Kintyred,
You put your finger unerringly on the nub of things. Many, many, many years ago I visited the AIB black museum in Shell-Mex House. Among the exhibits was a very ancient NRV with a luggage label attached. Our guide explained that it had been fitted to a Vickers Valencia (or some such) and crashed in the desert due fuel starvation.
The resulting inquiry found that this valve had been fitted the wrong way round, for the simple reason that it could be. As a result, all NRVs were made with different diameter inlet/outlet pipes thereafter so that it would not happen again.
Classic Flight Safety parable, which was just as well for we were all on a course for prospective FSO's. When that link was broken between cause, effect and cure we were sent back to the aviation stone age. It seems from the threads on this forum relating to airworthiness affected fatal military air accidents that we are still there.
Anyone got some flint?