Special
I do find it amazing however that such issues can be fixed in the latest 'Service Pack 4' or whatever its called !! Thank goodness for steam driven aircraft !
A bit of a non-sequeter there and your ludditism is showing!
The issue of control instability is (as with any control system) that if the gain of the control loop is greater than unity at the frequency where there is 180deg phase shift in the feedback, divergent instability results.
Gain in this case is related to "control power" which for a helicopter varies with rotor speed and with air density - ie slow rotors = little control power, fast rotors = lots of control power, thin air = little control power etc.
So with very thick air, control power/gain can be reduced back to below unity by slowing down the rotors. This is what your SP4 will give us - the FADEC will reduce the Nr when thick air is detected. At the moment it only takes it down to 100% but with SP4 it will go down to 97.7% (I think) thus allowing ops down to -6000'DA
So I say "thank goodness for software driven aircraft" - the software can be tweaked to fix an inherent structural/control problem.
HC