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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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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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. Because the water scavenge inlet was not co-located with the fuel scavenge inlets it is more likely for water to be ingested in 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 ic!
e) in the scavenge system could result in a low rate of scavenge or no fuel scavenge.
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 and 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.
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
Originally posted by Zaphod58
From my understanding of fuel pumps, once cavitation starts, it can stop on its own, BUT, you have damaged the fuel pump at that point and it may not pump as efficiently as it was before the cavitation started. Cavitation is caused when SOMETHING creates bubbles in the fuel,
"When bubbles in a liquid get compressed, the insides get hot -- very hot," said Ken Suslick, the Marvin T. Schmidt Professor of Chemistry at Illinois and a researcher at the Beckman Institute for Advanced Science and Technology. "Nobody has been able to measure the temperature inside a single collapsing bubble before. The temperature we measured -- about 20,000 degrees Kelvin -- is four times hotter than the surface of our sun."
I would like to remind everybody that, according to a senior avionics manager at Honeywell in a paper published in a major conference, a certain aircraft type came within days of having its airworthiness certificate withdrawn in the early 2000's because of irresoluble problems with data transfer on critical data buses (known as "Byzantine faults").
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"
Originally posted by stumason
reply to post by TheAvenger
Indeed!! An Indian work colleague of mine said had this been an airline such as India Air or Pakistan Airways, it would have plowed into the tarmac nose first. Apparently of all the airlines he's flown, with BA you hardly notice your landing.
even the passengers didn't even think they had crashed until the evacuation began, must have been that smooth a crash! I was once on a KLM flight from Heathrow to Schippol and it felt like the pilot really hated the tarmac when we got to Holland!
Originally posted by C0bzz
One accident in almost 15 years with a fleet of almost 1000 aircraft. Take a guess.
[edit on 19/3/2008 by C0bzz]