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B777 Fuel scavage system MEL

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B777 Fuel scavage system MEL

Old 30th Nov 2019, 16:28
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B777 Fuel scavage system MEL

Let's have B777-300ER which has Main Tank Jettison Pumps INOP according to MEL 28-31-03. That means I have to use MEL 28-22-04 and deactivate Fuel Scavage System. In Operations part for Main Jettison pump is written

Limit the takeoff weight to the maximum climb limited landing weight at the departure airport, plus center tank fuel (which would be jettisoned during return-to-land procedure), plus 2,450 kg (for fuel that would be burned during a return-to-land procedure).
If I'm right the problem is performance during go-around in case of return, right? If so how can I get RTOW? I think it is only language problem but the MEL looks (for me) like I should calculate performace and RTOW and then lower it by fuel weight in CWT and by 2450 kg. But I think it is nonsense.

Operations part of MEL for Fuel Scavage System is quite clear. Add 1225 kg of fuel from CWT to ZFW. If I got it correctly it is the amount of fuel I can not get out of CWT because Scavage system is not working.

To confuse it even more what happens when one fuel jettison pump is working?
Rarife is offline  
Old 1st Dec 2019, 02:21
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Rarife

I am a long time away from this stuff, but I think you are confusing the Jettison Pumps and the Scavange Pumps. Two seperate systems.
My copy of the DDG/MEL dated 2011, at 28.31.03 relates to the Jettison pumps and does not call for dectivation of the Fuel Scavange system.

Maui

Last edited by maui; 1st Dec 2019 at 02:22. Reason: out of date DDG/MEL
maui is offline  
Old 1st Dec 2019, 03:21
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You take the Landing Climb Limit weight from Performance Dispatch 22.3 and ADD the weight of the center tank fuel (because this fuel can be jettisoned) and 2,450 kg (the fuel that you would burn returning to the airport). This number is an additional restriction to your maximum takeoff weight, so you compare this weight to the regular runway-limited and climb-limited maximum takeoff weights and use the lowest as your operational maximum takeoff weight.

You are correct that you also have to consider the fuel scavenge system inop and need to consider the last bit of center tank fuel unusable and lower the maximum zero fuel weight . The MEL doesn’t make a distinction between one or both jettison pumps inop, so you need to use the same performance penalties in both cases.
Long Haul is offline  
Old 1st Dec 2019, 03:26
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Post 2011 Boeing made a design change to use motive flow from the main tank jettison pumps for the center tank scavenge system because it was a higher pressure source. If the main tank jettison pumps are inoperative on these airplanes, the center tank scavenge system won't work. There is a series of ADs requiring this change and other associated scavenge system changes due to freezing of the scavenge system trapping some fuel in the center tank (2015-07-07, 2016-11-03 and 2019-16-13).

The weight limit the OP quoted tells you to only take credit for center tank jettison, not main tank jettison, when calculating your return to land climb limited weight when the main tank jettison pumps are inop.
Dave Therhino is offline  
Old 1st Dec 2019, 06:36
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Dave: Thankyou for the update and correction. As I said, a long time away from.
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Old 1st Dec 2019, 10:03
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If your outfit uses the Boeing OPT:

Go to your IAC for the runway you’ll return on if needing to come back in a hurry. Check the missed approach climb gradient for that approach.

Check different approaches (RNAV etc.) to find the lowest missed approach climb gradient, weather dependant for that runway. You can also check the reciprocal runway should the wind permit, if a more favourable missed approach climb gradient can be found. Add 0.6% if a turn is involved.

Punch all the info into the ‘Landing Dispatch’ section of the OPT. Then hit ‘Calculate’, then ‘Airport Info’ then ‘Landing Details’. Add 2450kg to the Go-Around Limited Weight and there is your RTOW.
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