What is ASIL?

Definition

ASIL refers to Automotive Safety Integrity Level. It is a risk classification system defined by the ISO 26262 standard for the functional safety of road vehicles.

The standard defines functional safety as “the absence of unreasonable risk due to hazards caused by malfunctioning behavior of electrical or electronic systems.” ASILs establish safety requirements―based on the probability and acceptability of harm―for automotive components to be compliant with ISO 26262.

There are four ASILs identified by ISO 26262―A, B, C, and D. ASIL A represents the lowest degree and ASIL D represents the highest degree of automotive hazard.

Systems like airbags, anti-lock brakes, and power steering require an ASIL-D grade―the highest rigor applied to safety assurance―because the risks associated with their failure are the highest. On the other end of the safety spectrum, components like rear lights require only an ASIL-A grade. Head lights and brake lights generally would be ASIL-B while cruise control would generally be ASIL-C.

How do ASILs work?

ASILs are established by performing hazard analysis and risk assessment. For each electronic component in a vehicle, engineers measure three specific variables:

• Severity (the type of injuries to the driver and passengers)
• Exposure (how often the vehicle is exposed to the hazard)
• Controllability (how much the driver can do to prevent the injury)

Each of these variables is broken down into sub-classes. Severity has four classes ranging from “no injuries” (S0) to “life-threatening/fatal injuries” (S3). Exposure has five classes covering the “incredibly unlikely” (E0) to the “highly probable” (E4). Controllability has four classes ranging from “controllable in general” (C0) to “uncontrollable” (C3).

All variables and sub-classifications are analyzed and combined to determine the required ASIL. For example, a combination of the highest hazards (S3 + E4 + C3) would result in an ASIL D classification. 

What are the challenges of ASILs?

Determining an ASIL involves many variables and requires engineers to make assumptions. For example, even if a component is hypothetically “uncontrollable” (C3) and likely to cause “life-threatening/fatal injuries” (S3) if it malfunctions, it could still be classified as ASIL A (low risk) simply because there’s a low probability of exposure (E1) to the hazard.

ASIL definitions are informative rather than prescriptive, so they leave room for interpretation. A lot of room. ASIL vocabulary relies on adverbs (usually, likely, probably, unlikely). Does “usually” avoiding injury mean 60% of the time or 90% of the time? Is the probability of exposure to black ice the same in Tahiti as it is in Canada? And what about traffic density? Rush hour in Los Angeles vs. late morning on an empty stretch of road in the Australian Outback?

Simply put, ASIL classification depends on context and interpretation. 

How are ASILs evolving?

Given the guesswork involved in determining ASILS, the Society of Automotive Engineers (SAE) drafted J2980, “Considerations for ISO 26262 ASIL Hazard Classification” in 2015. These guidelines provide more explicit guidance for assessing Exposure, Severity, and Controllability for a given hazard. J2980 continues to evolve―the SAE published a revision in 2018.

With the evolution of the self-driving car, ISO 26262 will need to revisit the definition of “Controllability,” which currently pertains to the human driver. As the standard reads now, the absence of a human driver means that Controllability will always be C3, the extreme of “uncontrollable.” The other variables of Severity (injury) and Exposure (probability) will no doubt require re-examination as well.

What are the benefits of ASILs?

ISO 26262 is a goal-based standard that’s all about “preventing harm.” Despite their challenges, ASIL classifications are intended to “prevent harm” and help us achieve the highest safety rating possible for myriad automotive components across a long and often disjointed supply chain.

Key benefits include:

• Establishing safety requirements to mitigate risks to acceptable levels
• Managing and tracking safety requirements
• Ensuring that standardized safety procedures have been followed in the final product