Temperature and Ice Detection
Domain 3 covers temperature measurement in air and engines, and the detection and treatment of icing. The exam asks which sensor suits which range, why TAT differs from SAT, how anti-ice and de-ice systems differ, and what happens when a probe ices up.
Temperature sensors
- RTD (platinum resistance thermometer) - accurate, stable, wide range, positive coefficient; used for air temperature and oil.
- Thermistor - semiconductor, large non-linear change, usually negative coefficient; good for narrow ranges such as cabin or fuel temperature.
- Thermocouple - two dissimilar metals, Seebeck effect; produces a small voltage proportional to the difference between hot and cold junctions, so cold-junction compensation is essential. Used for EGT/TGT/ITT and fire detection.
- Bimetallic strips - simple and cheap, used in older gauges.
SAT, TAT and ram rise
The TAT probe sits in the airflow, is electrically heated, and has a recovery factor near 0.98. Ram rise follows TAT = SAT × (1 + 0.2 M²) with temperatures in kelvin for a recovery factor of 1. TAT is always higher than SAT, and TAT is what the engine and air data computer use.
Ice detection and protection
Vibrating-probe (magnetostrictive) detectors sense a mass change and trigger the ice-protection controller plus a cockpit advisory. Protection is either anti-ice (thermal bleed air on wings and engine inlets, electrothermal on probes and windshields) or de-ice (pneumatic boots that break the ice off). A blocked TAT probe corrupts SAT, and therefore TAS and Mach.
Worked example
SAT = −45 °C, M = 0.78. SAT = 228.15 K and M² = 0.6084, so 1 + 0.2 × 0.6084 = 1.1217. TAT = 228.15 × 1.1217 ≈ 255.9 K = −17 °C. The ram rise is about 28 °C, which is why a fast, high-flying aircraft needs a heated probe.
Traps that cost marks
- Thinking TAT can be lower than SAT.
- Inserting °C directly into the ram-rise formula - it needs absolute temperature.
- Believing a thermocouple measures absolute temperature; it measures a difference.
- Forgetting that probe heat failure degrades SAT, TAS and Mach, but not static pressure.