Definition
An airspeed tolerance notation indicating the published target airspeed may be reduced by up to 10 knots, but not exceeded. Used in instrument procedures (such as published holding speeds, approach speeds, or maximum airspeed restrictions) to define the allowable downward variation from a stated value while keeping the aircraft within the protected airspace or design assumptions of the procedure.
Plain English
The published speed is the ceiling. You can fly it slower by up to 10 knots, but you cannot fly it faster.
Context Anchor
Seen in instrument procedure airspeed discussions when a speed tolerance or allowed variation is being described.
Derivation
The dash is being used like a minus sign. “Knot” comes from an old way of measuring a ship’s speed with a rope tied with knots; in aviation, it means nautical miles per hour.
Why Pilots Care
Small speed changes like this directly affect descent rates, fuel use, and the ability to meet published crossing restrictions.
Intuition Check
Do not read the dash as decoration or a pause. Here it means “minus” or “less than the reference speed.”
Example Sentence 1
The maximum holding airspeed at this altitude is 230 knots —10 knots, so the pilot plans to enter the hold at 225 knots to stay safely under the limit.
Example Sentence 2
At —10 knots the aircraft arrived at the fix exactly on the published altitude.