Drift and Ground Track Control

Wind direction and velocity variations are the primary effects requiring corrections of the flightpath during ground reference maneuvers. Unlike an automobile, but similar to a boat or ship, wind directly influences the path that the airplane travels in reference to the ground. Whenever the airplane is in flight, the movement of the air directly affects the actual ground track of the airplane.

For example, an airplane is traveling at 90 knots (90 nautical miles per hour) and the wind is blowing from right to left at 10 knots. The airplane continues forward at 90 knots but also travels left 10 nautical miles for every hour of flight time. If the airplane, in this example doubles its speed to 180 knots, it still drifts laterally to the left 10 nautical miles every hour. The airplane travels within an often moving body of air, so traveling to a point on the surface requires compensation for the movement of the air mass.

Ground reference maneuvers are generally flown at altitudes between 600 and 1,000 feet above ground level (AGL). The pilot must consider the following when selecting the maneuvering altitude:
  • The lower the maneuvering altitude, the faster the airplane appears to travel in relation to the ground.
  • Drift should be easily recognizable from both sides of the airplane.
  • The altitude should provide obstruction clearance of no less than 500 feet vertically above the obstruction and 2,000 feet horizontally.
  • In case of an engine failure, the pilot must plan, consider, and be alert for forced landing areas while understanding that the lower the airplane’s altitude, the less time there is to configure the airplane for an emergency landing and the shorter the glide distance.
  • Any specific altitude required by test standards.

Correcting Drift During Straight-and-Level Flight

When flying straight and level and following a selected straight-line direct ground track, the preferred method of correcting for wind drift is to angle the airplane sufficiently into the wind to cancel the effect of the sideways drift caused by the wind. The wind’s speed, the angle between the wind direction and the airplane’s longitudinal axis, and the airspeed of the airplane determines the required wind correction angle. For example, an airplane with an airspeed of 100 knots, a 20 knot wind at 90° to the airplane’s longitudinal axis, and a 12° angle into the wind is required to cancel the airplane’s drift. If the wind in the above example is only 10 knots, the wind correction angle required to cancel the drift is six degrees. When the drift has been neutralized by heading the airplane into the wind, the airplane will fly the direct straight ground track.

To further illustrate this point, if a boat is crossing a river and the river’s current is completely still, the boat could head directly to a point on the opposite shore on a straight course to that opposite point without any drift; however, rivers tend to have a downstream current that must be considered if the captain wants the boat to arrive at the opposite shore using a direct straight path. Any downstream current pushes the boat sideways and downstream at the speed of the current. To counteract this downstream movement, the boat must move upstream at the same speed as the river is moving the boat downstream. This is accomplished by angling the boat upstream sufficiently to counteract the downstream flow. If this is done, the boat follows a direct straight track across the river to the intended destination point. The amount of angle required is dependent on the forward speed of the boat and the speed of the current. The slower the forward speed of the boat and/or the faster speed of the current, the greater the angle must be to counteract the drift. The converse is also true. [Figure 1]

Airplane flying, Ground Reference Maneuvers
Figure 1. Wind drift

As soon as an airplane lifts off the surface and levels the wings, if there is any crosswind, the airplane will begin tracking sideways with the wind. Any wind not directly on the nose or tail of the airplane will drift the airplane sideways at a speed up to the speed of the wind. A wind that is directly to the right or the left (at a 90° angle) drifts the airplane sideways at the speed of the wind; when the wind is halfway between the side and the nose of the airplane (at a 45° angle), it drifts the airplane sideways at just over 70 percent of the speed of the wind. It should be understood that pilots do not calculate the required drift correction angles for ground reference maneuvers; they merely use the references and adjust the airplane’s relationship to those references to cancel any drift. The groundspeed of the airplane is also affected by the wind. As the wind direction becomes parallel to the airplane’s longitudinal axis, the magnitude of the wind’s effect on the groundspeed is greater; as the wind becomes perpendicular to the longitudinal axis, the magnitude of the wind’s effect on the groundspeed is less. In general, When the wind is blowing straight into the nose of the airplane, the groundspeed will be less than the airspeed. When the wind is blowing from directly behind the airplane, the groundspeed will be faster than the airspeed. In other words, when the airplane is headed upwind, the groundspeed is decreased; when headed downwind, the groundspeed is increased.

Constant Radius During Turning Flight

In a no-wind condition, the pilot can perform a ground-based constant radius turn by accurately maintaining a constant bank angle throughout the turn; however, with any wind the complexities of maintaining a ground-based constant radius turn increase. When wind is present, during ground reference maneuvers involving turns, the pilot must correct for wind drift. [Figure 2] Throughout the turn, the wind is acting on the airplane from a constantly changing angle—increasing or decreasing the groundspeed in a manner similar to straight flight. To follow a circular, constant radius ground track, the bank angle must vary to compensate for wind drift throughout the turn. The airplane’s ground-based turn radius is affected by the airplane’s groundspeed: the faster the groundspeed, the steeper the airplane must be banked to maintain a ground-based constant radius turn. The converse is also true: the slower the groundspeed, the shallower the airplane needs to be banked to maintain a ground-based constant radius turn.

Airplane flying, Ground Reference Maneuvers
Figure 2. Effect of wind during a turn

For a given true airspeed, the radius of turn in the air varies proportionally with the bank angle. To maintain the constant radius over the ground, the bank angle is proportional to ground speed. For example, an airplane is in the downwind position at 100 knots groundspeed. In this example, the wind is 10 knots, meaning that the airplane is at an airspeed of 90 knots (for this discussion, we ignore true, calibrated, and indicate airspeed and assume that they are all the same). If the pilot starts a downwind turn with a 45° “steepest” bank angle, the turn radius is approximately 890 feet. Let’s assume the airplane is now upwind with a groundspeed of 80 knots. In order to maintain the 890-foot radius, the pilot must reduce the bank angle to a shallowest bank of approximately 33°. In another example, if the downwind is flown at an airspeed of 90 knots in a 10 knot tailwind with a desired turn radius of 2,000 feet, the “steepest” bank angle needs to be at approximately 24° and the upwind “shallowest” bank angle at approximately 16°.

To demonstrate the effect that wind has on turns, the pilot should select a straight-line ground reference, such as a road or railroad track. [Figure 3] Choosing a straight-line ground reference that is parallel to the wind, the airplane would be flown into the wind and directly over the selected straight-line ground reference. Once a straight-line ground reference is established, the pilot makes a 360° constant medium banked turn. As the airplane completes the 360° turn, it should return directly over the straight-line ground reference but downwind from the starting point. Choosing a straight-line ground reference that has a crosswind, and using the same 360° constant medium-banked turn, demonstrates how the airplane drifts away from the reference even as the pilot holds a constant bank angle. In both examples, the path over the ground is an elongated circle, although in reference to the air, the airplane flew a perfect continuous radius.
 
Airplane flying, Ground Reference Maneuvers
Figure 3. Effect of wind during turn

In order to compensate for the elongated, somewhat circular path over the ground, the pilot must adjust the bank angle as the groundspeed changes throughout the turn. Where groundspeed is the fastest, such as when the airplane is headed downwind, the turn bank angle must be steepest; where groundspeed is the slowest, such as when the airplane is headed upwind, the turn bank angle must be shallow. It is necessary to increase or decrease the angle of bank, which increases or decreases the rate of turn, to achieve the desired constant radius track over the ground.

Ground reference maneuvers should always be entered from a downwind position. This allows the pilot to establish the steepest bank angle required to maintain a constant radius ground track. If the bank is too steep, the pilot should immediately exit the maneuver and re-establish a lateral position that is further from the ground reference. The pilot should avoid bank angles in excess of 45°due to the increased stalling speed.

Tracking Over and Parallel to a Straight Line

The pilot should first be introduced to ground reference maneuvers by correcting for the effects of a crosswind over a straight-line ground reference, such as road or railroad tracks. If a straight road or railroad track is unavailable, the pilot will choose multiple references (three minimum) which, when an imaginary visual reference line is extended, represents a straight line. The reference should be suitably long so the pilot has sufficient time to understand the concepts of wind correction and practice the maneuver. Initially, the maneuver should be flown directly over the ground reference with the pilot angling the airplane’s longitudinal axis into the wind sufficiently such as to cancel the effect of drift. The pilot should scan between far ahead and close to the airplane to practice tracking multiple references.

When proficiency has been demonstrated by flying directly over the ground reference line, the pilot should then practice flying a straight parallel path that is offset from the ground reference. The offset parallel path should not be more than three-fourths of a mile from the reference line. The maneuver should be flown offset from the ground references with the pilot angling the airplane’s longitudinal axis into the wind sufficiently to cancel the effect of drift while maintaining a parallel track.

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