getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(69)

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than those expected from ideal solution (dashed line on plot). This graph also shows that
all the blends containing more than 28% of fuel H will have freeze points higher than
those of A or H individually.

Figure D7B
 involves fuels A (JET A1) and F (JP4). This is another example of freeze
point elevation. In this case, the freeze point of fuel F (-58.8°C) is much lower than that of
fuel A (-48.2°C). The maximum elevation was 2.8°C and it was observed in a blend
containing 25% of fuel A and 75% of fuel F.

Figure D7C
 shows that fuels B and E display distinct freeze point depression. The
maximum depression was 4.6°C. It was noticed that this depression behavior was related
to the exceptional solvency power of fuel B.

This study confirmed that mixing jet fuels of different origin gives a final mixture, whereby
the freezing point cannot be consistently predicted using a linear relationship.

Therefore, the only reliable way to obtain an accurate freeze point of a mixture of fuels is
to make an actual freeze point measurement. Without this, it is not possible to determine
the extent or the direction of non-linear solubility.
It would be comforting, if blending always provides freeze point depression. Unfortunately,
the reverse behavior, freezing point elevation, appears to be the more prevalent outcome.

136

Figure D7A - Freezing point as function of blend concentration for samples A and H

Figure D7B - Freezing point as function of blend concentration for samples A and F

Figure D7C - Freezing point as function of blend concentration for samples B and E
137

● Practical issues
Notwithstanding what is mentioned above, airlines operating transatlantic or transpacific routes generally have to have their own rules, because they have to continuously cope with the mixture of JET A generally delivered in USA and JET A1 elsewhere.
Some operators have the following pragmatic approach, which may be considered:
-
When the mixture contains less than 10% JET A, the fuel is considered as JET A1

-
When the mixture contains more than 10% JET A, the fuel is considered as JET A

Mixing all the residual JET A with all the refuel JET A1 to achieve maximum dilution is not considered practical. To practically achieve the best dilution, all the JET A should be placed in the inner wing tanks as these have the largest volume (by transfer of outer tanks JET A fuel into the inner tanks either during the previous flight or on ground before refueling). Depending on the aircraft model, inner tanks will receive fuel from the center tank early in the flight, further diluting the JET A. Placing all the JET A into the inner wing tanks potentially enables a maximum dilution but does not guarantee that the mixture will be homogenous. In reality, due to the compartmental structure of the inner wing tank and the fact that the residual JET A fuel will start at the inboard end of the tank, the concentration of JET A will be greater near the tank’s inboard end.
　
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