Dissimilar tools: Use cases and impact on tool qualification level

Frédéric Pothon

Frequently, the lack of existing qualifiable COTS tools raises the question whether two separately developed and thus dissimilar tools that satisfy the same operational requirements may be used instead of a single tool, either to avoid the need for qualifying that tool or to serve as an alternative method for tool qualification.

This question may be answered in different ways by different certification authorities, since no guidance or guidelines are provided in the applicable standards, such as DO-178C/ED-12C. DO-330/ED-215 identifies the use of dissimilar tools as an example of “alternative method for tool qualification” but without definition of the actual impact of such approach. DO we have to understand that the use of non-qualified dissimilar tools replaces the need for qualification of a single tool? Is this applicable for all TQL?

This paper presents several use cases based on the type of tools and certification credit, and examines the possible impact on the need for qualification and the applicable Tool Qualification Level of the use of dissimilar tools.

Disclaimer: Discussion of this topic in the Forum for Aeronautical Software (FAS) (see http://www.rtca.org/content.asp?pl=33&sl=170&contentid=170) raised controversial questions and different opinions. Therefore, it was difficult to reach a consensus on an Information Paper. It was decided that this topic is beyond the scope of the FAS working group, and the proposal was withdrawn. This document reflects the author’s point of view, based on his background and experience. It should help readers to provide an acceptable rationale for a software life cycle using dissimilar tools in place of a single qualified tool.

The author, Frédéric Pothon ACG Solutions, and several of the contributors and reviewers participated in the DO-178C/ED-12C working group and subcommittees.

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