Common-cause, particular-risk and zonal safety analysis

The common-cause analyses are where a safety case is most often quietly wrong. A fault tree can show two independent branches multiplying to a vanishingly small probability — but if a single common cause can fail both, the real number is orders of magnitude larger. ARP 4761A addresses this through three analyses, and Safety Lab Aero connects all three to the quantification they should influence.

Common Mode Analysis (CMA)

CMA examines whether the independence claimed by fault-tree AND gates actually holds. Link a CMA record to the gates it affects; when a CMA identifies a common mode, the gate independence is compromised, the quantification switches to a common-cause-aware treatment, and any requirements derived from the affected gate are flagged for re-verification. An append-only log records each independence-state change.

Particular Risk Analysis (PRA)

PRA addresses single external threats — rotor burst, fire, HIRF, lightning, bird, tire and ice — that can fail multiple items at once. Log each particular risk, the affected zones, the common-source impact and the mitigation; PRA records connect to the zones and functions they expose and produce zonal-protection requirements.

Zonal Safety Analysis (ZSA)

ZSA examines installation and interference effects within physical zones. For each zone, record the housed functions and equipment, interference effects and mitigations; zonal analysis connects housed functions to the particular risks affecting their zone and produces separation requirements.

Common-cause modeling in the quantification

For coupled basic events you define a common-cause group and its parameters (beta-factor and the multiple-Greek-letter terms). Safety Lab Aero integrates the common-cause contribution into the quantification — including across multi-page TRANSFER trees — so a single common-cause event correctly fails every group member at once instead of being under-counted.

Frequently asked questions

What is the difference between CMA, PRA and ZSA?

CMA checks whether claimed independence between failures actually holds; PRA addresses single external threats (fire, rotor burst, HIRF, lightning, etc.) that can fail several items at once; ZSA examines installation and interference effects within a physical zone. All three are part of the ARP 4761A process.

Does a common-mode finding actually change the fault-tree result?

Yes. When a CMA compromises a gate\u2019s independence, the quantification switches to a common-cause-aware treatment and the requirements derived from that gate are flagged for re-verification — it is not just a documentation step.