This can be -50degC. What I suggest is a combined cycle jet engine that is able to use this to improve engine efficiency.

Combined cycles are currently used in some power stations. They consist of both a gas turbine and a steam turbine hooked up to electrical generators. By using the exhaust gas from the gas turbine to heat the water in the steam turbine extra power can be scavenged. This can increase the efficiency from about 35% to about 60%.

However, steam must be well above 100deg C to power a turbine. This means we can't fully benefit from the low temperature of the high altitude atmosphere.

An alternative cycle exists called the kalina cycle. This is very similar to the conventional steam cycle except that it uses a mixture of ammonia and water. This allows the temperature at which the turbine can operate to be lowered.

If we go further and completely replace the steam cycle with an ammonia cycle we can then operate at much lower temperatures.

The engine is a turbofan that has radiators placed in the duct after the main fan. These contain thin copper pipes through which the ammonia is fed. This is the heat sink of the ammonia cycle. The heat source of the ammonia cyle is the exhaust of a conventional gas turbine.

This may be heavier than a conventional jet but if we can potentially double the efficency of the engine the weight of fuel saved on long flights might more than compensate.