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December 30, 2000
10-5
10.3 Software Safety Development Process
The process outlined below is briefly explained in this Handbook. Further guidance and specific instructions
can be obtained through a careful examination of the JSSSC Software System Safety Handbook, Dec. 1999
and DO-178B, Software Considerations in Airborne Systems and Equipment Certification, Dec. 1, 1992 at a
minimum.
10.3.1 Software Safety Planning and Management
Software system safety planning precedes all other phases of the software systems safety program. It is
perhaps the single most important step and should impose provisions for accommodating safety well before
each of the software life cycle phases: requirements, design, coding, and testing starts in the cycle. Detailed
planning ensures that critical program interfaces and support are identified and formal lines of
communication are established between disciplines and among engineering functions. The software aspects
of systems safety tend to be more problematic in this area since the risks associated with the software are
often ignored or not well understood until late in the system design.
Planning Provisions
The software system safety plan should contain provisions assuring that:
·  Software safety organization is properly chartered and a safety team is commissioned at
the beginning of the life cycle.
·  Acceptable levels of software risk are defined consistently with risks defined for the
entire system.
·  Interfaces between software and the rest of the system’s functions are clearly delineated
and understood.
·  Software application concepts are examined to identify hazards/risks within safetycritical
software functions.
·  Requirements and specifications are examined for hazards (e.g. identification of
hazardous commands, processing limits, sequence of events, timing constraints, failure
tolerance, etc.)
·  Design and implementation is properly incorporated into the software safety
requirements.
Planning
And
Management
10.3.1
Assign
Software
Criticality
10.3.2
Safety-Critical
Requirements
Derivation
10.3.3
Design
And
Analyses
10.3.4
Testing
10.3.5
Software Safety
Process Steps
FAA System Safety Handbook, Chapter 10: System Software Safety
December 30, 2000
10-6
·  Appropriate verification and validation requirements are established to assure proper
implementation of software system safety requirements.
·  Test plans and procedures can achieve the intent of the software safety verification
requirements.
·  Results of software safety verification efforts are satisfactory.
Software Safety Team
Software safety planning also calls for creating a software safety team. Team size and shape depends
commensurately on mission size and importance (see Figure 10-1). To be effective, the team should consist
of analytical individuals with sufficient system engineering background. Chapter 5 of this handbook
provides a comprehensive matrix of minimum qualifications for key system safety personnel. It applies to
software system safety provided professional backgrounds include sufficient experience with software
development (software requirements, design, coding, testing, etc.)
Figure 10-1: Example Membership of Software System Safety Team
Several typical activities expected of the team range from identifying software-based hazards to tracing
safety requirements, from identifying limitations in the actual code to developing software safety test plans
and ultimately reviewing test results for their compliance with safety requirements.
Management
Software System Safety program management begins as soon as the System Safety Program (SSP) is
established and continues throughout the system development. Management of the effort requires a variety
of tasks or processes from establishing the Software Safety Working Group (SwSWG) to preparing the
System Safety Assessment Report (SSAR). Even after a system is placed into service, management of the
software system safety effort continues to address modifications and enhancements to the software and the
system. Often, changes in the use or application of a system necessitate a re-assessment of the safety of the
software in the new application. Effective management of the safety program is essential to the effective
reduction of the system risk. Initial efforts parallel portions of the planning process since many of the
required efforts need to begin very early in the safety program. Safety management pertaining to software
　
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