Airbus- A310-300 Fuel pump.
 

What can be a probable reason for the OUTER TANKS both fuel pumps not showing any fault lights but, the tanks not emptying at the same rate as the right tank. Manual feed mode.

Weak pumps? Fuel pressure is shown on Ecam. On the ground try turning on one pump at a time on both sides and compare the result. It could also depend on one engine burning at a higher rate than the other.

Thanks tubby, scenario was the manual feed mode was being carried out. Fuel was being fed from the outer tanks but, the left outer tank depleting at a lot lessor rate I.e intermitant. No ECAM warnings either. Water accumulation in the tanks? Freezing at higher alt?

It could be water. I always treated fuel quantity indications in the A306 as an approximation rather than as a precise figure. I know of one aircraft that went into a sub-idle state on one engine because of water . We always used to take a ton of fuel from the outer tanks just so that we could put some warm fuel in it on the next turnround.Lufty used to empty the tank completely.

Another possibility is fuel leaking back into its own tank, a problem which changing pumps won't solve. Any maintenance done recently possibly by the same shift?

Blockage of fuel or jet pump by debris / tank seal matter?

Leaking pipe from pump to engine, which is returning fuel to the tank?

Is the system desinged to keep the outer tanks full for as long as possible to aid wing damping? If so, the pumps doing that refilling from the inner tank may be more efficient than the ones emptying the outer tank.

Leaks between the fuel-oil heat exchanger are not unknown. This is why the oil quantity gauges have white plastic bugs on them (CF6). If the quantity increases in flight it may be because of this.

The only maintenance action that was reported (tech log) after this was, water draining procedure, without mentioning if good amount of water was drained out or not. The outer tank pumps have valves which delays the fuel extraction until inner n center tank are empty. But, precisely the scenario I was talking about Is that both outer tanks were made to provide fuel to the engines by manual feed. The left tank ended up with an intermittent depletion and so the fuel remaining on the left tank was 1000 kgs more then the right outer, they are supposed to be depleting at the same rate. There was no local or ecam fault alert either.

the clack valve could be stuck open (one way valve allowing fuel to flow from inner to outer)

Sounds a lot like water but the probes are not very reliable and the aircraft are becoming antiques. I think that there was only one probe per tank.

For the update: One of the left outer pumps had a LO PR fault just 2 days after this flight. That probably means the pumps were giving out already. Ideas?

What was the position of the cross feed valve???

Over thinking this just a tad bit.

Crossfeed was inline.

The most probable reason could be that the pump out pressure on one side could be more then the other side.but outer tanks are generally used at the end of the flight most likely just prior to the top of descend.

Apparently yes RAM777, as mentioned one of the pumps had a malfunction 2 days after that.