Definition
True airspeed is the actual speed of the airplane through the surrounding air mass, obtained by correcting calibrated airspeed for the effects of altitude and non-standard temperature. As an airplane climbs, the air becomes less dense, so the airspeed indicator reads lower than the airplane is actually moving through the air; TAS accounts for this and gives the real speed through the air.
Plain English
How fast the airplane is really moving through the air around it, after accounting for the fact that the air gets thinner and warmer or cooler at altitude.
Context Anchor
Seen in performance planning, cruise flight, jet efficiency discussions, and flight computer calculations.
Derivation
True' here means 'actual' — the real speed through the air, as opposed to the indicated speed shown on the cockpit gauge, which is distorted by air density. The word 'true' is used in aviation in the same sense as 'true north' or 'true altitude': the actual value, not the displayed or apparent one.
Why Pilots Care
It determines actual aircraft performance, range, fuel consumption, and groundspeed when combined with wind.
Grounding Statement
At higher altitude, thinner air can make the same indicated airspeed correspond to a faster actual speed through the air.
Intuition Check
Do not assume “true” means the airplane’s speed over the ground. True airspeed is speed through the air; wind must still be added or subtracted to get groundspeed.
Example Sentence 1
At cruise altitude, the airspeed indicator showed 250 knots, but the true airspeed was closer to 410 knots because of the thinner air.
Example Sentence 2
The pilot used true airspeed in the flight computer to calculate accurate time and fuel to destination.