(Accident Investigation Regulations)
An occurrence is a notifiable accident (between the time anyone boards intending flight and the time all have disembarked) if someone is killed or seriously injured, the aircraft suffers damage beyond a defined minor list (excludes engine-internal damage, propeller/wingtip/tyre/fairing dents, etc., unless structural strength or performance is affected), or the aircraft is missing or completely inaccessible.
When that happens, the commander (or the operator if the commander is incapacitated) must notify the Chief Inspector of Air Accidents (AAIB) by the quickest means and the local police. The investigation belongs to the state in which the accident occurred, and its sole purpose is prevention, investigators do not apportion blame or liability.
Definitions to keep straight: a serious injury generally means hospitalisation for more than 48 hours (beginning within 7 days of the accident), a bone fracture (other than fingers/toes/nose), severe haemorrhage, internal-organ injury, or burns over 5% of the body. An incident is an occurrence other than an accident that affected or could have affected safety; a serious incident almost resulted in an accident. Separately, all bird strikes must be reported, and the listed events under the Mandatory Occurrence Reporting scheme are reported through the official portal; the confidential CHIRP scheme exists for human-factors reports.