Which temperature indicator predicts the operation of in-flight engine anti-icing without applying a correction factor?

The Total Air Temperature (T.A.T.) indicator is crucial because it reflects the temperature of the air around the aircraft, incorporating both static temperature and any increase in temperature due to the aircraft's speed. This makes T.A.T. the appropriate choice for predicting in-flight engine anti-icing performance without needing any correction factors.

In contrast, the Ram Air Temperature (R.A.T.) is influenced by the aircraft's speed and can vary based on airflow around the aircraft, thus not providing a true reflection of the air temperature encountered by the engines during anti-icing operations. The Outside Air Temperature (O.A.T.) simply measures the temperature outside the aircraft without accounting for the effects of speed, which also can lead to inaccuracies. Finally, the Fan Air Temperature (F.A.T.) pertains to the air entering the fan system within a turbo-fan engine and does not directly relate to external conditions affecting anti-icing.

Therefore, T.A.T. stands out as the most accurate and reliable indicator for the operation of engine anti-icing systems in-flight.