Definition
A specific occurrence in the operating life of an aircraft engine, propeller, or component that establishes the start of a time period used to determine when an inspection, overhaul, or replacement is required. The defining event sets the reference point from which time-in-service, calendar time, or operating cycles are counted toward a maintenance limit.
Plain English
An event — like an overhaul, a manufacture date, or a major repair — that starts the clock on how long a part can stay in service before it must be inspected, overhauled, or replaced.
Context Anchor
Seen in aviation accident and incident reports, safety summaries, and accident data systems.
Derivation
Define comes from a Latin word meaning “to set limits or make clear.” Event means “something that happens.” A defining event is the happening that sets the description or category for an accident or incident.
Why Pilots Care
Correctly identifying the defining event determines whether an occurrence is treated as an incident, accident, or training event and affects reporting and learning.
Intuition Check
Do not assume “defining event” means the root cause. It means the event used to describe or classify what happened.
Example Sentence 1
The last engine overhaul is the defining event for calculating time until the next required overhaul.
Example Sentence 2
Investigators identified the gear-up landing as the defining event that classified the occurrence as an incident rather than a minor event.