SMS Pro Aviation Safety Software Blog 4 Airlines & Airports

How to Define Acceptable Level of Safety (ALoS) in Aviation Safety

Posted by Tyler Britton on Jun 28, 2017 6:10:00 AM

What Is Acceptable Level of Safety (ALoS)

Defining Acceptable Level of Safety (ALoS)

The term Acceptable Level of Safety (ALoS) defines an aviation service provider’s minimum level of acceptable risk for a given safety issue. “Acceptable” describes the need for no further mitigatory actions on the part of the service provider for the safety concern in question. This valuation will be made based on the probability and severity of the safety concern in question.

Absolute safety is an impossible goal, yet service providers need to be able to set standards for how much risk is admissible. ALoS is the response to this dilemma. Because of this, ALoS is “exclusively concerned with safety performance measurement” (ICAO SSP: 6).

What this means is that, in terms of safety performance, ALoS marks the point at which, for a given hazard/potential mishap:

  • The current level of safety performance “good enough”;
  • Measures needed to further increase safety for hazard/mishap would be unreasonable/unrealistic to implement; and
  • Controls are strong enough that the residual risk is willingly taken on by the service provider.

It’s important to note that while defining a level of acceptable safety is a requirement for providers, it’s up to each provider to establish just what is and what isn’t acceptable. In this way, service providers define their ALoS, and then show (e.g., with safety data), that they operating within an acceptable range of risk.

Relationship between Risk Matrix and Acceptable Level of Safety

The Risk Matrix is the backbone of ALoS in terms of:

  • Drawing line between acceptability/unacceptability;
  • Ensuring consistent assessment of acceptable and not acceptable risk; and
  • Setting benchmarks for residual assessments of allowed exposure.

Service providers use initial risk assessments from safety issues to decide whether or not the concern meets the criteria for acceptable risk. For example, an organization might define acceptability as any assessment that is:

  • “Green,” low risk issues, and all other risk assessments as requiring further mitigatory action;
  • “Yellow” or “green” (medium or low) risk issues, with only high risk issues requiring further action; or
  • Medium-low issues, where the risk assessment is on the lower “half” of the risk matrix.

The reason there is variance between aviation service providers is either:

  • Differing willingness to take on more risk as being acceptable; and
  • Different defining criteria for severity and probability.

ALoS absolutely requires very clear criteria for probability and severity

Defining Risk Matrix Probabilities and Severities Criteria

Aviation SMS risk matrix

Airline SMS programs, Airport SMS programs, and other aviation service providers have the opportunity to define their own criteria for severity and probability. This kind of flexibility allows each service provider to define assessment criteria that best fits the needs and environment of each operation.

Criteria are simply the tangible “markers” of a particular level of severity/probability. For example, you might define specific criteria for a level of severity based on:

  • Financial loss;
  • Damage to equipment;
  • Injury/loss of life;
  • Effects on operations; and
  • Environmental damages.

The more criteria listed for each level of severity the better. Next, you might define specific criteria for a level of probability based on:

  • Number of times occurred in company/industry in past; or
  • Rate per number of flight operations expected in the future (such as on occurrence in 100k operations, however you define operations) given existing conditions and controls.

Establishing Acceptable Level of Safety

Organizations should create their criteria, and decide which combination of severity likelihood is the minimum requirement for acceptable. A good practice on a risk matrix that is swiftly being adopted is to label:

  • Low risk issue green;
  • All acceptable medium risk issues (if applicable) in yellow; and
  • All unacceptable medium risk issues (if applicable) in orange.

Of course, each aviation service provider will need to establish what is “acceptable” at the onset of aviation SMS implementation. However, as safety management systems are an organic, “living” entity, the criteria for acceptable should change over time as:

  • Oversight agencies express concern for current level of acceptability;
  • Current ALoS cannot be justified with safety data;
  • The SMS program is implemented;
  • A great number and more robust set of risk control measures are developed;
  • Growing database of safety data details a more specific picture of safety needs; and
  • Safety performance improves.

Basically, at certain points in the development of a risk management program it makes sense to intensify the requirement for acceptability in order to strive towards continuous improvement. These “intensifications” should be small/subtle adjustments, such as making a change that risk assessments of 2C are no longer within acceptable range (a 2b is now required).

Final Thought: Justifying Criteria for ALoS in Aviation Industry

As discussed, you need to justify your established criteria for level of acceptable risk. Justifying this involves using:

  • Safety performance indicators; and
  • Safety performance targets.

Targets and indicators are delivered through:

  • Safety data, such as from an aviation safety database;
  • Safety charts and metrics; and
  • Demonstration of “acceptable” issue safety performance.

A good way to do this is to use:

  • KPIs related to risk assessments;
  • Leading indicators related to risk assessments; or
  • Custom graphs that show hazard classifications based on risk assessments.

For a list of leading indicators that can help you get started with defining your acceptable level of safety, please see the following free resource:

Our list of 40 aviation SMS leading indicators is completely free!

Published June 2017. Last updated February 2019.

Topics: Quality-Safety Management

Site content provided by Northwest Data Solutions is meant for informational purposes only. Opinions presented here are not provided by any civil aviation authority or standards body.



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