Definition
A specific regulation within Title 14 of the Code of Federal Regulations (Part 91, General Operating and Flight Rules) that requires aircraft used to carry persons for hire, or used for flight instruction for hire when the operator provides the aircraft, to undergo an annual inspection plus a 100-hour inspection within each 100 hours of time in service. The 100-hour limit may be exceeded by up to 10 hours only to fly the aircraft to a place where the inspection can be performed, and that excess time must be included in computing the next 100 hours.
Plain English
If an aircraft is rented out with an instructor, or used to fly people for money, this rule says it must be inspected once a year AND every 100 hours of flight time. You can go up to 10 hours over the 100-hour limit only if you're flying it to the inspection, and those extra hours still count toward the next 100.
Context Anchor
Seen during preflight assessment when checking aircraft maintenance records, inspection status, or a checklist item that refers to required inspections.
Derivation
CFR stands for Code of Federal Regulations, the published collection of U.S. federal rules. Title 14 covers Aeronautics and Space. Part 91 contains general flight rules. Section 91.409 is the specific paragraph on inspections, and (b) is the sub-paragraph dealing with 100-hour inspections. Reading regulatory citations this way -- Title, Part, Section, sub-paragraph -- helps pilots find the exact rule quickly.
Why Pilots Care
Operating without meeting this requirement means the aircraft is not legally airworthy for hire operations, exposing the pilot to enforcement action and increased safety risk.
Intuition Check
Do not assume this rule makes every Part 91 aircraft need a 100-hour inspection. Section 91.409(b) applies to specific paid-carrying and paid-instruction situations, and the 100 hours are based on aircraft operating time, not calendar days.
Example Sentence 1
Before dispatching the rental Cessna, the flight school checked the maintenance logs to confirm the aircraft was within the 100-hour inspection limits required by 14 CFR part 91, section 91.409(b).
Example Sentence 2
Prior to the lesson, the instructor confirmed the aircraft had the required inspection under 14 CFR part 91, section 91.409(b) within the preceding 100 hours.