Pumping Iron… correction, Fuel.
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I had too much time on my hands and was playing with the fuel management in BMS. I focused on the trapped fuel case. In my test I took one bag on the right wing, a centre bag and nothing on the left side, double ugly in short. I set the Wing First fuel transfer and Feed to Norm. All this to trigger trapped fuel warning. Now according to Dash 34 I should get trapped fuel warning the moment the left wing internal wing tank starts to “empty into the fuselage tank”. With my configuration this should be an immediate trapped fuel warning but it wasn’t. I got the trapped fuel warning when the left internal fuel tank is empty and the aft fuselage tank is at 1400 lbs of fuel and of course, the right bag is still feeding. So it seems to be an incongruity with the manual. However, after I switched the fuel transfer back to normal, i.e. centre tank first, the centre bag emptied completely into wing and fuselage tanks within a minute or so. My question is whether the fuel pumps, pump the fuel irrespective of the actual engine consumption. Or is the flow dictated by the engine demand? Also, is it possible to transfer the fuel from the centre tank into an empty internal wing tank or is it lost fuel?
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Center external feeds the internal wings and external wing tanks. It’s not just possible, it’s mandatory. It uses the same fuel pipe from the AAR receptacle. There are various pumps in the system: the electrics are going to be independent of engine, the pressure fed ones use bleed air but I think they are regulated so probably the same regardless of bleed air pressure. Maybe at idle the bleed air is below the regulation pressure I don’t know.
Trapped fuel is based on fuselage fuel totals, the five tanks F-1, F-2, A-1, FWD RSVR, AFT RSVR, basically the sum of the two needles in NORM. Wing internal tanks aren’t included.
Trapped fuel warning has a bunch of and-type conditions:
FUEL QTY SEL knob NORM
Fuselage fuel quantity < capacity - 500 (> 30s)
Total fuel > fuselage capacity + 500 (> 30s)
FF < 18,000 pph (> 30s)There’s no reason that I can see that trapped fuel warning even “might” (as BMS -1 says) happen with wing tank emptying. It would take fuselage fuel being 500 lbs less than fuselage capacity for a warning to even be possible.