump will have a smallest NHSP (head succion power) than a normal one, and if there is a low fuel condition, and supposing that the pumps are nor checked frequently, this can add a hole so the swiss cheese.

The other question has to be: is there a way to know that the cavitation damage was produced on the last flight?

Thanks in advance.

All engine driven fuel pumps will show some cavitation damage (<10% of the pumping surface) during their life. It's very difficult to determine when the damage occurred With out knowing the time on the pump since overhaul, a guess as to how much cavitation damage you would expect is impossible. I could go into detail on positive displacement pumps but suffice it to say that any decrement in suction from cavitation damage would be negated by the boost operation. The boost pumps were operating normally, they pumped what ever fuel there was to pump. Suction from the main engine pump is of no consequence.

[Dmmoore]

However, you did not respond to.....Quote Hypersonic "I am also interested in how air is prevented from entering the pickup should one of the pickups become uncovered (above fuel level) due to aircraft attitude and the relatively "flat" nature of the tanks.

In the sketchy diagram I posted some slots above I ommited a check valve immediatly after each low pressure pump. Remember that this is what I understood about the fuel system from the explanations given in internet forums. So a confirmation from Don would be in order.

So how does the check valve work (suppose to).

No doubt it is supposed to stop reverse flow of fuel OK but how does it stop air being sucked up by the HP.
What is the HP pump suction pressure vs the boost pump output pressure.

How does the system stop air getting into the supply if one pickup is uncovered?
Does the pump auto stop if it is pumping air? i.e. pichup above fuel level. If not is this not a problem in its own right?

The system is designed to deliver fuel from the tanks to the engines. If air enters the system with the boost pumps operating, the tank is empty or the fuel pick up is damaged. If the boost pumps are inop, the check valves prevent air from entering the system. I don't know specifically on the 777 but Boeing usually installs a suction by pass valve at each boost pump. The valve opens if boost pump pressure fails and a small float will close the valve when the fuel level drops to the inlet.

However this is of no consequence, suction feed is not an issue, the boost pumps were operating normally.

[Gabriel]

In the sketchy diagram I posted some slots above I omitted a check valve immediately after each low pressure pump. Remember that this is what I understood about the fuel system from the explanations given in Internet forums. So a confirmation from Don would be in order.

So how does the check valve work (suppose to).

No doubt it is supposed to stop reverse flow of fuel OK but how does it stop air being sucked up by the HP.
What is the HP pump suction pressure vs the boost pump output pressure.

How does the system stop air getting into the supply if one pickup is uncovered?
Does the pump auto stop if it is pumping air? i.e. pichup above fuel level. If not is this not a problem in its own right?

What Don said, plus (Don, please correct me if I'm wrong):

The flow rate capability of any single low pressure pump is higher than the flow rate capability of the high pressure pump. That's why, as Don said, if one pump sucks air (or is inop) then the flow and pressure of the pump sucking fuel is enough to feed the engine plus to "try" to make fuel go backwards through the pump sucking air, but the check valve prevents it.

If both LP pumps are inop then the HP pump still has enough suction power to suck fuel through the inop pumps and their check valves.

If one LP pump is inop and the other LP pump's pickup is uncovered, I have no idea what's the plan B. The same if both LP pumps are inop and any of the pickups is uncovered.

If both pickups are uncovered, then you have cross-feed, unless the other two pickups are uncovered too in which case the plan B is whatever the procedure for a deadstick landing says.

[Robert Hilton]

The vent system interconnects the fuel tanks, if that was blocked for any reason that could affect the fuel flow to both engines in the manner described.

[Hypersonic]

The flow rate capability of any single low pressure pump is higher than the flow rate capability of the high pressure pump. That's why, as Don said, if one pump sucks air (or is inop) then the flow and pressure of the pump sucking fuel is enough to feed the engine plus to "try" to make fuel go backwards through the pump sucking air, but the check valve prevents it.
............
If one LP pump is inop and the other LP pump's pickup is uncovered, I have no idea what's the plan B. The same if both LP pumps are inop and any of the pickups is uncovered.

I had reasoned out the above but the interesting thing is whether
1. a single boost pump does IN FACT have a flow capability > the (suction) flow rate of the HP pump at full demand.
2. With one pickup uncovered and the other pump inop that there is no plan B. Thats what I was getting at.

I was trying to understand how the system worked (understanding) rather than suggest that this was how air got in since as Don has confirmed there is no suggestion that the wing tank boost pumps were not working.

So the pprune observation of frost pattern on the wing underside could be correct in confirming that there was ADEQUATE fuel in the wing tanks. It seemed very reasonable to me. The observed frost line would likely under indicate the amount of fuel remaining.

So we potentially face lack of useable fuel although the tanks had fuel in them!
.... with the boost and HP pumps working correctly IMHO you are only left with a blockage and "incorrect spec" fuel
Blockage could be ice/wax that melted post event or debris that was subsequently found.
Since no trace of water was found in the engine filters etc the only answer is simultaneous debris blockage on both sides of the aircraft (however unlikely) by the debris that was found
,,,or the fuel. Remember that -57C freezing point.

The point I was trying to made earlier about how the engines received fuel in the event of boost pump failure was also dismissed without discussion.
The amount of fuel left in the tanks must be almost the same as when the engines stopped turning since there was no suction available to lift the fuel into the pckups once the engines stopped turning.
So even with the open spar valves only a small amount of fuel would escape from the engine feed pipes.
The amount of fuel actually in the tanks post event MUST be known by the AAIB since the tanks have been examined for debris etc.

I remain intrigued.

[Giles]

was no suction available to lift the fuel into the pckups once the engines stopped turning.

correct me if i am wrong, but with all the asked and answered then asked again questions i may of missed something- the engines did not stop turning, but rather, they did not respond to a request for more thrust.

[Procede]

correct me if i am wrong, but with all the asked and answered then asked again questions i may of missed something- the engines did not stop turning, but rather, they did not respond to a request for more thrust.

They did respond initially, but reduced back to near idle a few seconds later.

[Giles]

correct me if i am wrong, but with all the asked and answered then asked again questions i may of missed something- the engines did not stop turning, but rather, they did not respond to a request for more thrust.

They did respond initially, but reduced back to near idle a few seconds later.

Thanks. However, they did not "stopped turning".

[AndyToop]

correct me if i am wrong, but with all the asked and answered then asked again questions i may of missed something- the engines did not stop turning, but rather, they did not respond to a request for more thrust.

They did respond initially, but reduced back to near idle a few seconds later.

Thanks. However, they did not "stopped turning".

Giles,

I think Hypersonic is talking about when the engines/fuel pumps stopped. i.e. after the crash and the crew pulled the fire handles and closed cut off the fuel flow, not about the cause of the power reduction.
He's saying that after the crash the fuel tanks did not empty themselves through gravity or syphoning, so measuring the residual fuel in the wing tanks after the crash should be an accurate measure of how much was in them while the problem was occuring.

[Hypersonic]

Thanks AndyToop. Maybe I wasn't very clear but I did mean stopped turning post crash.

However I originally asked how fuel got to the engines if the boost pumps were inop and asked whether it was by syphon since the fuel needs to go up into the pickup before it goes down to the engines.
However I was corrected by Don stating that it was not syphon action but suction from the HP pumps....plus the boost pumps were working OK so it was acadaemic.
Hence I drew the conclusion that fuel could not have escaped from the wing tanks post crash thru the open spar valves because no suction was present once the engines had stopped turning (post crash.
However maybe syphoning could occur with an open pipe below the pickup level once air has been expelled from the circuit.

I'm increasingly of the opinion that although the engines may well have rolled back after initially accelerating due to inadequate flow of fuel that there was adequate liquid (fuel?) in the wing tanks. I believe the frost pattern analysis on the wing underside is indeed very compelling.

So restricted fuel supply (debris) or something "strange" with the fuel i.e. propensity to wax and block pickup or low calorific value?

[Giles]

Giles,

I think Hypersonic is talking about when the engines/fuel pumps stopped.

Thanks Andy. Ill bow out now and wait for the final report. Too many "what-ifs" going on.

[Dmmoore]

The vent system interconnects the fuel tanks, if that was blocked for any reason that could affect the fuel flow to both engines in the manner described.

While "POSSIBLE" that event would require some very interesting conditions covering a minimum of two vent ports on the lower side of the wing close to the wing tips or blocking numerous vent ports inside both tanks.

To my knowledge, no event blocking the wing tip vent ports has Ever been recorded on any transport category aircraft. Anyone know any different?

To block these vents would require some very difficult to imagine events.

It's possible, but if it's suspected, I would have expected an all operators alert to have been released.

What is clear is that an adequate supply of fuel was not available to the engines. Why hasn't been answered but the reason seems to be unique to this aircraft.

[Hypersonic]

The vent system interconnects the fuel tanks, if that was blocked for any reason that could affect the fuel flow to both engines in the manner described.

While "POSSIBLE" that event would require some very interesting conditions covering a minimum of two vent ports on the lower side of the wing close to the wing tips or blocking numerous vent ports inside both tanks.

Wouldn't the tanks (wing surface) implode/distort if the tank vents were blocked for whatever reason?

[Robert Hilton]

The vent system interconnects the fuel tanks, if that was blocked for any reason that could affect the fuel flow to both engines in the manner described.

While "POSSIBLE" that event would require some very interesting conditions covering a minimum of two vent ports on the lower side of the wing close to the wing tips or blocking numerous vent ports inside both tanks.

To my knowledge, no event blocking the wing tip vent ports has Ever been recorded on any transport category aircraft. Anyone know any different?

To block these vents would require some very difficult to imagine events.

It's possible, but if it's suspected, I would have expected an all operators alert to have been released.

What is clear is that an adequate supply of fuel was not available to the engines. Why hasn't been answered but the reason seems to be unique to this aircraft.

Indeed, possible and of course it would require a number of conditions being met. It is however no more off the wall than some of the ideas being offered up in this thread. To reiterate some of the points made in the special bulletin by the AAIB.
The a/c was planned to arrive at heathrow with 6900 kg of fuel left.
The indicated amount was 10500 kg.
A significant amount of fuel was present at the site
The fuel indication system was tested and found to working within acceptable limits.
The engines didn't shut down they just didn't respond correctly to demand, caused by a lack of fuel flow.

So where does that leave us?
Either:
It ran out of fuel because the the fuel indication system was badly calibrated at some unspecified point in time. Even though the AAIB have stated in their report the last date the tanks were entered (some 2 1/2 years previous), the last date any maintenance was carried out (a month) and the last time the tanks were checked for water (2 days).

Something throttled the fuel flow. The possibilties here are I supposed endless. I'm offering one up for scrutiny that to me sounds plausible. How? The only thing I could think of would be ice forming at the outlets. How? That's where it gets very hazy. My only argument here would be the warning issued by the FAA over engine icing at operating altitudes.
I know it's a very thin straw, but it is worth at least picking it up and having a look.

[Robert Hilton]

Wouldn't the tanks (wing surface) implode/distort if the tank vents were blocked for whatever reason?

I think you might run out of useable fuel before it got that far.

[Gabriel]

Wouldn't the tanks (wing surface) implode/distort if the tank vents were blocked for whatever reason?

I think you might run out of useable fuel before it got that far.

Why? I tend to agree with Hyper.
It shouldn't take a lot of differential pressure for the wing tank to start to collapse. And I know of one car where exactly that happened due to a blocked cap vent (by no means I'm trying to make an analogy between the fuel systems of a B-777 and a Fiat 1500)

[flyboy2548m]

(by no means I'm trying to make an analogy between the fuel systems of a B-777 and a Fiat 1500)

And to think you were nevertheless this successful without even trying.

[Ancient Mariner]

(by no means I'm trying to make an analogy between the fuel systems of a B-777 and a Fiat 1500)

And to think you were nevertheless this successful without even trying.

Why not? Same thing. Tank, pump, ignite, burn. Airplanes are not as fancy as you might think just because you drive one.
Per