the upwind side of the reference line (side from which the  wind is blowing), crab the nose of the helicopter toward the  outside of the circle. During the turn on the downwind side  of the reference line (side of the reference line opposite to the  direction from which the wind is blowing), crab the nose of  the helicopter toward the inside of the circle. In either case, it  is obvious that the helicopter is being crabbed into the wind  just as it is when trying to maintain a straight ground track.  The amount of crab depends on the wind velocity and how  close the helicopter is to a crosswind position. The stronger  the wind is, the greater the crab angle is at any given position  for a turn of a given radius. The more nearly the helicopter  is to a crosswind position, the greater the crab angle. The  maximum crab angle should be at the point of each half circle  farthest from the reference line. The pilot can choose to use a road, a fence, or a railroad  for a reference line. Regardless of what is used, it should  be straight for a considerable distance and should extend as  nearly perpendicular to the wind as possible. The object of  S-turns is to fly a pattern of two half circles of equal size  on opposite sides of the reference line.  [Figure 9-16]  The  maneuver should be performed at a constant altitude between  500 and 800 feet above the terrain. As mentioned previously,  if the student pilot is having a difficult time maintaining the  proper altitude and airspeed, have him or her attempt the  S-turn at a lower altitude, providing better ground reference.  The discussion that follows is based on choosing a reference  line perpendicular to the wind and starting the maneuver with  the helicopter facing downwind.  A standard radius for S-turns cannot be specified, since the  radius depends on the airspeed of the helicopter, the velocity  of the wind, and the initial bank chosen for entry. The only  standard is crossing the ground reference line straight and  level and having equal radius semi-circles on both sides. As the helicopter crosses the reference line, immedi- ately establish a bank. This initial bank is the steepest  used throughout the maneuver since the helicopter is  headed directly downwind and the groundspeed is greatest  (position 1). Gradually reduce the bank, as necessary, to  describe a ground track of a half circle. Time the turn so  that, as the rollout is completed, the helicopter is crossing  the reference line perpendicular to it and heading directly  upwind (position 2). Immediately enter a bank in the opposite  direction to begin the second half of the “S” (position 3).  Since the helicopter is now on an upwind heading, this bank  (and the one just completed before crossing the reference  line) is the shallowest in the maneuver. Gradually increase  the bank, as necessary, to describe a ground track that is a  half circle identical in size to the one previously completed on  the other side of the reference line (position 4). The steepest  bank in this turn should be attained just prior to rollout when  the helicopter is approaching the reference line nearest the  downwind heading. Time the turn so that as the rollout is  complete, the helicopter is perpendicular to the reference  line and is again heading directly downwind (position 5). This training maneuver requires flying constant radius  turns around a preselected point on the ground using a bank  angle of approximately 30°–45°, while maintaining both  a constant altitude and the same distance from the point  throughout the maneuver.  [Figure 9-17]  The objective, as in  other ground reference maneuvers, is to develop the ability  to subconsciously control the helicopter while dividing  attention between flightpath, how the winds are affecting  the turn and ground references and watching for other air  traffic in the vicinity. This is also used in high reconnaissance,  observation, and photography flight. In summary, the angle of bank required at any given  point in the maneuver is dependent on the groundspeed.  The faster the groundspeed is, the steeper the bank is; the  slower the groundspeed is, the shallower the bank is. To  express it another way, the more nearly the helicopter is to a  downwind heading, the steeper the bank; the more nearly it  is to an upwind heading, the shallower the bank. In addition  to varying the angle of bank to correct for drift in order to  maintain the proper radius of turn, the helicopter must also  be flown with a drift correction angle (crab) in relation to its  ground track; except, of course, when it is on direct upwind  or downwind headings or there is no wind. The factors and principles of drift correction that are involved  in S-turns are also applicable to this maneuver. As in other  ground track maneuvers, a constant radius around a point  requires the pilot to change the angle of bank constantly  and make numerous control changes to compensate for  One would normally think of the fore and aft axis of the  helicopter as being tangent to the ground track pattern at  each point. However, this is not the case. During the turn on  9-16 Technique Turns Around a Point Common Errors 1.  Using antitorque pedal pressures to assist turns. 2.  Slipping or skidding in the turn. 3.  An unsymmetrical ground track during S-turns across  a road.  4.  Improper correction for wind drift. 5.  Failure to maintain selected altitude or airspeed.  6.  Excessive bank angles.