BA038 (B777) Thread
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Cavitation and water questions
A few questions related to cavitation and one on water content:
1. Have there been any other reports at all of cavitation damage in any HP pump used in 777 or any other similar Boeing 7 series aircraft? I assume that fuel which has not been degassed never normally causes cavitation in HP pumps during takeoff or climb.
2. From before TOD but particularly below flight level 170, what volume of air would be entrained (or possibly dissolved) in solution in the fuel in each of the main tanks as a result of the continued operation of the centre tank fuel scavenge pumps after the 900kg rump of fuel was scavenged?
3. Would degassing be inhibited by relative coldness of fuel or its relatively high boiling point (corresponding to relatively low fuel freezing temp)?
4. Given no cavitation damage in LP boost pumps, should cavitation be assumed to be due to either (a) the higher rotational speed and/or design of the HP pumps coupled with something causing nucleation, (b) the formation of a restriction downstream of the LP boost pumps but upstream of the HP pumps or (c) a restriction upstream of LP boost pumps insufficient to cause LP pump cavitation damage?
5. Would cavitation in the HP pumps, once developed, release sufficient gas that the flow downstream would be reduced without any other restriction upstream of the HP pumps, or is the cavitation merely a secondary effect, possibly exacerbating but otherwise only symptomatic, of restrictions upstream of the HP pumps?
6. Might the suction feeds be blocked in descent (as they can be in climb) through degassing?
7. What amount of water is “excessive” in the 5.1/5.4 tons or so of fuel in each main tank ... 5, 50, 500, 5000 grammes? I guess 134 gallons or so in the 80 T centre tank is “excessive” giving rise to an alert, but does that mean that less is not?!
1. Have there been any other reports at all of cavitation damage in any HP pump used in 777 or any other similar Boeing 7 series aircraft? I assume that fuel which has not been degassed never normally causes cavitation in HP pumps during takeoff or climb.
2. From before TOD but particularly below flight level 170, what volume of air would be entrained (or possibly dissolved) in solution in the fuel in each of the main tanks as a result of the continued operation of the centre tank fuel scavenge pumps after the 900kg rump of fuel was scavenged?
3. Would degassing be inhibited by relative coldness of fuel or its relatively high boiling point (corresponding to relatively low fuel freezing temp)?
4. Given no cavitation damage in LP boost pumps, should cavitation be assumed to be due to either (a) the higher rotational speed and/or design of the HP pumps coupled with something causing nucleation, (b) the formation of a restriction downstream of the LP boost pumps but upstream of the HP pumps or (c) a restriction upstream of LP boost pumps insufficient to cause LP pump cavitation damage?
5. Would cavitation in the HP pumps, once developed, release sufficient gas that the flow downstream would be reduced without any other restriction upstream of the HP pumps, or is the cavitation merely a secondary effect, possibly exacerbating but otherwise only symptomatic, of restrictions upstream of the HP pumps?
6. Might the suction feeds be blocked in descent (as they can be in climb) through degassing?
7. What amount of water is “excessive” in the 5.1/5.4 tons or so of fuel in each main tank ... 5, 50, 500, 5000 grammes? I guess 134 gallons or so in the 80 T centre tank is “excessive” giving rise to an alert, but does that mean that less is not?!
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4. Given no cavitation damage in LP boost pumps, should cavitation be assumed to be due to either (a) the higher rotational speed and/or design of the HP pumps coupled with something causing nucleation, (b) the formation of a restriction downstream of the LP boost pumps but upstream of the HP pumps or (c) a restriction upstream of LP boost pumps insufficient to cause LP pump cavitation damage?
) to overcome the compressor discharge air pressure and feed the burner. Thus any air or vapor content would be compressed to a very small volume, if not redissolved completely.And the HP and LP pumps are quite different in design. The HP is of necessity a positive displacement type - gear type or vane type etc. Since it is such, the inlet can readily suck enough vacuum to cavitate if the LP pump isn't performing.
The LP pump is designed to tolerate a certain degree of gas/vapor/air in its inlet, which is its assigned job in protecting the HP pump. I think it may be a axial or centrifugal, non-positive displacement type -- but I'll defer to someone with more detail.
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Low Pressure Pumps
Barit1
The low pressure pumps are something very much like this....
http://www.eatonaerospaceltd.com/fuel/Resources/031_2_Type9602boostpump777_300.pdf
I beleive there are non return valves fitted in both the inlets and outlets and it is one or more of these that I am suggesting as being complicit in my post above.
There are two such pumps on either side of the aircraft and a smaller very similar one (on the left side I think) which is specifically for the APU.
JG
The low pressure pumps are something very much like this....
http://www.eatonaerospaceltd.com/fuel/Resources/031_2_Type9602boostpump777_300.pdf
I beleive there are non return valves fitted in both the inlets and outlets and it is one or more of these that I am suggesting as being complicit in my post above.
There are two such pumps on either side of the aircraft and a smaller very similar one (on the left side I think) which is specifically for the APU.
JG
Tucked away 2 items below the Lufthansa HAM incident report in the BFU document is the following item
12.3.2008 13:10 CET
B777-300 foreign registration (operator not specified)
The thrust levers were moved backwards and forwards very rapidly on approach to Frankfurt/Main.
According to the pilots, there was a delay in thrust increase.
An analysis of the flight recorder did not indicate abnormal turbine response.
B777-300 foreign registration (operator not specified)
The thrust levers were moved backwards and forwards very rapidly on approach to Frankfurt/Main.
According to the pilots, there was a delay in thrust increase.
An analysis of the flight recorder did not indicate abnormal turbine response.
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To CONF iture:
That depends on the timing between the moment the spar valves were in transit toward their most restrictive (but not fully closed) position and the moment of engine thrust increase, demanding more fuel flow. If engine acceleration coincided with closing spar valves, the engines could have rolled back before the spar valves reached their most restrictive positions.
Auto throttle commanded the thrust levers to increase thrust to which the engines initially responded. When the engines rolled back and no longer responded to auto throttle, the crew intervened and moved the thrust levers forward manually. This could theoretically have occurred at the moment the spar valves were near or at their most restrictive positions.
This would also have been the most damaging moment to the HP pumps with regards to cavitation. With the fuel flow now drastically reduced and the fuel metering valves fully open, this could have resulted in an imbalance to the fuel flow in the engine fuel circulation circuit (any excessive fuel circulating between FMU to the HP pump and again to the FMU). Technically speaking, manually moving the thrust levers forward could have prolonged this imbalance compared to if the thrust levers were retarded, giving the system time to restore to a stable situation with spar valves transiting back to the full open position. In this scenario, the crew would not have sufficient time to diagnose the situation and did what any crew would have done which is to squeeze as much thrust from the engines as possible so close to “terra firma.”
The EECs (in normal mode) control EPR for engine thrust on a RR Trent 895-17. A stable 1.03 and 1.02 EPR does not necessarily imply that the other parameters, N1, N2, N3 and EGT were stable as well. When the thrust levers were advanced, the EECs would be attempting to reach commanded EPR but N3, N2, N1 and EGT might well have been at varying values while stable EPR values were retained. The AAIB only mentions the EPR values but not the other engine parameters and confirms fuel flow reduction:
“The reduction in thrust on both engines was the result of a reduced fuel flow and all engine parameters after the thrust reduction were consistent with this.”
Question remains what those other consistent engine parameters were indicating.
I stress, the spar valve scenario is purely theoretical. The AAIB reported that “the spar valves were serviceable and operated correctly during the flight.” This implies that the AAIB has evidence in the form of non-volatile memory and/or recorded data, in this case data parameters which would have to have continuously monitored the spar valve positions, in open, closed or in-transit. However, no explanation is given as to how the AAIB reached this conclusion and if indeed the spar valves are monitored and recorded during all circumstances. The text is nicely formulated, so that the public perception is such that sufficient data has been obtained without the need to explain the details.
Regards,
Green-dot
In normal circumstances, MMM should have been around one minute from touchdown, so I figure out it was only 30 to 40 seconds from impact on this day when right engine reduced to 1.03 EPR
According to AAIB, both EPRs reduced and maintained steady values, 1.03 and 1.02, therefore I don't see as plausible scenario that spar valves were constantly in transit, which would have produced, I think, anything but steady values.
According to AAIB, both EPRs reduced and maintained steady values, 1.03 and 1.02, therefore I don't see as plausible scenario that spar valves were constantly in transit, which would have produced, I think, anything but steady values.
That depends on the timing between the moment the spar valves were in transit toward their most restrictive (but not fully closed) position and the moment of engine thrust increase, demanding more fuel flow. If engine acceleration coincided with closing spar valves, the engines could have rolled back before the spar valves reached their most restrictive positions.
Auto throttle commanded the thrust levers to increase thrust to which the engines initially responded. When the engines rolled back and no longer responded to auto throttle, the crew intervened and moved the thrust levers forward manually. This could theoretically have occurred at the moment the spar valves were near or at their most restrictive positions.
This would also have been the most damaging moment to the HP pumps with regards to cavitation. With the fuel flow now drastically reduced and the fuel metering valves fully open, this could have resulted in an imbalance to the fuel flow in the engine fuel circulation circuit (any excessive fuel circulating between FMU to the HP pump and again to the FMU). Technically speaking, manually moving the thrust levers forward could have prolonged this imbalance compared to if the thrust levers were retarded, giving the system time to restore to a stable situation with spar valves transiting back to the full open position. In this scenario, the crew would not have sufficient time to diagnose the situation and did what any crew would have done which is to squeeze as much thrust from the engines as possible so close to “terra firma.”
The EECs (in normal mode) control EPR for engine thrust on a RR Trent 895-17. A stable 1.03 and 1.02 EPR does not necessarily imply that the other parameters, N1, N2, N3 and EGT were stable as well. When the thrust levers were advanced, the EECs would be attempting to reach commanded EPR but N3, N2, N1 and EGT might well have been at varying values while stable EPR values were retained. The AAIB only mentions the EPR values but not the other engine parameters and confirms fuel flow reduction:
“The reduction in thrust on both engines was the result of a reduced fuel flow and all engine parameters after the thrust reduction were consistent with this.”
Question remains what those other consistent engine parameters were indicating.
I stress, the spar valve scenario is purely theoretical. The AAIB reported that “the spar valves were serviceable and operated correctly during the flight.” This implies that the AAIB has evidence in the form of non-volatile memory and/or recorded data, in this case data parameters which would have to have continuously monitored the spar valve positions, in open, closed or in-transit. However, no explanation is given as to how the AAIB reached this conclusion and if indeed the spar valves are monitored and recorded during all circumstances. The text is nicely formulated, so that the public perception is such that sufficient data has been obtained without the need to explain the details.
Regards,
Green-dot
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Green-dot,
I personally find your technical analysis viable, but I like to go further on your other comments:
Thanks to Internet, we can see a huge interest from general public for this specific event as well as others, but still, access to datas is very controlled, limited, and restricted. Give full access to all these datas, and we've got a much better chance to solve this enigma.
But no, everything is confined to an official Authority, with the only participation of the Big corporations, the ones directly involved in this bizarre event ...
And what does Joe public ... ?
Listen ... comply ... and
Do we really want to trust and follow a few in Power without questioning ... Will we ever learn ?
I personally find your technical analysis viable, but I like to go further on your other comments:
The AAIB only mentions the EPR values but not the other engine parameters ...
Question remains what those other consistent engine parameters were indicating.
... However, no explanation is given as to how the AAIB reached this conclusion and if indeed the spar valves are monitored and recorded during all circumstances. The text is nicely formulated, so that the public perception is such that sufficient data has been obtained without the need to explain the details.
Question remains what those other consistent engine parameters were indicating.
... However, no explanation is given as to how the AAIB reached this conclusion and if indeed the spar valves are monitored and recorded during all circumstances. The text is nicely formulated, so that the public perception is such that sufficient data has been obtained without the need to explain the details.
But no, everything is confined to an official Authority, with the only participation of the Big corporations, the ones directly involved in this bizarre event ...
And what does Joe public ... ?
Listen ... comply ... and
Do we really want to trust and follow a few in Power without questioning ... Will we ever learn ?
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Originally Posted by johngreen
The low pressure pumps are something very much like this...
http://www.eatonaerospaceltd.com/fue...ump777_300.pdf
I beleive there are non return valves fitted in both the inlets and outlets and it is one or more of these that I am suggesting as being complicit in my post above.
There are two such pumps on either side of the aircraft and a smaller very similar one (on the left side I think) which is specifically for the APU.
JG
http://www.eatonaerospaceltd.com/fue...ump777_300.pdf
I beleive there are non return valves fitted in both the inlets and outlets and it is one or more of these that I am suggesting as being complicit in my post above.
There are two such pumps on either side of the aircraft and a smaller very similar one (on the left side I think) which is specifically for the APU.
JG
These are the boost pumps, sucking directly from the tanks, located in the wings, and supplying positive pressure to the fuel manifolds.
There are in fact 7 such pumps, two in each wing tank, supplying their respective manifolds, and two in the centre tank, each supplying one manifold. There is an additional, DC-powered, pump in the left wing tank, feeding the left manifold, that can be used to start the APU if both main generators are offline.
I do not know if it can create enough flow to supply one or both main engines.
When one refers simply to "low-pressure pump", one usually means the engine's LP pump, which is part of the engine, and directly driven by the engine, and not the (electrically driven) boost pump.
As far as I know, the engine LP pump is also of centrifugal/impeller type, but whether or not it does have a dedicated check (no-return) valve I do not know.
Bernd
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Bernd
The mechanically driven Fuel Pump mounted on the engine is "Low Pressure"? This is counter-intuitive to this mechanic, the HP pump must be by definition "last in line". Driven by a steel shaft which derives its power from the engine gearbox suggests High Pressure. Also, it is my understanding that the engines will run fine on "suction" alone. Back to school for me. (Read: suction/gravity). My impression also is that the high pressure pump is "geared" or lobed; impellors ("vanes") are more susceptible to cavitation.
Airfoil
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Often the gearbox-driven pump assembly contains both an LP pump along with the HP pump. The purpose of the LP pump is essentially the same as the tank boost/transfer pumps, i.e. to keep a positive pressure head on the HP inlet.
Any positive-displacement pump (whether gear or vane type) can cavitate under the right conditions. In fact a marine propeller can cavitate as well if driven hard enough.
And yes, gravity feed will work just fine if the flow demand doesn't exceed the plumbing capacity/vertical head/viscosity limitations. I know of one application that is always started by gravity feed, but switches to a positive LP boost before reaching idle. (There are probably many such aircraft...)
Any positive-displacement pump (whether gear or vane type) can cavitate under the right conditions. In fact a marine propeller can cavitate as well if driven hard enough.
And yes, gravity feed will work just fine if the flow demand doesn't exceed the plumbing capacity/vertical head/viscosity limitations. I know of one application that is always started by gravity feed, but switches to a positive LP boost before reaching idle. (There are probably many such aircraft...)
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I seem to remember a post soon after the crash about Peter Burkhill retracting a stage of flap at the last minute and this was criticised by the usual gob s h i t e know alls on here.
BA have concluded that this action gained the aircraft a number of feet and almost certainly saved it from the ILS aerial bank, thus saving everyone.
Would the know all idiot who posted some nonsense about negative lift now care to come back on and apologise to Peter?
No?
BA have concluded that this action gained the aircraft a number of feet and almost certainly saved it from the ILS aerial bank, thus saving everyone.
Would the know all idiot who posted some nonsense about negative lift now care to come back on and apologise to Peter?
No?
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airfoilmod,
apologies for the misleading wording.
There are two pumps in the engine, one low pressure, that ensures uninterrupted feed to the high-pressure pump, and often, as shown in the diagram posted by Jet II here a while ago, also pumps the fuel through the fuel/oil heat exchanger and the low-pressure fuel filter.
Some mentioned that the AAIB was concentrating on the FOHE as the location of a possible fuel blockage; this would be consistent then with the engine LP pump not showing cavitation signs, because (at least in the diagram) the FOHE, and thus the blockage, would have been downstream of the (engine) LP pump.
Bernd
apologies for the misleading wording.
There are two pumps in the engine, one low pressure, that ensures uninterrupted feed to the high-pressure pump, and often, as shown in the diagram posted by Jet II here a while ago, also pumps the fuel through the fuel/oil heat exchanger and the low-pressure fuel filter.
Some mentioned that the AAIB was concentrating on the FOHE as the location of a possible fuel blockage; this would be consistent then with the engine LP pump not showing cavitation signs, because (at least in the diagram) the FOHE, and thus the blockage, would have been downstream of the (engine) LP pump.
Bernd
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Flap change
Does anyone know how BA deduced that the flap change was beneficial? Is this an 'official' performance calculation, sim trial or just subjective? Most of us are certainly 'uncertain' of the balance of benefits and disadvantages at such a low height, and it would be useful background info to carry in our mental flight bags.
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Flaps settings
Generally speaking ... (I'm not qualified B777 ... by far).
There is a flaps setting called "approach flaps setting" that allows a N-1 go-around with 2+ % climb gradient.
That "approach" flaps setting causes much less drag than the "full landing" flaps setting while still providing a lot of lift (81% - and generally more - of the full flaps setting).
That approcach flaps setting is generally very close to the maximum take-of flaps setting.
So, reducing from landing flaps to approach flaps is a normal move in case of thrust problems. I would say : a normal, immediate move. The stall margin is reduced by less than 10%. Second immediate move : lower the nose, keep the speed.
In short : Know your aircraft, fly your aircraft.
Whatever the aircraft type.
There is a flaps setting called "approach flaps setting" that allows a N-1 go-around with 2+ % climb gradient.
That "approach" flaps setting causes much less drag than the "full landing" flaps setting while still providing a lot of lift (81% - and generally more - of the full flaps setting).
That approcach flaps setting is generally very close to the maximum take-of flaps setting.
So, reducing from landing flaps to approach flaps is a normal move in case of thrust problems. I would say : a normal, immediate move. The stall margin is reduced by less than 10%. Second immediate move : lower the nose, keep the speed.
In short : Know your aircraft, fly your aircraft.
Whatever the aircraft type.
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Yes - I am aware of that, but
- herein lies the dilemma. You don't give any clue as to what you are 'quailfied on' (or indeed 'if'), but would you suggest doing that at 200AGL/400/600 etc? It is not that simple! Lower the nose and increase speed by up to 10% you say. I wonder where they would have 'landed' doing it that way? What they did worked out. It would be nice to have a little more than that from a qualified source. Indeed on 10 and 9 April you called for the same. Do you now have the answer? M Mouse suggests that BA 'trials' prove there would have been a better way (9 April).
- is a new one to me. Can you substantiate that SOP for any carrier, and perhaps amplify 'thrust problems'? Also I am uncertain what "a N-1 go-around" is and I am also used to a 2.5% 'approach' gradient.
The stall margin is reduced by less than 10%. Second immediate move : lower the nose, keep the speed.
reducing from landing flaps to approach flaps is a normal move in case of thrust problems.
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Regarding water in the fuel. It's not at all uncommon for aircraft on long duration flights to land with fuel temps below 0C (or 32F). I stand to be corrected, but I don't see how a "not significant" amount of water in the fuel could have effected this flight any differently than many other "ordinary" long duration flights.
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Some details from the BBC
An internal British Airways probe into a crash-landing at Heathrow has revealed that the captain and first officer prevented a far worse accident.
It states that both officers tried to keep the plane in the air after it suffered a double engine failure.
The report also suggests an emergency evacuation alarm was too quiet for some of the cabin crew to hear.
The BA probe into the Boeing 777 crash in January will not be made public, but details have been seen by the BBC.
Flight BA 38 from Beijing was extensively damaged when it touched down on 17 January.
Some 136 passengers were safely evacuated from the plane.
The report states that the captain, Peter Burkill, changed the angles of the plane's wing flaps in order to reduce the drag on the plane.
This action helped the plane to maintain speed.
And Senior First Officer John Coward pushed the plane's nose towards the ground in order to increase speed before levelling out just before the Boeing crashed on to the grass. Concerns about the volume of the evacuation alarm is being investigated in the official inquiry into the crash
http://news.bbc.co.uk/2/hi/uk_news/e...on/7409625.stm
It states that both officers tried to keep the plane in the air after it suffered a double engine failure.
The report also suggests an emergency evacuation alarm was too quiet for some of the cabin crew to hear.
The BA probe into the Boeing 777 crash in January will not be made public, but details have been seen by the BBC.
Flight BA 38 from Beijing was extensively damaged when it touched down on 17 January.
Some 136 passengers were safely evacuated from the plane.
The report states that the captain, Peter Burkill, changed the angles of the plane's wing flaps in order to reduce the drag on the plane.
This action helped the plane to maintain speed.
And Senior First Officer John Coward pushed the plane's nose towards the ground in order to increase speed before levelling out just before the Boeing crashed on to the grass. Concerns about the volume of the evacuation alarm is being investigated in the official inquiry into the crash
http://news.bbc.co.uk/2/hi/uk_news/e...on/7409625.stm