Definition
When the term 'speed' is used in ATC instructions, it refers to airspeed — the speed of the aircraft through the surrounding air mass — unless ATC specifically states 'groundspeed.' Pilots comply with assigned speeds using indicated airspeed (IAS) on the airspeed indicator.
Plain English
If a controller tells you a speed, they mean how fast you're moving through the air, not how fast you're moving across the ground. You read it off your airspeed indicator.
Context Anchor
You see airspeed on the airspeed indicator and in operating instructions for takeoff, climb, approach, and landing.
Derivation
Airspeed is built from air and speed. The important point is the reference: this speed is measured against the air the aircraft is moving through, not against the ground below.
Why Pilots Care
Determines lift, stall margins, and structural limits; incorrect airspeed leads to loss of control or damage.
Analogy
A boat moving through a river has a speed through the water, even if the river itself is moving. Airspeed is like that: it is the airplane’s speed through the moving air.
Intuition Check
Airspeed does not mean the airplane’s speed over the ground. Wind can make those two speeds different.
Example Sentence 1
Approach instructed us to maintain a speed of 210 knots until the final approach fix.
Example Sentence 2
True airspeed increases with altitude even when indicated airspeed stays constant.