I believe I've been told that the un-reliability of a commercial jet has to be less than 10^-18 (I don't have a reference for that).
The interpretation I understood (again no reference) was that it meant; airplane equipment failures sufficient to render the aircraft un-flyable or un-landable during the course of a single flight would occur no more than 1 time in 1,000,000,000,000 flights. I'm also under the impression that no single human can work to the 10^-18 number (been told that, again no reference).
It seems to me that even if you build an aircraft that reliable, the process for ensuring sufficient fuel at every departure also needs to be at least that good for the results to be predictable. Nowadays there is a lot of economic pressure to put less fuel on things. That means bean counters and all sorts of half trained self styled efficiency heroes come into the picture. Does anyone do a formal numeric system safety assessment of fueling processes for an airline and come up with a reliability number?
No fuzz on it, I wouldn't pretend to know where to start, I'm probably overloading my butt when I get into this stuff.