S-Turns Across A Road

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.
The straight line may
be a road, fence, railroad, or section line that 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.

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 flight
along a reference line before the student attempt 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 flightpath with ground references, follow an
assigned ground track, arrive at specified 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.

To accomplish a constant radius ground track requires
a changing roll rate and angle of bank to establish the
wind correction angle. 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° 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
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 wind correction 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° 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 wind correction angle will be at the
maximum when the airplane is headed directly crosswind.

After turning 90°, 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° of the semicircle, so that the wind correction
angle is removed completely and the wings become
level as the 180° 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 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 airplane is turning from an upwind to a
downwind heading, the groundspeed will increase
and after turning 90°, 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° 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.

Throu

ghout 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° turn before re-crossing 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.

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

• 
  Failure to adequately clear the area.
  
• 
  Poor coordination.
  
• 
  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.