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An incident is defined as:
An occurrence, other than an accident, associated with the operation of an aircraft which affects or could affect the safety of operation.
Note.— The types of incidents which are of main interest to the International Civil Aviation Organization for accident prevention studies are listed in Attachment C to Annex 13.
A serious incident is defined as:
An incident involving circumstances indicating that there was a high probability of an accident and associated with the operation of an aircraft which, in the case of a manned aircraft, takes place between the time any person boards the aircraft with the intention of flight until such time as all such persons have disembarked, or in the case of an unmanned aircraft, takes place between the time the aircraft is ready to move with the purpose of flight until such time as it comes to rest at the end of the flight and the primary propulsion system is shut down.
Note 1.— The difference between an accident and a serious incident lies only in the result.
(ICAO Annex 13 - Aircraft Accident and Incident Investigation)
ICAO Annex 13: Aircraft Accident and Incident Investigation. ICAO Doc 9156: Accident/Incident Reporting Manual.