Absolute safety is generally an unachievable and very expensive goal. Therefore the concept of acceptable safety has been adopted in risk bearing industries, including aviation. The term "acceptable risk" describes an event with a probability of occurrence and consequences acceptable to the society, i.e. the society is willing to take or be subjected to the risk that the event might bring. It is the role of the safety regulatory authorities to translate the society expectations and perceptions into a qualitative or quantitative target level of safety.
“The acceptable level of safety expresses the safety goals of an oversight authority, an operator, or a services provider. From the perspective of the relationship between oversight authorities and operators/services providers, it provides the minimum safety objective(s) acceptable to the oversight authority to be achieved by the operators/services providers while conducting their core business functions.” (ICAO Annex 11, Attachment E).
Traditionally, in many industries including aviation, safety regulation has been carried out prescriptively, i.e. the regulator defines the rules and standards to be followed and uses audit and inspection to check compliance with them. This approach requires a great deal of specialist resource on the part of the regulator and is often over-constraining for the regulated entity, particularly in the introduction of new processes and technologies.
Recognition of these difficulties has led to an objective-based approach to safety regulation, in which safety is much more clearly the responsibility of the operator/service provider, the regulator’s role being mainly to ensure that the service provider discharges his responsibilities properly. The regulator sets objectives for the achievement and demonstration of safety - acceptable (or tolerable) safety levels - and the service provider has to show (by argument and evidence) that those objectives have been met.
In aviation, the acceptable level of safety is generally defined in terms of the probability of an aircraft accident occurring. It is defined individually for each operator/service provider on the basis of the target level of safety set by the regulator. An array of factors such as the complexity of operations, the operational context, past safety performance, existing safety regulatory framework, applicable safety standards, etc. are taken into account. “Each agreed established level of safety should be commensurate with the complexity of individual operator/service providers’ operational contexts, and the level to which safety deficiencies can be tolerated and realistically addressed.” (ICAO Annex 11, Attachment E).
The concept of acceptable level of safety is expressed by two specific metrics, namely safety performance targets and safety performance indicators.