FAR 91.421 — Rebuilt Engine Records
FAR 91.421 explains zero-time rebuilt engine records: who may rebuild, what must be logged, and how a rebuilt engine differs from an overhauled one.
FAR 91.421 lets the owner or operator start a new maintenance record with zero time for an aircraft engine that has been rebuilt by the manufacturer or by an agency the manufacturer has approved. This is a big deal because no other shop can grant zero time — only an overhaul, which keeps the original total time in service.
When the manufacturer (or approved agency) rebuilds the engine and grants zero time, they must enter in the new record:
- A signed statement of the date the engine was rebuilt;
- Each change made to comply with airworthiness directives (ADs); and
- Each change made per manufacturer's service bulletins, when the bulletin specifically requests the entry.
A rebuilt engine is a used engine that has been completely disassembled, inspected, repaired as necessary, reassembled, tested, and approved to the same tolerances and limits as a new engine. Parts may be new or used, but all parts must meet new-part production tolerances or be approved oversize/undersize dimensions.
Operationally, this matters when buying an aircraft or reviewing logs: a zero-timed rebuilt engine resets the maintenance history, while a field overhaul does not.