snanceki said
For ice to have formed, to the extent required to restrict flow, a considerable amount of water would need to be present in the supply from the fuel tanks and in addition some mechanism would have to be present to make this situation common to both LH and RH systems.
A small amount of water coming out of solution due to super-cold temperatures can form a sizeable ice-block if it all sinks to (or aggregates at) a common (left and right side) low point in the system. What could cause water to exit solution at a particular point (and later melt from its attachment point and migrate during the warming descent)? Perhaps it's a function of:
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long-term exposure in the cruise to super-low temperature as compounded by.....
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pressure drops at constrictive flow-points (such as in a venturi) and .....
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a very cold area (not all areas outside the pressure hull stay at a homogenous temperature). Aerodynamic stagnation points can cause significant temperature variations, particularly where piping runs along or immediately adjacent to the aircraft's skin.
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Reflect upon what water does in another fluid (i.e. air) when it's cold enough. It will form freezing rain (that hits and sticks and accumulates upon impact). It will also form snow and hail, depending upon relative humidity, the temperature strata (lapse-rates), condensation nuclei and the recirculation found in strong convective flows. Atmospheric water will form rough shapes upon wings and fuselage as clear or rime icing, particularly in areas where the airflow tends to be slowed or stagnates. The equivalent outcome within fuel tanks is dependent upon the thermal and fluid dynamics that are in internal play. If the fuel-feed obstruction was actually a coalescing mass of "not so small" ice particles (i.e. an icing "tumour" or growth), rather than a detaching mobile lump, we should not be amazed.