The tragic nature of safety-critical software failure's consequences makes high quality and extreme reliability requirements in such types of software of paramount importance. Far too many accidents have been caused by software failure error or where such failure/error was part of the problem. Safety-critical software is widely applied in diverse areas, ranging from medical equipment to airborne systems. Currently, the trend in the use of safety-critical software in the aerospace industry is mostly concentrated on avionic systems. While standards for certification and development of safety-critical software have been developed by authorities and the industry, very little research has been done to address safety-critical software quality. In this paper, we study safety-critical software embedded in airborne systems. We propose a lifecycle specially modeled for the development of safety-critical software in compliance with the DO-178B standard and a software quality assurance (SQA) model based on a set of four acceptance criteria that builds quality into safety-critical software throughout its development.