What is the correct phraseology to instruct an aircraft to reduce speed?

• A. <Callsign>, slow down to <speed> knots
• B. <Callsign>, reduce speed to <speed> knots
• C. <Callsign>, decrease speed to <speed> knots
• D. <Callsign>, speed control: reduce to <speed> knots

Answer: (B)

Standard ATC phraseology uses a precise command to set a target airspeed, using the verb reduce followed by the speed value in knots. Saying “reduce speed to” knots communicates a clear, unambiguous instruction to slow to an exact target, which is essential in busy radar environments to avoid misinterpretation.

Using “slow down” is informal and can be interpreted variably, which is not ideal when you need a specific speed. “Decrease speed” is understandable but not the standard operational phraseology, and “speed control” with a colon isn’t how you issue a normal speed instruction.

So, the best way to tell an aircraft to reduce speed is: “Callsign, reduce speed to knots.”