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Originally Posted by lomapaseo
If you support grounding, then just how far do you go?
That answers the question "how far do you go?": maybe all the way, depending. PBL |
What Boeing is doing
Boeing doesn't know the answer, apparently, but it knows what it's looking for:
http://www.flightglobal.com/articles...h-landing.html SF |
From the URL above:
"Boeing's chief pilot flight operations safety division Capt Dave Carbaugh says the fact that the fuel pumps, according to the UK Air Accidents Investigation Branch (AAIB), had been cavitating not long before the impact" Is my mind going? I only remember a statement in the latest AAIB report that there was damage caused by cavitation, not anything about when it might have occured. |
Cats, be assured your mind's not going. Your link to the AAIB's statement proves that. But had you wondered whether Capt Carbaugh might know something you don't?
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Question 1: If we cracked open a 100 average fuel pumps on a 777, how many would show signs of cavitating?
Question 2: How could one tell that the cavitation was recent, and if recent how recent (aka this flight vs flight before)? |
My (layman's) understanding of the AAIB investigation process is that the manufacturer (and airline for that matter) will be closely involved at all stages. It seems likely, therefore, that Boeing and their chief pilot would have inside information about the direction the investigation is going.
What surprises me is that he "went public". Bobbsy |
To Question 2: perhaps the output pressure would show fluctuations which (depending on the sample rate) could indicate that the pump is cavitating?
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Just remember that the cavitation is an indication that the pump had been operating with low inlet pressure, the pump itself would only have its performance mildly affected- ie not enough to cause the event
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It's fuel
SuperT
I'm not sure we can imply that a problem with cavitation is only a function of low inlet pressure on a pump - with or without nucleation within the liquid phase, although I agree, nucleation will help considerably. Presumably you're imply waxing or freezing of the fuel thereby restricting flow rates to the donkeys, lowering fuel pump inlet pressures and causing the cavitation. My own non specialist view is that the waving/freezing hypothesis died the death aeons ago on this most worthy of threads in the light of the published AAIB data showing fuel fpt to be < - 50 C - a suspiciously low figure. The following, somewhat familiar by now hypothesis, alleges no failure of the pumps. I'm rusty on this but cavitation can easily produce a mixture of true gas phase mixture fuel components (which may or may not contain air as well) from the mixture alone by mechanical induction in a liquid phase mixture if component(s) in the fuel mixture have a low enough Mr or levels of intermolecular bonding and the kinetic energy input is high enough. I'm guessing, but I reckon any gas phase component's t½ at say +80C will be long enough for it to be a nuisance. And and ……if the fuel has stratified in the tanks as well, with the low Mrs with their lower densities at the top of the tanks, they'll be fed to the donkeys at higher concentrations just when the FADECs go with throttle up so to speak, at the end of the flight. As an aside, the pumps are defined as serviceable by the AAIB interim report. Now I take that to mean that they're within spec and I presume spec refers to volumetric efficiency against a liquid phase. If the poor things are trying to pump lots of eminently compressible of bubbles we will have considerably reduced flow rates and outa-gas symptoms. Just what were the additives? What was their Mr? It's already been said and it's still true, AAIB have done their job entirely to spec and we have to wait for the full report, but hey this is PPRuNe after all! :ok: |
Fuel system tests.
What Boeing is doing Boeing doesn't know the answer, apparently, but it knows what it's looking for: http://www.flightglobal.com/articles...h-landing.html SF Reading the article, i assume the tests will not be limited to fuel properties alone, as the article suggests, but fuel system tests including failure simulations of fuel system components upstream of the engine pumps? I would not be surprised they will do tests equivalent to what i have posted in post #590. In this case for 2 donks instead of one. Regards, Green-dot |
Cavitation
Just to build a little on the last few posts:
The investigation reports that the fuel pumps are serviceable. I take that to mean that the flow/pressure characteristics, when the pumps are bench tested on fuel, are within specification. But when the cavitation damage occurred (and, depending on the causes, the design of the flow paths, and the metallurgy, that might have occurred relatively quickly), we can speculate that the output from the pump(s), would have been some dual phase mixture. If such a mixture was being passed by the pumps at the time the FADEC requested a higher fuel flow in response to the increased power demand, it is possible that the fuel flow rate within such a mixture would be insufficient. |
My (layman's) understanding of the AAIB investigation process is that the manufacturer (and airline for that matter) will be closely involved at all stages |
Being involved in automotive design for many years may I answer a question from a day or so ago, cavitation damage can appear very rapidly, even to the point of destroying a pump in a matter of minutes in extreme cases.
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But when the cavitation damage occurred (and, depending on the causes, the design of the flow paths, and the metallurgy, that might have occurred relatively quickly), we can speculate that the output from the pump(s), would have been some dual phase mixture. If such a mixture was being passed by the pumps at the time the FADEC requested a higher fuel flow in response to the increased power demand, it is possible that the fuel flow rate within such a mixture would be insufficient. |
Pump Damage Rate
smuff2000 wrote: "Being involved in automotive design for many years may I answer a question from a day or so ago, cavitation damage can appear very rapidly, even to the point of destroying a pump in a matter of minutes in extreme cases."
And I would expect that, since the pressures and flow rates generated by these pumps are many times that ever found in any automotive applications, the damage rate would be even faster. |
Chris weston
We've been waiting for someone like you to enhance our knowledge of the 'phenomonen' of cavitation of our high pressure fuel pumps. But then you confuse us by introducing Mr and Mrs without explaining what that happens to be. Help!! I know a little more than most about cavitation having been triggered into researching the subject after having an irretrievable flame out on a Canberra's left Avon engine followed by the loss of the remaining right engine on short final. Fortunately we just made it to the under run and rolled out on the runway, much to the relief of the nav who had been tempted to leave via his ejection seat. But that incident was caused by the failure of all four variable stroke piston pumps which rapidly disintegrated when they started to pump water instead of fuel. There were some signs of earlier cavitation damage discovered. Experience in those days soon revealed that short periods of operation of the HP pumps at high altitude soon had HP pumps cavitating and then we were learning that jet fuel starts to boil/gassify/produce vapour as the suction side of HP pumps reaches some low value not far above a vacuum. Presumably there will be a temperature variable. I asked in a previous post whether anyone could produce some feel or appreciation for the likely pressure below which one could expect cold fuel, with some water content, to start vapourising which would consequently mark the onset of HP pump inlet cavitation. Such a number will set a bench mark as to the extent of fuel flow restriction necessary to substantially reduce output flow. I would expect that fuel cavitating at the pump intake would instantly return to total liquid at the considerably higher pressures at its outlet with its flow then moderated by the FADEC demand. Can you elaborate and give us an enhanced comprehension as to the operation of the conventional HP pumps particularly under marginal conditions. Apart from opening up further instructive speculation this will prompt pilots to a better appreciation of the train of events which result when they move those thrust levers and the booster pump switches. |
A thought:
What is the situation with EROPS/ETOPS certification? It would seem that there should be some reassessment in view of the uncertainty. |
If both donks dead, forget about ETOPS.
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What is the situation with EROPS/ETOPS certification? It would seem that there should be some reassessment in view of the uncertainty. |
Mr and Mrs Cavitation
Milt,
I must needs disappoint and apologise to you if you feel I have mislead you and others, engineer I ain't and I can't advise on something as safety critical as the operation of conventional HP pumps in likely cavitation situations. I can speculate that the 777 had a fuel mixture problem probably revolving around additives. I have great respect for those people such as yourself for whom cavitation is a not merely an armchair and academic exercise. I'm simply a pure-ish chemist who has the privilege to work with some competent Chem Eng people who, amongst other things, pump a lot of hydrocarbons around. We agree that any gas phase hydrocarbons et al will go back to liquid phase more or less instantly and that their half life (t ½) will be short. Mr is the Mass of a Molecule on the C scale and I am further to blame for not rendering it as such in my text. I typed the r s as the subscripts they should be at the time but on cutting and pasting, the site rendered them as "Mr" and in the plural as "Mrs" - I forgot the understandably limited word processing on the site, Word it ain't. I also typed +8 (superscript) O Centigrade which became +80C ………. a singularly startling LHR Jan temperature. My fault, not Danny's, I know the site does this, mea culpa. CW |
additives?
Chris Weston said
I can speculate that the 777 had a fuel mixture problem probably revolving around additives. . The scenario below describes Boeing's awareness of a problem but poor analysis of it and absolutely no extrapolation to a situation where the inflight temperatures were largely irrelevant - simply because the water in the center-section tank had been frozen (beijing overnight temps being below 7 degrees Celsius), therefore not able to be sumped (i.e. drained) and accumulating over a lengthy period. Furthermore Boeing's engineers failed to foresee or entertain any scenario wherein thrust could be affected. Boeing simply addressed the undesirability of the icy accumulations making inroads into the planned reserve fuel. SUBJECT: 777-300ER/777-200LR Failure to Scavenge Fuel /A/ Service Related Problem 777-SRP-28-0118 /B/ Fleet Team Digest Article 777-FTD-28-07002 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - SUMMARY: Note: This message contains important information relevant to flight operations and airplane dispatch, please distribute accordingly. Several 777-300ER operators have reported intermittent occurrences of airplanes landing with as much as 2200 lbs/1000kgs/300 gallons of fuel in the center tank. Boeing theorizes that this is an indication that the fuel scavenge system has malfunctioned. A failure such as this of the fuel scavenge system reduces the range of the airplane and could potentially lead to fuel exhaustion in the event additional failures occur which require use of all planned reserve fuel. To address this concern, Boeing recommends that 777-300ER and 777-200LR operators review their fuel reserve policy to ensure adequate reserves exist for each mission. - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - DESCRIPTION: The scavenge system is designed to transfer fuel from low areas of the center wing tank to the main tanks after the override pumps are shut off. Scavenging this additional fuel from the center tank increases the fuel available for engine use. The 777-300ER and 777-200LR airplanes have incorporated scavenge system design changes intended to increase the amount of fuel scavenged and reduce the amount of trapped unusable fuel in the center tank to approximately 3 gallons. These changes included relocating the fuel scavenge inlet further inboard, while the water scavenge inlet location remained unchanged. Additionally, the fuel scavenge outlet and float valve were moved further outboard to allow fuel scavenge to be initiated earlier in flight. Several 777 -300ER operators have reported intermittent occurrences of airplanes landing with as much as 2200 lbs/1000kgs/300 gallons of fuel in the center tank. Boeing theorizes that this is an indication that the fuel scavenge system has malfunctioned. These instances have only occurred on long routes originating from colder climates and have led to the conclusion that an excessive amount of water is entering the fuel scavenge system and is freezing during scavenge operations (and for sumping?). Because the water scavenge inlet was not co-located with the fuel scavenge inlets it is more likely for water to be ingested into the scavenge system. Additionally, as the outlet float valve location is further outboard in the main tank than previous, the scavenged center tank fuel has more exposure to the cold soaked main fuel tank prior to reaching the scavenge discharge. Indications are that the water in the scavenge system is freezing prior to discharging in the main tank. Frozen water (or ice) in the scavenge system could result in a low rate of scavenge or no fuel scavenge. (But ice in the form of melting slush?) Failure of the fuel scavenge system could result in airplanes landing with as much as 2200 lbs (1000 kgs) of fuel in the center tank. During mission planning and dispatch, this fuel in the center tank was considered usable fuel. However, failure of the fuel scavenge system in flight renders this 2200 lbs (1000 kgs) of fuel as unusable. There is no indication to the flight crew that the scavenge system has failed nor that the fuel is unusable. Failure of the fuel scavenge system essentially reduces the range of the airplane and could potentially lead to fuel exhaustion in the event additional failures occur which require the use of all planned fuel reserves. Boeing review has determined that the failure to completely scavenge the center tank is the result of system configuration changes unique to the 777-300ER and 777-200LR airplanes. This issue has been placed in our Service Related Problem (SRP) process for resolution and is the subject of the REF /B/ Fleet Team Digest article. DESIRED ACTION =============== Boeing recognizes each operator establishes its own fuel reserve policy. Some operators choose to add additional conservatism to existing regulatory fuel reserve requirements. In addition, we note that not all routes and/or operators have shown a susceptibility to this condition. This may be because of environmental conditions, individual airline water sumping policies, or different operator fuel system procedures. Boeing suggests 777-300ER and 777-200LR operators review their operation for exposure to trapped center tank fuel and their maintenance policy related to water sumping. We recommend operators establish a policy to monitor center tank fuel quantity upon arrival of each flight. If trapped center tank fuel above 400 lbs (200 kgs) is discovered, we recommend a further review of fuel reserve and maintenance policies as noted above. If operators chose to address this issue by uploading additional fuel, Boeing recommends operators notify their flight crews that additional fuel has been loaded to mitigate the potential for up to 2200 lbs (1000 kgs)of unusable fuel following failure of the scavenge system. For operators who have seen the trapped center tank fuel condition and chosen to adjust their fuel reserve policy, we recognize it may be possible for this condition to be resolved on future flights due to a change in environmental conditions or maintenance practices.. If this situation arises, we believe it appropriate to adjust fuel reserve policies to original levels provided they continue the monitoring policy on a flight by flight basis for trapped center tank fuel. Although these failure to scavenge occurrences have only been reported on the 777-300ER, any Boeing recommendations should also be applied to the 777-200LR as it has an identical center tank fuel scavenge system. If further information is needed regarding the subject, please contact your local Boeing Field Service Representative. If your local Field Service Representative is unavailable, you may contact the appropriate Airline Support Manager or call the BCA Operations Center at (206) 544-75 |
:eek: QUOTE]Boeing doesn't know the answer, apparently, but it knows what it's looking for:[/QUOTE]
Yeah - It's called GAS, as in AVTUR ;) |
because the water in the center-section tank had been frozen (beijing overnight temps being below 7 degrees Celsius), therefore not able to be sumped (i.e. drained) and accumulating over a lengthy period. The temperature in Beijing was -7 deg. Celcius according to AAIB Bulletin S1/2008, page 3. I assume the poster intended to mention it as -7 deg. C. On page 5 of the same bulletin the AAIB states: "The aircraft's fuel tanks were last checked for water in the fuel on the 15 january 2008 at Heathrow; this was prior to its refuelling for the outboard sector to Beijing." For the record i just checked: temperatures at Heathrow on 15 january were between +6 and +11 degrees Celcius, depending on the time of day. Warm enough to drain water. For sure if the airplane had been at Heathrow for a considerable time. I don't know the aircraft's flight history prior to the outbound sector to Beijing so alot depends on the aircraft's turnaround time between flights. But if it had an overnight stay, temperatures were well above freezing at Heathrow the day before (the whole week before, for that matter). Any water drained in such conditions is all the water accumulated in the tanks prior to refuelling. Fuel samples taken from the aircraft after the crash have sofar ruled out contamination or unusual levels of water content, neither did sump samples taken from the main wing tanks reveal "significant quantities of water" according to AAIB Bulletin S1/2008, page 4. Apparently water did not accumulate over a lengthy period. Green-dot |
Just 3 questions to the experts:
1 If there is sign of cavitation in the pumps, I presume that it involves wear on the impellers. What happens to that wear or debris; does it travel downstream or is there a filter to capture it before it enters the cans? 2 When the propeller on my boat cavitates the RPM increases but the boat goes nowhere; the prop effectively stalls. Does the same happen to the fuel pumps? 3 If temperature of fuel is an issue why hasn't anyone restricted the altitude or fuel temp of the 777 as a "safety first" prudent measure or are they looking for another incident to verify their research? |
Originally Posted by sky9
1 If there is sign of cavitation in the pumps, I presume that it involves wear on the impellers. What happens to that wear or debris; does it travel downstream or is there a filter to capture it before it enters the cans? Having said that, there is a high-pressure filter specifically to prevent debris from the HP pump to enter the engine and damage it permanently. It also has no bypass: if the HP pump fails, the engine would not work anyway. 2 When the propeller on my boat cavitates the RPM increases but the boat goes nowhere; the prop effectively stalls. Does the same happen to the fuel pumps? 3 If temperature of fuel is an issue why hasn't anyone restricted the altitude or fuel temp of the 777 as a "safety first" prudent measure or are they looking for another incident to verify their research? There are fuel temperature limits, but just how far above the known freezing point would you consider "prudently safe"? Is 13 degrees C enough? In this incident fuel temperature was still very far in the "safe" region. Freezing point according to spec would have been -47C or lower, actual freezing point was -57C, lowest recorded fuel temperature was -34C. Bernd |
lowest recorded fuel temperature was -34C
I agree that it's unlikely the actual temperature of the fuel on flight BA38 ever got below what are currently understood to be safe limits, but the fact that there is only one fuel temp sensor in one location of one wing of the B777 does introduce at least the potential for a single point failure.
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Why were a lot of flights that day on similar routes descending to FL250 and this flight recorded a low of -34C fuel temp? ISA-20 was the temp aloft that day according to a previous post. Once installed is the fuel temp sensor ever calibrated or checked?
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So just one fuel temp sensor, and what proportion of the remaining fuel does it measure? Is the wing tip the coldest position? What are the fluid dynamics of hydrocarbons in a tank at critically low temperatures? Does it become a little like a soft boiled egg i.e. would the temperature be homogenous throughout the tank or would Apollo 13 type tank stirrers be required to make it so?
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I am interested re the tested fuel f/p of -57.
That seems unusual for traditional jet fuel. I have experienced unusual engine exhaust conditions using jet fuel in a third world country. Turns out they were short of jet fuel so they blended in a substantial amount of avgas. |
RF
At the risk of being shot down, my own view is that this was a 'proof of concept'. no evidence just a feeling. sometimes that's all that's needed though.
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I am interested re the tested fuel f/p of -57. That seems unusual for traditional jet fuel. |
Volatile fuel mix vapour pressure at moderate temperatures
moderator ,did you block my message yesterday?
This thread leads to what I was saying about fuel being ok for not waxing at extremely low temps when tested,, but light enough to boil at low above zero temps when subject to pump suction |
wilyflier, I'm not the moderator but will save him from answering you. Your post was not blocked but deleted together with my reply to your ridiculous theories. Give it a rest mate, the AAIB are working on it. There is nought you can suggest that hasn't been looked at by experts in the field which most probably have no test tube freaks amongst them.:ugh:
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Is there any possible way the fuel could have been Russian TS-1, which has a freeze point of -57C, min low flash point of 28C, and has properties and test methods similar to Jet A-1?
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Pinkman and Flight Safety. Thanks, I can find technical references to TS-1 with f/p of -50 (see below). The only one that fits close to a f/p of -57 is obsolete JP-3 (further below). Excuse the format, the graphs do not copy properly.
Selected Specification Properties of Jet Fuels Fuel Jet A Jet A-1 TS-1 Jet B Specification ASTM D 1655 DEF STAN 91-91 GOST 10227 CGSB-3.22 Acidity, mg KOH/g 0.10 0.015 0.7 (mg KOH/100ml) 0.10 Aromatics, % vol, max 25 25.0 22 (% mass) 25.0 Sulfur, mass% 0.30 0.30 0.25 0.40 Sulfur, mercaptan, mass% 0.003 0.003 0.005 0.003 Distillation, °C: Initial boiling point — Report 150 Report 10% recovered, max 205 205 165 Report 50% recovered, max Report Report 195 min 125; max 190 90% recovered, max Report Report 230 Report End point 300 300 250 270 Vapor pressure, kPa, max — — — 21 Flash point, °C, min 38 38 28 — Density, 15°C, kg/m3 775–840 775–840 min 774@20°C 750–801 ------------------------------------------------------------- ..............................Jet A...A-1..TS-1...................Jet B Freezing Point, °C, max –40 –47.0 –50 (Chilling point) –51 ------------------------------------------------------------- Viscosity, –20°C, mm2/sec, max 8 8.0 8.0 @ –40°C — Net Heat of combustion, MJ/kg, min 42.8 42.8 42.9 42.8 Smoke point, mm, min 18 19.0 25 20 Naphthalenes, vol%, max 3.0 3.00 — 3.0 Copper corrosion, 2 hr @ 100°C, max rating No. 1 No. 1 Pass (3 hr @ 100°C) No. 1 Thermal stability Filter pressure drop, mm Hg, max 25 25 — 25 Visual tube rating, max <3 <3 — <3 Static test 4 hr @ 150°C, mg/100 ml, max — — 18 — Existent gum, mg/100 ml, max 7 7 5 Freeze Point Flash Point Fuel Introduced Type RVP, psi °C max °C min Comments JP-1 1944 kerosine –60 obsolete JP-2 1945 wide-cut 2 –60 obsolete JP-3 1947 wide-cut 5–7 –60 obsolete JP-4 1951 wide-cut 2–3 –72 U.S. Air Force fuel JP-5 1952 kerosine –46 U.S. Navy fuel JP-6 1956 kerosine -54 XB-70 program, obsolete JPTS 1956 kerosine –53 Higher thermal stability JP-7 1960 kerosine –43 Lower volatility, higher thermal stability JP-8 1979 kerosine –47 U.S. Air Force fuel JP8+100 1998 kerosine –47 U.S. Air Force fuel containing an additive that provides improved thermal stability JP stands for Jet Propulsion. |
Pinkman. Interestingly many of the chinese and soviet higher freeze point fuels also have lower flash points. But I dont think thats relevant in the current case. |
Flight Safety: Is there any possible way the fuel could been Russian TS-1, which has a freeze point of -57C, min low flash point of 28C, and has properties and test methods similar to Jet A-1? GOST Standard 10227-86 lists four grades of fuel, TS-1, T-1, T-2 and RT. Each has a category of quality with TS-1 having both a higher and first category, of which only the first category appears to be in use. An update of Table 1 of this specification, listing TS-1, RT and two Military fuels, T-8V and T-6, requires an antioxidant and a lubricity improver in RT, T-8V and T-6. Another amendment noted is a change in the Crystallization Point (similar to our freezing point) in TS-1 and RT to -50 °C with -60 °C and -55 °C respectively being produced by user demand. |
This quote is from this link on the Shell website, aviation fuels for civil aircraft.
http://www.shell.com/home/content/av..._10081004.html Former Soviet Union and East European Jet Fuels Soviet kerosine type jet fuels are covered by a wide range of specification grades reflecting different crude sources and processing treatments used. The grade designation is T-1 to T-8, TS-1 or RT. The grades are covered either by a State Standard (GOST) number, or a Technical Condition (TU) number. The limiting property values, detailed fuel composition and test methods differ quite considerably in some cases from the Western equivalents. The principle grade available in Russia (and members of the CIS) is TS-1. The main differences in characteristics are that Soviet fuels have a low freeze point (equivalent to about -57 degrees C by Western test methods) but also a low flash point (a minimum of 28 degrees C compared with 38 degrees C for Western fuel). RT fuel (written as PT in Russian script) is the superior grade (a hydrotreated product) but is not produced widely. TS-1 (regular grade) is considered to be on a par with Western Jet A-1 and is approved by most aircraft manufacturers. Eastern European countries have their own national standards with their own nomenclature. Many are very similar to the Russian standards but others reflect the requirements of visiting international airlines and are similar to Western Jet A-1 in properties and test methods. |
From Air BP:
Jet Fuel No 3 (formerly known as Grade RP-3) – is similar to Jet A-1 and is the grade supplied at all civil airports in mainland China. From Exxon Mobil: Footnote to TS-1 (2) Temperature for start of crystallisation. TS-1 fuels with freezing point not above -50°C intended for use in all climatic zones except zone 11 (GOST16350-80).In zone 11 TS-1 fuel with freezing point above -50°C may be used when ground temperature is below - 30°C for 24 hours before take-off. TS-1 fuel with freezing point not above -60°C intended for use in zone 11 shall be produced as required by the consumers. (3) In case of dispute, the heat of combustion shall be determined by GOST 21 Fuel System Icing Inhibitors (FSII) Water dissolved in fuel can come out of solution at low temperatures in the form of very fine droplets. Although the amounts are small, the droplets formed can freeze at altitude and cause filter plugging. Fuel system icing inhibitors have been developed to protect the system from this problem. The most widely used additive is diethylene glycol monomethyl ether (DiEGME). The use of FSII is required in UK and US military jet kerosine and although optional in many civilian specifications is very seldom used. |
Flight Safety;
Thanks. I have been researching several different sites (but not Shell yet) and fuel companies. There seems to be a variation in specifications between sites/companies. I also note that China imports approximately 30% of their jet fuel. |
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