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Ballast fuel

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Registered: May 2009
Posts: 5140
Good evening,

as we know, the 744 combi has a ballast fuel alert.

The alert level can be set on the FMC's PERF INIT page in line 3R. When the center wing tank quantity drops below this level, the alert is triggered and the pilot should deactivate the center wing tank pumps.

The center wing tank sits slightly forward of the center of gravity. This tank is the counterforce when the combi has heavy cargo in its tail, so it won't get tail-heavy.

On this subject there's a question about the scavenge pump:

The scavenge pump may autostart when the reserves-to-main-tank transfer begins. Normally, at this time, the center wing tank is nearly empty and the scavenge pump just draws the rest of the soup from the tank floor.

Does anybody know if the scavenge start is inhibited when a fuel ballast value is entered in the FMC? It looks strange when the crew wants to keep fuel in the center tank and the scavenge ignores this desire.

Also unknown is whether the >FUEL OVD CTR L & R messages are inhibited if the crew deselects CTR L & R pump switches when the quantity is below the ballast level.



Registered: May 2009
Posts: 5140
Do combis have scavenge pumps?

If so, why?
Registered: May 2009
Posts: 5140
OK, combis, too, can be empty :-)

(No load in the tail and no need for ballast fuel.)

By the way, the answer on this one ...
Also unknown is whether the >FUEL OVD CTR L & R messages are inhibited if the crew deselects CTR L & R pump switches when the quantity is below the ballast level.

... is: Yes, inhibited.
Registered: May 2009
Posts: 5140
While I'm talking to myself ... thinking aloud ...

The hydromechanical scavenge starts only if the CWT quantity is below 3000 LBS. So, keeping the ballast above 3000 will inhibit the scavenge start.

For the old electrical scavenge, however, there must be a special inhibit function on the fuel management cards, so that the scavenge valves won't open if ballast fuel must remain in the CWT.

Registered: May 2010
Posts: 843
Boeing Service Bulletin SB 747-28-2255 Revison 2 (July 15, 2004) refers to this.

As a result of fuel management procedures given by Airworthiness Directive AD 2002-24-52* back in the year 2002, some airplanes with hydro-mechanical (jet pump) CWT fuel scavenge systems were not scavenging fuel (up to 3,300 lbs (1,500kgs) was remaining in the CWT).

*basically, turn off your CWT pumps before they show low pressure

SB 747-28-2255 Revision 2 has operators (line numbers 972 through 1320 and 1323) lockwiring the CWT (3000lb level ?) float valves open. Now only the float valves in the wings had to operate to get scavenge ops.

Revision 2 included Combis because they needed to keep ballast fuel in the CWT. The combi operators either had to deactivate ballast fuel ops, or deactivate the scavenge pump system!

I don't know if the Combi operators/Boeing have since come up with a fix. Accessing the tank and deactivating the scavenge system every time you need to use ballast fuel seems a little inconvenient :mrgreen:

FAA AD 2007-13-04 supersedes SB 747-28-2255. IDS-504 software now includes the new fuel system EICAS messages and, with modifications to various components, the fuel management procedures outlined in SB 747-28-2255 are no longer necessary.


Having said that, I don't know if the lockwiring SB was cancelled for Combi operators who require ballast fuel. Our non-Combi fleet still seems to have their CWT float valves lockwired open.

Clear as mud )


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