Definition
A section of the Federal Aviation Regulations that sets maximum indicated airspeed limits for aircraft operating in U.S. airspace. The core limits are: 250 knots below 10,000 feet MSL; 200 knots in Class B airspace beneath the lateral limits of Class B or in a VFR corridor through Class B; and 200 knots within 4 nautical miles of a Class C or Class D primary airport at or below 2,500 feet AGL. If an aircraft's minimum safe operating speed is higher than these limits, it may be flown at that minimum safe speed.
Plain English
The rule that caps how fast you can fly in certain airspace. Below 10,000 feet you stay at or under 250 knots. Closer to busy airports, the cap drops to 200 knots. If your aircraft can't safely fly that slow, you're allowed to fly the slowest speed it can safely handle.
Context Anchor
Seen in airspeed, descent planning, instrument procedures, and operations below 10,000 feet or near controlled airport airspace.
Derivation
CFR stands for Code of Federal Regulations. Part 91 is the section covering general operating and flight rules for civil aircraft in the United States. The number 91.117 simply identifies the specific rule within Part 91 that deals with aircraft speed.
Why Pilots Care
Compliance prevents regulatory violations and reduces collision risk in congested airspace.
Intuition Check
Do not read “Part” here as an aircraft component. In this citation, “Part” means a numbered group of federal aviation rules.
Example Sentence 1
Descending through 11,000 feet, the crew slowed to 250 knots before crossing 10,000 feet to comply with 14 CFR 91.117.
Example Sentence 2
During the arrival briefing the crew reviewed 14 CFR Part 91 — 91.117 restrictions that apply inside Class B airspace.