Definition
The temperature of the outside air surrounding the aircraft in flight, measured away from any heat sources or surfaces that could distort the reading. It is the ambient air temperature used as an input for altimeter corrections, true airspeed calculation, and density altitude.
Plain English
The temperature of the air the aircraft is flying through, measured outside and uncontaminated by engine or cabin heat.
Context Anchor
Seen in altimeter and performance discussions when temperature is used to explain how the atmosphere differs from standard conditions.
Derivation
‘Free’ here means unobstructed or undisturbed — air that is moving freely past the aircraft, not air that has been warmed by contact with the engine, skin, or cabin. The phrase distinguishes the true outside air temperature from any reading affected by the aircraft itself.
Why Pilots Care
Used to compute density altitude, true airspeed, and aircraft performance limits.
Grounding Statement
Picture measuring the air out in the open, away from a warm engine cowling or a sun-heated surface.
Intuition Check
“Free air” does not mean air that is available or unrestricted for flying. Here it means the outside air in its natural condition, before the airplane or its motion changes the temperature reading.
Example Sentence 1
Before calculating true airspeed, the pilot noted the free air temperature from the outside air temperature gauge.
Example Sentence 2
At cruise, the free air temperature of minus fifteen degrees Celsius helped determine true airspeed.