What condition activates TAT heaters?

The condition that activates TAT (Total Air Temperature) heaters is when the Outside Air Temperature (OAT) is less than 10°C with the engine running. This is crucial because the TAT heaters are designed to prevent ice formation on the aircraft’s pitot-static sensors and ensure accurate airspeed and altitude readings in cold weather conditions.

When the OAT falls below this critical threshold, there's an increased risk of ice accumulation, which can interfere with the aircraft's instrumentation. The activation of the TAT heaters in this scenario helps maintain safe operational conditions by ensuring that the temperature around these sensors remains above freezing, allowing for reliable performance of critical flight instruments. Furthermore, the requirement that the engine must be running suggests that sufficient power is available to operate the heaters, as they rely on the electrical systems of the aircraft.

This combination of conditions ensures that the aircraft remains safe and operable in potentially hazardous weather situations.