Vortex Ring State  Common Errors 11-9 Helicopters with low inertia rotor disks settle immediately.  Keep a level attitude and ensure a vertical descent with cyclic  control while maintaining heading with the pedals. Any lateral  movement must be avoided to prevent dynamic rollover. As  rotor rpm decays, cyclic response decreases, so compensation  for the winds will require more cyclic input. At approximately  1 foot AGL, apply upward collective control, as necessary,  to slow the descent and cushion the landing without arresting  the rate of descent above the surface. Usually, the full amount  of collective is required just as the landing gear touches the  surface. As upward collective control is applied, the throttle  must be held in the idle detent position to prevent the engine  from re-engaging. The idle detention position is a ridged stop  position between idle and off in which the idle release button  snaps into, prevent accidental throttle off. Vortex ring state (formerly referenced as settling-with- power) describes an aerodynamic condition in which a  helicopter may be in a vertical descent with 20 percent up to  maximum power applied, and little or no climb performance.  The previously used term settling-with-power came from the  fact that the helicopter keeps settling even though full engine  power is applied. Helicopters with high-inertia rotor disks settle more slowly  after the throttle is closed. In this case, when the helicopter has  settled to approximately 1 foot AGL, apply upward collective  control while holding the throttle in the idle detent position  to slow the descent and cushion the landing. The timing of  collective control application and the rate at which it is applied  depend upon the particular helicopter being used, its gross  weight, and the existing atmospheric conditions. Cyclic control  is used to maintain a level attitude and to ensure a vertical  descent. Maintain heading with antitorque pedals. In a normal out-of-ground-effect (OGE) hover, the helicopter  is able to remain stationary by propelling a large mass of air  down through the main rotor. Some of the air is recirculated  near the tips of the blades, curling up from the bottom of  the rotor disk and rejoining the air entering the rotor from  the top. This phenomenon is common to all airfoils and is  known as tip vortices. Tip vortices generate drag and degrade  airfoil efficiency. As long as the tip vortices are small, their  only effect is a small loss in rotor efficiency. However, when  the helicopter begins to descend vertically, it settles into its  own downwash, which greatly enlarges the tip vortices. In  this vortex ring state, most of the power developed by the  engine is wasted in circulating the air in a doughnut pattern  around the rotor. When the weight of the helicopter is entirely resting on  the landing gear, cease application of upward collective.  When the helicopter has come to a complete stop, lower the  collective pitch to the full-down position. The timing of the collective movement is a very important  consideration. If it is applied too soon, the remaining rpm may  not be sufficient to make a soft landing. On the other hand,  if it is applied too late, surface contact may be made before  sufficient blade pitch is available to cushion the landing.  The collective must not be used to hold the helicopter off  the surface, causing a blade stall. Low rotor rpm and ensuing  blade stall can result in a total loss of rotor lift, allowing the  helicopter to fall to the surface and possibly resulting in blade  strikes to the tail boom and other airframe damage such as  landing gear damage, transmission mount deformation, and  fuselage cracking.  In addition, the helicopter may descend at a rate that exceeds  the normal downward induced-flow rate of the inner blade  sections. As a result, the airflow of the inner blade sections is  upward relative to the disk. This produces a secondary vortex  ring in addition to the normal tip vortices. The secondary  vortex ring is generated about the point on the blade where the  airflow changes from up to down. The result is an unsteady  turbulent flow over a large area of the disk. Rotor efficiency  is lost even though power is still being supplied from the  engine.  [Figure 11-3] A fully developed vortex ring state is characterized by an  unstable condition in which the helicopter experiences  uncommanded pitch and roll oscillations, has little or no  collective authority, and achieves a descent rate that may  approach 6,000 feet per minute (fpm) if allowed to develop. A vortex ring state may be entered during any maneuver  that places the main rotor in a condition of descending in a  column of disturbed air and low forward airspeed. Airspeeds  1.  Failure to use sufficient proper antitorque pedal when  power is reduced. 2.  Failure to stop all sideward or backward movement  prior to touchdown. 3.  Failure to apply up-collective pitch properly, resulting  in a hard touchdown. 4.  Failure to touch down in a level attitude.  5.  Failure to roll the throttle completely to idle. 6.  Failure to hover at a safe altitude for the helicopter  type, atmospheric conditions, and the level of training/ proficiency of the pilot. 7. Failure to go around if not within limits and specified  criteria for safe autorotation.