CHAPTER 9. Ground Reference Maneuvers

S-Turns Across a Road

An S-turn across a road is a practice maneuver in which the aircraft’s ground track describes semicircles of equal radii on each side of a selected straight line on the ground. Reference Figure 9-6 throughout this S-turn across the road section. The straight line may be a road, fence, railroad, or section line that lies perpendicular to the wind and should be of suffi cient length for making a series of turns. A constant altitude should be maintained throughout the maneuver.

S-turns across a road present one of the most elementary problems in the practical application of the turn and in the correction for wind drift in turns. While the application of this maneuver is considerably less advanced in some respects than the rectangular course, it is taught after the student has been introduced to that maneuver in order that the student may have a knowledge of the correction for wind drift in straight fl ight along a reference line before the student attempts to correct for drift by playing a turn.

The objectives of S-turns across a road are to develop the ability to compensate for drift during turns, orient the fl ightpath with ground references, follow an assigned ground track, arrive at specifi ed points on assigned headings, and divide the pilot’s attention. The maneuver consists of crossing the road at a 90° angle and immediately beginning a series of 180° turns of uniform radius in opposite directions, re-crossing the road at a 90° angle just as each 180° turn is completed. The maneuver can be started with either a left hand turn or a right hand turn to go in either direction. Figure 9-6 starts the turn in a left hand turn as an example.

Accomplishing a constant radius ground track requires a changing roll rate and angle of bank to establish the wind correction angle. Both increase or decrease as the groundspeed increases or decreases.

The bank must be steepest when beginning the turn on the downwind side of the road and must be shallowed gradually as the turn progresses from a downwind heading to an upwind heading. On the upwind side, the turn should be started with a relatively shallow bank and then gradually steepened as the aircraft turns from an upwind heading to a downwind heading. In this maneuver, the aircraft should be rolled from one bank directly into the opposite just as the 90° reference line on the ground is crossed.

Before starting the maneuver, a straight ground reference line or road that lies 90° to the direction of the wind should be selected, then the area checked to ensure that no obstructions or other aircraft are in the immediate vicinity.

The road should be approached from the upwind side at the selected altitude on a downwind heading. When directly over the road, the fi rst turn should be started immediately. [Figure 9-6, position 1 and Figure 9-7] With the aircraft headed downwind, the groundspeed is greatest and the rate of departure from the road is rapid; the roll into the steep bank must be fairly rapid to attain the proper wind correction angle. [Figure 9-6, position 2] This prevents the aircraft from fl ying too far from the road and from establishing a ground track of excessive radius.

During the latter portion of the fi rst 90° turn, when the aircraft’s heading is changing from a downwind heading to a crosswind heading, the groundspeed becomes less and the rate of departure from the road decreases.

[Figure 9-6, position 2 to 3, and Figure 9-8] The wind correction angle is at the maximum when the aircraft is headed directly crosswind. [Figure 9-6, position 3]

After turning 90°, the aircraft’s heading becomes more and more an upwind heading, the groundspeed decreases, and the rate of closure with the road becomes slower. If a constant steep bank were maintained, the aircraft would turn too quickly for the slower rate of closure and would prematurely be headed perpendicular to the road. Because of the decreasing groundspeed and rate of closure while approaching the upwind heading, it is necessary to gradually shallow the bank during the remaining 90° of the semicircle, so that the wind correction angle is removed completely [Figure 9-9] and the wings become level as the 180° turn is completed at the moment the road is reached. [Figure 9-6, position 4]

At the instant the road is being crossed at 90° to it, a turn in the opposite direction should be started. Since the aircraft is still fl ying into the headwind, the groundspeed is relatively low. Therefore, the turn must be started with a shallow bank to avoid an excessive rate of turn that would establish the maximum wind correction angle too soon. The degree of bank should be that which is necessary to attain the proper wind correction angle so the ground track describes an arc the same size as the one established on the downwind side.

Since the aircraft is turning from an upwind to a downwind heading, the groundspeed increases and after turning 90° the rate of closure with the road increases rapidly. [Figure 9-6, position 5] Consequently, the angle of bank and rate of turn must be progressively increased so that the aircraft has turned 180° at the time it reaches the road. Again, the rollout must be timed so the aircraft is in straightand- level fl ight directly over and perpendicular to the road. [Figure 9-6, position 6]

Throughout the maneuver a constant altitude and airspeed should be maintained, and the bank should be changing constantly to effect a true semicircular ground track.

Common errors in the performance of S-turns across a road are:

  • Failure to adequately clear the area.
  • Creating too small of a radius/too high of a banked turn during the start of the maneuver.
  • Creating banked turns too high to complete the maneuver.
  • Poor coordination creating variations in airspeeds.
  • Gaining or losing altitude.
  • Inability to visualize the half circle ground track.
  • Poor timing in beginning and recovering from turns.
  • Faulty correction for drift.
  • Inadequate visual lookout for other aircraft.
  • Inability to judge closure rates to the road and adjust the bank angle so the semi-circle is completed at 90° to the reference road.
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