Kaitak From Ireland, joined Aug 1999, 7674 posts, RR: 18 Reply 1, posted (8 months 7 hours ago) and read 16089 times:
The AAIB seems to have traced it to this: signs that the fuel pressure at the inlet to the engine high pressure (HP) fuel pumps had dropped because the pumps showed signs of “unusual and fresh cavitation damage”.
This would seem to exonerate the crew, which is good news. Hopefully Capt. Burkill and FO Coward can be returned to flight status asap.
"What are we going to do tonight, Brain?" "Same thing we do every night, Pinky. Try to take over the world".
Ikramerica From United States, joined May 2005, 15875 posts, RR: 49 Reply 2, posted (8 months 7 hours ago) and read 16033 times:
So the fuel was waxy or frozen but not contaminated it would seem. As for exonerating the flight crew, lets wait and see. There were reported warning lights before the flight started, we don't know about warnings in the air or variations in flight procedures, and there is no part of this article that discusses how the final seconds of the flight were handled (other threads with pilot input indicate a lack of consensus on whether the PIC made the right or wrong choices in those seconds).
Of all the things to worry about... the Wookie has no pants.
GRIVely From United States, joined Dec 2006, 95 posts, RR: 0 Reply 5, posted (8 months 5 hours ago) and read 15444 times:
Having just flown back from LHR yesterday (11 May) to IAD onboard UA925, a 777, I have been following this matter closely and was interested to read the AAIB bulletin. Having done so carefully I realize we still don't know anything more than we did before the bulletin was issued. I am sure we would all like to know what sorts of "restrictions" occurred between the fuel tanks and the HP fuel pumps as well as the "unusual and fresh cavitation damage".
Perhaps one of these days we will actually learn what caused these vexing conditions.
Litz From United States, joined Dec 2003, 1473 posts, RR: 0 Reply 7, posted (8 months 3 hours ago) and read 14164 times:
Cavitation happens when some kind of fluidic blade (ie: a propeller or impeller) tries to spin in a liquid and generates bubbles.
This can happen because the blades are moving faster than the fluid can be moved, or because there is insufficient fluid (e.g. low pressure), and the action of moving the fluid causes 'voids'.
Either way, you get areas of extreme differences in pressure which can cause damage to the blades.
This happens w/impellers in pumps when the fluid level is low, and in boats/ships/subs/etc when the propeller spins too fast.
See Hunt for Red October for good examples about cavitating propeller blades and their effects.
SEPilot From United States, joined Dec 2006, 3155 posts, RR: 9 Reply 8, posted (8 months 2 hours ago) and read 14121 times:
Quoting Rbgso (Reply 6):
What does this mean in laymen's terms?
It means that the pump was turning but not as much liquid was reaching it as it wanted, which caused the fuel that did reach it to essentially boil, and the gas bubbles that formed left visible damage on the pump parts. This happens on boat propellers that turn too fast, or are partially out of the water when turning fast.
The problem with making things foolproof is that fools are so doggone ingenious...
ZANL188 From United States, joined Oct 2006, 1347 posts, RR: 0 Reply 9, posted (8 months 2 hours ago) and read 14030 times:
Quoting Rbgso (Reply 6): What does this mean in laymen's terms?
The pumps were starved of fuel and began to cavitate.
Cavitation from Wiki:
"Cavitation is defined as the phenomenon of formation of vapour bubbles of a flowing liquid in a region where the pressure of the liquid falls below its vapour pressure.Cavitation is usually divided into two classes of behavior: inertial (or transient) cavitation and non-inertial cavitation. Inertial cavitation is the process where a void or bubble in a liquid rapidly collapses, producing a shock wave. Such cavitation often occurs in pumps, propellers, impellers, and in the vascular tissues of plants. Non-inertial cavitation is the process where a bubble in a fluid is forced to oscillate in size or shape due to some form of energy input, such as an acoustic field. Such cavitation is often employed in ultrasonic cleaning baths and can also be observed in pumps, propellers etc."
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Rivet42 From United Kingdom, joined Aug 2005, 704 posts, RR: 1 Reply 10, posted (8 months 1 hour ago) and read 13232 times:
Quoting Hotelmode (Reply 4): The fuel never got anywhere near its proven freezing point
I don't see how you can make that assumption - all we know is that samples of the fuel froze in tests at -57'c, the lowest recorded temperature during the flight was -45'c, but the Met Office reported atmospheric tempuratures down to -76'c. It is therefore perfectly plausible that portions of the fuel may well have reached 'near-freezing', and it wouldn't have to actually freeze for the density and fluidity to change - imagine a tank of fuel with semi-solid 'lumps' swimming around inside it, and then consider how this might affect - at random - the flow of fuel towards the engines. I'm not speculating on anything here, I'm just trying to show that your 'conclusion' is in itself speculation not born out by the evidence.
Roseflyer From United States, joined Feb 2004, 5321 posts, RR: 17 Reply 11, posted (8 months 1 hour ago) and read 12782 times:
Each wing tank has a forward and aft boost pump. Inlets are on the forward and aft side of the tanks for the pumps. There are three pick up points in each wing tank. The fuel is then sent to the engines from there. It can go from either tank to either engine.
It amazes me that the pumps could all cavitate. That pretty much means that there was no fuel or restricted flow in those lines. What could have caused it for all the lines? It will be interesting to see. At least we know there was fuel in the tanks and it was of an acceptable quality now.
Ikramerica From United States, joined May 2005, 15875 posts, RR: 49 Reply 13, posted (8 months ago) and read 12375 times:
Quoting Awthompson (Reply 12): I just watched the TV review of tomorrow's newspapers and one major newspaper is reporting "fuel freeze" as the cause of this crash. Views?
Not an expert, but sounded like it from the start, or the after effects of such a freeze, including uneven expansion and separation after unfreeze and damage to the systems from the "lumps". What caused the freeze was the question for me, the cold temps (one of the coldest winters ever known in the region the plane flew), contaminated fuel (what I thought would end up being a contributing factor), the wrong flight path chosen in light of conditions, error on the part of the crew when dealing with warnings, etc.
but also, what lead to the crash that occurred vs. a worse crash vs. not having a crash at all? Again, we don't know yet, but to say the pilots were not in any way at fault, it's too soon to say. Maybe they did such a good job they averted death to all. Maybe they made a critical mistake that led to a crash instead of a skin of the teeth landing. We don't know yet.
Of all the things to worry about... the Wookie has no pants.
Baw716 From United States, joined Nov 2003, 1816 posts, RR: 32 Reply 14, posted (8 months ago) and read 12375 times:
Quoting Rivet42 (Reply 10): imagine a tank of fuel with semi-solid 'lumps' swimming around inside it, and then consider how this might affect - at random - the flow of fuel towards the engines. I'm not speculating on anything here, I'm just trying to show that your 'conclusion' is in itself speculation not born out by the evidence.
Question:
Are the HP pumps in the actual tank itself or are they outside the tank with lines leading from the tank to the pumps?
I have a theory, but I don't want to share it until I get this question answered.
thanks
baw716
David L. Lamb, fmr Area Mgr Alitalia SFO 1998-2002, fmr Regional Analyst SFO-UAL 1992-1998
BristolFlyer From United Kingdom, joined May 2004, 1849 posts, RR: 0 Reply 15, posted (8 months ago) and read 12349 times:
Quoting Awthompson (Reply 12): one major newspaper is reporting "fuel freeze"
Which newspaper was this? I read an article in the Scum (sorry, Sun) from a journalist that was encouraging people to boycott BA 'cos he suspected the crash was caused by fuel running out. T*sser. I was on a BA flight the next day.
BAW716 From United States, joined Nov 2003, 1816 posts, RR: 32 Reply 16, posted (8 months ago) and read 12245 times:
Quoting BristolFlyer (Reply 15): Which newspaper was this? I read an article in the Scum (sorry, Sun) from a journalist that was encouraging people to boycott BA 'cos he suspected the crash was caused by fuel running out. T*sser. I was on a BA flight the next day.
Ah, jounalistic expertise at it's finest (eh, um)...
That's about as silly a notion as I've ever heard...perhaps the journalist would like to observe a 777 engine operating up close?
baw716
David L. Lamb, fmr Area Mgr Alitalia SFO 1998-2002, fmr Regional Analyst SFO-UAL 1992-1998
Pellegrine From United States, joined Mar 2007, 207 posts, RR: 0 Reply 17, posted (8 months ago) and read 12231 times:
Quoting Rivet42 (Reply 10): It is therefore perfectly plausible that portions of the fuel may well have reached 'near-freezing', and it wouldn't have to actually freeze for the density and fluidity to change - imagine a tank of fuel with semi-solid 'lumps' swimming around inside it, and then consider how this might affect - at random - the flow of fuel towards the engines.
The properties of the refined hydrocarbon at question here, Jet A-1, don't really reflect that from the information I've read. "Freeze point" isn't the temperature at which the fuel freezes solid, it's the temperature at which the last solid crystal melts AFTER the fuel has already been frozen. When they conduct a spot test, they drop the temp of the sample down to -100-120*C, let it warm up, and mark the temperature as the last solid disappears. The pour point, under which the fuel does not flow appreciably as a liquid, is a few degrees below freeze point. The fuel tested with an even wider margin than required. I can't really see how fuel icing could have been a problem, especially since the lack of thrust happened on approach not during cruise or TOD (top of descent). By that point the fuel temp could have gone up a degree or two (input from one of the long-haul pilots on here would be welcome).
Crjfixer From United States, joined Mar 2008, 86 posts, RR: 0 Reply 18, posted (7 months 4 weeks 1 day 22 hours ago) and read 11504 times:
Also aircraft fuel is heated by a fuel/oil heat exchanger ( cools the engine oil while warming the fuel ). so i highly doubt freezing fuel could have been a factor.
JetMech From Australia, joined Mar 2006, 1958 posts, RR: 36 Reply 19, posted (7 months 4 weeks 1 day 22 hours ago) and read 11412 times:
Quoting Baw716 (Reply 14): Are the HP pumps in the actual tank itself or are they outside the tank with lines leading from the tank to the pumps?
The fuel tanks have electrically powered boost pumps ( usually centrifugal impellers ) which deliver fuel to the engine driven fuel pump. The engine driven fuel pump is mechanically driven off the accesory gearbox, and has two stages, a low pressure stage ( usually a centrifugal impeller ), and a high pressure stage ( usually a gear type pump ).
The system can continue to operate if the electric boost pumps fail, as there is sufficient suction in the low pressure side of the engine driven fuel pump, which combined with gravity, will still allow fuel to reach the engine.
Regards, JetMech
[Edited 2008-05-12 18:57:31]
The universe revolves around engineers as we choose the co-ordinate system!
Tdscanuck From Canada, joined Jan 2006, 4005 posts, RR: 28 Reply 20, posted (7 months 4 weeks 1 day 20 hours ago) and read 10850 times:
Quoting GRIVely (Reply 5): Having done so carefully I realize we still don't know anything more than we did before the bulletin was issued.
Sure we do. We know it wasn't contaminated fuel, an aircraft or engine malfunction, or water in the fuel. That's a lot of potential causes that have been eliminated.
Quoting Rivet42 (Reply 10): It is therefore perfectly plausible that portions of the fuel may well have reached 'near-freezing', and it wouldn't have to actually freeze for the density and fluidity to change - imagine a tank of fuel with semi-solid 'lumps' swimming around inside it
Pellegrine beat me to it in Reply 17...if you've got semi-solid "lumps" swimming around in the fuel tank, you're already way below the freeze point. Freeze point on jet fuel is when you *first* start getting solids (or when the last solid melts, if going the other direction), not when it goes solid.
Quoting Roseflyer (Reply 11): Each wing tank has a forward and aft boost pump. Inlets are on the forward and aft side of the tanks for the pumps. There are three pick up points in each wing tank. The fuel is then sent to the engines from there. It can go from either tank to either engine.
It amazes me that the pumps could all cavitate.
As I read it, the cavitation was on the HP pumps, not the boost pumps. There are only two HP pumps, one on each engine, and the fuel from all boost pumps is co-mingled long before it gets to the HP pump.
Quoting Baw716 (Reply 14): Are the HP pumps in the actual tank itself or are they outside the tank with lines leading from the tank to the pumps?
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