Definition
An ATC instruction directing a pilot to slow the aircraft to a specified airspeed, expressed in knots. The pilot is expected to adjust to and maintain that speed until further instruction, while still complying with any applicable regulatory speed limits.
Plain English
Controller is telling you to slow down to a specific number of knots and hold that speed until told otherwise.
Context Anchor
Heard in radio calls from air traffic control during arrivals, approaches, and traffic sequencing when controllers need aircraft to keep safe spacing.
Why Pilots Care
Following the instruction keeps proper separation from other aircraft and avoids conflicts in busy airspace.
Intuition Check
Do not read this as “slow down some.” It means slow to the specific speed the controller stated. If you cannot do that safely, tell the controller.
Example Sentence 1
Approach instructed us, 'Reduce speed to one eight zero,' so we pulled the throttles back and set 180 knots.
Example Sentence 2
On arrival the pilot heard 'Reduce speed to one eight zero' and complied to stay behind the aircraft ahead.