affected. Therefore, any inconsistencies between the applications of the two guidance  documents must be rectified.  CNS/ATM software may be justified to AL4 when there is no failure effect on airborne systems. However, when the safety assessment requires RTCA DO-178C development to Level C, and  the CNS/ATM software may cause a potential hazard to the aircraft such as message  corruption, then the CNS/ATM software must be developed to at least AL3. This is necessary to  instill an acceptable level of confidence that an anomaly in the CNS/ATM software will not result  in unacceptable behavior of the airborne system.  The use of Commercial Off-the-Shelf (COTS) software has been widely adopted in software  development projects for CNS/ATM systems and equipment. Examples of COTS software  include operating systems, real-time kernels, user-interface software, application  software/configuration items, communication and telecommunication protocols, runtime  libraries, and data management systems. COTS software can be purchased alone or in  conjunction with COTS hardware—such as workstations and communication and network  equipment—or hardware items such as memory, storage, and input/output devices. There may  be instances in which the use of COTS software is impractical to avoid, such as when a library  code is associated with certain compilers. It is essential that the level of confidence for COTS  software be the same as for any other software used along a CNS/ATM systems and equipment  chain. RTCA DO-278A in its entirety provides a means for evaluation and acceptance of  CNS/ATM software. In particular, RTCA DO-278A, Section 12.4,  Commercial Off-the-Shelf  Software , describes the framework for COTS compliance and approval. This framework  includes:  The legacy NAS and the associated developmental processes in place prior to March 14, 2005,  were accepted as a baseline prior to the transition to the SMS. Any changes to the NAS after  the establishment of the baseline must be SMS compliant; therefore, any change to the NAS  baseline software after March 14, 2005, must be (1) considered for its contribution to identified  • Additional objectives for COTS software lifecycle processes,  • A description of activities and considerations for achieving those objectives,  • A description of evidence that demonstrates that the objectives have been met, and  • Some alternative strategies to provide assurance for COTS software that may have only  partial (or no) evidence of compliance with the RTCA DO-278A objectives.  6.3  Legacy Systems  6.2  Commercial Off-the-Shelf Software  RTCA DO-278A AL RTCA DO-178C Software Level AL 1  A  AL 2  B  AL 3  C  AL 4  No Equivalent  AL 5  D  AL 6  E  J_SRMGSA_202003 Originally published March 2020  Uncontrolled copy when downloaded J-6  Table J.2: Correlation of CNS/ATM ALs and Airborne Software Levels 