1. Risk

The failure to address software common cause failures in the software and avionics design poses a critical risk to the project. This risk is characterized by the high probability of major software failures resulting in Loss of Vehicle (LOV), mission failure, or even the loss of human life in manned missions. Common cause failures arise from faults in software or systems that impact multiple redundant or independent components simultaneously, undermining the benefits of redundancy typically built into aerospace systems. This could occur due to coding/logic errors, processor resource overruns, database errors, or malicious software (e.g., computer viruses).

The risk is especially heightened during dynamic mission phases, such as ascent, entry, descent, and landing (EDL), when systems must perform reliably under conditions with short time-to-effect or faster-than-human response requirements. During these time-critical scenarios, any failure in the software's ability to operate fault-tolerantly can lead to catastrophic results, including complete loss of control of the vehicle.

Understanding Software Common Cause Risks

Software common cause failures carry a unique risk profile because they:

1. Affect Multiple Systems: A single software failure can propagate through multiple redundant or independent systems (e.g., flight control, navigation, communication), bypassing the safeguards that physical redundancy is designed to provide.
2. Create Fail-Silent or Erroneous-Output States: In the fail-silent case, the system fails to respond to critical events when required, while in the erroneous-output case, the system provides incorrect outputs that can lead the vehicle into unsafe states.
3. Introduce High-Risk Scenarios in Human Spaceflight: In human spaceflight systems, a single flight software failure can cascade into a catastrophic loss-of-vehicle-control hazard, jeopardizing payloads, crew safety, and mission objectives.

Without addressing software common cause risks in the early design of both software and avionics, the project exposes itself to unacceptable levels of residual risk that cannot be effectively mitigated later in the lifecycle.

Key Risk Factors

1. Dynamic and Time-Critical Operations: Software common cause failures are especially dangerous during mission phases that require rapid responses (e.g., launch, EDL, or high-G maneuvers), where even minimal delays in decision-making or control can lead to irreversible mission failure.
2. Complex Software Architectures: Modern aerospace software systems are increasingly complex, creating numerous potential points of failure that are difficult to isolate quickly in dynamic phases.
3. Shared Dependencies: Dependencies on shared resources (e.g., processors, memory, communication buses) create single points of failure that can propagate errors across redundant systems, eliminating the inherent safety intended by those redundancies.
4. Human-In-the-Loop Constraints: Faster-than-human response times required during critical failure scenarios add additional complexity, as failures in the software cannot be mitigated by manual intervention in time.

Impact of Software Common Cause Failures

If left unaddressed, software common cause failures can result in:

1. Loss of Vehicle (LOV): A single fault propagating through redundant systems during critical dynamic phases can result in complete vehicle failure.
2. Loss of Mission (LOM): A software failure impacting navigation, propulsion, or other primary systems can lead to an inability to complete mission objectives, resulting in total mission loss.
3. Loss of Crew (LOC): For human spaceflight systems, software failures can lead to loss-of-life hazards, making prevention of these risks an unequivocal priority for mission success.
4. Erosion of Confidence in System Design: A failure to address common cause risks undermines confidence in the software and avionics architecture, increasing testing, maintenance, and operational costs, and putting project schedules at risk.

2. Mitigation Strategies

Mitigation Strategies for Software Common Cause Risks

To effectively address the risk of software common cause failures, the following strategies should be incorporated into both software and avionics design:

1. Diverse Redundancy:

   • Implement software diversity in redundant systems to ensure that errors in one software implementation do not propagate across the system. Diverse designs can include alternate coding languages, algorithms, or execution paths.
   • Use functionally independent backups such as Backup Flight Software (BFS) to ensure continued operational capability in the event of a primary software failure.

2. Safe Mode Coverage:

   • Implement reliable Safe Mode mechanisms for quiescent on-orbit operational phases to achieve system stability during fault conditions, providing time for diagnostics and recovery.
   • Safe Mode should be autonomous and robust enough to return the vehicle to a safe state without requiring human intervention.

3. Rigorous Fault Management Systems:

   • Include robust fault detection, isolation, and recovery (FDIR) systems to monitor for anomalies, isolate the impact of faults, and implement automated recovery procedures.
   • Design FDIR systems with fail-safe mechanisms that minimize the likelihood of common cause failure propagation.

4. Thorough Testing and Verification:

   • Employ system-level testing to validate the interaction of software components and ensure that software errors do not propagate across interfaces.
   • Conduct scenario-based testing during dynamic mission phases to simulate realistic fault scenarios and evaluate the system’s ability to recover.

5. Partitioning of Critical Resources:

   • Use hardware and software partitioning techniques to isolate critical processes and resources. Partitioning ensures that faults in one process do not cascade into failures in other processes.

6. Cybersecurity Protections:

   • Harden the software against external threats such as computer viruses and unauthorized access through robust cybersecurity measures, including secure coding practices, penetration testing, and monitoring for malicious activity.

7. Analysis for Common Mode Errors:

   • Perform thorough common mode analysis during system design to identify shared dependencies or vulnerabilities, and address identified risks through architectural adjustments.

Recommendations for Implementation

To ensure this risk is adequately addressed:

• Include Common Cause Analysis in Early Design Phases: Analyze the software and avionics architecture from the outset to identify and mitigate points of potential common cause failure.
• Adopt Safety-Critical Standards: Ensure that software development adheres to relevant safety standards, such as NASA’s NPR 7150.2, RTCA DO-178C, or ISO 26262.
• Incorporate Dedicated Backup Systems: The inclusion of diverse backup systems, such as the BFS, ensures system continuity even under the presence of primary software faults during critical phases of the mission.
• Invest in System-Integrated Testing: Rigorous testing procedures must be implemented to identify system-level interactions and potential fault propagation paths.

Conclusion

Software common cause risks are among the most challenging and consequential issues in the development of avionics systems for safety-critical applications. These risks, if left unaddressed, can lead to catastrophic outcomes, including the loss of vehicles, missions, and crew. By embedding diverse redundancy mechanisms, rigorous fault management, and proactive risk identification into the software and avionics design from the beginning, these risks can be mitigated to acceptable levels. Addressing software common cause failures with the highest priority is essential to ensuring mission success, maintaining adherence to safety-critical standards, and safeguarding human lives and system assets.

3. Resources

3.1 References