Guys,
Quite straightforward. The AP100 rules were quite clear that STFs were for trials only, and not to be released to operations. Nothing to do with peacetime, or cold war, or anything. STFs were for trials only. End of.
For ops, the mandated route was SEMs. Yes, it's possible and even probable that SEMs were abused, but the fact was that by the late 90s, STFs were endemic as a 'work around' the slightly more onerous SEM route. They were kept on RAF aircraft for years, while the RN EAs were having to justify the retention of NSMs (Navy equivalent of SEMs) every 6 months to a very hard nosed two star engineer. Note that the RN never even had such a thing as an STF.
I'm not making a 'dark blue vs light blue' point, by the way. My point is that the abuse of STFs became an institutionally approved practice, which should have been recognised and stopped long before it was, by issue of the new JAP100 regulations on service modifications. The RAF (and the various PTs) are still clearing up the results of that abuse.
Best regards as ever
Engines