Modified 777 Center Tank Scavenge (circa 2013)
Here is my explanation of the Modified 777 Center Tank Scavenge System.
It is applicable to airplane line numbers 1099 and-on.
(see background info below).
It is based on information from the 777F AMM and my own operating experience.
The OM-B has old information about the original system which 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.
The reason that I am so familiar with all of this is the fact that because of the modification, the fuel system management computer will occasionally throw a status message
FUEL JETT PUMP L or R due to a software bug (how often have we heard that!) and shut down a main tank jettison pump.
(Other causes of the status msg are mentioned below.)
The maintenance fix is to set the offending FUEL JETT PUMP inop.
And as a result, the MEL for the CTR TANK SCAVENGE SYSTEM must also be applied.
My problem was that I couldn't figure out the relationship between the main tank FUEL JETT PUMPS and the CTR TANK SCAVENGE system.
It didn't make sense to me because our OM-B still had the old system description for the center tank scavenge sys.
That's when I started to dig into our 777F AMM Chap 28.
So here's what I found out.
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.
At 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 4hrs, scavenging the center tank.
This goes unnoticed by the crew (no change on the FUEL SYNOPTIC page) other than the fact that the CTR TANK quantity decreases.
You can monitor the pump status by openning the MAINT INFO > DISPLAY via the CTR MCDU and the select the FUEL MANAGEMENT page.
There you will see that the MAIN TANK JETTISON pumps show PRESS, indicating that they are operating.
Now each L/R MAIN TANK JETTISON pump scavenges the CTR TANK via motive flow jet pumps.
The 45 minute waiting period ensures that the main tanks have room for the fuel to be scavenged.
NOTE:
Cycling the switches will reset the 45 minute timer.
I know this, I've tried it!
The MEL does say to plan on having 1200kgs as unusable fuel, however my experience has shown that if only one pump is set INOP accd to the Maint Proc. the other pump will scavenge the center tank fuel which will result in a slight fuel imbalance.
Another consideration is that if they do set a MAIN TANK JETTISON PUMP inop, it will not be available for dumping fuel and so dumping will take a bit longer and you will have some fuel balancing issues to deal with.
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.