Originally Posted by
bearfoil
I think it used to be "per sob". Soul on board. Each butt in the chair, each time, each belt fasten. I've thought this a long time, but I formed this knowledge years ago, and have faith in it only because no one has corrected me yet, Please do. It inspires faith in me when I fly, so be gentle in one's "disabuse".
bearfoil,
There are statistics and statistics.
You're right... when YOU step on board, you want to know what are YOUR chances in stepping off the flight with your body and soul still in the same place.
So some statistics will give YOU that answer, in terms of fatalities per million passenger miles (or similar figures).
For "us engineers", those statistics are not much use.
We look at many
events, from the simple failure of a relay, to the catastrophic disintegration of a turbine disk.
Those too get assigned statistics.
If you have to use something with an MTBF (mean time between failures) of something like 5000 hours (like a relay) in a design, you make damn sure any single failure will have no further effect other than a change-over to another system and a maintenance alert.
When you get to really catastrophic failures, you try to design in such a way that those have only a small chance of happening in the life of the fleet... which often means you're talking about 10e-9 to 10e-12/hour.
And sometimes, Murphy himself intervenes.
It's not that we engineers don't care about the 'souls on board'.
We try to stop the events that makes the 'souls on board' becoming an issue.
For that, we have to use different statistical tools, though.
CJ