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?
