Will the ICDs Please Stand Up? An attempt to reason about subsystem interfaces in avionics system integration

System integrators for commercial aircraft reason about the subsystems they must integrate using Interface Control Documents (ICDs). Although ICD format standards exist in this specific domain, avionics suppliers express their ICDs in different ways, making the integrator's work challenging. As software engineering researchers, we are trying to define a common vocabulary for ICDs in the context of avionics systems integration. This paper describes this effort and the associated challenges. Our experience is that concepts such as architectural interfaces and abstraction, among others, are challenged in an avionics context. Moreover, understanding the concept of ICD, coming from a software engineering perspective, is not easy. Examples of ICDs in avionics are few, as real ICDs convey proprietary information which aircraft builders and their suppliers do not share publicly. We share our experience by describing an attempt to model ICDs for subsystems of a small avionics system, based on open standards, using both federated and IMA architectures. Our attempts lead us to conclude that it is required to model subsystems to a certain level of detail before establishing what their ICDs are (i.e. a bottom-up approach). The Architecture Analysis and Design Language (AADL) appears to be an appropriate language for this undertaking.