'Phase advance filter'
Bloke told me in a pub (so it has to be right) about a low rotor rpm warning system incorporating a phase advance filter. This warning sounded when the rate of rrpm decay reached a certain value, rather than when rrpm themselves reached a certain level. As a result it gave extra warning time after a complete engine failure.
This is good news for us low-inertia folks. Does anyone know if this system exists, and if so, where I can get one?