No decimal point . . . .
Quoting Steamchicken:
"In the B777-200, the center tank message is shown at approximately 14.5 gallons.
14 *decimal* 5. Does anyone else think the "138 gallons H20" thing is a missing decimal point? Otherwise the difference between the -200 and the -200ER is just shy a factor of 10; and I doubt the -200ER's centre tank is 10 times as big as the -200's."
The 138 gallons is correct. There is no decimal point missing. As mentioned on previous posts, the 14.5 gallons applies to the -200 center wing tank which is located in the inboard wingbox of the left and right wing. The center wing dry bay divides the center tank in two parts. Two interconnect tubes connect the two halves. The center tank bottom surface has a slope where water can easily collect at the lowest point near the wing root (basically identical to the main wing tanks). This is where the water detector is located for the -200 configuration.
On the -200 ER, the center tank is in the wing center section and in the inboard wingbox of the left and right wings. The bottom surface of the tank in the wing center section is a much larger, flat, horizontal surface, measured from left to right side of the tank. The water detector is located on the right side of the center wing. This is why the alert level is set at 138 gallons because at that value the entire bottom surface (from left to right) will be covered with water and reach the detector.
Quoting Tanimbar, post #289:
"water detection message for a particular tank to show". There was no such message shown during the flight, therefore there was no water, greater than 7 gallons, present in the bottom of the wing tanks and the water scavenge jet pumps did not operate."
Tanimbar, the messages show up on the maintenance pages, which are not directly visible to the pilots. Only if the crew had reason to select a maintenance page they would have seen such a message. I am not suggesting these messages showed up during the subject flight. From what i have read from the AAIB report sofar is that the water content in any of the tanks never came close to a level for the messages to display.
The water scavenge pumps operate continuously (fuel scavenge pumps do not).
Below i have copied a post of mine from another thread (now closed) regarding the scavenge system:
There are four water scavenge jet pumps and two center tank fuel scavenge jet pumps.
Main tanks:
Each main tank has one water scavenge jet pump.
Center tank:
Each side of the center tank has a water scavenge jet pump and a fuel scavenge jet pump.
Fuel scavenge:
The fuel scavenge jet pumps take fuel from the low points in the center tank and send it to the main tanks. Float-operated shutoff valves prevent fuel scavenge when the main tanks are full. Inlet float-operated shutoff valves prevent motive flow to the jet pump until the center tank is almost empty. This prevents the fuel from flowing to the main tank too early if the outlet float-operated shutoff valve fails. A check valve in the jet pump prevents fuel movement from the main tank to the center tank.
Water scavenge:
The water scavenge jet pumps (continuously) take fluid from the low points in the tanks and send it to the fuel pump inlets. This prevents water from collecting in the bottom of the tanks.
Operation:
The scavenge pumps (both fuel and water) operate automatically when the fuel pumps are on. They use fuel from the fuel pumps as motive fuel. The flow of the motive fuel through the jet pump causes suction that takes fluid from the low point in the tanks.
Regards,
Green-dot
Last edited by Green-dot; 22nd February 2008 at 22:49.