Years ago, I was being shown around a Hercules (it must have been a long time ago - it was still in that 2 shades-of-$hit camouflage). The guy showing me around showed me a 'bleed air pressure gauge' (or something like that) and told me it was an essential item to monitor whether there could be a hot air leak....
Roll forward several years and I was on the mighty Tin Triangle. I asked how one could be certain that, with all the engine air switches off, the duct was depressurised as there was no gauge. I was told that it wasn't possible; however, I invented a check of my own involving a check of no Air Ventilated Suit flow with engine airs off. This was easy, as the engines were brought up above 80%, just press the valve on the Personal Equipment Connector and check that there was no airflow. If there was, throttle back each engine in turn to find the culprit. One night I did this as we were about to set off for Cyprus - and we found a stuck air valve as a result. This would have meant that, in the event of a hot air leak, we wouldn't have been able to stop the leak apart from shutting down the associated engine.... So the captain wisely elected to shut down the offending engine and taxi back in.
Some months later, a Waddington (second division) crew were flying to Goose, then Offutt. They received the classic symptoms of a hot air leak, but seemingly didn't know how to confirm whether the engine airs were working properly. However, they pressed on.... On the way into Goose the fuel readings were all over the place - some tanks had seemingly gained 1000 lb of fuel. Later, serious damage was found to have occurred; I think that the aircraft was later donated to Goose as a mascot.
Hot air leaks, and the inability to detect whether a bleed air duct is pressurised, are very serious. The old excuse of 'not practicable due to impending OSD' was often trotted out in V-force days - yet the aged jets kept going for years after the TSR2 was supposed to have been in service.
It seems that the lessons of the past have not been properly 'identified', let alone learned.
Keep up the good work, T's D - and don't let the squirming airships wriggle off the hook!