|CHAPTER 9. Ground Reference Maneuvers
Maneuvering by Reference to Ground Objects
Ground track or ground reference maneuvers are performed at relatively low altitudes while applying wind drift correction as needed to follow a predetermined track or path over the ground. These maneuvers are designed to develop the ability to control the aircraft and to recognize and correct for the effect of wind, while dividing attention among other matters. This requires planning ahead of the aircraft, maintaining orientation in relation to ground objects, fl ying appropriate headings to follow a desired ground track, and being cognizant of other air traffi c in the immediate vicinity.
Ground reference maneuvers should be fl own at an altitude of
approximately 500 to 1,000 feet above ground level (AGL).
The actual altitude will depend on the ability to reach a safe
landing area if there is an engine failure during the maneuver
and the type of air in which the maneuvers are being fl own.
If there is signifi cant vertical movement of the air, higher
altitudes should be used to avoid the possibility of
Overall, the following factors should be considered in determining the appropriate altitudes for ground reference maneuvers:
During these maneuvers, both the instructor and the student should be alert for available forced-landing fi elds. The area chosen should be away from communities, livestock, or groups of people to prevent becoming an annoyance or hazard. Due to the altitudes at which these maneuvers are performed, there is little time available to search for a suitable fi eld for landing in the event the need arises.
Drift and Ground Track Control
Whenever an object is free from the ground, it is affected by the medium surrounding it. This means that a free object moves in whatever direction and speed that the medium moves.
For example, if a powerboat were crossing a still river, the boat could head directly to a point on the opposite shore and travel on a straight course to that point without drifting. However, if the river were fl owing swiftly, the water current would require consideration. That is, as the boat progresses forward on its own power, it must also move upstream at the same rate the river is moving it downstream. This is accomplished by angling the boat upstream suffi ciently to counteract the downstream fl ow. If this is done, the boat follows the desired track across the river from the departure point directly to the intended destination point. If the boat is not headed suffi ciently upstream, it would drift with the current and run aground at some point downstream on the opposite bank. [Figure 9-1]
As soon as an aircraft becomes airborne, it is free of ground friction. Its path is then affected by the air mass in which it is fl ying; therefore, the aircraft (like the boat) does not always track along the ground in the exact direction that it is headed. When fl ying with the longitudinal axis of the aircraft aligned with a road, it may be noted that the aircraft gets closer to or farther from the road without any turn having been initiated by the pilot. This would indicate that the air mass is moving sideward in relation to the aircraft. Since the aircraft is flying within this moving body of air (wind), it moves or drifts with the air in the same direction and speed, just like the boat moved with the river current.
When fl ying straight and level and following a selected ground track, the preferred method of correcting for wind drift is to head the aircraft (wind correction angle) suffi ciently into the wind to cause the aircraft to move forward into the wind at the same rate the wind is moving it sideways. Depending on the wind velocity, this may require a large wind correction angle or one of only a few degrees. This wind correction angle is also commonly known as the crab angle. When the drift has been neutralized, the aircraft follows the desired ground track.
To understand the need for drift correction during fl ight, consider a fl ight with a wind velocity of 20 knots from the left and 90° to the direction the aircraft is headed. After 1 hour, the body of air in which the aircraft is fl ying has moved 20 nautical miles (NM) to the right. Since the aircraft is moving with this body of air, it too has drifted 20 NM to the right. In relation to the air, the aircraft moved forward; but in relation to the ground, it moved forward as well as 20 NM to the right.
There are times when the pilot needs to correct for drift while in a turn. [Figure 9-2] Throughout the turn, the wind is acting on the aircraft from constantly changing angles. The relative wind angle and speed govern the time it takes for the aircraft to progress through any part of a turn. This is due to the constantly changing groundspeed. When the aircraft is headed into the wind, the groundspeed is decreased; when headed downwind, the groundspeed is increased. Through the crosswind portion of a turn, the aircraft must be turned suffi ciently into the wind to counteract drift.
To follow a desired circular ground track, the wind correction angle must be varied in a timely manner because of the varying groundspeed as the turn progresses. The faster the groundspeed, the faster the wind correction angle must be established; the slower the groundspeed, the slower the wind correction angle may be established. It can be seen then that the steepest bank and fastest rate of turn should be made on the downwind portion of the turn and the shallowest bank and slowest rate of turn on the upwind portion.
The principles and techniques of varying the angle of bank to change the rate of turn and wind correction angle for controlling wind drift during a turn are the same for all ground track maneuvers involving changes in direction of fl ight.
When there is no wind, it should be simple to fl y along a ground track with an arc of exactly 180° and a constant radius because the fl ightpath and ground track would be identical. This can be demonstrated by approaching a road at a 90° angle and, when directly over the road, rolling into a medium-banked turn. Then, maintaining the same angle of bank throughout the 180° of turn. [Figure 9-2]
To complete the turn, the rollout should be started at a point where the wings become level as the aircraft again reaches the road at a 90° angle and is directly over the road just as the turn is completed. This would be possible only if there were absolutely no wind and if the angle of bank and the rate of turn remained constant throughout the entire maneuver.
If the turn were made with a constant angle of bank and a wind blowing directly across the road, it would result in a constant radius turn through the air. However, the wind effects would cause the ground track to be distorted from a constant radius turn or semicircular path. The greater the wind velocity, the greater the difference between the desired ground track and the fl ightpath. To counteract this drift, the fl ightpath can be controlled by the pilot in such a manner as to neutralize the effect of the wind and cause the ground track to be a constant radius semicircle.
The effects of wind during turns can be demonstrated after selecting a road, railroad, or other ground reference that forms a straight line parallel to the wind. Fly into the wind directly over and along the line and then make a turn with a constant medium angle of bank for 360° of turn. [Figure 9-3] The aircraft returns to a point directly over the line but slightly downwind from the starting point, the amount depending on the wind velocity and the time required to complete the turn. The path over the ground is an elongated circle, although in reference to the air it is a perfect circle. Straight fl ight during the upwind segment after completion of the turn is necessary to bring the aircraft back to the starting position.
A similar 360° turn may be started at a specifi c point over the reference line, with the aircraft headed directly downwind. In this demonstration, the effect of wind during the constant banked turn drifts the aircraft to a point where the line is re-intercepted, but the 360° turn is completed at a point downwind from the starting point.
Another reference line which lies directly crosswind may be selected and the same procedure repeated. If wind drift is not corrected, the aircraft is headed in the original direction at the completion of the 360° turn, but has drifted away from the line a distance dependent on the amount of wind.
From these demonstrations, it can be seen where and why it is necessary to increase or decrease the angle of bank and the rate of turn to achieve a desired track over the ground. The principles and techniques involved can be practiced and evaluated by the performance of the ground track maneuvers discussed in this chapter.
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