DO-178C

DO-178C, Software Considerations in Airborne Systems and Equipment Certification is the title of the recently published document from RTCA, Incorporated, in a joint effort with EUROCAE. This replaces DO-178B as the primary document by which the certification authorities such as FAA, EASA and Transport Canada will approve all commercial software-based aerospace systems. The new document is called DO-178C/ED-12C and was completed in November 2011 and approved by the RTCA in December 2011. It became available for sale and use in January 2012.^[1][2][3]

The FAA approved AC 20-115C^[4] on 19 Jul 2013, making DO-178C a recognized "acceptable means, but not the only means, for showing compliance with the applicable airworthiness regulations for the software aspects of airborne systems and equipment certification."

Background

Since the release of DO-178B, there have been strong calls by DERs (FAA Designated Engineering Representatives) for clarification/refinement of the definitions and boundaries between the key DO-178B concepts of high-level requirements, low-level requirements, and derived requirements and a better definition of the exit/entry criteria between systems requirements and system design (see ARP4754) and that of software requirements and software design (which is the domain of DO-178B). Other topics such as what does verification mean in a model-based development paradigm and can model simulation or formal methods replace some or all software testing activities. The release of DO-178C and the companion documents DO-278A (Ground Systems), DO-248C (Additional information with rational for each DO-178C objective), DO-330 (Tool Qualification), DO-331 (Modeling), DO-332 (Object Oriented), and DO-333 (Formal Methods) were created to address the issues noted. The SC-205 members worked with the SAE S-18 committee to assure that APR4754A and the above noted DO-xxx documents provide a unified and linked process with complimentary criteria.

Overall, DO-178C keeps most of the DO-178B text, which has raised concerns that issues with DO-178B, such as the ambiguity about the concept of low-level requirements, may not be fully resolved.^[5]

Committee organization

The RTCA/EUROCAE joint committee work was divided into seven Subgroups:

• SG1: SCWG Document Integration
• SG2: Issues and Rationale
• SG3: Tool Qualification
• SG4: Model Based Development and Verification
• SG5: Object-Oriented Technology
• SG6: Formal Methods
• SG7: Safety Related Considerations

The Model Based Development and Verification subgroup (SG4), was the largest of the working groups. All work is collected and coordinated via a web-site that is a collaborative work management mechanism.^[6] Working artifacts and draft documents were held in a restricted area available to group members only.

The work was focused on bringing DO-178B/ED-12B up to date with respect to current software development practices, tools, and technologies.^[7][8]

Software level

The Software Level, also known as the Design Assurance Level (DAL) or Item Development Assurance Level (IDAL),^[9] is determined from the safety assessment process and hazard analysis by examining the effects of a failure condition in the system. The failure conditions are categorized by their effects on the aircraft, crew, and passengers.

• Catastrophic - Failure may cause multiple fatalities, usually with loss of the airplane.
• Hazardous - Failure has a large negative impact on safety or performance, or reduces the ability of the crew to operate the aircraft due to physical distress or a higher workload, or causes serious or fatal injuries among the passengers.
• Major - Failure significantly reduces the safety margin or significantly increases crew workload. May result in passenger discomfort (or even minor injuries).
• Minor - Failure slightly reduces the safety margin or slightly increases crew workload. Examples might include causing passenger inconvenience or a routine flight plan change.
• No Effect - Failure has no impact on safety, aircraft operation, or crew workload.

DO-178C alone is not intended to guarantee software safety aspects. Safety attributes in the design and as implemented as functionality must receive additional mandatory system safety tasks to drive and show objective evidence of meeting explicit safety requirements. The certification authorities require and DO-178C specifies the correct DAL be established using these comprehensive analyses methods to establish the software level A-E. "The software level establishes the rigor necessary to demonstrate compliance" with DO-178C.^[9] Any software that commands, controls, and monitors safety-critical functions should receive the highest DAL - Level A.

The number of objectives to be satisfied (some with independence) is determined by the software level A-E. The phrase "with independence" refers to a separation of responsibilities where the objectivity of the verification and validation processes is ensured by virtue of their "independence" from the software development team. For objectives that must be satisfied with independence, the person verifying the item (such as a requirement or source code) may not be the person who authored the item and this separation must be clearly documented.^[10]

Traceability

Diagram illustrating the required tracing between certification artifacts, as required by the RTCA DO-178C standard. Red-colored traces are required only for Level A. Purple-colored traces are required for Levels A, B, and C. Green-colored traces are for Levels A, B, C, and D. Level E does not require any tracing. The references on each trace arrow represent references to the standard for the objective, the activity, and the review/verification, respectively.

DO-178 requires a documented connection (called a trace) between the certification artifacts. For example, a Low Level Requirement (LLR) traces up to a High Level Requirement (HLR). A traceability analysis is then used to ensure that each requirement is fulfilled by the source code, that each requirement is tested, that each line of source code has a purpose (is connected to a requirement), and so forth. Traceability ensures the system is complete. The rigor and detail of the certification artifacts is related to the software level.

Differences with DO-178B

SC-205 was responsible for revising DO-178B/ED-12B to bring it up to date with respect to current software development and verification technologies. The structure of the document remains largely the same from B to C. Example changes include:^[12]

• Provide clearer language and terminology, provide more consistency
• More objectives (for Levels A, B, and C)
• Clarified the "hidden objective", applicable to Level A, which was implied by DO-178B in section 6.4.4.2b but not listed in the Annex A tables. This objective is now explicitly listed in DO-178C, Annex A, Table A-7, Objective 9: "Verification of additional code, that cannot be traced to Source Code, is achieved."^[13]
• Parameter Data Item Files - Provides separate information that influences the behavior of an executable object code (without changing it). An example would be a configuration file that sets up the schedule and major time frames of a partitioned operating system. The parameter data item file must be verified together with the executable object code, or else it must be tested for all possible ranges of the parameter data items.
• DO-330 "Software Tool Qualification Considerations, a new “domain independent, external document”, was developed to provide guidance for an acceptable tool qualification process. While DO-178C was used as the basis of the development of this new document, the text was adapted to be directly and separately applicable to tool development and to address all tool aspects. Tool Qualification guidance was removed in DO-178C, replaced with guidance for deciding when to apply DO-330 tool qualification guidance to tools used in a DO-178C context.^[14]
• Technology supplements were added to extend the guidance of the DO-178C document to specific techniques. Rather than expanding the prior text to account for all current and future software development techniques, supplements are made available to explicitly add, delete, or otherwise modify the guidance of the core standard for application to specific techniques or technologies. All guidance in these supplements are written in the context of the affected guidance elements in DO-178C and so should be considered as at the same level of authority as that core document.^[15]
  • DO-331 "Model-Based Development and Verification Supplement to DO-178C and DO-278" - addressing Model-Based Development (MBD) and verification and the ability to use modeling techniques to improve development and verification while avoiding pitfalls inherent in some modeling methods
  • DO-332 "Object-Oriented Technology and Related Techniques Supplement to DO-178C and DO-278A" - addressing object-oriented software and the conditions under which it may be used
  • DO-333 "Formal Methods Supplement to DO-178C and DO-278A" - addressing formal methods to complement (but not replace) testing

Guidelines vs. Guidance

DO-178B was not completely consistent in the use of the terms Guidelines and Guidance within the text. "Guidance" conveys a slightly stronger sense of obligation than "guidelines". As such, with the DO-178C, the SCWG has settled on the use of “guidance” for all the statements that are considered as "recommendations", replacing the remaining instances of "guidelines" with “supporting information” and using that phrase wherever the text is more "information" oriented than "recommendation" oriented.

The entire DO-248C/ED-94C document, "Supporting Information for DO-178C and DO-278A", falls into the “supporting information” category, not guidance.^[16]

Sample difference between DO-178B and DO-178C

Chapter 6.1 defines the purpose for the software verification process. DO-178C adds the following statement about the Executable Object Code: "The Executable Object Code is robust with respect to the software requirements that it can respond correctly to abnormal inputs and conditions." As a comparison, DO-178B states the following with regard to the Executable Object Code: "The Executable Object Code satisfies the software requirements(that is, intended function) and provides confidence in the absence of intended functionality." The additional clarification fills a gap that a software developer may encounter when interpreting the document.^[17]

See also

• DO-178B
• Modified_condition/decision_coverage

References

External links

• Charlotte Adams (2010-10-21). "Safety-critical software for mission-critical applications to get boost with release of DO-178C". Military & Aerospace Electronics. Retrieved 2014-02-04. 
• Charlotte Adams (2010-09-01). "DO-178C Core changes". Avionics Intelligence. Retrieved 2010-10-23. 
• Bill StClair and Nat Hillary (2010). "DO-178C: Improved certification for cost-effective avionics systems". VME and Critical Systems. Retrieved 2010-10-23. 
• John McHale (2009-10-08). "Upgrade to DO-178B certification, DO-178C, to address modern avionics software trends". Avionics Intelligence. Retrieved 2010-10-23. 
• Frederic Pothon (2012). "DO-178C/ED-12C vs DO-178B/ED-12B: Changes and Improvements". Open DO. Retrieved 2010-10-23.