1. Main tank water scavenge
If centre tank fuel scavenge pipes can ice up, perhaps the main tank water scavenge ones could in certain conditions too? Maybe a scenario with cold-soaked fuel in each main tank at critical icing temperature at end of flight with a relatively high concentration of water after centre tank fuel scavenge and then, during cold stopover, a very cold water/ice mass accumulated in and around water scavenge inlets (near bottom of main tank), blocking them? As long as no more than 7 gallons of free water was in main tank at any one time, no excess water message would be recorded about this issue.
If each main tank's water scavenge lines became restricted by ice before the return flight, would that also explain how there was a very low level of suspended/entrained water to be found in fuel in main tank as it would not continue to be mixed into the fuel by the water scavenge and tend to separate out?
2. Icing
It almost seems easier to explain icing if there is some form of cascade effect where:
- first, relatively minor icing over time restricts narrower and colder scavenge pipes.
- then, unforeseen major condition such as lack of water scavenge and/or fuel scavenge develops.
- then, unlikely accumulation of free water/ice where it is not expected.
- then, accumulation goes on to cause bigger and worse things downstream.
And has to be readily duplicated on either side. And lack of warning messages and tell-tales such as water ppm accounted for.