AIM Text
The separate digits of the speed followed by the word “KNOTS.” Except, controllers may omit the word “KNOTS” when using speed adjustment procedures; e.g., “REDUCE/INCREASE SPEED TO TWO FIVE ZERO.”
The separate digits of the Mach Number preceded by “Mach.”
Source: FAA Aeronautical Information Manual · current edition · paragraph 4-2-11.
The short answer
This AIM section is FAA advisory guidance on stating speeds in radio communications. It explains that airspeed is given as separate digits followed by the word "KNOTS" (which controllers may omit during speed adjustments), and Mach number is given as separate digits preceded by "Mach," supporting standardized Part 91 ATC phraseology.
Research Notes
Common Questions
How do I state airspeed on the radio?
State the separate digits of the speed followed by the word "KNOTS"—for example, 250 is "two five zero knots" and 190 is "one niner zero knots."
When can a controller leave out the word "knots"?
Controllers may omit the word "KNOTS" when using speed adjustment procedures—for example, "REDUCE/INCREASE SPEED TO TWO FIVE ZERO."
How is a Mach number spoken?
State the separate digits of the Mach number preceded by "Mach"—for example, 1.5 is "Mach one point five," 0.64 is "Mach point six four," and 0.7 is "Mach point seven."