Definition
A pitot-static flight instrument that displays the aircraft's speed through the surrounding air, typically in knots. It works by comparing ram air pressure (collected by the pitot tube) with static air pressure (collected from a static port), and mechanically converting the difference into a speed reading on a dial.
Plain English
A cockpit instrument that shows how fast the aircraft is moving through the air around it.
Context Anchor
Seen on the instrument panel. Before engine start, the pilot checks that the ASI reads zero; in flight, the pilot uses it during takeoff, climb, approach, and landing.
Derivation
Straightforward: 'air' + 'speed' + 'indicator' (something that indicates or shows). The key idea is that it shows speed through the air — not speed over the ground.
Why Pilots Care
Maintaining correct airspeeds prevents stalls, ensures safe takeoff and landing performance, and avoids exceeding structural limits.
Analogy
It is like a car speedometer, but for air instead of road travel. A car speedometer shows speed over the road; the ASI shows speed through the air, so wind can make it different from the airplane’s speed over the ground.
Intuition Check
Do not read the ASI as map speed. It shows speed through the air around the airplane, not how fast the airplane is moving over the ground.
Example Sentence 1
During the takeoff roll, the pilot called out 'airspeed alive' as the ASI needle began to move.
Example Sentence 2
During cruise, the ASI read 110 knots with the throttle set for level flight.