Definition
A federal aviation regulation that prohibits any person from operating an aircraft equipped with an anti-collision light system unless that system is lighted, except when the pilot in command determines that, due to operating conditions, it is in the interest of safety to turn the lights off.
Plain English
If your aircraft has anti-collision lights (the bright flashing or strobe lights), they must be on whenever you are operating the aircraft. The only exception is when the pilot decides leaving them on would be unsafe — for example, if the strobes are reflecting off clouds at night and disorienting the pilot.
Context Anchor
Seen in discussions of aircraft lighting, preflight checks, night operations, and when deciding whether anticollision lights should be on during ground or flight operations.
Derivation
14 CFR means Title 14 of the Code of Federal Regulations, which is the section of U.S. federal law that covers aeronautics and space. Part 91 contains the general operating and flight rules. Section 91.209 is the specific rule about aircraft lights. The (b) refers to paragraph (b) of that section, which deals specifically with anti-collision lights.
Why Pilots Care
Ensures legal compliance and improves aircraft visibility to other traffic, reducing mid-air collision risk.
Intuition Check
Do not read “91.209(b)” as a page number or a checklist step. It is a specific federal rule paragraph that tells pilots when anticollision lights must be used.
Example Sentence 1
Per 14 CFR part 91 section 91.209(b), the pilot turned the strobes on before taxiing and left them on through engine shutdown.
Example Sentence 2
The preflight checklist included a check that the airplane satisfied 14 CFR part 91 section 91.209(b) requirements for anticollision lighting.