| An "S-turn across a road" is a practice maneuver
in which the airplane's ground track describes semicircles of equal radii
on each side of a selected straight line on the ground (Fig. 11-5). The
straight line may be a road, fence, railroad, or section line which lies
perpendicular to the wind, and should be of sufficient length for making
a series of turns. A constant altitude should be maintained throughout
the maneuver. The altitude should be low enough to easily recognize drift
but in no case lower than 500 feet above the highest obstruction.
The objectives are to develop the ability to compensate for drift during turns, orient the flightpath with ground references, and divide the pilot's attention. The maneuver consists of crossing the road at a 90 degree angle and immediately beginning a series of 180 degree turns of uniform radius in opposite directions, recrossing the road at a 90 degree angle just as each 180 degree turn is completed.
Since turns to effect a constant radius ground track require a changing roll rate and angle of bank to establish the crab needed to compensate for the wind, both will increase or decrease as 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 airplane turns from an upwind heading to a downwind heading.
In this maneuver the airplane should be rolled from one bank directly into the opposite just as the reference line on the ground is crossed.
Before starting the maneuver, a straight ground reference line or road that lies 90 degrees 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 no less than 500 feet AGL on a downwind heading. When directly over the road, the first turn should be started immediately. With the airplane headed downwind, the groundspeed is greatest and the rate of departure from the road will be rapid; so the roll into the steep bank must be fairly rapid to attain the proper crab angle. This prevents the airplane from flying too far from the road and from establishing a ground track of excessive radius.
During the latter portion of the first 90 degrees of turn when the airplane'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. The crab angle will be at the maximum when the airplane is headed directly crosswind.
After turning 90 degrees, the airplane's heading becomes more and more an upwind heading, the groundspeed will decrease, and the rate of closure with the road will become slower. If a constant steep bank were maintained, the airplane would turn too quickly for the slower rate of closure, and would be headed perpendicular to the road prematurely. Because of the decreasing groundspeed and rate of closure while approaching the upwind heading, it will be necessary to gradually shallow the bank during the remaining 90 degrees of the semicircle, so that the crab angle is removed completely and the wings become level as the 180 degree turn is completed at the moment the road is reached.
At the instant the road is being crossed again, a turn in the opposite direction should be started. Since the airplane is still flying into the headwind, the groundspeed is relatively slow. Therefore, the turn will have to be started with a shallow bank so as to avoid an excessive rate of turn which would establish the maximum crab angle too soon. The degree of bank should be that which is necessary to attain the proper crab so the ground track describes an arc the same size as the one established on the downwind side.
Since the airplane is turning from an upwind to a downwind heading, the groundspeed will increase and after turning 90 degrees, the rate of closure with the road will increase rapidly. Consequently, the angle of bank and rate of turn must be progressively increased so that the airplane will have turned 180 degrees at the time it reaches the road. Again, the rollout must be timed so the airplane is in straight and level flight directly over and perpendicular to the road.
Throughout the maneuver a constant altitude should be maintained, and the bank should be changing constantly to effect a true semicircular ground track.
Often there is a tendency to increase the bank too rapidly during the initial part of the turn on the upwind side, which will prevent the completion of the 180 degree turn before recrossing the road. This is apparent when the turn is not completed in time for the airplane to cross the road at a perpendicular angle. To avoid this error, the pilot must visualize the desired half circle ground track, and increase the bank during the early part of this turn. During the latter part of the turn, when approaching the road, the pilot must judge the closure rate properly and increase the bank accordingly, so as to cross the road perpendicular to it just as the rollout is completed.