Definition
Three sections of the Code of Federal Regulations (Title 14, Aeronautics and Space) that together govern the airworthiness of civil aircraft in the United States. Part 21 covers the certification procedures for aircraft and parts. Part 43 covers maintenance, preventive maintenance, rebuilding, and alteration. Part 91 covers the general operating and flight rules. Compliance with all three is a condition of an aircraft's airworthiness certificate remaining in effect.
Plain English
These are three sets of federal rules that, taken together, decide whether an aircraft is legally safe to fly. One set covers how aircraft and parts get approved, one covers how they must be maintained, and one covers how they must be operated. If any of the three is ignored, the aircraft is no longer considered airworthy.
Context Anchor
You will see this term in aircraft document discussions, especially when reading about an airworthiness certificate and what must be followed for that certificate to remain valid.
Derivation
CFR stands for Code of Federal Regulations, the official collection of U.S. federal rules. Title 14 is the aviation title. The numbers 21, 43, and 91 simply identify the specific parts within Title 14. The numbering is administrative, not descriptive — the part numbers themselves do not tell you what is inside.
Why Pilots Care
An aircraft must meet the standards in these parts to hold a current airworthiness certificate and remain legal to fly.
Analogy
Think of Title 14 as the aviation rulebook, and Parts 21, 43, and 91 as three specific chapters: one for approval, one for maintenance, and one for operation.
Intuition Check
“Parts” here does not mean physical aircraft parts like tires or spark plugs. It means numbered sections of the federal aviation regulations.
Example Sentence 1
The airworthiness certificate remains valid only when the aircraft is maintained and operated in accordance with 14 CFR parts 21, 43, and 91.
Example Sentence 2
During the preflight document check the owner pointed out the records showing adherence to 14 CFR parts 21 43 and 91.