What is the first condition needed for TATs to be heated?

The correct answer pertains to the operation of the TAT (Total Air Temperature) sensors and their heating mechanisms. The first condition for the TATs to begin heating is indeed when the Outside Air Temperature (OAT) is below 10°C with the engines running. This is crucial because TATs are designed to prevent ice formation and ensure accurate temperature readings in colder conditions. When the ambient temperature is low enough, the system automatically activates the heating function to maintain proper functionality and accuracy of the sensor.

This condition ensures that when the aircraft is likely to encounter icing conditions, the TAT will be kept free from ice build-up, which could impede its ability to function effectively. Heating is a preventive measure to ensure that the system stays operational even in suboptimal temperatures, which is critical for safe flight operations.

Other conditions, such as whether the engines are off, the aircraft's airspeed, or whether the aircraft is on the ground, do not directly trigger the heating function of the TAT sensors in the same manner as the specified temperature condition with the engines running.