That “ one in a billion” requirement sounds to me that the standard does not call for absolute safety.
It looks as as if there are compromises made for affordability reasons.
That is in in some cases CASA and regulators throughout the world put the ultimate cost of safety improvements as the most important consideration.
Just a fact of of life that is admitted in other countries and reflected in legislation.
But not in Canberra. They have “ safety” as the most important consideration- more important in all cases than cost.
Why they live this lie is behind me.
In in the meantime with the “one in a billion” parameter when is it likely that a twin will end up in the ocean?