Definition
The maximum indicated airspeed permitted for civil aircraft operating below 10,000 feet MSL under 14 CFR 91.117(a), and the speed threshold used to distinguish two categories of Military Training Routes: IR (IFR Military Training Route) and VR (VFR Military Training Route) segments flown at or below 1,500 feet AGL at speeds of 250 knots or less are designated with four-digit identifiers, while routes with any segment above 1,500 feet AGL or flown faster than 250 knots are designated with three-digit identifiers.
Plain English
250 knots is both the everyday speed limit for all aircraft below 10,000 feet and the dividing line the military uses to label its training routes. Slower, lower routes get four-digit numbers; faster or higher routes get three-digit numbers.
Context Anchor
Seen in chart and handbook discussions of Military Training Routes, where military aircraft may fly low and fast.
Derivation
Knot comes from old ship navigation. Sailors measured speed with a rope that had knots tied in it; the number of knots that ran out over a set time showed the ship’s speed. In aviation, a knot still means one nautical mile per hour.
Why Pilots Care
It tells civilian pilots the highest speed they may encounter from military traffic on that route segment.
Grounding Statement
If you are near a Military Training Route, traffic can appear and pass through your area much faster than you may expect.
Intuition Check
Do not read 250 knots as 250 miles per hour. A knot is a nautical mile per hour, so 250 knots is about 288 miles per hour.
Example Sentence 1
Because the route on the chart was marked IR-107, a three-digit identifier, the pilot knew military aircraft might be transiting at speeds greater than 250 knots.
Example Sentence 2
Expect military aircraft on this route to remain at or below 250 knots during training.