What might be an exception to the 250 knots restriction below 10,000 feet?

The exception to the 250 knots restriction below 10,000 feet is particularly relevant when coordination with Air Traffic Control (ATC) for Department of Defense (DoD) routes is involved. This is because there are specific operational requirements and protocols for military aircraft that may necessitate flying at higher speeds in certain designated areas.

Military operations often operate under different rules compared to typical civilian air traffic, allowing for more flexible speed regulations when expressly authorized by ATC. The coordination ensures safe and effective execution of missions while still maintaining compliance with overall airspace management. This stipulation allows military aircraft to perform necessary maneuvers and maintain operational readiness, which may include prohibitions against the typical speed limit imposed on civilian aircraft in these specific airspace scenarios.

In contrast, flying over mountains, normal training flights, and night flights do not inherently justify an exception to the 250 knots restriction, as these circumstances do not provide the same level of operational necessity that is recognized under military guidelines in coordination with ATC.