The heater of a TAT sensor has an effect on the temperature reading. Specification MIL-P-27723E allows a maximum heater error of up to 1.0° at an external flow condition of Mach 1.0. The actual heater error of the designed probe is much less than this and of course it varies with the flow rate. The effect highest when there is "no flow" because there is no airflow to take away heat.

The error due to deicing heat remains very low (less than 0.4° C) at high flight speeds typical of turbine powered aircraft. Significant errors occur only at conditions which yield a low internal mass flow. These conditions cause the boundary layer on the interior surfaces of the heated housing to thicken enough to contact portions of the sensing element. As can be seen in the figure below total air temperature sensors are sensitive to variable flight conditions at values of “Z” as defined below.

• Z = M (ρ1 / ρ0)

Where

• M = Flight Mach Number
• ρ1 = air density at flight condition
• ρ0 = air density at sea level standard condition

Z Heater Error (°C) .04 7 ± 2 .06 3.4 ± 1.3 .08 2.0 ± 0.9 .10 1.3 ± 0.7 .12 1.00 ± 0.60 .14 0.75 ± 0.45 .16 0.60 ± 0.40 .20 0.40 ± 0.30 .30 0.20 ± 0.15 .40 0.20 ± 0.15 .50 0.20 ± 0.15 .60 0.20 ± 0.15 .80 0.20 ± 0.15 1.00 0.20 ± 0.15 2.00 0.20 ± 0.15

These above figures are provided for reference only. Wind tunnel testing is necessary to produce a guaranteed heater error versus airflow correction curve.