ICAO severity classifications for occurrences

The International Civil Aviation Organization classifies occurrences into five categories depending on their severity.

A: Serious Incident

An incident involving circumstances indicating that an accident nearly occurred. Note: The difference between an accident and a serious incident lies only in the result.

B: Major Incident

An incident associated with the operation of an aircraft, in which safety of aircraft may have been compromised, having led to a near collision between aircraft, with ground or obstacles (i.e., safety margins not respected which is not the result of an ATC instruction).

C: Significant Incident

An incident involving circumstances indicating that an accident, a serious or major incident could have occurred, if the risk had not been managed within safety margins, or if another aircraft had been in the vicinity.

D: Not determined

Insufficient information was available to determine the risk involved or inconclusive or conflicting evidence precluded such determination.

E: No safety effect

An incident which has no safety significance.