Accident Severity Categorization

Definitions

Accident Severity Categorization

the safety work product that categorizes:

Accidents in terms of accident severity level
Hazards in terms of hazard severity level

Accident Severity Level (ASL)

a category of accidents based on the maximum severity of harm that they can credibly cause

Hazard Severity Level (HSL)

a category of hazards based on the maximum severity of harm that their associated accidents can credibly cause

Objectives

The typical objectives of the accident severity categorization are to:

Divide this continuum of accident (and hazard) severities into a small number of manageable and useful levels that can be treated as equivalence classes when dealing with safety risk and risk mitigation.

Benefits

The typical benefits of the accident severity categorization are to:

Support hazard and safety risk analysis.
Help determine mandatory risk mitigation approaches.
Promote communication about safety.

The typical contents of the accident severity categorization are:

Severity Levels
Associated definitions of ranges of harm

Stakeholders

The typical stakeholders of the accident severity categorization are:

Producer:
- Safety Team
Evaluators:
- Safety Evaluation Team
Approvers:
- Project Management Team
- Customer Representative
Maintainers:
- Safety Team
Users:
- Safety Team, which uses the accident severity categorization to produce hazard indices, which in turn are used to produce safety assurance levels and safety evidenc assurance levels.

Phases

Business Strategy: Not Applicable
Business Optimization: Not Applicable
Initiation: Completed
Construction: Maintained
Delivery: Maintained
Usage: Maintained
Retirement: Archived

Preconditions

The accident severity categorization typically can be started if the following preconditions hold:

The initiation phase is started.
The safety team is adequately staffed and trained in safety program planning.

Inputs

The accident severity categorization typically has the following inputs:

Work Products:
- Safety Program Plan
Stakeholders:
- Customer Representatives
- Safety Domain Experts

Guidelines

Accident and hazard severity is a function of the safety architecture, design, and implementation. By modifying the safety architecture, design, and implementation (e.g., redesigns, adding safeguards), the consequences of accidents can be reduced or eliminated, thus lowering their severity.
Depending on the system, the negative consequences of an accident can vary more or less continuously from catastrophic to inconsequential. To make the number of severity levels small and manageable, keep the number of severity levels small (e.g., from 3 to 6).
The number and definitions of the individual severity levels may be system specific, specific to a given application domain, or they may be determined based on the use of international standards.
Start with an accident severity categorization from some international standard.
Modify the standard accident severity categorizations to meet the needs of the endeavor.
Create hazard severity layers by grouping hazards by the maximum severity of their associated accidents.

Conventions

The accident severity categorization is typically constrained by the following conventions:

Content and Format Standard
MS Word Template
XML DTD
Inspection Checklist

As illustrated by the following example tables, the number and definitions of severity levels varies by application domain and international standards. Example accident severity categorization tables from various international standards include:

Commercial Aviation
European Space Agency
Medical Equipment
Railways
United Kingdom Ministry of Defense
United States Department of Defence

Commercial Aviation

The commercial aviation standard, Software Considerations in Airborne Systems and Equipment Certification (RTCA/DO 178B: 1992), defines accident/hazard severity levels as follows:

Accident/Hazard Severity Categorization
Severity Level	Definition
Catastrophic	• Failure conditions, which prevent the continued safe flight and landing of the aircraft
Severe-Major	• Failure conditions, which reduce the capability of the aircraft or the ability of the crew to cope with adverse operation conditions • Serious or potentially fatal injuries to some passengers
Major	• Failure conditions, which reduce the capability of the aircraft or the ability of the crew to cope with adverse operating conditions • Discomfort and possible injury to the passengers
Minor	• Failure conditions, which do not cause a significant reduction in aircraft safety
No Effect	• Failure conditions, which do not effect the operational capability of the aircraft or increase the crew’s workload

European Space Agency

The European Space Agency (ESA) standard, Space Product Assurance: Safety (ECSS-Q-40A: 1996), defines accident/hazard severity levels as follows:

Accident/Hazard Severity Categorization
Severity Level	Definition
Catastrophic	• Loss of life • Life threatening or permanently disabling injury • Loss of a manned flight system or launch system, and/or • Long term detrimental environmental effects
Critical	• Temporary disabling, but not life threatening, injury • Loss of or major damage to flight systems, major system elements or ground facilities • Loss of or major damage to public or private property, and/or • Short term detrimental environmental effects

Medical Equipment

The International Electrotechnical Commission (IEC) standard, Medical Electrical Equipment - Part 1: General Requirements for Safety (IEC 601-1-4: 1996), defines accident/hazard severity levels as follows:

Accident/Hazard Severity Categorization
Severity Level	Definition
Catastrophic	Potential of multiple deaths or serious injuries
Critical	Potential of death or serious injury
Marginal	Potential of injury
Negligible	Little or no potential of injury

Railways

The European Community standard, Railway Applications: Software for Railway Control and Protection Systems (CENELEC EN 50128: 1997), defines accident/hazard severity levels as follows:

Accident/Hazard Severity Categorization
Severity Level	Definition
Catastrophic	• Fatalities and/or multiple severe injuries • Loss of one or more systems
Critical	• Single fatality or severe injury • Loss of a major system
Marginal	• Minor injury • Several systems damaged
Insignificant	• Possible single minor injury • System damaged

United Kingdom Ministry of Defense

The United Kingdom Ministry of Defense military standard, Safety Management Requirements for Defence Systems Containing Programmable Electronics: Part 1 - Requirements (DEF STAN 00-56 (Part 1)/Issue 2: 1996), defines accident/hazard severity levels as follows:

Accident/Hazard Severity Categorization
Severity Level	Definition
Catastrophic	Multiple deaths
Critical	A single death and/or multiple severe injuries or severe occupational illnesses
Marginal	A single severe injury or occupational illness and/or multiple minor injuries or minor occupational illnesses
Negligible	At most a single minor injury or minor occupational illness

United States Department of Defence

The US Department of Defense military standard, Mishap Risk Management (MIL-STD-882D: 1998), defines accident/hazard severity levels as follows:

Accident/Hazard Severity Categorization
Severity Level	Definition
Catastrophic	• Death or permanent total disability, • Property/financial loss exceeding $1,000,000, and/or • Irreversible severe environmental damage that violates a law or regulation
Critical	• Permanent partial disability, • Injuries, or occupational illness that may result in hospitalization of at least three individuals, • Financial loss exceeding $200,000, and/or • Reversible environmental damage that violates a law or regulation
Marginal	• Injury or occupational illness resulting in one or more lost work day(s), • Financial loss exceeding $10,000, and/or • Mitigatible environmental damage where restoration can be accomplished without violation of a law or regulation
Negligible	• Injury or illness not resulting in a lost work day, • Financial loss less than $10,000, and/or • Minimal environmental damage without violation of a law or regulation