FAA Aviation Mechanic Airframe (AMA)Airframe Electrical and InstrumentsHard
A technician is troubleshooting a total air temperature (TAT) probe on a turbine-powered aircraft. During cruise flight, the TAT indication reads noticeably higher than the calculated static (outside) air temperature for that altitude, even though the probe and system are functioning correctly. What is the primary cause of this expected difference?
- AIce contamination partially blocking the probe's sensing element
- BCalibration error requiring a compass swing of the temperature system
- CRecovery of kinetic (ram) heating as high-velocity air is brought to near-zero velocity inside the probe
- DResistance heating generated by the probe's anti-ice element circuit
Show answer & explanationAnswer & explanation
Correct answer: C. Recovery of kinetic (ram) heating as high-velocity air is brought to near-zero velocity inside the probe
As air is decelerated to near-zero velocity inside the TAT probe, its kinetic energy converts to heat (adiabatic compression), producing a temperature rise above the true static air temperature. This 'ram rise' is a normal, expected characteristic of TAT measurement, not an error condition.
Why the other options are wrong
- A. Ice contamination would typically cause erratic or low readings, not a predictable, consistent higher reading.
- B. A compass swing corrects magnetic compass deviation and has no relation to a temperature probe.
- D. Anti-ice heating elements are not normally active except in icing conditions and would not explain a routine cruise reading.
Total Air Temperature (TAT) / Ram Rise
TAT is the temperature of air after it is brought to a near-complete stop at the probe, which is warmer than static air temperature (SAT) due to kinetic (ram) heating from air deceleration.
- TAT = SAT + ram temperature rise from air compression
- Ram rise increases with airspeed (more pronounced at high Mach)
- Used to correctly compute true airspeed and engine performance parameters
Memory trick: Stop the air, and its speed turns to heat — that's your ram rise.