Originally Posted by violator
While that calculation is good, you're assuming that hydraulic system failures are independent events
Correct. Explicitly. So now we are getting into what the figures can mean. They are a decision guide. In order to let such figures guide one in making decisions, one does indeed need a feel for what such calculations can say, and what they can't. And the independence assumption is the trickiest of the lot.
There are obviously people here who don't know what such calculations can say and what they can't, and I don't see how to give a feel for this which can be packed into a dozen words.
What the figures are good at showing is that, even if you know about or have experienced a simultaneous failure of two systems, that does not necessarily mean you or anyone else needs seriously to worry about a failure of three.
Originally Posted by violator
but they're not.
I think you mean that not all hydraulic failures need be caused by independent subsystem failures. Correct. Being hit by a missile might be a common cause failure of all three hydraulic systems. The calculation obviously does not account for failures caused by external events, such as missiles or mid-air collisions or such.
It is also the case that there have been features of certain architectures that slipped through the regulators, such as the common-cause failure near Sioux City. But that was a glaring design error which should have been caught at review time by the hazard analysis. Throwing blades was not exactly an unknown event. And when doing the hazard analysis obviously either nobody had asked what the worst outcome could have been when number 2 throws a blade, or had done so and not answered the question correctly.
Calculations of likelihood don't help when significant design-analysis errors are made
On the other hand, when it comes down to it I don't actually know what hazard analysis techniques were current when that AC was designed.
Originally Posted by violator
Most (all?) of A320 G&Y failures have been caused by inadventent PTU selection.
Note that I took account of that specific common-cause failure, and it played a conservative role in the argument.
[/QUOTE]All total hydraulic failures on commercial aircraft have been caused by structural damage.[/QUOTE]
Right, as far as I know.
PBL