Definition
A federal aviation regulation that prohibits any person from operating an aircraft below 10,000 feet MSL at an indicated airspeed of more than 250 knots (288 mph), unless otherwise authorized by the Administrator.
Plain English
When you are flying anywhere below 10,000 feet above sea level, you are not allowed to fly faster than 250 knots indicated airspeed. This is a hard speed limit set by the FAA.
Context Anchor
Seen in discussions of climb, descent, and general operating speed limits under FAA rules.
Derivation
CFR stands for Code of Federal Regulations. Title 14 covers Aeronautics and Space. Part 91 covers General Operating and Flight Rules. Section 91.117 is the specific rule on aircraft speed, and (a) is the first paragraph within it. The numbering tells you exactly where to find the rule in the regulations.
Why Pilots Care
Compliance prevents enforcement action and reduces mid-air collision risk in busy low-altitude airspace.
Intuition Check
Do not read this as a suggestion or a best-practice guideline. It is a federal operating rule that sets a legal limit.
Example Sentence 1
During the initial climb, the pilot kept the airspeed below 250 knots until passing 10,000 feet MSL to comply with 14 CFR part 91, section 91.117(a).
Example Sentence 2
The instructor reminded the student to recheck the speed limit under 14 CFR part 91, section 91.117(a) before descending below 10,000 feet.