Since the glider is in a skidding turn during the crossed- control condition, the wing on the outside of the turn increases  speed and produces more lift than the inside wing, and the  glider starts to increase its bank. The down aileron on the  inside of the turn helps drag that wing back, slowing it and  decreasing its lift. This further causes the glider to roll. The  roll may be so fast that it is possible the bank will be vertical  or past vertical before it can be stopped. Minimum sink airspeed is defined as the airspeed at which  the glider loses the least altitude in a given period of time.  Minimum sink airspeed varies with the weight of the glider.  Glider manufacturers publish altitude loss in feet per minute or  meters per second (e.g., 122 ft/min or 0.62 m/sec) at a specified  weight. Flying at minimum sink airspeed results in maximum  duration in the absence of convection in the atmosphere. For the demonstration of the maneuver, it is important that it  be entered at a safe altitude because of the possible extreme  nose-down attitude and loss of altitude that may result. Before  demonstrating this stall, the pilot should clear the area for  other air traffic. While the gliding attitude and airspeed are  being established, the glider should be retrimmed. When the  glide is stabilized, the glider should be rolled into a medium  banked turn to simulate a final approach turn that would  overshoot the centerline of the runway. During the turn,  excessive rudder pressure should be applied in the direction  of the turn but the bank held constant by applying opposite  aileron pressure. At the same time, increased back-elevator  pressure is required to keep the nose from lowering. The minimum sink airspeed given in the GFM/POH is based  on the following conditions. All of these control pressures should be increased until the  glider stalls. When the stall occurs, releasing the control  pressures and simultaneously decreasing the AOA initiates  the recovery. In a crossed-control stall, the glider often stalls  with little warning. The nose may pitch down, the inside  wing may suddenly drop, and the glider may continue to  roll to an inverted position. This is usually the beginning of  a spin. It is obvious that close to the ground is no place to  allow this to happen. While flying in a thermaling turn, the proper airspeed is the  minimum sink airspeed appropriate to the load factor, or  G-load, that the glider is undergoing. The glider’s stall speed  increases with load factor. The minimum sink speed needs to  be increased with an increase in load factor. Another factor  that always needs to be considered is that the gross weight and  stall speed of the glider can vary if equipped with water ballast,  which affects the minimum sink airspeed. For most gliders,  the following weights will be of greatest interest: Recovery must be made before the glider enters an abnormal  attitude (vertical spiral or spin); it is a simple matter to  return to wings-level, straight flight by coordinated use of  the controls. The pilot must be able to recognize when this  stall is imminent and must take immediate action to prevent  a completely stalled condition. It is imperative that this type  of stall not occur during an actual approach to a landing,  since recovery may be impossible prior to ground contact  due to the low altitude. The effect of only weight on stall speed can be expressed by  using the basic lift formula that is manipulated and simplified to  derive the necessary information. The modified lift formula is: Common errors during advanced stalls include: V$_{S}$ 2 = √W2 V$_{S}$ 1 W1 V$_{S}$ 1 = stall speed corresponding to some gross weight, W1 V$_{S}$ 2 = stall speed corresponding to a different gross weight, W2 •  Improper pitch and bank control during straight-ahead  and turning stalls. •  Rough or uncoordinated control procedures. •  Failure to recognize the first indications of a stall. •  Failure to achieve a stall. •  Poor recognition and recovery procedures. 1.  Maximum gross weight with full water and both seats  occupied 2.  Maximum gross weight with both seats occupied and  no water ballast 3.  One pilot on board 7-36 •  The glider is wings level and flying a straight  flightpath; load factor is 1.0 G. •  The glider flight controls are perfectly coordinated. •  Wing flaps are set to zero degrees and air brakes are  closed and locked. •  The wings are free of bugs or other contaminants. •  The glider is at a manufacturer-specified weight. Operating Airspeeds Minimum Sink Airspeed •  Excessive altitude loss or airspeed or encountering a  secondary stall during recovery.