Jet A-1 fuel can have a maximum temperature of about 49°C. But what is, when the plane is i.e. in Phoenix, ...

Question

.. where the temperature can raise sometimes over 50° C? Because the aircraft is on the ground, the true air temperature (TAT) is equal to the outside air temperature (OAT), so TAT=OAT.... So what should you do then?

Answer 1

The OAT will only be this high for a very short period of time, and will not be able to raise the temperature of the large volume of fuel in an airplane this high.  

Just consider how long that it takes to boil water even when applying extremely high heat directly to it!

Answer 2

The temperature is referring to the liquid's temperature, not OAT. So at 50c OAT, the liquid itself is much more cooler, way below the normal flash point.