Forced Landings.
I've been fortunate enough in my career to do a fair bit on Tiger Moths, - and over a long time.
In connection with this flying of "old"`aeroplanes, an "old" mate of mine, G-KEST to the cognicenti, gave me a piece of advice, more years ago than I care to remember. that I have never forgotten.
" Don't sit there with yer thumb up yer b*m and yer mind in neutral. Always have somewhere to go if it all goes wrong, even if you've just passed it."
That translated a couple of years ago to getting a Tiger back to Old Warden when the engine suffered a bout of indigestion due no doubt due to its intense dislike of 100LL.
A couple of valves decided to stick when I was about four miles away.
Anyway, suffice it to say, that after the initial shock, we made it back to Old Warden with the engine limited to a max. of 1100rpm, having had the choice of an amazing number of previously identified fields should the problem have exacerbated itself.
The engine gave up as we rolled to a stop.
This also supports the message of a previous contributor, that it is comparitively rare for an engine to pack up either instantly or completely. As BigendBob says, there is usually something that will give some prior indication that all may not be well.
Sleeve.