getting to grips with COLD WEATHER OPERATIONS 寒冷天气运行(70)

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The poor dilution of the JET A in the inner wing tank and its concentration near the inboard end of the tank has a potentially positive consequence. This is because the fuel near the inboard end of the inner wing tank tends to be consumed first by the engines. Thus, the concentration of the remaining JET A fuel on board, later in flight, when low fuel temperatures might be encountered in the case of low OATs, will be less than at takeoff. This gives a higher confidence margin that low concentrations of JET A in JET A1 will have a freeze point similar to JET A1 and can thus be treated as JET A1 with respect to the cold fuel alert.
D6 LOW TEMPERATURE BEHAVIOR OF FUEL
The low temperature properties of fuels must be controlled to guarantee adequate system operation. Basic fuel properties, such as freezing point and viscosity, are important factors in fuel pumpability.
Nevertheless, it may be of some interest to have an idea of the expected behavior of the fuel both at its freezing point and below.
D6.1 Pumpability limit
Fuels are a mixture of hydrocarbons. They do not completely solidify at a fixed temperature, as do simple liquids, such as, water. They do not have a single .freezing
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point., but a range of temperatures below which they contain a higher and higher proportion of solidified fuel.
For the purpose of routine testing and specification requirements, certain stages in this gradual transition are selected and closely defined (See figure D8):
.
 The .freezing point. is represented as the temperature at which the last wax crystal melts on warming, the fuel having previously been cooled down with stirring. (Different definition, as mentioned before under GOST method)

.
 The .cloud point., which occurs at nearly the same temperature as the .freezing point., is the temperature at which a visible cloudiness appears when fuel is cooled down without stirring.

.
 The .pour point. is the temperature at which the fuel just pours from a standard glass cylinder, the fuel being not stirred.

Tests conducted on certain fuels showed that the pumpability limit ranged from 4°C to
16°C below the freezing point, and from 1°C to 7°C below the pour point, depending on
the nature of the fuel.
Thus, the pour point seems to be a better criterion for pumpability limit, but one of its
drawbacks is that it is not very precise.

However, test results show that the .pumpability limit. of a fuel is not, in any way,
connected with the first appearance of wax crystals.
Therefore, none of the existing laboratory tests can predict the minimum temperature of
pumpability.

Ground level

Until now, the .freezing point. has been the only test to survive in fuel specifications as a
control on low temperature pumpability.
The advantages of using this parameter include the relative ease with which it can be
　
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