The cause of the crash of the U.S. military helicopter during the assault on Osama bin Laden’s compound in Pakistan was reportedly a unique helicopter aerodynamic phenomenon that pilots call “settling with power.” The more the pilot pulled up on the collective handle in his left hand asking for more lift, the more lift degraded and the helicopter continued down.
The official term for settling with power is vortex ring state, and the term pretty well describes what is happening.
The main rotor blades of helicopters are actually wings, not propeller blades. Like any wing, some of the high-pressure air under the blade escapes at the tip. The air moves rapidly upward and twists itself into a vortex. It is the wing tip vortex trailing behind an airplane that can be powerful enough to upset smaller airplanes that fly through it. The vortex is often called a wake, but it is the rotating air in the vortex that is the danger to following airplanes.
In the case of a hovering helicopter, the vortex can rotate up and over and be drawn back down into the air flowing into the main rotor disk. In other words, the vortex forms a ring and flows out from under the blade tip and back down into the rotor.
As you can imagine, the air in the ring vortex that is rapidly moving downward as it enters the main rotor disk, which dramatically degrades the lift production of the rotor. The air in the vortex ring is disturbed and jumbled, robbing the main rotor blades of lift generating efficiency. Because the air in the vortex ring is moving down more rapidly, the angle of attack of the main rotor blade is also changed.
The reason pilots call vortex ring state “settling with power” is because the more you pull up on the collective – which collectively increases the angle of all blades in the main rotor at every point on their rotation – the more lift you are requesting. Lift takes power so helicopter pilots use the term “power” as a general way to describe the need for more lift. But in the vortex ring state situation, asking for more lift only makes the vortex more powerful.
The stronger vortex created by the main rotor blades creating more lift actually is more disruptive, so the rotor actually loses additional lift. It is a vicious circle of the need for more lift – power – creating an ever stronger vortex that causes the main rotor to lose lift instead of gain it.
Recovery from settling with power is very simple: Get the helicopter moving. As soon as the helicopter begins moving forward the ring vortex is left behind just like the vortex trails the wingtip of an airplane. With the helicopter moving, clean undisturbed air is drawn into the top of the main rotor disk and lift is fully restored.
But the pilots hovering over bin Laden’s compound had nowhere to go with the buildings and high walls. It’s even likely that the walls contributed to formation of the vortex ring state by changing the normal outflow of the rotor downwash. Hovering in a confined area has many risks, and encountering vortex ring state is one of them.
Lower air density is also a contributing factor, so if it was warm in Pakistan that night the risk was higher. And the weight of the helicopter is a factor. The higher the weight, the more lift each main rotor blade must produce, and the stronger the vortex created by the rotor tip. Fixed-wing airplane pilots know this because separation between heavy airplanes must be greater because the vortex produced by the heavy airplane is always more powerful.
The pilots over bin Laden’s compound clearly did a great job of making the best of a bad situation, because nobody was killed or seriously injured as the helicopter settled under power. Helicopter pilots train for settling under power at a safe altitude. The normal training procedure is to bring the helicopter to a hover, then begin a slow descent in the hover.
With the helicopter settling you pull power on the collective and, if all conditions are right, vortex ring state will develop and the sink rate increases. Once you have established that the vertical speed is increasing as you add power, you dump the nose down to get moving forward and escape the vortex ring state. Easy at 500 feet, but close to the ground, at night, and surrounded by obstructions, all you can do is manage the sink rate the best you can and hope not to hit too hard.
The magic of helicopters is their ability to hover, and to move in any direction including backward. But the mechanisms and aerodynamics of hovering are almost unbelievably complicated. When a helicopter is moving forward there are many analogies to a fixed-wing airplane. When the helicopter is hovering, it’s a unique aircraft with both the machine and the pilot performing the most demanding task in flying that I can think of.