2 Common Mistakes

Failure to Lower Collective all the way down
If the pilot forgets to lower collective and this is a real engine failure, it's a fatal mistake. Lowering collective is the most important part of doing an autorotation. If you remember to do that, you will probably walk away from the landing. Some pilots only put the collective pitch part of the way down. They get to "know" where it belongs. The only problem with this is that the position the collective needs to go to depends on many factors such as pitch link rigging, gross weight, and density altitude. These things can change from day to day. This method also delays recovery of rotor RPM, and there is no good reason to do that. The best method is to lower collective all the way, and as RPM starts to build back up some collective should be raised to stop the RPM somewhere in the operating range.

Failure to trim with anti-torque pedals Pilots will either forget to push right pedal, or push too much, or even sometimes push the left pedal. In any case, the aircraft should be autorotated in trim, and the pilot can do this by putting in the correct amount of right pedal when the engine fails.

Allowing the nose to drop
Do not let the nose drop during the entry. Whatever attitude the helicopter is in, enter the autorotation in that attitude, and then after the autorotation is established the pilot can make any attitude adjustments required for proper airspeed. Allowing the nose to pitch down delays the recovery of RPM (it's like an anti-flare) plus it is not uncommon for pilots to overspeed the rotor by waiting until the airspeed builds to 80 knots or more, and then suddenly trying to fix it by yanking back on cyclic. The result is an almost instantaneous rotor overspeed.

Failure to control Rotor RPM with collective
Most helicopters are rigged so that at normal weights the collective will have to be raised somewhat to keep rotor RPM in the normal operating area. Common mistakes are either to leave the collective full down so long that a rotor overspeed occurs, or to overcontrol the collective, moving it up and down during the entire glide. The proper way to manipulate collective is to lower it full down during the entry to autorotation. Then, as RPM starts to increase toward the normal operating area raise enough collective to stop the RPM from changing. Wait a few seconds until it stabilizes, and make one final adjustment to place the RPM exactly where it is desired. Normally no further manipulation of the collective will be required during the glide. One exception is that during turns, especially at high speed, some collective may be required to prevent the RPM from climbing too high. Rolling out of the turn, the pilot should put the collective back to where it was before the turn was entered. By performing turns at lower airspeeds, little or no collective will be required.

Failure to maneuver to the point of intended landing
Many pilots get quite proficient at autorotating to the runway at their home airport, but have more trouble when trying to make a specific landing area in the off-airport environment. It is best to set up a (tight) traffic pattern to the landing area, just as is done at an airport. The pilot should figure out the wind, and therefore where "final" will be. Then the pilot should figure out where he currently is with respect to the traffic pattern (is he already on downwind, base, or final?). Once he knows what leg he is on, he can manipulate the length of the remaining legs to arrive on final at the proper altitude. A very short final is suggested. The longer final is, the bigger the chance is of over or undershooting, with no easy way to correct once the under or overshoot is recognized. Instead, fly a very tight base and time your turn onto short final to give you the desired distance to the touchdown spot. If you are a little low, turn final slightly early. If you are a little high, delay the turn to final, overshoot the centerline somewhat, and use up the additional altitude on base. For gross errors, S-turns or zero (or negative) airspeed may be required. One final rule I have is never do a 360 degree turn. You lose track of your approach angle for too long. Instead, if you have massive amounts of altitude to lose, perform a figure-8 pattern on final. This way the spot is always visible, and you can turn back onto final when the angle begins to look right.

Flaring at the wrong altitude
Each helicopter has a range of altitudes it needs to be flared at. The altitude will change from flight to flight based on gross weight, density altitude, wind, and airspeed. Generally, aircraft with higher disk loadings require a higher flare. If the pilot flares too high, the helicopter will stop its descent too high above the ground to make a safe landing. If the pilot flares too low, he will be forced to level the helicopter (get rid of the flare) too early (to avoid hitting the tail on the ground). The result will be a high rate of descent (which he can probably fix by raising collective) and high forward ground speed (which he can't fix, so he'll slide hundreds of feet). Assuming a perfect flare cannot be made, which way the pilot should err depends on the surface being landed on. If the surface is firm and level, some slide probably won't hurt, and it would be best to be a little bit low to give a soft touchdown, followed by a little slide. If the surface does not appear to allow a slide (swamp or such which will cause the skids to dig in) the flare should probably be a little high to insure removal of all forward speed. The touchdown may be a little harder, but, by being more vertical, the chance of rolling over is reduced. One caveat is that human beings do not take vertical accelerations well, so to avoid back injuries, the flare should not be too high.

Flaring too aggressively or not aggressively enough
The speed with which the nose of the aircraft needs to be pitched up is related to gross weight, density altitude, wind, and airspeed. Generally if gross weight is high, a more aggressive flare will be required. If density altitude is high, a more aggressive flare is required. If wind is high, a less aggressive flare is required. And if airspeed is high, a less aggressive flare is required. Pilots can adjust for minor airspeed deviations by flaring at different altitudes, or with different amounts of aggressiveness. For instance, if the airspeed is 10 knots below optimal, a more aggressive flare will help to make up for this. Of course there are limits to the amount of correction that is possible.

Failure to level the aircraft
Some aircraft land in a slightly tail low attitude, but with many others it is critical to have the landing gear level before touchdown. Failure to do so can result in tail boom strikes and porpoising (where you hit on the heels, and then roll up onto the toes and flip over forward).

Failure to maintain heading during the slide
There are a couple reasons that heading might not be maintained during any ground slide. One is just that the pilot fails to manipulate the pedals correctly, the other is that if rotor RPM gets too low the tail rotor may lose effectiveness. Failure to maintain heading can cause a skid gear to catch and roll the aircraft over on its side. Most aircraft can perform fairly high speed slides if the skids are pointed in the direction the aircraft is moving.

Moving the cyclic aft during the slide
It's human nature to want to stop the slide as early as possible, but moving the cyclic aft has two problems. One is that the main rotor is probably not generating much thrust at this point, so it won't help much anyway. The other is that flapping is at maximum because RPM is low, and moving the cyclic aft moves the rotor blades even closer to the tailboom. The rotor blades hitting the tailboom is a very real possibility.

Reference http://www.copters.com

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