What is DO-178B?
DO-178B, officially RTCA DO-178B / EUROCAE ED-12B and titled Software Considerations in
Airborne Systems and Equipment Certification, is a software certification standard for
airborne systems on commercial aircraft. Published in December 1992 (and revised as DO178C in December 2011), DO-178B contains guidance for the planning, development, and
verification of airborne software. The guidance comprises several elements:
A series of objectives keyed to the various software life cycle processes,
The specified activities for accomplishing these objectives, and
The required artifacts (life cycle data) that serve as evidence that the objectives have been
met.
The intent behind DO-178B is to achieve a degree of confidence in a software component
proportional to the component’s criticality, i.e., the impact of a software anomaly on the
continued safe operation of the aircraft. Criticality is reflected in the component’s software
level:
The number of objectives, and the rigor required for showing that they have been satisfied,
depend on the software level. For example, at Level A there are 66 objectives to be
achieved. Different system components may be at different software levels, provided that
absence of interference can be demonstrated.
DO-178B identifies a number of software life cycle processes, with associated objectives,
activities, and life cycle data:
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Software Planning Process
Integral Processes
o Software Verification
o Software Configuration Management
o Software Quality Assurance
o Certification Liaison
Software Development Processes
o Software Requirements
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o
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Software Design
Software Coding
Integration
A major emphasis of DO-178B is on software verification, which consists of reviews,
analyses, and tests. Testing needs to be based on the software requirements, and coverage
must be demonstrated in multiple senses:
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All requirements must be covered (i.e., for each requirement there must be some
test that checks that the requirement is met), and
The requirements-based tests must completely cover the source code structure (i.e.,
there can be no “dead” code that is present but does not correspond to a
requirement)
DO-178B places different demands on structural coverage based on the software level. For
Level C, only statement coverage needs to be shown. For Level B, both statement and
decision coverage need to be shown. (Decision coverage implies, among other things, that
Boolean expressions need to be exercised with tests for both True and False outcomes.)
Level A further requires Modified Condition/Decision Coverage (MC/DC), which entails
additional tests for subexpressions of Boolean expressions.
Supplementing its guidance keyed to the various software life cycle processes, DO-178B
includes “Additional Considerations” for several topics:
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The use of previously developed software,
The requirements for qualifying a tool, when DO-178B processes are “eliminated,
reduced or automated … without [the tool’s] output being verified” as otherwise
specified in the document
The use of alternative methods to satisfy DO-178B objectives, for example formal
methods
DO-178B is more of a correctness standard than a safety standard; i.e., it is focused on
demonstrating that the software meets its requirements as opposed to satisfying specific
safety properties. The safety analysis is performed as part of the system life cycle processes,
for example following SAE Aerospace Recommended Practice standards ARP 4754A and ARP
4761. These processes determine the software level and any safety requirements allocated
to software, which then serve as input to the software life cycle processes defined in DO178B.
DO-178B is not really specific to aviation and could, in principle, be applied to other highintegrity domains. The document does not dictate any particular development process or
approach to hazard assessment. DO-178B also says very little about programming
languages, but objectives such as source code accuracy and consistency are best met by
languages such as Ada with strong data typing and other features amenable to static
analysis / early error detection.
A variety of additional official material relates to DO-178B:
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RTCA DO-248B / EUROCAE ED-94B, Final Report for Clarification of DO-178B/ED12B, comprising a list of errata, a FAQ section, and several discussion papers
Certification Authority Software Team (CAST) Papers, offering detailed discussions of
various subjects treated in DO-178B