An aviation accident is defined by the Convention on International Civil Aviation Annex 13 as an occurrence associated with the operation of an aircraft, which takes place between the time any person boards the aircraft with the intention of flight until all such persons have disembarked, where a person is fatally or seriously injured, the aircraft sustains damage or structural failure or the aircraft is missing or is completely inaccessible. If the aircraft is destroyed or severely damaged so that it must be written off, it is further defined as a hull loss accident. Annex 13 further defines an aviation incident as an occurrence, other than an accident, associated with the operation of an aircraft which affects or could affect the safety of operation.