Originally Posted by
JohnDixson
The aero guys at SA in answer to an internal query on an accident investigation some decades ago, did some simulation and opined that there is an Nr number, where at least from the lift vector and angle of attack calculations, the Nr can reach a point where the decay is unrecoverable, and hence ( and this was assumed at full low collective ) you need to put the throttle(s) forward or use your chute. Number was in the 60% range. So there is some margin between where the qualification test crews operate and the point at which the situation turns really dangerous. With an Nr decay rate of 10%/second, though, one does not have all day.
John: I have heard similar claims of lower numbers and it provides some comfort to know that there may be a small contingency available in the Nr recovery range.
However (and needless to say) one's objective must be to do all possible to maintain Nr within the green.