744 Override Jettison Pump Problem

[NSEU]

On one of our CF6-powered 744's, a Technical Log report noted that in cruise the #3 Fore and Aft Override Jettison fuel pumps came on automatically with the CWT still containing 25 tonnes of fuel.

This seems to defy airplane logic. The wiring schematics show that each pump is controlled by separate relays and the logic in the Fuel System Management Card shows that a single logic gate controls four individual logic gates which control their respective O/J pump relays. If the card is getting false information, then all four O/J pumps should turn on.

Fuel balancing was required later on in the flight, so it wasn't just an indication error.

During normal ops, the O/J pumps require either of the following to operate:
1) Both CWT pumps turned off manually
2) Either CWT pump showing low pressure (after at least 5 minutes operation)

Any suggestions?

Thnx
NSEU

[gas path]

Tried changing the cards yet, just in case?
Failing that try some input monitoring.

[NSEU]

Yep.... Just as a stab in the dark, I changed FSMC "A". Not sure if it has worked yet... The problem didn't happen on every flight.

Cheers.
NSEU.

[NSEU]

Both FSM cards have now been replaced and plugs on the #3 OJ pumps pressure sensors have been cleaned... Still broken...

[chemical alli]

try looking at the single point sensor in the cwt it has alot to do with stab transfer

[NSEU]

... but what has Stab Transfer to do with Wing Tank O/J pump ops???

[chemical alli]

25 t sounds about single point sensor switch over for stab transfer.i know your asking about 2 and 3 oj pumps but if the fqis cant see the single point sensor then they may come on with fuel remaining in ctr

[NSEU]

Please note that only the #3 O/J pumps are coming on. The wiring schematics indicate that if the logic is correct for the OJ pumps to come on, then all 4 will come on.

[gas path]

Strange that BOTH pumps are coming on, as you say the cards should command all four. Something somewhere is giving a ground to pull in the relays. How about the FJCC? I know they don't have power until commanded BUT.....
Anything logged in the CMC?
I take it that the indications are normal prior to flt. with the 'blue' pumps commanded but not on and the green indications when running?

How about swarf down in the card files

[jettison valve]

B747 override jettison pump... Isnīt that a Crane-Hydroaire one that has caused so many headache in the past...???

Cheers,
J.V.

[Max Arets]

I've had this happen on a flight. We CRM'd it, called our MW people and asked for a confirmation of our suspicions and our intentions. We were LOSING fuel (totaliser), the qty in tank 2 was about 800 kg more than the full tank we started with, the O/J pumps in tank 2 indicated on with flow lines also green. The CWT had some 30 tonnes (from memory) and the CWT pumps were on. All main tank pumps were ON and tank 3 O/J pumps were armed (blue). No EICAS messages at all!
Our suspicions were that it might be a non-return valve problem on the output side of the O/J pump(s) in tank 2, allowing high pressure fuel to enter tank 2 via the still armed O/J pumps, and the full tank venting fuel overboard. We thought perhaps the pressure in the O/J output lines may cause the pumps to indicate ON (green).
We suggested a fix to MW. This was a temporary measure to hopefully stop any further fuel loss. We proposed switching off the CWT pumps and allowing the O/J pumps in tanks 2 and 3 to reduce quantities in tanks 2 & 3 by about 2 tonnes each (and balanced of course). Our next move was to restore normal fuel configuration, and see what happened.
They consulted, and got back to us to proceed. We did the above and all went normally afterwards.
As it says in the QRH foreword......you might have to improvise.

[NSEU]

Thanks Max. I believe there was also a recent entry about fuel venting (although I can't remember which side).

I'm still a little puzzled, however, as both non-return valves would have to suddenly fail to get the pressure lights on both O/J's and I don't understand why the fuel in our #3 tank would be decreasing faster than the #2.

Anyway, return valves are something to look at.

Gas Path: There didn't seem to be any relevant CMC messages and, as far as I know, indications were normal earlier. Also, I would have thought that swarf would have caused this problem at all times, rather than at a particular point in flight.

Anyway, thanks! (to all)

Cheers.
NSEU

[gas path]

I believe there was also a recent entry about fuel venting (although I can't remember which side).

That would open up a whole new can of worms!
Fuel venting: most common cause, a failed vent valve.
But would be interesting to know if the imbalance was causing an INCREASE in another tanks qty. This would have to be a failure (internal leak) of 2 valves, a jettison transfer valve and a refuel valve. (Or a split manifold!)
However back to the original defect. As already posted in order to get the pressure switches made in both O/Jett pumps then BOTH pumps would have to have a failed NRV.
Interesting one that!