737NG minor fuel imbalance
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The limit of 453 kgs is just that (1000lbs in American units) and anything less is not worth the distraction.
Mikehotel152,
My two cents worth. Firstly, I reckon Skyjob and vapilot20014 are onto it. Fuel is being pumped from the R tank and into the Centre tank through the centre tank r/h boost pump due to a failed or partially failed boost pump outlet check valve. Crossfeed valve position (open/closed) is irrelevant due to the valve being mounted between the centre tank boost pumps in the engine fuel feed manifold, so even if it was open, fuel would still flow through the r/h pump and into the centre tank.
You did not mention anything about your first two sectors, so I can only assume that you arrived with <4tons of fuel both times and zero in the centre tank. You have to have <2tons in the l/h tank in order for centre tank fuel scavenge to operate.
You then tanker fuel and arrive with 4tons and 60kgs in the centre. So with 4tons in the wings no scavenge occurred and hence 60kgs in the centre. As Skyjob pointed out, very low fuel levels in the centre can cause erroneous indications. That 60kgs could have been 100kgs for all we know.
Next sector depart with 6tons and approx 4tons at TOC so scavenge pump kicks in and at next port we have a fuel imbalance but no fuel in the centre.
Mikehotel152 and its easy,
I am very surprised that your respective engineers did not carry out a stick check of the wing tanks to confirm ACTUAL tank quantities compared to indicated.
Lightning5,
Close but no banana. Fuel will drain into whatever tank that you are refuelling at the time when you finish refuelling.
Hope this helps.
My two cents worth. Firstly, I reckon Skyjob and vapilot20014 are onto it. Fuel is being pumped from the R tank and into the Centre tank through the centre tank r/h boost pump due to a failed or partially failed boost pump outlet check valve. Crossfeed valve position (open/closed) is irrelevant due to the valve being mounted between the centre tank boost pumps in the engine fuel feed manifold, so even if it was open, fuel would still flow through the r/h pump and into the centre tank.
You did not mention anything about your first two sectors, so I can only assume that you arrived with <4tons of fuel both times and zero in the centre tank. You have to have <2tons in the l/h tank in order for centre tank fuel scavenge to operate.
You then tanker fuel and arrive with 4tons and 60kgs in the centre. So with 4tons in the wings no scavenge occurred and hence 60kgs in the centre. As Skyjob pointed out, very low fuel levels in the centre can cause erroneous indications. That 60kgs could have been 100kgs for all we know.
Next sector depart with 6tons and approx 4tons at TOC so scavenge pump kicks in and at next port we have a fuel imbalance but no fuel in the centre.
Mikehotel152 and its easy,
I am very surprised that your respective engineers did not carry out a stick check of the wing tanks to confirm ACTUAL tank quantities compared to indicated.
Lightning5,
Close but no banana. Fuel will drain into whatever tank that you are refuelling at the time when you finish refuelling.
Hope this helps.
Thread Starter
McHale,
Your helpful input encouraged me to get out the FCOM2, Pat Boone's MRG, Brady's technical guide and Bulfer's book and I found a note in the latter which stated that an inadvertent transfer of fuel into the centre tank can occur if a check valve doesn't seat properly. Not much else in any of those titles to help solve the riddle.
As for figures, I have the following to hand:
Sector 1: Dep 7.8 Arr 4.5.
Sector 2: Dep 6.0 Arr 2.9
Sector 3: Dep 7.8 Arr 4.5
Sector 4: Dep 6.2 Arr 2.8
On our second sector I shut down engine no.2 for the taxy to stand and I do recall that there was a smaller 100-150kg imbalance at that time. The main imbalance occurred on Sector 4. Interestingly, the imbalances seemed to have occurred on similar length sectors.
By the way, the fuel in the centre tank was not evident on arrival. It only appeared after fuelling the aircraft.
Your helpful input encouraged me to get out the FCOM2, Pat Boone's MRG, Brady's technical guide and Bulfer's book and I found a note in the latter which stated that an inadvertent transfer of fuel into the centre tank can occur if a check valve doesn't seat properly. Not much else in any of those titles to help solve the riddle.
As for figures, I have the following to hand:
Sector 1: Dep 7.8 Arr 4.5.
Sector 2: Dep 6.0 Arr 2.9
Sector 3: Dep 7.8 Arr 4.5
Sector 4: Dep 6.2 Arr 2.8
On our second sector I shut down engine no.2 for the taxy to stand and I do recall that there was a smaller 100-150kg imbalance at that time. The main imbalance occurred on Sector 4. Interestingly, the imbalances seemed to have occurred on similar length sectors.
By the way, the fuel in the centre tank was not evident on arrival. It only appeared after fuelling the aircraft.
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152.
In a desperate attempt to try to find out what was happening to the NGs I was driving and as occupational therapy to keep me "alert" on the Hurghada and similar rotations, l took a piece of graph paper and plotted accurate fuel readings every 10 mins. (I know, sad b'std!)
I plotted at the same event, FMS fuel off the cdu-optimistic, guage totals to nearest 10kgs and finally ramp fuel minus engine totalizers at the same moment.
Assuming the pre takeoff apu burn was a constant and did not significantly affect the result, EVERY 'frame I flew showed an alarming "loss" of up to 200kgs at around the time the ctr tank pumps were selected off iaw SOPs.
Later in the sector, the "missing" fuel mysteriously recovered and despite presenting a number of graphs to the engineers, nobody could put forward a reasonable explanation.
All this was done whilst checking bowser uplift in litres against the kg uplift converted with SG at the time, an old Dan-Air precaution which has been forgotten by the newbies in flt ops management on the basis " not invented here, so we don't do that".
Apart from keeping you awake during the dogwatch, this plotting may provide an answer?
However, unless the observed imbalance/errors cause a sphincter-tightening moment or two, personally I'd use the spare mental capacity to look out of the window, or to practise the black art of weather radar tweaking or planning where I'd go in a hurry when something goes 'twang'. Or asking your (junior?) oppo in the rhs what would he/she planned to do if it all went pear-shaped whilst you were in the loo?
There again, it depends if/how soon one reaches the boredom threshold, if at all............
In a desperate attempt to try to find out what was happening to the NGs I was driving and as occupational therapy to keep me "alert" on the Hurghada and similar rotations, l took a piece of graph paper and plotted accurate fuel readings every 10 mins. (I know, sad b'std!)
I plotted at the same event, FMS fuel off the cdu-optimistic, guage totals to nearest 10kgs and finally ramp fuel minus engine totalizers at the same moment.
Assuming the pre takeoff apu burn was a constant and did not significantly affect the result, EVERY 'frame I flew showed an alarming "loss" of up to 200kgs at around the time the ctr tank pumps were selected off iaw SOPs.
Later in the sector, the "missing" fuel mysteriously recovered and despite presenting a number of graphs to the engineers, nobody could put forward a reasonable explanation.
All this was done whilst checking bowser uplift in litres against the kg uplift converted with SG at the time, an old Dan-Air precaution which has been forgotten by the newbies in flt ops management on the basis " not invented here, so we don't do that".
Apart from keeping you awake during the dogwatch, this plotting may provide an answer?
However, unless the observed imbalance/errors cause a sphincter-tightening moment or two, personally I'd use the spare mental capacity to look out of the window, or to practise the black art of weather radar tweaking or planning where I'd go in a hurry when something goes 'twang'. Or asking your (junior?) oppo in the rhs what would he/she planned to do if it all went pear-shaped whilst you were in the loo?
There again, it depends if/how soon one reaches the boredom threshold, if at all............
Mikehotel152,
Thankyou for the additional info re..sector figures. This has encouraged me to also reach for my "special" book titled.......Fuel Gremlins 101!!! A truly perplexing defect.
Thankyou for the additional info re..sector figures. This has encouraged me to also reach for my "special" book titled.......Fuel Gremlins 101!!! A truly perplexing defect.
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Lots of comments about phantom fuel.
Maybe it's time to think not in terms of phantom fuel but fuel that exists due to A/B/C, e.g. due leaking seals etc, but subsequently is actually used as per design using the scavenge pumps.
Unexplained fuel in a centre tank does not mean phantom fuel which implies that it does not exist.
There may be a perfect reason for those tanks to indicate fuel, as explained for various technical reasons, the amount of fuel of which can be indicating a different amount at varying stages of flight due pitch attitude, acceleration/deceleration, turns and subsequent time required to settle again for accurate measuring.
Maybe it's time to think not in terms of phantom fuel but fuel that exists due to A/B/C, e.g. due leaking seals etc, but subsequently is actually used as per design using the scavenge pumps.
Unexplained fuel in a centre tank does not mean phantom fuel which implies that it does not exist.
There may be a perfect reason for those tanks to indicate fuel, as explained for various technical reasons, the amount of fuel of which can be indicating a different amount at varying stages of flight due pitch attitude, acceleration/deceleration, turns and subsequent time required to settle again for accurate measuring.
Thread Starter
But Skyjob, phantom fuel, as with the existence of the 'Mary Celeste' criss-crossing the Seven Seas to this day is not in doubt. It's how and why it got there, left and returned is a curious phenomenon which I suspect we've done to death. 
Barkingmad, you live up to the expectation sown in the mind by your pseudonym. What fun. I don't think 220 kgs is quite enough to encourage me to such graphic antics. Funnily enough, I left the FD to 'check my make-up' (he didn't get the joke either) on this morning's Scandinavian adventure and left my 2-striper to see how he got along with the lovely Valentina in my absence. The disappointment at my untimely return was audible...
Thank you all for your helpful insights. Unless someone comes up with a wholly new angle on this, perhaps we can move on.
Barkingmad, you live up to the expectation sown in the mind by your pseudonym. What fun. I don't think 220 kgs is quite enough to encourage me to such graphic antics. Funnily enough, I left the FD to 'check my make-up' (he didn't get the joke either) on this morning's Scandinavian adventure and left my 2-striper to see how he got along with the lovely Valentina in my absence. The disappointment at my untimely return was audible...
Thank you all for your helpful insights. Unless someone comes up with a wholly new angle on this, perhaps we can move on.
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So yesterday We took off with 10.4 in the tanks. Wings were full and rest in the center.
After about an hour of flying at a sat of about -40 at .78 I totaled the Burn and Remaining whihc I found to be only 10.2
After about an hour of flying at a sat of about -40 at .78 I totaled the Burn and Remaining whihc I found to be only 10.2
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Apart from the previously mentioned inaccuracies, one thing I note:
Burn is measured near the nozzles in the engine.
Remaining is what's in the tanks.
In between you've got a bit of fuel in the pipeline. After shutdown you can 'see' this fuel pouring back by a slight increase in fuel quantity over a minute or so. I believe also the reason why FMC fuel onboard is more than adding the fuel gauges together.
I totaled the Burn and Remaining
Remaining is what's in the tanks.
In between you've got a bit of fuel in the pipeline. After shutdown you can 'see' this fuel pouring back by a slight increase in fuel quantity over a minute or so. I believe also the reason why FMC fuel onboard is more than adding the fuel gauges together.
172 driver,
Is not fuel burn a derivative of fuel flow, eg...400kg/h? In all my years I have never heard of fuel burn being "measured near the nozzles in the engine". Could you please elaborate.
Also, I would suggest that the "slight increase" in fuel quantity after shutdown is due to the fuel settling after sloshing around in the tanks due to landing/taxiing and not "fuel pouring back" due to the tank boost pump non return check valves.
Is not fuel burn a derivative of fuel flow, eg...400kg/h? In all my years I have never heard of fuel burn being "measured near the nozzles in the engine". Could you please elaborate.
Also, I would suggest that the "slight increase" in fuel quantity after shutdown is due to the fuel settling after sloshing around in the tanks due to landing/taxiing and not "fuel pouring back" due to the tank boost pump non return check valves.
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Is not fuel burn a derivative of fuel flow, eg...400kg/h? In all my years I have never heard of fuel burn being "measured near the nozzles in the engine". Could you please elaborate.
Regarding 172's comment on draining/returned fuel causing a higher reading, I am not so sure about that one either Cap. I've always understood the change in levels was due to the FQIS averaging out the probe readings more accurately with the aircraft still and level on the ground - with the main tanks having more variation than the center.
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Is not fuel burn a derivative of fuel flow, eg...400kg/h? In all my years I have never heard of fuel burn being "measured near the nozzles in the engine". Could you please elaborate.
About the pouring back , could be an old wives' tale I have just taken for granted. The technical description does not seem to support it.
vapilot2004,
Totally agree with your interpretation of fuel indication with the aircraft at a standstill. But what is a "HMU output transmitter" and what does it transmit and to where?
Totally agree with your interpretation of fuel indication with the aircraft at a standstill. But what is a "HMU output transmitter" and what does it transmit and to where?
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Apologies for the inexact wording I used Cap M. I should have said the full and proper phrase, "fuel flow transmitter" which is not far down the pipe from the fuel output of the HMU. It measures the flow rate coming out of the HMU on the way to the nozzles in the cans. It is an electromechanical device, converting fuel flow to an analogue signal.
The output of the transmitter goes to the EEC, located on the fan case, which communicates this flow rate via the ARINC-429 bus to the EICAS computers and fuel totalizer which sends this figure to the FMC for displaying and comparing CALCULATED to TOTALIZER fuel. For the classics, the fuel flow transmitter signal is monitored directly by the EFIS and totalizer units.
The output of the transmitter goes to the EEC, located on the fan case, which communicates this flow rate via the ARINC-429 bus to the EICAS computers and fuel totalizer which sends this figure to the FMC for displaying and comparing CALCULATED to TOTALIZER fuel. For the classics, the fuel flow transmitter signal is monitored directly by the EFIS and totalizer units.
vapilot2004,
Many thanks for that, now I see the picture. This has been a great thread (as has others) for all to maybe gain a bit of deeper knowledge about the toys we play with. Again many thanks.
Many thanks for that, now I see the picture. This has been a great thread (as has others) for all to maybe gain a bit of deeper knowledge about the toys we play with. Again many thanks.
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Cheers Cap!
I too have learned much about many things here at pprune as well! Often, things I have wondered about myself, or sometimes its something I would have never even thought about until coming upon a thread by a fellow member.
I too have learned much about many things here at pprune as well! Often, things I have wondered about myself, or sometimes its something I would have never even thought about until coming upon a thread by a fellow member.
