Stochastic or Not?
Perhaps I can help a bit here. (I'm an engineer, too).
I'd agree (sorry PDR1) that using the word 'stochastic' is really asking a bit too much of the average pilot. As an engineer, I always made sure that when I was explaining technical matters to ANY audience, I did so in a way that they could understand. That said, there's never any excuse for lack of manners and sarcasm, from either side. Forums like PPrune should be informal, but rude? No.
Actually, I had to look up 'stochastic', and I got this (fairly clear) definition:
Situations or models containing a random element, hence unpredictable and without a stable pattern or order. All natural events are stochastic phenomenon. And businesses ... are stochastic systems because their internal environments are affected by random events in the external environment. Stochastic is often taken to be synonymous with probabilistic but.... stochastic conveys the idea of (actual or apparent) randomness whereas probabilistic is directly related to probabilities and is therefore only indirectly associated with randomness.
So, as I read it, 'stochastic' refers to random events, as opposed to 'probable' events. However, that does rather cut across PRD1's statement that:
'...the foundation of all the analyses is reliability data, and reliability data is statistical*, based on the probability of events occurring in fleets over large numbers of missions/years or whatever.'
I absolutely agree with PDR1 on that last statement. Where I would, very respectfully, part with him would be an assertion that an ILS/LSA analysis is 'stochastic'. In my experience, it's normally probability based, but if the newer analyses have found a way to include randomness then, hey, fill yer boots.
OK, explanation for non-experts (like me). Any ILS/LSA analysis for a new aircraft (or any new system) has to use a large number of assumptions and predictions, particularly those associated with anticipated usage and the predicted reliability of components and systems. So here's the unvarnished truth - any initial spares/support package is liable to be really, really wrong. What smart project managers do is take the output from from ILS/LSA analyses and apply a good stiff dose of common sense to it. Then ensure that when the stuff enters service, it is subjected to formal fault reporting, with a comprehensive 'blacklist' to make sure that key (high cost and/or mission critical) components assessed as U/S are properly investigated to find out what went wrong.
Then, they regularly adjust their support/spares packages to make sure that what they're sending to the front line matches what is actually happening. Here's the problem. Many PMs now working in DE&S don't even know what a 'blacklist' is, never mind a defect reporting form. So kit enters service without the essential feedback loop to adjust the spares provisioning. This is not an opinion, I've seen it happen on multiple (and recent) programmes.
To the subject. Dismantling aircraft for spares is only ever economic when you have surplus aircraft you don't need any more. And even then it's marginal, given the costs to actually remove the kit, get it reconditioned and certified, repackaged and reloaded on to the ILS system. Oh, and the parts you get out of this process are 'part lifed', unless you pay for them to be reset to zero hours. That said, given the state of the MoD budgets these days, even a marginal pay back is worth it. Yeah, strip the mothers.
This also shows that the Typhoon fleet is now too big for the job it's now required to do. The T1 buy was probably driven by the same 'broad brush' (i.e. bum) calculations I saw being used to justify Harrier T10 numbers on JFH. That, plus a realisation that sim hours can be far more useful that burning holes in the sky (or providing a handy 'hack' for the CO to fly around in).
Best regards as ever to all those juggling the fleets, it's never easy with less money,
Engines