Definition
A section of Title 14 of the Code of Federal Regulations dealing with rebuilt engine maintenance records. It permits the owner or operator of an aircraft engine that has been rebuilt by the manufacturer or by an agency approved by the manufacturer to use a new maintenance record showing zero time-in-service, replacing the engine's prior service history.
Plain English
This is the federal rule that lets a properly rebuilt engine start its maintenance records over from zero hours, as if it were brand new, but only when the rebuild is done by the manufacturer or someone the manufacturer has approved.
Context Anchor
Seen in aircraft maintenance records, engine logbooks, and discussions of rebuilt engines or “zero-time” engine records.
Derivation
14 CFR means Title 14 of the Code of Federal Regulations, the section of federal law covering aviation. Part 91 covers general operating and flight rules. Section 91.421 is the specific paragraph within that part. The numbering is a locator, not a name.
Why Pilots Care
Proper compliance ensures the engine remains legally airworthy and that its maintenance history is clear for future owners, insurers, and inspectors.
Intuition Check
Do not read this as just a page reference. In aviation use, “14 CFR part 91, section 91.421” points to a specific federal rule that has legal effect.
Example Sentence 1
Because the engine was rebuilt by the manufacturer under 14 CFR part 91, section 91.421, the logbook was started fresh with zero time-in-service.
Example Sentence 2
During the annual inspection the IA checked that all entries met 14 CFR part 91, section 91.421 before approving the rebuilt engine for return to service.