Definition
A cockpit instrument that displays the temperature of the outside air surrounding the aircraft in flight. It uses a probe mounted on the exterior of the aircraft, exposed to the airflow, and reads the ambient air temperature unaffected by cabin or engine heat.
Plain English
A gauge in the cockpit that tells the pilot how hot or cold the air is outside the aircraft.
Context Anchor
Seen in flight and navigation equipment discussions, especially when outside temperature matters for icing awareness, altitude corrections, and aircraft performance.
Derivation
"Free air" means air that is not enclosed or influenced by the aircraft's interior or engine heat — it's the air freely flowing past the aircraft. The name simply describes what it measures: the temperature of the open, undisturbed air outside.
Why Pilots Care
Accurate readings are required for true airspeed, density altitude, Mach calculations, and icing risk assessment.
Grounding Statement
If the airplane is flying through very cold air, the indicator gives the pilot a direct clue that icing or cold-temperature altitude effects may matter.
Intuition Check
Free-air does not mean the air is free of cost or completely untouched. Here it means outside air that is as unaffected by the aircraft as practical.
Example Sentence 1
Cruising at 8,000 feet, the pilot checked the free-air temperature indicator and noted it was reading minus four degrees Celsius — cold enough to watch for icing.
Example Sentence 2
At cruise altitude the free-air temperature indicator read minus thirty-five degrees, confirming the aircraft was below the freezing level.