Chris Weston said
I can speculate that the 777 had a fuel mixture problem probably revolving around additives.
(Does this include an icy slush from the nether regions of the center-section tank mixed in with the fuel-feed (and emanating from the situation described below)?:
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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
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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.
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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
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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