I agree completely with Blender and SASless, that cookbook stuff better be left for kitchens and that every auto merits its own approach depending on various factors as mentioned. Comfort can be found by reading some books on helicopter aerodynamics and how various factors will affect the autorotation.
I DEFINITELY also agree that pilot's should know where the bottom stop of the helicopter collective pitch lever is set, especially so for that altitude the helicopter operates at mostly.
I had an incident during autorotation training at 5000' AMSL at 25 degrees Celsius, with 2 crew and approximately 500 pounds of fuel onboard a Bell 407. After the entry, the throttle was rolled to the idle stop. Immediately the Rotor RPM (RRpm) started decaying and continued to decay to well below 95% when the horn activated with the collective pitch lever fully down at 65 - 70 kts. During further experimentation at 1000’ AGL and a higher than normal IAS, not even doing a harsh flare produced sufficient RRpm to ensure a safe landing and the only way to regain RRpm to within limits was to open the throttle.
Now was this a real case or the helicopter was at sea level, this could have had disastrous consequences, as it was nearly impossible to recover from the low and lost RRpm.
Setting bottom stops should be done after routine maintenance, every 100 hr inspection or if any adjustments were made on the rotor head, including adjustments to pitch change rods after track and balance flights etc or if main rotor blades were replaced. All it takes to check if the stabilised RRpm is correct for a specific Weight, Altitude and Temperature, is to do an auto with the throttle at idle at a given weight and speed as stated in the Maintenance Manual for your specific helicopter type. It is easier to raise the collective pitch lever than it is to try "manufacture" RRpm from a low RRpm state.
Also remember that it is virtually impossible to recover from catastrophic blade stall due to excessively low RRpm. (For more information:
http://www.helicopteros.com.br/sn24box.htm)
So high to low, careful go means more than just flying from meteorological high pressure to low pressure areas. It also means have the required adjustments made or check it at your destination if you are going from say 5000’ AMSL to Sea Level and vica versa to ensure RRpm can be adequately maintained should it be required in case of an engine failure.
And to agree with SASless one more time, Rotor RPM is life! An autorotation is a living, changing animal, to be approached with respect and knowledge and only perfected with lots of training. But once in your pocket, it gives one great confidence.
