737 Fuel anomaly.
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Theory
The only explanation to it after monitoring for a long time is that it isn't about what reads the fuel it is about how it is read and measured by the corresponding capcitators , so just consider the body angle and the attitude with the existing CG and what phase of flight you are in? And you will figure out why with more nose up attitudes you get more fuel indicated and vice versa , effect of center tank fuel consumption on the above factors and why we have more fuel indicated by the begining of the flight and less than what was indicated by it's end .
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Just another theory
Originally Posted by Barkingmad #1
Subsequent checks showed the ramp minus totaliser readings had crossed over to above the tank total line, which eventually managed to recover some of its "lost" fuel as more was consumed.
Originally Posted by Skyjob #6
Scavenge Pump Inlet located most aft of tank, this should (over time) be drawing all remaining center fuel out, but this takes time, and sometimes this pump is not effective enough to drain the tank remains before landing, then resulting in an apparent fuel quantity in the center tank which is yet to be used.
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Doesn't that explain it? HN39 - no.
- I had followed that line too but BM is quite clear - the 'usable' ctre tank fuel is ZERO at that moment - gauge shows ZERO. Yes, we know there IS fuel to be scavenged but it is not 'usable' as far as the gauging is concerned - at least until it arrives in Tank 1.
total fuel seems to reduce by 100-200 kgs at the time the centre tank empties
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RAMP FUEL MINUS TOTALISER
Skyjob, Many thanks for your info, it increases our knowledge of how the system works but my observations have been made in the cruise, with steady pitch/roll attitudes, no climbing and obviously without the complications of engine shutdown, fuel sloshing around galleries and indicated values changing after shutdown.
Alas my recent flights since the thread started have been with lower fuel amounts, with centre tank switchoff occurring in the climb, thereby invalidating any attempt to record the STEADY STATE fluctuations which are the main point of the posting.
My proposed datums are the PLOG/OFP stated figures, whose accuracy I have no info about but have to assume they are good and linear, and the ramp-minus-used figure which I assume is more accurate, though it might show a variation due to the decreasing fuel temp. I accept that APU useage since refuel is a constant for that sector and therefore will not change with subsequent readings.
That said, anyone trying this monitoring should notice the consistent and possibly alarming drop on tank totals at or near the changeover to wing tank useage.
It may be that the 2.5% tolerance kicks in and changes to minus rather than plus, equalling virtually 100kgs per wing tank, but this in itself would be an odd characteristic?
Longer flights permitting, I will restart plotting a graph of the relevant values and report back in due course.
A colleague has told me that tolerating flashing ctr pump lo-press annunciators until they illuminate steady lasts for up to 6 minutes and reasons that if the lo-press light extinguishes even momentarily, then there is fuel present to provide pressure and 6 mins at cruise settings approximates to 200 kgs.
I notice the auto shutoff logic requires 15 secs of continuous illumination, so would this be where the "missing" 200kgs are hiding and the ctr tank indication indicates zero early? Subsequently this fuel will become "available" at scavenging stage (approx 1900 per wing tank), but by then the diversion decision may have been made with a 4+ tonne reserves figure?
Alas my recent flights since the thread started have been with lower fuel amounts, with centre tank switchoff occurring in the climb, thereby invalidating any attempt to record the STEADY STATE fluctuations which are the main point of the posting.
My proposed datums are the PLOG/OFP stated figures, whose accuracy I have no info about but have to assume they are good and linear, and the ramp-minus-used figure which I assume is more accurate, though it might show a variation due to the decreasing fuel temp. I accept that APU useage since refuel is a constant for that sector and therefore will not change with subsequent readings.
That said, anyone trying this monitoring should notice the consistent and possibly alarming drop on tank totals at or near the changeover to wing tank useage.
It may be that the 2.5% tolerance kicks in and changes to minus rather than plus, equalling virtually 100kgs per wing tank, but this in itself would be an odd characteristic?
Longer flights permitting, I will restart plotting a graph of the relevant values and report back in due course.
A colleague has told me that tolerating flashing ctr pump lo-press annunciators until they illuminate steady lasts for up to 6 minutes and reasons that if the lo-press light extinguishes even momentarily, then there is fuel present to provide pressure and 6 mins at cruise settings approximates to 200 kgs.
I notice the auto shutoff logic requires 15 secs of continuous illumination, so would this be where the "missing" 200kgs are hiding and the ctr tank indication indicates zero early? Subsequently this fuel will become "available" at scavenging stage (approx 1900 per wing tank), but by then the diversion decision may have been made with a 4+ tonne reserves figure?
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More graphs later and it now transpires that the apparent decrease does manifest itself BEFORE the CTR pumps are turned off.
Latest expanded scale plot shows the "dip" started at 8400kgs, so back to the drawing board for me.
Curiously some of my RHS colleagues had observed this apparent dip, others who look at the FMS figure minutes before the waypoint and record it as such, have not noticed.
I really must get outa the cockpit more..............................................
Latest expanded scale plot shows the "dip" started at 8400kgs, so back to the drawing board for me.
Curiously some of my RHS colleagues had observed this apparent dip, others who look at the FMS figure minutes before the waypoint and record it as such, have not noticed.
I really must get outa the cockpit more..............................................
