One of the more confusing subjects for helicopter students to fully understand is known as the vortex ring state, also (correctly or incorrectly – depends on who you ask) referred to as settling with power or power settling. Much of the confusion comes from the terminology as these terms are used interchangeably in many textbooks. Yet, there are instructors who teach that settling with power is very different from the vortex ring state. This is true when settling with power is defined as simply not having sufficient power to hover, thus causing the helicopter to descend or settle when power required exceeds power available. However, many textbooks do not use that definition – hence the confusion.
Regardless of what labels you use, the important point to know is that there are two very different situations that can affect a helicopter in an out of ground effect hover or a steep approach. One is simply running out of power as described above, the other is the vortex ring state, which is an aerodynamic condition that forms when a helicopter is allowed to descend into its own downwash. It can happen even when a helicopter has more power available than needed.
To understand the vortex ring state imagine a helicopter hovering at 1,000 feet. The rotor system is drawing air from above and accelerating a large amount of air downward in a column underneath the helicopter. If the pilot allows the helicopter to descend vertically at too high a rate of descent into the column of downward moving air, the air that is now above the rotor system will still be moving downward. As the rotor system tries to draw air that is moving downward a re-circulation of air forms, causing the rotor tip vortices to become much bigger. (Normal rotor tip vortices are small and only cause a small loss of rotor efficiency.) Additionally, a secondary vortex ring will form near roots of the rotor blades.
In a well-developed vortex ring state, most of the engine power is consumed by accelerating air in a circular pattern around the rotor system. The vortex ring state causes turbulent rotational flow across the blades with increasing roughness and possible loss of control. The helicopter continues to descend and a natural reaction is to increase power by raising the collective control. This merely increases the strength of the vortex ring and the helicopter will settle even faster.
To recover from this situation, the pilot needs to remove the helicopter from the column of air. This can be done in any direction, however, it is best to accelerate forward and reduce the collective pitch slightly. This will normally result is a minimum altitude loss. In the early stages of development, a large application of power (if available) might be sufficient to overcome the upward moving air and initiate a recovery. Theoretically, entering autorotation would change the airflow and result in a recovery, although it would also produce a large loss of altitude.
The best plan is to understand what type of conditions can cause the vortex ring state and avoid them. The three basic conditions are: a vertical descent rate greater than 300 fpm (the actual descent rate required may be higher depending on density altitude and aircraft weight); the rotor system must be consuming 20 to 100 percent of available power; and airspeed less than about 15 knots.
Some of the maneuvers that are susceptible to encountering the vortex ring state are steep approaches (especially downwind) or hovering out of ground effect at high density altitude. To stay out of trouble, keep approach angles shallower than 30 degrees, and when performing steeper approaches keep your rate of descent no more than 300 fpm and don’t let airspeed get too slow.