I have read the manual several times and am still confused about how cross-feed should work. I understand the centre tank and feeder tanks but my question is about balancing left and right - is that a thing?
Trigger for the query was after I left the APU running for about an hour which feeds from left inner and ended up with about 50kg less in left than right. I tried various combinations of the cross-feed switches but never saw any rebalancing done.
Probably not an issue for 50kg but does it ever happen, and if so at what threshold and do I need to do anything?
I'll double check with our 146 pilots but I'm not aware of any procedure to balance wing tank fuel, or any method of transferring from one wing tank to another, the fuel transfer system only allowing centre fuel to transfer out to each wing equally, and the auxiliary tanks (if fitted) into the respective wing tanks.
The fuel burn from the APU is relatively low so maybe it was just considered unnecessary as any imbalance would be quite limited.
Balancing can only be accomplished on the ground with the manual defuel valve opened at the wing fuel panel, and the "lower" tank refuel/defuel valve selected open. You then use AC pumps on the higher volume tank side to move fuel through the refuel/defuel manifold and into the "lower" volume tank.
Lets say after take off you wish to balance the fuel:
In cruise - all 4 engines operating (hopefully!)
Select Common Feed Selector on "high" side top Open.
Fuel Crossfeed selected "Open"
Turn off Fuel Pumps for "low" side.
Fuel will now draw for the low side engines from the high side.
Once fuel is balanced reverse the above and you're good to go.
@martyn I've noticed a bug with the APU feed.
Typically if an aircraft is to parked with the APU operating for a long time (ie: in cold or temperate areas outside with packs running) we may have an engineer keep an eye on the fuel.
The APU can be fed from the right side in this case by selecting the R Common Feed open, with the X Feed selected open. Then fuel pressure will be provided to the left manifold for the APU.
I tried this procedure with the MSFS 146 for fun and I noticed the APU LOW FUEL pressure annunciation doesn't extinguish if configured in this manner - it seems to be tied directly to the L INNER fuel pump switch.
Now it may be a bit excessive to model that systems depth so i'll leave it to you!
There's no way for flight crew to transfer fuel between tanks. But we can configure the Fuel panel to use fuel from a particular tank.
The procedure is as folows:
Common feeds open
AC Pumps. ON, on the high side and OFF on the low side. Both Inner and Outers.
Tto stop fuel balancing, you effectively reverse the procedure. In the CRZ, you can expect the Fuel inbalance to reduce by about 30kg a minute.
@plhought during the balancing procedure, we turn off the AC Pumps on one side. These pumps are used to create "motive flow" through the jet pumps. So without the AC Pumps, we need to use the STBY pump to maintain motive flow.
The reason we turn both STBY pumps on is because if one is on, it will cause a STBY LO PRESS caution due to their logic. This LO PRESS situation is removed if both are switch on. These STBY pumps are actually redundancy for motive flow as a result of electrical failures.
@flightstrike Hmm seems a little redundant. The Standby Hyd pump only provides motive flow ti the jetpumps just before rib 13 which only move fuel from the main wing into the inner feed tank. The inner feed tank wont be drawn from - if the other sides' AC pumps are on and pressurizing the engine feed manifold. There's a check valve upstream and downstream of the common feed valve and in between the AC & Standby pump input anyways -so with the greater pressure coming from the other sides' AC pumps - so all running the Standby pump will do is recirculate fuel between the main tank and inner feed tank on that side.
Gotta love the redundancy on redundancy with the Hyd driven Stby pumps.
But alas, if that is what BAe wants yeah to do - I guess to ensure the feed tanks stay full? Give the Standby Pumps a warm up every once a while?
@plhought keeping the feed tanks full is important, otherwise you can run into fuel starvation issues if the levels reduce too much in certain phases of flight. Ensuring motive flow is still operating is critical, hence we have the STBY pumps.