Definition
A regulatory speed limit of 250 knots indicated airspeed that, under 14 CFR 91.117, applies to all aircraft operating below 10,000 feet MSL. The speed is read directly from the airspeed indicator without correction for instrument or position error, density altitude, or wind.
Plain English
Below 10,000 feet above sea level, you are not allowed to fly faster than 250 knots as shown on your airspeed indicator. Above 10,000 feet, this limit no longer applies.
Context Anchor
Seen in instrument procedure discussions, airspeed limits, and operating rules that specify a maximum or required speed by indicated airspeed.
Derivation
‘Knots’ comes from the old sailing practice of measuring a ship’s speed by counting knots tied along a rope thrown overboard — one knot equals one nautical mile per hour. ‘Indicated’ means what the instrument shows, before any corrections. So ‘250 KIAS’ literally means ‘250 nautical miles per hour as the airspeed indicator reads it.’
Why Pilots Care
This is the standard maximum speed limit below 10,000 feet MSL; exceeding it violates regulations.
Intuition Check
Do not read 250 KIAS as 250 knots over the ground. It means 250 knots shown on the airspeed indicator; wind and altitude can make the actual speed across the ground different.
Example Sentence 1
Passing through 10,000 feet on descent, the pilot reduced power to comply with the 250 KIAS limit.
Example Sentence 2
ATC cleared the flight to maintain 250 KIAS until passing the waypoint.