ALARP principle tolerable risk

The ALARP principle is applied for events in the intermediate area. For those near the broadly acceptable limit, the risks are considered tolerable if the cost of risk reduction would exceed the improvement gained. For those near the maximum tolerable hmit, the risks are considered tolerable only if risk reduction is impracticable or implementation of risk reducing measures would lead to disproportionate costs compared with safety benefits gained. 

There are a number of recognized principles for managing risks and achieve target values for tolerable risks of accidents with injuries or casualties within the railway industry. Typically, different countries have recognized different principles. Thus, MEM is mainly practiced in Germany, ALARP in the UK, and GAMAB/GAME in France. The principles are shortly described below. 

A key development in system safety is the ALARP principle that states that the residual risk of a system shall be as low as reasonably practicable (ALARP principle) and was codihed through the UK Health and Safety at Work Act of 1974. The concept asserts that safety-critical systems and operations should be safe as far as reasonably practicable without risks to health and safety. This is important because it forces the overt decision to balance the realized safety benehts to the actual costs to implement in other words, residual risks are tolerable and thus do not need further mitigations. 

Having established a SIL target it is insufficient merely to assess that the design will meet the Maximum Tolerable Risk target. It is necessary to establish whether further improvements are justified and thus the principle of ALARP (as low as reasonably practicable) is called for as good practice. In the UK this is also arguably necessary in order to meet safety legislation ( all that is reasonably practicable is called for in the Health Safety at Work Act 1974). 

There are no safe states when aircraft are airborne thus the ATM system cannot be regarded as a protection system in the simplest sense. It must therefore be regarded as a control system although its goal is risk reduction to a level which is tolerable and As Low As Reasonably Practicable (ALARP). The ALARP principle was not traditionally applied to ATM but has become increasingly prominent in recent years and is recommended in the ICAO Safety Management Manual for ATM (ICAO 2009) and in the EUROCONTROL regulatory requirements (EUROCONTROL 2000). 

The concept of tolerable risk is illustrated by the following diagram showing what is known as the principle of ALARP. 

ALARP principle. The principle that no risk in the tolerability region can be accepted unless reduced As Low As Reasonably Practicable . 

Contemporary risk management follows a maturing path to the establishment, acceptance and management of a level of risk that is deemed tolerable and as low as reasonably practicable (ALARP). The recent issue of military standards [MoD 2004] describes six processes for risk management hazard identification, hazard analysis, risk estimation, risk and ALARP evaluation, risk reduction and risk acceptance. Whilst these are not the universal descriptions of the processes involved, the underlying principles are consistent with other procedures and handbooks, for example lEC 61 SOS, JSP 4S4 and Mil Stan 882D. 

The principle of ALARP describes the way in which risk is treated legally and by the HSE in the UK, and also applied in some other countries. The concept is that all reasonable measures will be taken in respect of risks which lie in the tolerable (ALARP) zone to reduce them further until die cost of further risk reduction is grossly disproportionate to the benefit. 

When it comes to evaluating risk, the popular question Ts it safe has to be rephrased Are the risks low enough for the public/authorities to tolerate Safety management is not about removal of all risks. Rather, the key principle is that risk is reduced to as low as reasonably practicable (ALARP). 

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