Air temperatures at cruise altitudes range between -70 and -55 Celsius, the lowest values usually found in the Northern hemisphere in winter time. At such temperatures, the jet fuel having long enough time would freeze. For example, the freezing temperature of Jet A1 is -40 Celsius. Some other advanced fuels have freezing temperatures -f -47 Celsius. So, what happens?

First of all, "freezing" does not necessarily mean block freezing. However, even small solid parts in the fuel feed lines are sufficient to create damage, because they can reduce and even stop the fuel flow. To limit these effects, fuel in the more critical tanks is burned first; each airplane has an optimal sequence of fuel use in flight, following procedures established by the aircraft manufacturers. Water content in the fuel can be catastrophic.

In any case, this even is extremely rare, because the fuel can be used as a heat sink, and it can be "warmed" by using waste heat from other services. Even when this is not possible, freezing is normally entirely avoided.

Let us show a practical example. A Boeing B777 on a long range cruise flight has a bulk jet temperature as shown in the graph below, extracted from the Flight Data Recorder (FDR in the graph).