Definition
The federal regulation that prohibits any person from acting or attempting to act as a crewmember of a civil aircraft while under the influence of alcohol or any drug that affects the person's faculties contrary to safety, within 8 hours of consuming alcohol, or while having a blood alcohol concentration of 0.04 or greater. It also requires crewmembers to submit to drug or alcohol testing on request and to provide test results to the FAA when requested.
Plain English
This is the FAA rule that says you can't fly if you've been drinking or using drugs that affect your ability to fly. You must wait at least 8 hours after your last drink, your blood alcohol must be under 0.04, and you can't be impaired by any drug. If asked, you have to take a test.
Context Anchor
You will see this citation in discussions of pilot fitness, alcohol use, medication use, and legal requirements before acting as a flight crew member.
Derivation
CFR stands for Code of Federal Regulations. Title 14 covers Aeronautics and Space. Part 91 contains the General Operating and Flight Rules that apply to nearly every flight in U.S. civil aviation. Section 91.17 is the specific rule within Part 91 that addresses alcohol and drugs. The shorthand '14 CFR 91.17' simply tells you exactly where to find the rule in the federal code.
Why Pilots Care
Violating this rule can lead to certificate action, legal penalties, and increased accident risk from impaired performance.
Intuition Check
Do not read this as only a “no illegal drugs” rule. It also covers legal alcohol, prescription medicine, over-the-counter medicine, and any drug effect that makes flying unsafe.
Example Sentence 1
After a late dinner with wine, the pilot delayed her morning departure to make sure she complied with 14 CFR 91.17.
Example Sentence 2
During preflight planning the instructor reminded the student that 14 CFR part 91, section 91.17 applies to every operation conducted under Part 91.