However, it is far from clear what relationship exists between the laboratory .freezing
point. and the flow behavior of fuel in the aircraft at low temperatures encountered during
prolonged flights at high altitude. In the absence of anything more representative, it is
obviously better than nothing, but its shortcomings have been acknowledged, and
aviation fuel research is developing tests which are specifically designed to correlate with
pumpability.

The main interest of such a .pumpability limit. test, in place of the .freezing point. test,
is its economic consequences, as it would avoid any waste of potential yield from crude
oil.

D6.2 Protection against wax
D6.2.1 Heating the fuel
Heating the whole fuel in the aircraft tanks would prevent wax from forming. Nevertheless, the penalties in weight and the complications make such a system unattractive.
Some aircraft (A319/A320/A321/A340) fitted with a fuel re-circulation system could theoretically take benefit of such a system. But this system is designed to cool the IDG oil, and not to heat the fuel. From a practical standpoint, it is not operative when very low temperatures are encountered.
Increasing the aircraft speed provides a marginal TAT increase (in the order of 0.5 to 1°C for 0.01 M increase) and thus a small fuel temperature increase, at the expense of a significant increase in fuel consumption.
Decreasing altitude generally provides a SAT increase (about 2°C per 1000 ft). Nevertheless, whenever the tropopause is substantially low, decreasing the altitude may not provide the corresponding expected SAT and, thus, TAT increase.
D6.2.2 Thermal insulation of tanks
Weight penalties of thermal insulation of tanks make this system impractical.
Nevertheless, it may be worth noticing that the fuel itself provides a free bonus, if unexpectedly low ambient temperatures are encountered, so that the fuel temperature falls below its pour point.
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In such rare cases, because the temperature gradient from the tank surfaces inwards, solidification of fuel begins at the tank surface and thereby provides its own thermal insulation. The thermal conductivity of frozen kerosene is approximately the same as that of rubber, so that, as it grows in thickness, it becomes an increasingly effective barrier to further heat losses through the tank walls.
Figure D9 illustrates the relatively slow rate of solidified fuel growth in a typical tank, assuming a TAT of -65°C and a fuel temperature that has fallen below its pour point of -55°C.

The fact that this insulating layer provides a natural protection cannot, of course, be exploited in any way. But, the knowledge that it would be there, working in one’s favor, should such conditions ever be met, is reassurance that the result would not be detrimental. Moreover, the solidified fuel would not be lost for that flight, due to temperature increase after descent to warmer conditions.
　
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