A friend of mine had an engine failure while doing sling load operations with a 100-ft. line. He instantly entered an autorotation and flared seconds before the helicopter crashed. The impact was brutal and the aircraft was heavily damaged, but luckily he walked away with only minor injuries.
If you have an engine failure in a low inertia rotor system helicopter and your airspeed is below 60 kt., you need at least 500 ft. of altitude to increase airspeed and get the rpm back into the green to autorotate safely to the ground. Any time you have an engine failure, you have a very short "window of opportunity" to do exactly the right thing, at the right time, to be successful.
In the Robinson R22 helicopter, the rate of descent is approximately 1400 fpm in the glide, depending on weight, density altitude, wind, pressure altitude, humidity and pilot performance. The slower the airspeed, the higher the rate of descent. In a zero speed autorotation, you will descend at 2000 fpm.
If you enter an autorotation from 500 ft., you will be down to a 40-ft. flare altitude in approximately 19 seconds. If the engine quits while hovering at low altitude, enter an autorotation quickly by lowering the collective full down, straight and level. Then nose the helicopter over to gain the minimum airspeed for your particular helicopter. The lower you are when the engine stops, the faster you have to enter an autorotation and the more you have to push the cyclic forward to increase to the appropriate airspeed. If you don't get the speed back, you will not have the airspeed energy needed to perform a safe landing. If you are both low and slow, a very dangerous condition, you might end up in a catastrophic situation. Always keep safe autorotation airspeed and rotor rpm in the green--all the way down if possible.
If the engine fails in flight, there are three energy fields that are needed for you to be able to walk away--altitude, airspeed, and rotor rpm.
Let's assume you are rescuing a person from a river with a 300 ft. line. You are headed into the wind. Suddenly the engine stops. Enter the autorotation fast by lowering the collective full down. Keep a level attitude. Then pitch the nose down with forward cyclic to gain airspeed. Even if you push the cyclic forward hard to get your airspeed back, the rpm will not go down below safe limits.
If, in fact, the caution light for low rpm and the warning horn comes on, lower the collective to increase rpm into the green to increase blade speed.
As you increase airspeed, rpm increases and you might have to lift collective slightly to prevent an rpm over-speed. If you lift collective too much, too fast, rpm decreases rapidly. The "Golden Rule" is that if you lift the collective fast, you have to be ready to lower the collective fast. And if you lower the collective fast, you have to be ready to lift it fast.
Quick reaction and full understanding of how the rpm is controlled is very important in this scenario. In the glide it is also important to keep the aircraft in trim, to prevent too much parasitic drag, which will lessen your glide distance. Do whatever it takes to keep rpm, airspeed and trim in control.
When you have an engine failure, the chances that the ground will be perfect for landing is slim. If it is of the worst sort, stay cool, calm, and confident.
At approximately 30-40 ft. of altitude, depending on the surface, in the Robinson helicopter (other helicopter types might require a higher flare altitude) flare the helicopter to a full stop and execute a hovering autorotation from approximately 3-5 ft. of altitude. If you have a road, you can start your flare at 30 ft. and do a running landing with effective translational lift airspeeds.
The R22 has approximately five seconds of stored energy in the rotor system before the blades stall in the flare. The idea with the flare is to stop forward airspeed and decrease the rate of descent at approximately 30-40 ft., and allow the aircraft to descend to a 3-5 ft. hover altitude.
From 3-5 ft., lift collective fully to cushion the landing.
Let's assume that you are hovering at approximately 700 ft AGL, above the only safe landing zone there is. If the engine stops, you enter the autorotation, then move your cyclic backwards to get some distance from your landing area. Lower the collective to prevent rpm loss. Autorotating backwards feels somewhat uncomfortable, but it is okay. You are descending with the nose slightly up and tail down. After a short while, when you have enough distance from the landing zone, push the cyclic forward to regain airspeed, using the same technique with cyclic and collective as for zero speed autorotation. Depending on terrain, flare the ship either to a full stop autorotation or a running landing touch down.
There is also another autorotation technique that you might want to know, called the "Quick stop entry auto". Let's say that you are flying at low altitude 400 ft AGL, with 75 kt. airspeed, in a tailwind. Suddenly the engine quits. Enter the autorotation by lowering collective full down, at the same time doing a smooth quick-stop. As the airspeed decreases slightly, perform a 180 deg. turn into the wind with right cyclic, keep the trim and lower the collective to prevent a low rpm situation. Next increase your airspeed with forward cyclic. You must lower the nose quite abrupt in order for you to get it back. It's all about timing. Always keep your rpm and airspeed. Never allow the rpm and airspeed to decrease. Don't go out and practice these kinds of autorotationss on your own. Do it with a high time advanced instructor pilot.