2csonTriple7
10th Sep 2017, 18:38
UPDATE (2017)
777 Center Tank Fuel Scavenge System
The fuel scavenge system uses the main tank fuel jettison pumps (as a motive-flow source) to transfer the fuel remaining in the center tank when the center tank pumps are not operating.
To ensure the non-normal procedure FUEL LEAK may be accomplished, fuel scavenge is inhibited for 45 minutes after the FUEL CENTER PUMPS switches are Off.
Following the 45 minute inhibit, fuel scavenge operates for a maximum of four hours or until the airplane is on the ground.
Explanation of the Modified 777 Center Tank Scavenge System
Applicable to airplane line numbers 1099 and-on.
(see background info below).
Based on information from the 777F AMM and operating experience.
Original system used the main tank pumps as the motive flow source for the center tank scavenge jet pumps.
The new, modified system, uses the main tank jettison pumps as the motive flow source.
HARDWARE/SOFTWARE MALFUCNTIONS
The fuel system management computer will occasionally throw a status message:
FUEL JETT PUMP L or R and shut down a main tank jettison pump.
(see causes mentioned below by Boeing)
The MEL dispatch fix is for maintenance to set the offending FUEL JETT PUMP inop.
As a result, the MEL for the CTR TANK SCAVENGE SYSTEM must also be applied.
Plan on having 1200kgs as unusable fuel.
Remember to also add this to the PERF INIT > RESERVES fuel.
Experience has shown that if only one pump is set INOP according to the MEL Maint Procedure, the other pump will scavenge the center tank fuel which will result in a slight fuel imbalance
Another consideration is that with a MAIN TANK JETTISON PUMP inop, it will not be available for dumping fuel.
Dumping will take a bit longer and will result in fuel imbalance.
Normal Operations
When the center tank quantity decreases to ~1200kgs, the EICAS message FUEL LOW CENTER comes on.
The crew then switches off the CTR TANK OVRD JETT pumps.
Exactly 45 minutes after switching OFF those pumps, the MAIN TANK L/R JETTISON pumps are automatically switched ON by the fuel management computer and run for approx 4-hrs, scavenging the center tank fuel into the respective main tanks.
This goes unnoticed by the crew (no change on the FUEL SYNOPTIC page) other than the fact that the CTR TANK quantity decreases.
The pump status can be monitored in flight by opening the MAINT INFO > DISPLAY via the CTR MCDU and selecting the FUEL MANAGEMENT page.
The MAIN TANK JETTISON pumps show CMND OFF (dump switches are not commanded ON by the crew) and PRESS, indicating that they are operating.
Now each L/R MAIN TANK JETTISON pump scavenges the CTR TANK via motive flow jet pumps.
NOTE:
Cycling the switches will reset the 45 minute timer.
Background info:
Circa 2013, Boeing made a design change to the fuel scavenge system to improve its operational reliability under cold temperatures.
With this design change incorporated, the jettison fuel pumps in the main fuel tanks (ed: formerly a main tank pumps were used as the motive flow source) are operated every flight as part of the fuel scavenge system. Then the jettison fuel pumps are automatically shut off after four hours of operating the fuel scavenge system, or when a low pressure condition of the jettison fuel pump is detected under failure conditions such as a fuel leak. It was also determined that the jettison pump inlets can be uncovered during normal fuel scavenge operation depending on the flight duration (less than four hours) and fuel loading in the main fuel tanks. In addition, the automatic shutoff system can fail in a latent manner. If the automatic shutoff system fails and the jettison pump inlets are uncovered as expected during normal fuel scavenge operation on short-range flights of less than four hours, the jettison pump will run dry for an extended period of time. Extended dry running of the jettison fuel pumps can be a potential ignition source inside the main fuel tanks, and could cause a fuel tank fire or explosion in the event that the jettison pump overheats or has an electrical fault.
Boeing stated that the fuel jettison pumps that are the subject of this AD are not a potential fuel tank ignition source because the pumps in question have been qualified to run dry without causing adverse pump operating temperatures for 600 hours. The jettison pump design includes redundant safety features to prevent fuel tank ignition. Boeing also stated that, based on service history and given the number of flight hours accrued by Model 777 airplanes, a conservative analysis shows the chance of a jettison pump running dry and causing a fuel tank ignition is less than extremely improbable.
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777 Center Tank Fuel Scavenge System
The fuel scavenge system uses the main tank fuel jettison pumps (as a motive-flow source) to transfer the fuel remaining in the center tank when the center tank pumps are not operating.
To ensure the non-normal procedure FUEL LEAK may be accomplished, fuel scavenge is inhibited for 45 minutes after the FUEL CENTER PUMPS switches are Off.
Following the 45 minute inhibit, fuel scavenge operates for a maximum of four hours or until the airplane is on the ground.
Explanation of the Modified 777 Center Tank Scavenge System
Applicable to airplane line numbers 1099 and-on.
(see background info below).
Based on information from the 777F AMM and operating experience.
Original system used the main tank pumps as the motive flow source for the center tank scavenge jet pumps.
The new, modified system, uses the main tank jettison pumps as the motive flow source.
HARDWARE/SOFTWARE MALFUCNTIONS
The fuel system management computer will occasionally throw a status message:
FUEL JETT PUMP L or R and shut down a main tank jettison pump.
(see causes mentioned below by Boeing)
The MEL dispatch fix is for maintenance to set the offending FUEL JETT PUMP inop.
As a result, the MEL for the CTR TANK SCAVENGE SYSTEM must also be applied.
Plan on having 1200kgs as unusable fuel.
Remember to also add this to the PERF INIT > RESERVES fuel.
Experience has shown that if only one pump is set INOP according to the MEL Maint Procedure, the other pump will scavenge the center tank fuel which will result in a slight fuel imbalance
Another consideration is that with a MAIN TANK JETTISON PUMP inop, it will not be available for dumping fuel.
Dumping will take a bit longer and will result in fuel imbalance.
Normal Operations
When the center tank quantity decreases to ~1200kgs, the EICAS message FUEL LOW CENTER comes on.
The crew then switches off the CTR TANK OVRD JETT pumps.
Exactly 45 minutes after switching OFF those pumps, the MAIN TANK L/R JETTISON pumps are automatically switched ON by the fuel management computer and run for approx 4-hrs, scavenging the center tank fuel into the respective main tanks.
This goes unnoticed by the crew (no change on the FUEL SYNOPTIC page) other than the fact that the CTR TANK quantity decreases.
The pump status can be monitored in flight by opening the MAINT INFO > DISPLAY via the CTR MCDU and selecting the FUEL MANAGEMENT page.
The MAIN TANK JETTISON pumps show CMND OFF (dump switches are not commanded ON by the crew) and PRESS, indicating that they are operating.
Now each L/R MAIN TANK JETTISON pump scavenges the CTR TANK via motive flow jet pumps.
NOTE:
Cycling the switches will reset the 45 minute timer.
Background info:
Circa 2013, Boeing made a design change to the fuel scavenge system to improve its operational reliability under cold temperatures.
With this design change incorporated, the jettison fuel pumps in the main fuel tanks (ed: formerly a main tank pumps were used as the motive flow source) are operated every flight as part of the fuel scavenge system. Then the jettison fuel pumps are automatically shut off after four hours of operating the fuel scavenge system, or when a low pressure condition of the jettison fuel pump is detected under failure conditions such as a fuel leak. It was also determined that the jettison pump inlets can be uncovered during normal fuel scavenge operation depending on the flight duration (less than four hours) and fuel loading in the main fuel tanks. In addition, the automatic shutoff system can fail in a latent manner. If the automatic shutoff system fails and the jettison pump inlets are uncovered as expected during normal fuel scavenge operation on short-range flights of less than four hours, the jettison pump will run dry for an extended period of time. Extended dry running of the jettison fuel pumps can be a potential ignition source inside the main fuel tanks, and could cause a fuel tank fire or explosion in the event that the jettison pump overheats or has an electrical fault.
Boeing stated that the fuel jettison pumps that are the subject of this AD are not a potential fuel tank ignition source because the pumps in question have been qualified to run dry without causing adverse pump operating temperatures for 600 hours. The jettison pump design includes redundant safety features to prevent fuel tank ignition. Boeing also stated that, based on service history and given the number of flight hours accrued by Model 777 airplanes, a conservative analysis shows the chance of a jettison pump running dry and causing a fuel tank ignition is less than extremely improbable.
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