TAT =
Total air temperature - Wikipedia, the free encyclopedia
In aviation, stagnation temperature is known as total air temperature and is measured by a temperature probe mounted on the surface of the aircraft. The probe is designed to bring the air to rest relative to the aircraft.
i.e. its is not designed to take into account the temperature effects of ice formation....
Thus the aircraft engine receives incorrect TAT measurements which can effect its operation including producing engine surges since its trying to operate using the measure TAT value rather than the actual effective TAT value.
Incorrect TAT values probably relates to incorrect calculated air induction volumes which leads to incorrectly calculated air/fuel mixtures.