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Experience
Regarding BA038 and to a greater extent, other investigations, it's important to note that there are as many perspectives as there are stakeholders in the "outcome". Perhaps closest to the pure neutral position is AAIB, as folks back away from their point of view, parochial vestings start to appear. I think that is why those who annoyingly post "Wait for the Report, etc." have an inarguable point. Long ago, when media was more limited, had fewer outlets and showed a small amount of judgment, the public was less interested, simply because information and venue were so limited. A friend died at DFW in the Delta 1011 windshear crash, another lost #2 (727) on takeoff, still another landed with left main gear collapsed. Incidents inform as well as titillate, and that should be the goal of AAIB BA038. I don't take this thread too seriously (with certain few exceptions), but the expanse of the web permits, no, encourages amateurish opinion and rant. Having been involved in many investigations, (albeit working for folks with a huge stake in the outcome), I can safely say in my opinion AAIB at this stage is well ahead of anyone here, it is the nature of the exercise. Rant off, but armed.
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Parliamentary Reply
I found this in the House of Lords Hansard - Written Answers - 8 May
Lord Bassam of Brighton: The Air Accidents Investigation Branch (AAIB) investigation has identified that when the aircraft was at a height of approximately 700 feet, an increase power demand was received at both engine control units. Despite the correct operation of all engine control functions and sufficient fuel on board the aircraft, the engines did not respond. The focus of the investigation continues to be the fuel system both in the aircraft and on the engines. Under the direction of the AAIB, extensive and challenging full engine testing continues at Rolls-Royce, Derby and some equally, if not more so, challenging fuel system testing is ongoing at Boeing, Seattle, USA. These tests are collectively aimed at understanding and, if possible, replicating the system/engine performance experienced during the accident. Fuel experts and statisticians are also involved in the investigation, reviewing and analysing many thousands of data points recorded on the Boeing 777 fleet of aircraft to see if any trends of statistical significance can further contribute to an understanding of the accident. The Federal Aviation Administration, the European Aviation Safety Agency and the Civil Aviation Authority are being kept fully briefed on the progress of the investigation. |
Originally Posted by airfoilmod
Regarding BA038 and to a greater extent, other investigations, it's important to note that there are as many perspectives as there are stakeholders in the "outcome". Perhaps closest to the pure neutral position is AAIB, as folks back away from their point of view, parochial vestings start to appear. I think that is why those who annoyingly post "Wait for the Report, etc." have an inarguable point. Long ago, when media was more limited, had fewer outlets and showed a small amount of judgment, the public was less interested, simply because information and venue were so limited. A friend died at DFW in the Delta 1011 windshear crash, another lost #2 (727) on takeoff, still another landed with left main gear collapsed. Incidents inform as well as titillate, and that should be the goal of AAIB BA038. I don't take this thread too seriously (with certain few exceptions), but the expanse of the web permits, no, encourages amateurish opinion and rant. Having been involved in many investigations, (albeit working for folks with a huge stake in the outcome), I can safely say in my opinion AAIB at this stage is well ahead of anyone here, it is the nature of the exercise. Rant off, but armed.
In the end, the facts will determine. :ok: In the meantime, it is a worthwhile mental exercise trying to put the pieces together. Even though we know that the jigsaw puzzle is missing a number of important bits. |
More mis-information
Lord Bassam of Brighton:
The Air Accidents Investigation Branch (AAIB) investigation has identified that when the aircraft was at a height of approximately 700 feet, an increase power demand was received at both engine control units. Despite the correct operation of all engine control functions and sufficient fuel on board the aircraft, the engines did not respond. The engines initially responded but, at a height of about 720 ft, the thrust of the right engine the left engine to a similar level.reduced. Some seven seconds later, the thrust reduced on |
AAIB Special Report S-3 Issued May 12
Note the issuance of Special Report S-3 by the AAIB on 12 May.
http://www.aaib.dft.gov.uk/latest_ne...in_s3_2008.cfm |
This should answer a few of the theories postulated on this thread.
http://www.flightglobal.com/articles...-crash-la.html The possiblities for the incident are being narrowed. |
I note the report needs a little editing.
It contains the words :- "...... The autothrottles commanded an increase in thrust from both engines and the engines initially responded. However, at a height of about 720 ft the thrust of the right engine reduced to approximately 1.03 EPR (engine pressure ratio): some seven seconds later the thrust of the left engine reduced to approximately 1.02 EPR. 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. ....." and "...... and the engines, in order to understand why neither engine responded to the demand increase in power when all of the engine control functions operated normally. ....." --------- But seriously, I am amazed that the reduced thrust was so similar 1.03 v. 1.02 EPR ! |
New Special Bulletin.
Today the AAIB issued a new Special Bulletin, updating information of previous reports.
Of note: - No evidence of: . EMI . Core Engine Icing . Bird Strike . Wake Vortex . Fuel Contamination - Actual EPR values: Left 1.03 EPR, Right 1.02 EPR. - Reduced Fuel flow was detected (could be previously inferred, since the previous report stated that the EEC "reacted" to reduced fuel flow) - Flow stayed low after FMV was fully open (refuting "low-energy fuel" and "foaming fuel" theories) - Spar valves operated correctly (previously only stated that they had not been operated by the crew, before the accident) - The cavitation damage on the high-pressure pumps was fresh (I consider this the most interesting new information) - No recommendations by either AAIB, Boeing or RR. Findings so far - Cause of low engine thrust: low fuel flow - Cause of low fuel flow: Not the faintest clue. The upshot: complex test rigs are used at Boeing and Rolls-Royce to recreate the conditions of the flight and hopefully stumble upon something useful. Data recordings of this and other flights are being examined statistically in the hope of finding unusual parameter combinations, since all parameters by themselves appear to be well within the operating envelope of the aircraft. Bernd |
After S3/2008....
Evironmental factors.... fuel flow restriction.... but 'operated within flight envelope'.... as a SLF, I find it puzzling that no SB or general warning to 777 operating companies is deemed necessary: perhaps nature itself will replicate all contributing factors before the AAIB, RR or Boeing manage to do so. taking account of public interest is a good thing for a public body (that is reading PPrune quite thouroughly and dismissing many theories), but this bulletin is rather worrying: no precautionary measures whatsoever - not very reassuring for a frequentt flyer, neither for some 777 crew I suppose. |
Borghha, what would the AAIB recommend? Since the cause is not known yet (and this cause is a real mystery resulting in a difficult investigation), how would you suggest avoiding a repeat?
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well Flight Safety,
how would you suggest avoiding a repeat? |
No Evidence
Originally Posted by borghha
[...] made very clear that there were no mechanical/electrical/electronic failures, nor RFI etc suggests to me that they suspect a link between the prolonged cold soak and the incident.
But since no evidence whatsoever of what went wrong has been found, it may have been one of those things, just without leaving any evidence that could be discovered with the methods employed so far. Bernd |
Narrowing it down to low fuel pressure and low flow from the tanks and fresh cavitation to the hp fuel pumps and the isa temp at altitude was -20 that day causing other flights on that route to descend to FL250 that day continues to point to super cold fuel causing fuel waxing to not let the engines come up to commanded power. How often do they check the 777 fuel sensor circuit for accuracy once it is installed? Obviously their low fuel temp light didn't come on in cruise but why did the others?
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What I hope the AAIB doesn't eventually issue is a "probable cause" result for this accident, meaning "we don't know what caused it, but we think it was this".
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Bubbers44, it's easy enough to test temp probe accuracy in the current configuration. Just instrument the fuel tank at multiple locations and go fly it at cold levels.
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Phraseology
I note with interest the phrasing of the AAIB update:
Begins: Under the direction of the AAIB, extensive full scale engine testing has been conducted at Rolls-Royce, Derby, and fuel system testing is ongoing at Boeing, Seattle. Ends. So, to me that suggests that whatever has been done at Derby now is complete, and that the work at Seattle is continuing. I can imagine that had the work at Derby shown there to be an issue with the engines' fuel systems, then that would have been reflected in today's AAIB update. But... apparently not. So, it looks as if summats up with the aircraft's fuel system, for whatever reason. |
Mr. Gollin
The more similar the results at each engine of a fault in ETOPS the deeper the threat to the concept. By this I mean such "identical" EPR's suggest a "single" fault. Moving in reverse, the further back upstream the fault, the more troubling the implication if it can be shown that it caused simultaneous fuel starvation. Conversely, the more individual the result, the safer the format is from criticism. My first reaction to the report of the short landing was, it's Etops, engine failure is nearly impossible, and dual engine failure is out of the question. Simultaneous dual engine failure? The language in this last report also mentions no "excessive" water in the fuel. That isn't exactly encouraging.
Bsieker- The theory about cavitation being "fresh" was offered by the Boeing Safety Pilot (3-14-08): "not long before the incident". Though it is "fresh" as related to AAIB findings. borgha- I agree the lack of any useful remedial suggestions is troubling. |
Yes it would be possible to do that for certification but very expensive to do that for recurring checks once certified. No one is going to spend thousands of dollars to have maintenance go to max altitude to check one light to see if it comes on at an extreme temp especially if you have to cold soak for hours in the right conditions to make it happen. Most polar flights the light doesn't come on. On this day several flights had it happen and descended to FL250 to get it out. If they had a ground check to calibrate I would be interested in how they would do it, sensor being in the fuel tank submerged with fuel and all. It seems they could check it easily now since they could just cut into the tank and test it in a controlled temperature box and see if it was accurate.
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Apart from the suspiciously similar EPR of the two engines (1.02 and 1.03) there is an interesting line in the report :-
"....... The evidence to date indicates that both engines had low fuel pressure at the inlet to the HP pump. Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows is suspected. ....." I find it almost incredible that two different systems could suffer almost exactly the same restrictions HAVING INITIALLY RESPONDED, but I await the reasons with great interest. . |
I can imagine that had the work at Derby shown there to be an issue with the engines' fuel systems, then that would have been reflected in today's AAIB update. But... apparently not. So, it looks as if summats up with the aircraft's fuel system, for whatever reason. Restrictions in the fuel system between the aircraft fuel tanks and each of the engine HP pumps, resulting in reduced fuel flows, is suspected. NoD |
From AAIB report:
No individual parameter from the flight of G‑YMMM has been identified to be outside previous operating experience. The analysis is concentrating on identifying abnormal combinations of parameters This may not be a specific RR/Boeing problem and could possibly apply to large variety of turbine aircraft that are operated in a certain way... |
An excellent, carefully crafted update from the AAIB.
So, since we are down to a transitory fuel flow reduction affecting both engine subsystems, brought on in conjunction with lower than typical fuel temps, I am at last compelled to point out the following: Toothpaste: Toothpaste counterfeited with poison: Over the years, Chinese counterfeiters have found it profitable to substitute diethylene glycol for its chemical cousin, glycerin, which is usually more expensive. Antibiotics: China’s top drug regulator gets death sentence In one instance, an antibiotic approved by Zheng’s agency killed at least 10 patients last year before it was taken off the market. Zheng Xiaoyu [took] bribes in cash and gifts worth more than $832,000 when he was director of the State Food and Drug Administration...The U.S. Food and Drug Administration also warned consumers not to buy or eat imported fish from China labeled as monkfish because it might actually be pufferfish, which contains a potentially deadly toxin called tetrodotoxin.The warning came days after three southern U.S. states banned imports of catfish from China because they contained traces of antibiotics the FDA says have never been approved for use in aquaculture. Dog food: Chinese authorities acknowledged for the first time that ingredients exported to make pet food contained a prohibited chemical, stepping up their probe of two Chinese companies' roles in one of the USA's largest animal-food recalls. While pledging cooperation with U.S. authorities investigating the recall, the Chinese government in a statement Thursday also disputed that the chemical — melamine, which is used to make plastic — was responsible for harming pets.... "There is no clear evidence showing that melamine is the direct cause of the poisoning or death of the pets," the Ministry of Foreign Affairs in Beijing argued in a prepared statement. Heparin: Federal drug regulators believe that a contaminant detected in a crucial blood thinner that has caused 81 deaths was added deliberately, something the Food and Drug Administration has only hinted at previously. “F.D.A.’s working hypothesis is that this was intentional contamination, but this is not yet proven,” Dr. Janet Woodcock, director of the Food and Drug Administration’s drug center, told the House Subcommittee on Oversight and Investigations in written testimony given Tuesday. A third of the material in some batches of the thinner heparin were contaminants, “and it does strain one’s credulity to suggest that might have been done accidentally,” Dr. Woodcock said. Two weeks ago, Food and Drug Commissioner Andrew C. von Eschenbach told a Senate subcommittee that the contamination was done “by virtue of economic fraud,”... Lead paint: n 2007, the U.S. Consumer Product Safety Commission issued a recall of 1.5 million Thomas toys made in China because they contained unsafe amounts of lead paint. Electronic components: The FBI announced Friday that an investigation into counterfeit network components made in China and sold to the U.S. government has recovered about 3,500 fake devices with a value of $3.5 million. The criminal probe, code-named Operation Cisco Raider, was prompted by concerns that counterfeit network components could give hackers access to government databases. Bolts and fasteners: In 2003, more than 66% of goods seized at ports of entry into this country were traced to China. What are Counterfeiting and Piracy Costing the American Economy. National Chamber Foundation, 2005. http://www.ebearing.com/counterfeit.htm Golf clubs, car parts,books, prunes, razors, film, running shoes... The World's Greatest Fakes -Chinese Copies Are Making Their Way Back To U.S. "We don't want to ignore counterfeiting, but for those foreign companies, when they enter the Chinese market, I'm afraid they should also pay some cost due to the realities of China," says Feng Are we so naive that we don't think the Chinese would doctor up some fuel to pass all bench tests, but still not be quite what it seems? I sure hope a full GC mass spec analysis is being run on the retained fuel samples. |
How would you explain every other flight from PEK to Europe that night having no other issues with fuel?
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"How would you explain every other flight from PEK to Europe that night having no other issues with fuel?"
A quite reasonable question, of course. My best SPECULATIVE answer: 1) Their not having followed the same flight regime (including a host of environmental and operational factors) 2) Them not all being B-777, with perhaps a unique vulnerability in the piping configuration. 3) Their not having received the exact fuel load, from the same reservoir , that BA-038 got In other HIGHLY SPECULATIVE words, I think that substandard fuel was probably a key factor, but one that needed one (or several) other elements to be in place in order to cause the response that occurred -- partial occlusion of the input lines to both HP fuel pumps during decent. Again, a complete GC mass spec analysis should be cheap, easy, and definitive. |
There was 3 tonnes of fuel left in the tanks when the AAIB got hold of the aircraft. They've already checked the fuel and found it to be entirely within spec.
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Plumbing query
Where does the fuel for the APU come from? Is the supply from a specific tank; is it from any and all, with its own complex of valves and pumps; or is it take off the fed to one or both main engines? Does the pipework between the supply point and the APU pumps remain full at all times, or do the pipes drain during normal flight?
Hypothesis: if there are empty pipes leading to the APU, and the APU supply is taken from some point in the system downstream of the first pumps in the path to a main engine, then starting up the APU will cause a low-pressure transient in the supply system. The short term effect, possibly for a few seconds, would be for almost all fuel to take the path of least resistance and so starve the main engines. I understand from some posts too far back to find easily that the APU start sequence was initiated before impact. Was it before of after the start of the final "glider" phase. On a different sub-thread, would flying through and knocking down the boundary fence, rather than hopping over it, have had any significant effect on the outcome? |
Probably not. Raising the flaps got them over the fence and closer to the runway and probably kept the sink rate from increasing with the decrease in drag. The crew did everything right and probably the lesser sink rate on impact saved the day. Whatever they come up with as the reason for no throttle response, the pilots did everything they could, thinking outside the box, to get that 777 down the best way they could.
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Heathrow Crash caused by fuel freeze
WE are discussing this article and the interim report issued monday, may 12, 2008.
Heathrow crash landing 'caused by fuel freeze' By David Millward, Transport Editor Last Updated: 2:32AM BST 13/05/2008 The crash landing of a British Airways Boeing 777 at Heathrow may have been caused by abnormally cold conditions over Russia, air investigators have said. Image**of* REUTERS Experts have homed in on fuel flow as the key issue which led to the crash landing of the Boeing 777 Flight BA38 was forced to make an emergency landing at Heathrow when it lost thrust in both engines at 600ft coming in to land on January 17. Witnesses described seeing the airline bank sharply to the left and miss the tops of houses by 200ft as First Officer John Coward glided the aircraft to safety. It missed the perimeter fence by inches before touching down on the grass, leaving one passenger seriously injured and 12 others slightly hurt. Article continues advertisement On Monday, in an interim report, the Air Accident Investigations Branch (AAIB) said the drop in temperatures to -105F (-76C) may have caused the fuel in the aircraft to thicken during the flight which meant it was unable to get the additional thrust needed to land. Further tests will be carried out to establish precisely what happened. The circumstances leading to the worst aviation accident at Heathrow in more than 30 years will raise some concerns over the safety of the Boeing 777. However, neither Britain's safety regulators, Boeing, nor Rolls Royce, the engine's manufacturers, have recommended operational changes. There are 667 Boeing 777s in service – with 40 in the British Airways fleet – and it is regarded as one of the safest airliners in the world. None has been lost since the plane first flew in 1995. In its report the AAIB has focused on the "region of particularly cold air" between the Urals and Eastern Scandinavia during the 10-hour flight from Beijing to Heathrow. It found that temperatures plummeted far lower than would have been expected for the region. As a result, AAIB experts are examining what this would have done to the fuel and whether this would have caused a change in its consistency. Although the weather was unusually cold, it was not unprecedented and such problems have never been reported before. The AAIB has established that the fuel used on the aircraft was of high quality. While the average freezing temperature of aviation fuel is -53F (-47C), tests showed that the fuel used on the airliner does not turn to ice until -71F (-57C). Tests also found that the fuel temperature throughout the flight never dropped below -29F (-34C). Even though the fuel did not become frozen it could have thickened to an unusual extent, which could have restricted its flow. Fresh tests on fuel are being carried out both at Rolls-Royce's engine plant in Derby and Boeing's factory in Seattle. A number of other theories have been ruled out by the AAIB. They include birds flying into the engine or ice blocking the engine intake. David Learmount, the operations and safety editor at Flight International, said all the evidence was starting to point towards the consistency of the fuel. "There might have been an issue with viscosity – with the fuel becoming thicker and flowing less well," he said. You are here: Telegraph News UK News About us Contact us Forgotten your password? Advertising Press office Promotions Archive Today's news © Copyright of Telegraph Media Group Limited 2008. Terms & Conditions of reading. Commercial information. Privacy and Cookie Policy. |
Is it possible for cold-soaked fuel to start depositing wax on the interior of fuel feed lines, in much the same way plaques gradually fur-up your arteries?
Just a thought that occurred - during the cruise phase, a high flow-rate may prevent this from happening. Throttle back for a descent, lower flow rate, less friction, lower velocity=higher effective pressure (iaw Bernouilli). Cold-soaked fuel may start building wax somewhere along the inside of a feed line, hence restriction. After the fact, wax buildup melts, evidence vanishes. Just throwing a possibility out there - I'm sure somebody much more versed in fluid dynamics than I could critique (read - shoot down!) this theory. |
Intermittent . . .?
Or it was something of an intermittent nature that was not recorded.
Either because something migrated but was restored before reaching a recording parameter. Or it was not on the list of recording parameters at all? The AAIB states: "The spar valves and the aircraft fuel boost pumps were serviceable and operated correctly during the flight." This would imply for the spar valves, to make the AAIB reach that conclusion, that both open and closed positions are continuously recorded and any migration from the open to the closed position but not reaching the closed position is also sensed and continuously, accurately recorded. If so, then the spar valves are ruled out as suspect. Green-dot |
APU Fuel Path
The APU is fed from the left engine fuel manifold, which is supplied by either left boost pump (main tank aft, main tank forward, center tank left), or by an additional DC pump in the left main tank. When, as the AAIB said they did, the boost pumps are working, there is positive pressure in the fuel manifolds.
Starting up the APU (for which there is no evidence) would not lead to a sufficient pressure drop to starve the main engines. The APU fuel demand is tiny compared to the supply capacity of the boost pumps. Bernd |
Yet another idea!
The latest AAIB report has indeed focussed our understanding and has allowed different weighting to be applied to different potential scenarios.
Now the significant new bit of data (IMHO) is the similarity of the actual EPR at WOT (wide open throttle). What could cause such similar limiting EPR on BOTH engine installations? I believe we have to be talking of shortcoming in either Design Concept or more likely Standards. i.e. everything was within the agreed operating parameters which is why nothing untoward can be found. I therefore make the following contention. As fuel temp decreases its propensity to wax increases but this transition (I believe) is not clear cut being dependent upon the degree of molecular disturbance/agitation. Now the fuel must have been flowing adequately to maintain cruise requirements which I assume would have been similar to that demanded during the ill fated finals. However during the descent the reduced fuel demand at flight idle MAY have allowed already very cold fuel to transition to wax due to reduced molecular agitation. This would fit with the indication from the AAIB that (low temperature?) flow testing is being undertaken. In addition I have always been "suspicious" about the fuel. Its wax point has been verified at -57C, a full 10 degrees better than specification. I keep asking. Why? So maybe the fuel was within specification but does anybody have any idea how this test is conducted (i.e. Is the fuel placed (statically) in a vessel in a lab freezer or is the fuel agitated in someway during the test?) I also remain surprised that no directive has been issued requiring a temporary tightening of minimum fuel temperature permissible. I look forward to your response and further debate. |
http://www.paclp.com/product/ISL/lit_isl/FZP%205G2s.pdf
"The freezing point of jet fuel is defined as the temperature at which the last crystal melts when warming a fuel that has been previously cooled until hyrdocarbon crystals form" http://www.expotechusa.com/catalogs%...g/05-Fuels.pdf Page 94 The sample is cooled in the test chamber with constantly stirring. The sophisticated dynamic measurement system emits a light pulse every 0.58C from a coaxial fiber optic cable positioned above the test sample. The light pulse is then reflected off the silvered-bottom test jar to an optical sensor. The advanced software package analyzes the response of the light pulse. The initial appearance of crystallization is monitored by light scattering. The sample is then warmed up, and the temperature at which the hydrocarbon crystals disappear is recorded as the freezing point. |
bsieker says (re May 12 report )
- Flow stayed low after FMV was fully open (refuting "low-energy fuel" and "foaming fuel" theories) Please explain the logic of your dismissive comment. |
Originally Posted by arcniz
Originally Posted by bsieker
- Flow stayed low after FMV was fully open (refuting "low-energy fuel" and "foaming fuel" theories)
Please explain the logic of your dismissive comment. I assume the Fuel Flow Transmitter measures volume flow, and not mass flow. I am also unsure foam would remain foamy in the high pressure conditions downstream of the HP pump. Foam anywhere else, contributing to a fuel flow restriction, and allowing only a minor amount of (liquid) fuel to be pumped to the nozzles, would still be a possibility. I am not a fuel chemist, so I won't comment on the likelihood or mechanics of that. |
I assume the Fuel Flow Transmitter measures volume flow, and not mass flow. |
Mass Flow Transmitters
hetfield,
thanks a lot. I've been reading up a little and it appears that due to the widely varying temperatures (I'd reasonably expect somewhere between +40 and -40 degrees C) and densities of jet fuel, mass-flow transmitters are commonly used, probably in RR engines, too. Energy content per mass unit is almost constant, but varies per volume unit, so that makes sense. That would also mean that fuel foam at full volume flow would still register low (mass) fuel flow, contrary to my earlier comments. Bernd |
Ref: Fuel freezing point, Actual vs Spec
A specification is set out by the purchaser or authority over an industry for any given chemical substance (including aviation fuel). The specification sets out for some parameters the lowest acceptable quality of the substance, and for other parameters the widest acceptable range. Having worked in the industrial chemical business for the past 14 years, I know it is not unusual for a chemical substance to be of a far higher standard than that set out on the Specification Sheet.
The reason the spec is put together this way is to allow for variations of product characteristics due to slight differences in manufacturing processes, slight differences in constituent products, slight differences in ambient conditions during the manufacturing processes and various other deviations during manufacture. Someone more familiar with the cracking of hydrocarbon fuels will no doubt provide the specifics of these variances and their corresponding tolerances within the spec. In this instance, the lowest acceptable quality of the fuel is a freezing point of -47C, but the fuel itself has an actual freezing point of -57C (therefore it's quality is above spec). While this does not rule out the presence of a contaminant, it is certainly not a positive indicator of one. |
Fuel waxing in cold temperatures
This may have no relevance to aviation fuel, however:
I lived in Germany for 25 years and during that time we had some extremely cold surface temperatures - I remember -27ºc on several occasions. Having always driven diesel cars - from before the time when "winter grade diesel" was commonly available, I occasionally experienced waxing of fuel with consequent inability to start the car. The waxing either builds up in or causes blockage of the extremely small passages in the injector pump, at a time when fuel in the supply pipeline is still flowing easily. The wax persists for long after the vehicle has been towed into a heated garage: the rule-of-thumb used to be 24hrs of heat-soak before trying to restart the car. |
The heat exchanger is the only part still being studied as a contributing cause, according to today's Washington Post.
But the recorders quickly disproved that theory, revealing no errors in the computer signals sent from the cockpit to the engines, according to reports and sources. Meanwhile, investigators were looking at the plane's fuel supply. An empty gas tank would have led to a thrust reduction, but the plane's tanks still contained plenty of high-quality fuel, investigators said in reports. When they took apart the engines, investigators found evidence on pumps that the engines had been starved for fuel in the moments before the crash. That led them to conclude that the fuel supply had become blocked somewhere between the plane's tanks and its engines. Sources familiar with the probe said engineers suspect that ice collected in or near a fuel-oil heat exchanger on each engine, blocking the fuel supply. The heat exchanger uses cold fuel to cool hot engine oil, and the hot engine oil to warm fuel before it is injected into the engines. The heat exchanger is the only point in the system that engineers have not yet eliminated as the potential bottleneck, the sources said. But the ice theory is not perfect. Plenty of jets fly through cold weather, and the 777's fuel contained additives designed to prevent it from freezing under such conditions. Outside experts noted that the ice theory seems implausible for another reason: Each engine should have been drawing fuel from a separate tank. The chances of ice breaking off in separate fuel tanks and blocking the fuel supply in each engine at nearly the same moment is almost too tiny to comprehend, said John Goglia, a former member at the National Transportation Safety Board, which investigates air crashes in the United States. "This isn't supposed to happen," Goglia said. "These are two independent systems." |
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