Accuracy of FQIS
In response to the question on FQIS accuracy.
I believe the minimum designed accuracy of fuel quantity indication systems is 5%. However most modern systems are accurate to 3%. Some systems also have error indications that will advise you if the system is gauging to an accuracy of less than 3%.
In practice geater accuracies are hard to achieve as the "accuracy" of the fuel is only around 1% (as its properties vary a lot).
FQIS systems are on condition. There may be a serviceability check in major servicings, but mostly the maintenance processes will rely on pilot-reported problems. There is no accuracy limits for maintenance in terms of percentages; but there will be limits in the standard tests. based on mass indications.
In practice modern systems are very robust and the digital systems will remove the inputs from suspect probes. The older summing systems do carry errors though, and often maintenance crews calibrate out the errors, which is not really how the system should work.
The best way to maintain accuracy of capacitance systems is to drain the water regularly, and for pilots to report accurately any percieved errors and fluctuations.
It isn't easy to work on the systems, particularly when in-tank maintenance is required. Too often, to avoid the cost, simple measures like replacing the indicator (repeatedly) is carried out to keep the plane flying.
It's not all doom though. As the errors are %-based, the system normally gets more accurate as you burn fuel (1% of 5000kgs vs 1% of 20,000kgs). Unless fuel draining isn't carried out regularly, whereby fungus can form on the probe causing it to overead.