Definition
The federal regulation that prohibits a person from acting or attempting to act as a crewmember of a civil aircraft while under the influence of alcohol, while using any drug that affects faculties contrary to safety, or within 8 hours of consuming alcohol or with a blood alcohol concentration of 0.04 percent or greater. It also prohibits the carriage of intoxicated persons except in specific medical or law-enforcement circumstances.
Plain English
This is the FAA rule that says you cannot fly drunk, hungover, or impaired by drugs. You must wait at least 8 hours after your last drink, must be below 0.04% blood alcohol, and must not have any drug in your system that affects your ability to fly safely.
Context Anchor
You will see this citation in FAA discussions of alcohol, medication, drugs, and pilot fitness before flight.
Derivation
CFR stands for Code of Federal Regulations, the system that organizes U.S. federal rules. Title 14 covers Aeronautics and Space, Part 91 contains the General Operating and Flight Rules, and Section 91.17 is the specific paragraph dealing with alcohol and drugs.
Why Pilots Care
Failure to follow this regulation can result in FAA enforcement action, certificate suspension or revocation, and significantly raises the risk of an accident.
Intuition Check
Do not assume this rule is only about illegal drugs or being obviously drunk. A legal medication can still be a problem if it affects your ability to fly safely.
Example Sentence 1
He scrubbed the morning flight because he had been at a wedding the night before and could not meet the 8-hour requirement under 14 CFR 91.17.
Example Sentence 2
During the safety briefing the instructor emphasized that 14 CFR part 91 section 91.17 applies to every flight, not only those conducted for hire.