Chief Technology Officer at Advalange
5 common traps can hurt your DO-178C project seriously if recognized late
DO-178C, Software Considerations in Airborne Systems and Equipment Certification by RTCA, regulates the process of software development in the aerospace domain. The document, however, does not provide any clear recipe of getting things done in a given project. Each particular team has its own way of interpreting DO-178C objectives and limitations. Below are five cases that, in my experience, have caused the most arguments and misunderstandings. I call these aspects “traps” because choosing the wrong way in implementing them will have a tangible, negative impact on your project.
#1. Waterfall lifecycle trap
The opinion that DO-178C mandates a heavyweight waterfall-like life cycle is wrong. Any methodology is good, even an agile one, if you follow transition criteria and fulfill the objectives. Choose what fits your project and organizational culture best and complement it with additional steps to get the compliance. Do not exclude any activities that bring value to your product just because their conformance to the standard is questionable. Simply do not take formal credit from such activities. And vice versa, don’t be scared to bring in extra tasks to meet formal requirements. Often a week spent on dedicated formal tasks saves a month of ineffective and useless work on artificial life cycle stages.
#2. Trap of the undefined robustness
Often people recall about robustness at late stages of verification. It may look like: “Ouch, we need to add some kind of robustness testing to pass through SOI audit.” Software made this way is nothing more than a colossus with feet of clay. You must consider robustness from the very beginning and transform it into appropriate outputs throughout project life cycle: identify abnormal situations and containment actions in the requirements, insert corresponding features into your design and do not let untraceable defensive code ooze into your implementation. Robustness testing should be just another portion of a requirements-based test set if you do it right.
#3. Structural testing trap
DO-178C stands on a requirements-based testing concept. Nevertheless, a word combination “MC\DC tests” often circulates in engineers’ conversation. Forget such word combinations. Tests should check the requirements. MC\DC decision or statement coverage is just a measure that shows overall mutual consistency and completeness between requirements, code and tests. Having a coverage gap does not necessarily mean that a test is bad. Often requirements may be inadequate and lack details or you may have untraceable additional code. The worst mistake you can make is adding a synthetic test case just to exercise certain combinations of software variables.
#4. Overzealous tool qualification trap
DO-178C pays additional attention to tool qualification. Dedicated standard’s supplement DO-330 regulates this aspect. However, there is no need to qualify every single software tool. Moreover, tool qualification is an expensive process and you need to choose your qualification strategy wisely to balance your efforts. You need to understand tool qualification criteria and evaluate costs for each tool qualification level. Sometimes it is even more efficient to introduce additional activities, such as reviews or analysis, rather than to qualify a tool. Another mistake is over-reliance on the tool qualification support package from the tool vendor. You can rarely take the package as is and provide it as qualification evidence. Tune-up to a project’s environment, qualification test runs and results analysis may consume considerable time. You need to take this effort into account when figuring out your tool qualification strategy.
#5. Reckless use of external components trap
Introducing a third party or open source component into your software often seems like an attractive idea. However, any single portion of your code must be DO-178 compliant. This means that you cannot link a library to your project without further actions. The statement that “the community uses this library for ages” will not work for a formal credit. You need to have a solid plan of adaptation of such components into your project. You may use a certification support package or go through a reengineering cycle to capture requirements for a code library and to conduct all needed verification. Consider this extra effort when you are thinking about an external component. Sometimes developing your dedicated library is more efficient than working out a common third-party component.
The list above is not exhaustive. There are many more technological, design or process traps you can face along the way. Nonetheless, these five mistakes will be costly if revealed in the late stages of your project. My advice is: keep an eye on these aspects from the very beginning and don’t forget to negotiate your approach with your DER.