Definition
A pitot-static flight instrument that displays the aircraft's speed through the surrounding air, expressed in knots or miles per hour. It works by measuring the difference between ram air pressure (from the pitot tube) and static air pressure (from the static port), and converting that pressure differential into an indicated airspeed reading on the dial.
Plain English
The cockpit gauge that tells you how fast you are moving through the air around you. It compares the pressure of air being pushed into a forward-facing tube against the pressure of still air outside the aircraft, and shows the result as a speed.
Context Anchor
Seen on the instrument panel during taxi checks, takeoff, climb, cruise, approach, and landing.
Derivation
Straightforward: 'air' + 'speed' + 'indicator.' Worth noting that it shows speed through the air, not speed over the ground. A 100-knot indication tells you how fast the air is flowing past the aircraft, which can be very different from how fast you are travelling across the earth below.
Why Pilots Care
Pilots rely on it to maintain safe operating speeds, prevent stalls, avoid structural damage from overspeed, and follow performance charts for takeoff, climb, cruise, descent, and landing.
Grounding Statement
Picture the ASI as the cockpit gauge that answers, “How fast is the airplane moving through the air right now?”
Intuition Check
Do not read the Airspeed Indicator as ground speed. It shows speed through the surrounding air; wind can make the airplane’s speed over the ground different.
Example Sentence 1
On the takeoff roll, the pilot called out 'airspeed alive' as the ASI needle began to move off its resting position.
Example Sentence 2
In cruise flight the airspeed indicator showed 120 knots, confirming the airplane was operating within its normal flight envelope.