Independent failures, represented by an AND gate in a fault tree analysis, have a low probability of occurring in the same flight.
CCA and ZSA are used to find and eliminate or mitigate common causes for multiple failures.
In those areas of the aeroplane where multiple systems and components are installed in close proximity, it should be ensured that the zonal analysis would identify any failure or malfunction which by itself is considered sustainable but which could have more serious effects when adversely affecting other adjacent systems or components.
[1] Aircraft manufacturers divide the airframe into zones to support airworthiness regulations, the design process, and to plan and facilitate maintenance.
It also considers failure modes and maintenance errors that could have a cascading effect on systems,[3] such as: Potential problems are identified and tracked for resolution.
On July 19, 1989, United Airlines Flight 232, a McDonnell Douglas DC-10-10, experienced an uncontained failure of its No.
The lack of independence of the three hydraulic systems, although physically isolated, left them vulnerable to a single failure event due to their close proximity to one another.
Cabin air rushed into the unpressurized fuselage cavity, overpressurizing the area and causing failure of the auxiliary power unit (APU) firewall and the supporting structure for the vertical fin.