From the AAIB website
2. Definition of a Serious Incident
“Serious Incident” means an incident involving circumstances indicating that there was a high probability of an accident and is 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 it comes to rest at the end of the flight and the primary propulsion system is shut down.
The incidents listed below are typical examples of serious incidents. The list is not exhaustive and only serves as a guide to the definition of ‘serious incident’.
A near collision requiring an avoidance manoeuvre or when an avoiding manoeuvre would have been appropriate to avoid a collision or an unsafe situation.
Controlled flight into terrain (CFIT) only marginally avoided.
An aborted takeoff or a takeoff using a closed or engaged runway, a taxiway or unassigned runway.
A landing or attempted landing on a closed or engaged runway, a taxiway or unassigned runway.
Gross failure to achieve predicted performance during takeoff or initial climb.
All fires and/or smoke in the cockpit, in the passenger compartment, in cargo compartments or engine fires, even though such fires were extinguished with extinguishing agents.
Any events which require the emergency use of oxygen by the flight crew.
Aircraft structural failure or engine disintegration, including uncontained turbine engine failure, which is not classified as an accident.
Multiple malfunctions of one or more aircraft systems that seriously affect the operation of the aircraft.
Any case of flight crew incapacitation in flight.
Any fuel state which would require the declaration of an emergency by the pilot.
Runway incursions classified with severity A. The ‘Manual on the Prevention of Runway Incursions’ (Doc 9870) contains information on the severity classifications.
Takeoff or landing incidents, such as undershooting, overrunning or running off the side of runways.
System failures, weather phenomena, operation outside the approved flight envelope or other occurrences which caused or could have caused difficulties controlling the aircraft.
Failure of more than one system in a redundancy system which is mandatory for flight guidance and navigation.
The unintentional or, as an emergency measure, the intentional release of a slung load or any other load carried external to the aircraft.