An OGE (out of ground effect hover) hover takes the most power of any maneuver to maintain altitude. The biggest concern is getting into settling with power. There are three requirements for getting into settling with power- 1. airspeed less than 10 knots 2. 20% power or more going to the rotor. 3. greater than 300 ft per minute descent rate. If any of these are lacking, you won't get into settling with power. So how does settling with power occur, what are the events that lead up to it, well lets talk about that!!
Any wing producing lift has a vortex produced on the tip, usually the vortex is small and has a negligable effect on lift , as the angle of attack on that wing increases the size of the vortex increases. So let's say you are in your helicopter in level flight at 1000 ft. and 35 knots and it takes 20 inches of manifold pressure to maintain level flight. So you decide to slow down to a hover. As you slow down the efficiency of the rotor disc decreases, requiring you to pull more collective to maintain altitude, this increases the angle of attack and the vortices on the end of the blades gets bigger, but as long as a descent is not allowed to develop the vortex still has a relatively small effect on lift production. As long as you have power available to maintain blade rpm as you pull additional collective, AND YOU DON'T GET INTO A DESCENT, then life is good. But let's look at what happens when you allow a descent to develop at the same time.
As the aircraft descends this increases the angle of attack. (Why? Well instead of the airflow coming pretty much straight at the leading edge of the blade because of the rotation of the blade, the descent changes the relative wind as it descends and angle of attack increases, the higher the rate of descent the greater change in relative wind and the greater the increase in angle of attack) As the angle of attack increases the size of the vortex on the tip of the blade increases, the vortex on the tip of the blade gets bigger and decreases the lift. As you are now producing less lift, the descent rate increases, this further increases your angle of attack, the vortex gets bigger further reducing lift. If this is allowed to continue the more the descent increases the greater the increase in the size of the vortex, the less lift produced the greater the descent and so on. This effect becomes critical at a descent rate of 300 ft/min or so, and the sink rate increases exponentially. Descent rates of several thousand feet per minute can develop.
Remember the tip of the rotor blade is exponentially more important than the root of the blade. (as speed doubles lift increases by 4-fold) the roational speed of the blade increases as we move from the root to the tip, so those "lift killing vortices" are decreasing lift on the section that is the most critical, specifically the tips.
So what are some situations leading to the development of settling with power? Commonly, as pilots will slow to an OGE hover if they don't increase collective enough to prevent a descent from developing they can get into it. Just like coming to a hover in ground effect as you slow to below ETL the amount of power increases signigicantly, if you are lazy on getting the collective up, you can inadvertantly get into a descent which leads to settling with power. IF YOU ARE SLOWING TO AN OGE HOVER PAY ATTENTION TO THE VSI, AND DON'T GET INTO A DESCENT, IF ANYTHING YOU WANT TO MAINTAIN A SLIGHT CLIMB, SAY 100FT/MIN. AS YOU SLOW TO A HOVER, DON'T GET LAZY ON THE COLLECTIVE.
Another situation occurs when the winds are relatively light but you get a wind shift from the time you take off to the time you land, and you don't recognize it. This happens most often in light winds of say 5-7 knots. When the winds are blowing 25 knots it's easy to recognize what direction the winds are from!!! So here's the scenario, the winds have swapped 180 degrees and you come in to land and don't realize the swap has occurred. On approach you find that you are tending to overshoot the landing so you lower the collective to increase your descent, but that alone isn't quite enough so you raise the nose to slow down. Your speed across the ground is still about 20 knots, but you haven't been watching the airspeed indicator, so you failed to notice that your airspeed is now about 5 knots (by the way they don't read well below 20 knots) and your lowering the collective earlier has increased your descent rate to 500 ft/min.- CONGRATULATIONS, YOUR GOING TO MAKE THE EVENING NEWS. You just got into settling with power at low altitude, which is nearly unrecoverable. By the time you recognize it you ran out of altitude and luck at the same time.