DO-178 was developed in the late 1970s and originally released in 1982. The original standard provided a prescriptive set of design assurance processes for software developed for airborne systems and focused on testing and documentation.
DO-178 was updated to DO-178A in the 1980s, with the update for the first time defining different levels of activities depending on the criticality of the system for which the software was written.
In 1992, DO-178B was released and was a total re-write of the existing DO-178 standard. Rather than the previously prescriptive processes described, DO-178B detailed a combination of activities and objectives that a design assurance process must meet. By defining activities and objectives only, airborne software developers had more flexibility in terms of development approaches. DO-178B did, however, specify key attributes for a design assurance process, derived from airworthiness regulations. For example, identifying unintended function and specific verification of software behavior whilst running on the final target hardware.
DO-178B also introduced the concept of the Design Assurance Level (DAL). DAL categorization determines the amount of rigor required by the design assurance process. DAL categorization itself is determined by the impact that a specific system's failure could have in terms of Aircraft Safety. The more critical the DAL, the more activities and objectives are required. DAL categorization continues to be used in DO-178C.
*The final DAL is DAL E, which confers no failure rate condition or objectives.
In 2012, DO-178C was released, partly due to the advancement of software development technologies which were not accounted for in previous iterations of the standard. DO-178C clarified details, removed inconsistencies and included supplements to provide guidance for the use of specific technologies in DO-178C projects.
The basic structure of a Design Assurance process consists of three components:
“Stage Of Involvement” (SOI) reviews are typically used by a certification authority to determine compliance to DO-178C guidance in a given project. The four review stages are:
For each of these review stages a specific aspect of the design assurance process followed is evaluated. Typically, certification authorities require that each SOI is completed in order. SOI stages represent key milestones in a DO-178C project.
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DO-178C guidance is designed to ensure that clear best practices are defined and followed by avionics system developers. DO-178C guidance also prescribes specific software testing measures that are dependent on the criticality of the system in question.
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