1. Purpose and scope
Conceptual design is where most of the cost and risk of an avionics program is fixed. The architect chooses how each function is built: in a programmable device, a custom ASIC, a commercial processor or simple discrete logic. The software architect decides how software is structured, scheduled and partitioned so items of different assurance levels cannot interfere. Component engineering checks that every candidate part can be bought, in a suitable grade, for as long as the aircraft fly.
Each choice is judged against safety as well as performance. If the preliminary safety assessment lowered an item's level because two channels are independent, the architecture must show that independence: separate devices, power, clocks and software, and no common design error. The updated preliminary safety assessment and a first common cause analysis are outputs of this stage, not afterthoughts. DO-254 calls this the conceptual design process; for software, DO-178C treats architecture as part of the design process, and it is reviewed with the same care.
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