Definition
A federal regulation that prohibits any person from operating an aircraft in U.S. airspace equipped with an anti-collision light system unless that system has lighted anti-collision lights. The rule allows the pilot in command to turn the lights off when the pilot determines that, because of operating conditions, it would be in the interest of safety to do so.
Plain English
If your aircraft has anti-collision lights (such as a red rotating beacon or white strobes), you must have them on whenever you are operating the aircraft. The only exception is when the pilot in command judges that turning them off is safer in the moment.
Context Anchor
Seen in discussions of aircraft lighting, night operations, preflight checks, and when deciding whether beacon or strobe lights should be on.
Derivation
This is a legal citation. “14 CFR” means Title 14 of the Code of Federal Regulations, the main body of U.S. federal aviation rules. “Part 91” covers general operating and flight rules, and “section 91.209(b)” points to the specific paragraph about anti-collision lights.
Why Pilots Care
Directly affects legal compliance and collision avoidance at night while giving the pilot final authority to adjust lights for actual conditions.
Intuition Check
Do not read this as just a page number or a maintenance label. It is a specific regulation that tells the pilot when anti-collision lights must be used and when they may be turned off for safety.
Example Sentence 1
Per 14 CFR part 91, section 91.209(b), the pilot turned the rotating beacon on before engine start and left it on until shutdown.
Example Sentence 2
The handbook cites 14 CFR part 91, section 91.209(b) to explain when a pilot may legally turn off anti-collision lights.