Download Chapter 6 Ground Reference Maneuvers and more Schemes and Mind Maps Geography in PDF only on Docsity!
Introduction
Initial pilot training requires that a pilot understand the relationship of the various flight controls pressure inputs to the resulting attitudes of the airplane. This allows a pilot to develop a sense of feel and understand the various indications of airplane performance, such as pitch, roll, and yaw attitudes. With sufficient competency in this environment, the pilot is ready to apply these skills and place the airplane, not only in the correct attitude and power configuration, but also in orientation to specific ground-based references. These skills are the basis for traffic patterns, survey, photographic, sight- seeing, aerial application (crop dusting) and various other flight profiles requiring specific flightpaths referenced to points on the surface.
Ground Reference Maneuvers
Chapter 6
A pilot must develop the proper coordination, timing, and attention to accurately and safely maneuver the airplane with regard to the required attitudes and ground references. Ground reference maneuvers are the principle flight maneuvers that combine the four fundamentals (straight-and-level, turns, climbs, and descents) into a set of integrated skills that the pilot uses in their everyday flight activity. A pilot must develop the skills necessary to accurately control, through the effect and use of the flight controls, the flightpath of the airplane in relationship to the ground. From every takeoff to every landing, a pilot exercises these skills in controlling the airplane. The pilot should be introduced by their instructor to ground reference maneuvers as soon as the pilot shows proficiency in the four fundamentals. Accomplishing the ground reference maneuvers requires that the pilot competently manipulate the flight controls without any undue attention to mechanical flight control inputs—the pilot applies the necessary flight control pressures to affect the airplane’s attitude and position by using the outside natural horizon and ground-based references with brief periods of scanning the flight instruments.
Maneuvering by Reference to Ground
Objects
The purpose of ground reference maneuvers is to train pilots to accurately place the airplane in relationship to specific references and maintain a desired ground track. Such precision requires that a pilot simultaneously evaluate the airplane’s attitude, reference points along the desired path, and the natural horizon. Vision is the most utilized sense in maneuvering in orientation to ground-based references; however, all senses are actively involved at different levels. For example, touch provides tactile feedback as to the required flight control pressures to overcome flight control surface forces that indirectly indicate the airplane’s airspeed and aerodynamic load. It is a common error for beginning pilots to fixate on a specific reference, such as a single location on the ground or the natural horizon. To be effective, the pilot must scan between several visual references to determine relative motion and to determine if the airplane is maintaining, or drifting to or from, the desired ground track. A pilot fixating on any one reference eliminates the ability to determine rate, which significantly degrades a pilot’s performance. Visual scanning across several references allows the pilot to develop the important skill of determining the rate of closure to a specific point. Consider a skilled automobile driver in a simple intersection turn; the driver does not merely turn the steering wheel some degree and hope that it will work out. The skilled driver picks out several references, such as an island to their side, a painted lane line, or the opposing curb, and they use those references to make almost imperceptible adjustments to the amount of deflection on the steering wheel, as well as the pressure on the accelerator pedal to smoothly join the lane into which they are turning. In the same manner, multiple references are required to precisely control the airplane in reference to the ground. Not all ground-based references are visually equal and some understanding of those differences is important for their selection and use. For example, larger objects or references may appear closer than they actually are when compared to smaller objects or references. Also, prevailing visibility has a significant effect on the pilot’s perception of the distance to a reference. Excellent visibilities with clear skies tend to make an object or reference appear closer than when compared to a hazy day with poor visibility. Another example is that rain can alter the visual image in a manner that an illusion of being at a higher altitude may be perceived, and brighter objects or references may appear closer than dimmer objects. Being aware of typical visual illusions helps a pilot select the best references for ground reference maneuvers. It is best, however sometimes impracticable, to find ground-based references that are similar in size and proportion. Ground-based references can be numerous. Excellent examples are breakwaters, canals, fence lines, field boundaries, highways, railroad tracks, roads, pipe lines, power lines, water-tanks, and others; however, choices can be limited by geography, population density, infrastructure, or structures. Selecting a ground-based reference requires prior consideration, such as the type of maneuver being performed, altitude at which the maneuver will be performed, emergency landing requirements, density of structures, wind direction, visibility, and the type of airspace. Division of attention is an important skill that a pilot must develop. A pilot must be able to fly the airplane affecting the flight controls in a manner they will place the airplane in the needed attitude while tracking a specific path over the ground. In addition, the pilot must be able to scan for hazards such as other aircraft, be immediately prepared for an emergency landing should the need arise, and scan the flight and engine instruments at regular intervals to ensure that a pending situation, such as decreasing oil pressure, does not turn into an unexpected incident. Safety is paramount in all aspects of flying. Awareness and practice of safety-enhancing procedures must be constantly exercised. Ground reference maneuvers place the airplane in an environment where heightened awareness is needed. Pilots should be looking for other aircraft, including helicopters, radio towers, and assessing locations for emergency landings. Pilots should always clear the area with two 90°
Figure 6-1. Wind drift. Wind Wind Current Current No current, no drift. (^) downstream unless corrected.With a current, the boat drifts With proper correction, the boatstays on intended course. No wind, no drift. With any wind, the airplane driftsdownwind unless corrected. With proper correction, the airplanestays on intended course. 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 6-2] 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. 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 6-3] 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
Figure 6-2. Effect of wind during a turn. Figure 6-3. Effect of wind during turn. Intended ground path Actual ground path No wind 20 knot wind A B C D No wind Wind Wind Wind 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. 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
Figure 6-4. Rectangular course.
Wind
Track with no wind correction Track with no wind correction
Base
Crosswind
Downwind
Upwind
No wind correction Turn more than 90° Complete turn at boundary Start turn at boundary Exit Turn into wind Complete turn at boundary No wind correction Start turn at boundary Start turn at boundary Turn into wind Complete turn at boundary Start turn at boundary Turn less than 90° Enter 45° to downwind Complete turn at boundary Turn less than 90° rollout with wind correction established Turn more than 90° rollout with wind correction established the nose of the airplane resulting in a direct headwind and decreased groundspeed; however, a real-world situation results in some drift correction. The pilot should roll the airplane into a medium banked turn with coordinated aileron and rudder pressures. As the airplane turns onto the upwind leg, the crosswind lessens and becomes a headwind, and the bank angle is gradually reduced with coordinated aileron and rudder pressures. Because the pilot was angled into the wind on the base leg, the turn to the upwind leg is less than 90°. The next leg is where the airplane turns from an upwind leg position to the crosswind leg. The pilot should slowly roll the airplane into a shallow-banked turn, as the developing crosswind drifts the airplane into the inside of the rectangular course with coordinated aileron and rudder pressures. As the airplane turns onto the crosswind leg, the headwind lessens and becomes a crosswind. As the turn nears completion, the bank angle is reduced with coordinated aileron and rudder pressures. To compensate for the crosswind, the pilot must angle into the wind, toward the outside of the rectangular course, which requires the turn to be less than 90°. The final turn is back to the downwind leg, which requires a medium-banked angle and a turn greater than 90°. The groundspeed will be increasing as the turn progresses and the bank should be held and then rolled out in a rapid, but not excessive, manner using coordinated aileron and rudder pressures. For the maneuver to be executed properly, the pilot must visually utilize the ground-based, nose, and wingtip references to properly position the airplane in attitude and in orientation to the rectangular course. Each turn, in order to maintain a constant ground-based radius, requires the bank angle to be adjusted to compensate for the changing groundspeed—the higher the groundspeed, the steeper the bank. If the groundspeed is initially higher and then decreases throughout the turn, the bank angle should progressively decrease throughout the turn. The converse is also true, if the groundspeed is initially slower and then increases throughout the turn, the bank angle should progressively increase throughout the turn until rollout is started. Also, the rate for rolling in and out of the turn should be adjusted
Figure 6-5. Turns around a point. Do wnw indhalfofcir cle Up wind halfofcirc le Wind Shallowest bank Shallower bank Steepest bank Steeper bank to prevent drifting in or out of the course. When the wind is from a direction that could drift the airplane into the course, the banking roll rate should be slow. When the wind is from a direction that could drift the airplane to the outside of the course, the banking roll rate should be quick. The following are the most common errors made while performing rectangular courses:
- Failure to adequately clear the area above, below, and on either side of the airplane for safety hazards, initially and throughout the maneuver.
- Failure to establish a constant, level altitude prior to entering the maneuver.
- Failure to maintain altitude during the maneuver.
- Failure to properly assess wind direction.
- Failure to establish the appropriate wind correction angle.
- Failure to apply coordinated aileron and rudder pressure, resulting in slips and skids.
- Failure to manipulate the flight controls in a smooth and continuous manner.
- Failure to properly divide attention between controlling the airplane and maintaining proper orientation with the ground references.
- Failure to execute turns with accurate timing.
Turns Around a Point
Turns around a point are a logical extension of both the rectangular course and S-turns across a road. The maneuver is a 360° constant radius turn around a single ground- based reference point. [Figure 6-5] The principles are the same in any turning ground reference maneuver—higher groundspeeds require steeper banks and slower ground speeds require shallower banks. The objectives of turns around a point are as follows:
- Maintaining a specific relationship between the airplane and the ground.
- Dividing attention between the flightpath, ground- based references, manipulating of the flight controls, and scanning for outside hazards and instrument indications.
- Adjusting the bank angle during turns to correct for groundspeed changes in order to maintain a constant radius turn; steeper bank angles for higher ground speeds, shallow bank angles for slower groundspeeds.
- Improving competency in managing the quickly changing bank angles.
- Establishing and adjusting the wind correction angle in order to maintain the track over the ground.
- Developing the ability to compensate for drift in quickly changing orientations.
- Developing further awareness that the radius of a turn is correlated to the bank angle.
- Failure to establish a constant, level altitude prior to entering the maneuver.
- Failure to maintain altitude during the maneuver.
- Failure to properly assess wind direction.
- Failure to properly execute constant radius turns.
- Failure to manipulate the flight controls in a smooth and continuous manner.
- Failure to establish the appropriate wind correction angle.
- Failure to apply coordinated aileron and rudder pressure, resulting in slips or skids.
S-Turns
S-turns is a ground reference maneuver in which the airplane’s ground track resembles two opposite but equal half-circles on each side of a selected ground-based straight- line reference. [Figure 6-6] This ground reference maneuver presents a practical application for the correction of wind during a turn. The objectives of S-turns across a road are as follows:
- Maintaining a specific relationship between the airplane and the ground.
- Dividing attention between the flightpath, ground- based references, manipulating the flight controls, and scanning for outside hazards and instrument indications.
- Adjusting the bank angle during turns to correct for groundspeed changes in order to maintain a constant radius turn—steeper bank angles for higher ground speeds, shallow bank angles for slower groundspeeds.
- Rolling out from a turn with the required wind correction angle to compensate for any drift cause by the wind.
- Establishing and correcting the wind correction angle in order to maintain the track over the ground.
- Developing the ability to compensate for drift in quickly changing orientations.
- Arriving at specific points on required headings. With the airplane in the downwind position, the maneuver consists of crossing a straight-line ground reference at a 90° angle and immediately beginning a 180° constant radius turn. The pilot will then adjust the roll rate and bank angle for drift effects and changes in groundspeed, and re-cross the straight-line ground reference in the opposite direction just as the first 180° constant radius turn is completed. The pilot will then immediately begin a second 180° constant radius turn in the opposite direction, adjusting the roll rate and bank angle for drift effects and changes in groundspeed, again re- crossing the straight-line ground reference as the second 180° constant radius turn is completed. If the straight-line ground reference is of sufficient length, the pilot may complete as many as can be safely accomplished. In the same manner as the rectangular course, it is standard practice to enter ground-based maneuvers downwind where groundspeed is greatest. As such, the roll into the turn must be rapid, but not aggressive, and the angle of bank must be steepest when initiating the turn. As the turn progresses, the bank angle and the rate of rollout must be decreased as the groundspeed decreases to ensure that the turn’s radius is constant. During the first turn, when the airplane is at the 90° point, it will be directly crosswind. In addition to the rate of rollout and bank angle, the pilot must control the wind correction angle throughout the turn. Controlling the wind correction angle during a turn can be complex to understand. The concept is best understood by comprehending the difference between the number of degrees that the airplane has turned over the ground verses the number of degrees that the airplane has turned in the air. For example, if the airplane is exactly crosswind, meaning directly at a point that is 90° to the straight-lined ground reference. If the wind, in this example, requires a 10° wind correction angle (for this example, this is a left turn with the crosswind from the left) the airplane would be at a heading that is 10° ahead when directly over the 90° ground reference point. In other words, the first 90° track over the ground would result in a heading change of 100° and the last 90° track over the ground would result in 80° of heading change. As the turn progresses from a downwind position to an upwind position, the pilot must gradually decrease the bank angle with coordinated aileron and rudder pressure. The pilot should reference the airplane’s nose, wingtips, and the ground references and adjust the rollout timing so that the airplane crosses the straight-line ground reference with the wings level, and at the proper heading, altitude, and airspeed. As the airplane re-crosses the straight-lined ground reference, the pilot should immediately begin the opposite turn—there should be no delay in rolling out from one turn and rolling into the next turn. Because the airplane is now upwind, the roll in should be smooth and gentle and the initial bank angle should be shallow. As the turn progresses, the wind changes from upwind, to crosswind, to downwind. In a similar manner described above, the pilot should adjust the bank angle to correct for changes in groundspeed. As the groundspeed increases, the pilot should increase the bank angle to maintain a constant radius turn. At the 90° crosswind position, the airplane should also have the correct wind correction angle. As the airplane turns downwind, the groundspeed increases;
the bank angle should be increased so that the rate of turn is used to maintain a constant radius turn. The following are the most common errors made while performing S-turns across a road:
- Failure to adequately clear the area above, below, and on either side of the airplane for safety hazards, initially and throughout the maneuver.
- Failure to establish a constant, level altitude prior to entering the maneuver.
- Failure to maintain altitude during the maneuver.
- Failure to properly assess wind direction.
- Failure to properly execute constant radius turns.
- Failure to manipulate the flight controls in a smooth and continuous manner when transitioning into turns.
- Failure to establish the appropriate wind correction angle.
- Failure to apply coordinated aileron and rudder pressure, resulting in slips or skids.
Elementary Eights
Elementary eights are a family of maneuvers in which each individual maneuver is one that the airplane tracks a path over the ground similar to the shape of a figure eight. There are various types of eights, progressing from the elementary types to very difficult types in the advanced maneuvers. Each eight is intended to develop a pilot’s flight control coordination skills, strengthen their awareness relative to the selected ground references, and enhance division of attention so that flying becomes more instinctive than mechanical. Eights require a greater degree of focused attention to the selected ground references; however, the real significance of eights is that pilot must strive for flight precision. Elementary eights include eights along a road, eights across a road, and eights around pylons. Each of these maneuvers is a variation of a turn around a point. Each eight uses two ground reference points about which the airplane turns first in one direction and then the opposite direction—like a figure eight. Eights maneuvers are designed for the following purposes:
- Further development of the pilot’s skill in maintaining a specific relationship between the airplane and the ground references.
- Improving the pilot’s ability to divide attention between the flightpath and ground-based references, manipulation of the flight controls, and scanning for outside hazards and instrument indications during both turning and straight-line flight. - Developing the pilot’s skills to visualize each specific segment of the maneuver and the maneuver as a whole, prior to execution. - Developing a pilot’s ability to intuitively manipulate flight controls to adjust the bank angle during turns to correct for groundspeed changes in order to maintain constant radius turns and proper ground track between ground references. Eights Along a Road Eights along a road is a ground reference maneuver in which the ground track consists of two opposite 360° adjacent turns with the center of each 360° turn and the adjacent turn point perpendicular or parallel to the straight-line ground reference (road, railroad tracks, fence line, pipeline right-of-way, etc.). [Figure 6-7] Like the other ground reference maneuvers, its objective is to further develop division of attention while compensating for drift, maintaining orientation with ground references, and maintaining a constant altitude. Although eights along a road may be performed with the wind blowing parallel or perpendicular to the straight-line ground reference, only the perpendicular wind situation is explained since the principles involved are common to each. The pilot should select a straight-line ground reference that is perpendicular to the wind and position the airplane parallel to and directly above the straight-line ground reference. Since this places the airplane in a crosswind position, the pilot must compensate for the wind drift with an appropriate wind correction angle. The following description is illustrated in Figure 6-7. The airplane is initially in a crosswind position, perpendicular to the wind, and over the ground-based reference. The first turn should be a left turn toward a downwind position starting with a steeping bank. When the entry is made into the turn, it requires that the turn begin with a medium bank and gradually steepen to its maximum bank angle when the airplane is directly downwind. As the airplane turns from downwind to crosswind, the bank angle needs to be gradually reduced since groundspeed is decreasing; however, the groundspeed only decreases by 1 ⁄ 2 of its velocity during the first 2 ⁄ 3 of the turn from downwind to crosswind. The pilot must control the bank angle as well as the rate at which the bank angle is reduced so that the wind correction angle is correct. Assuming that the wind is coming from the right side of the airplane, the airplane heading should be slightly ahead of its position over the ground. When the airplane completes the first 180° of ground track, it is directly crosswind, and the airplane should be at the maximum wind correction angle.
Figure 6-8. Eights across a road. Wind Shallowest bank Shallower bank Steepest bank Steeping bank Steeper bank Shallowest bank Steepest bank Shallower bank To successfully perform eights along a ground reference, the pilot must be able to smoothly and accurately coordinate changes in bank angle to maintain a constant radius turn and counteract drift. The speed in which the pilot can anticipate these corrections directly affects the accuracy of the overall maneuver and the amount of attention that can be directed toward scanning for outside hazards and instrument indications. Eights Across A Road This maneuver is a variation of eights along a road and involves the same principles and techniques. The primary difference is that at the completion of each loop of the figure eight, the airplane should cross an intersection of a specific ground reference point. [Figure 6-8] The loops should be across the road and the wind should be perpendicular to the loops. Each time the reference is crossed, the crossing angle should be the same, and the wings of the airplane should be level. The eights may also be performed by rolling from one bank immediately to the other, directly over the reference. Eights Around Pylons Eights around pylons is a ground-reference maneuver with the same principles and techniques of correcting for wind drift as used in turns around a point and the same objectives as other ground track maneuvers. Eights around pylons utilizes two ground reference points called “pylons.” Turns around each pylon are made in opposite directions to follow a ground track in the form of a figure 8. [Figure 6-9] The pattern involves flying downwind between the pylons and upwind outside of the pylons. It may include a short period of straight-and-level flight while proceeding diagonally from one pylon to the other. The pylons should be on a line perpendicular to the wind. The maneuver should be started with the airplane on a downwind heading when passing equally between the pylons. The distance between the pylons and the wind velocity determines the initial angle of bank required to maintain a constant turn radius from the pylons during each turn. The steepest banks are necessary just after each turn entry and just before the rollout from each turn where the airplane is headed downwind and the groundspeed is highest; the shallowest banks are when the airplane is headed directly upwind and the groundspeed is lowest. As in other ground reference maneuvers, the rate at which the bank angle must change depends on the wind velocity. If the airplane proceeds diagonally from one turn to the other, the rollout from each turn must be completed on the proper heading with sufficient wind correction angle to ensure that after brief straight-and-level flight, the airplane arrives at the point where a turn of the same radius can be made around the other pylon. The straight-and-level flight segments must be tangent to both circular patterns. Common errors in the performance of elementary eights are:
- Failure to adequately clear the area above, below, and on either side of the airplane for safety hazards, initially and throughout the maneuver.
- Poor selection of ground references.
Figure 6-9. Eights around pylons. Wind Shallowest bank Shallower bank Steepest bank Steeper bank (^) Steepest bank Steeper bank Shallowest bank Shallower bank
- Failure to establish a constant, level altitude prior to entering the maneuver.
- Failure to maintain adequate altitude control during the maneuver.
- Failure to properly assess wind direction.
- Failure to properly execute constant radius turns.
- Failure to manipulate the flight controls in a smooth and continuous manner.
- Failure to establish the appropriate wind correction angles.
- Failure to apply coordinated aileron and rudder pressure, resulting in slips or skids.
- Failure to maintain orientation as the maneuver progresses. Eights-on-Pylons The eights-on-pylons is the most advanced and difficult of the ground reference maneuvers. Because of the techniques involved, the eights-on-pylons are unmatched for developing intuitive control of the airplane. Similar to eights around pylons except altitude is varied to maintain a specific visual reference to the pivot points. The goal of the eights-on-pylons is to have an imaginary line that extends from the pilot’s eyes to the pylon. This line must be imagined to always be parallel to the airplane’s lateral axis. Along this line, the airplane appears to pivot as it turns around the pylon. In other words, if a taut string extended from the airplane to the pylon, the string would remain parallel to lateral axis as the airplane turned around the pylon. At no time should the string be at an angle to the lateral axis. [Figure 6-10] In explaining the performance of eights-on-pylons, the term “wingtip” is frequently considered as being synonymous with the proper visual reference line or pivot point on the airplane. This interpretation is not always correct. High-wing, low-wing, sweptwing, and tapered wing airplanes, as well as those with tandem or side-by-side seating, all present different angles from the pilot’s eye to the wingtip. [Figure 6-11]
Figure 6-12. Speed versus pivotal altitude. Approximate Pivotal Altitude Groundspeed Knots 87 91 96 100 104 109 113 MPH 100 105 110 115 120 125 130 670 735 810 885 960 1050 1130 airplane flies a prescribed path over the ground and the pilot attempts to maintain the track by correcting for the wind. With eights-on-pylons, the pilot maintains lateral orientation to a specific spot on the ground. This develops the pilot’s ability to maneuver the airplane accurately while dividing attention between the flightpath and the selected pylons on the ground. An explanation of the pivotal altitude is also essential. First, a good rule of thumb for estimating the pivotal altitude is to square the groundspeed, then divide by 15 (if the groundspeed is in miles per hour) or divide by 11.3 (if the groundspeed is in knots), and then add the mean sea level (MSL) altitude of the ground reference. The pivotal altitude is the altitude at which, for a given groundspeed, the projection of the visual reference line to the pylon appears to pivot. [Figure 6-12] The pivotal altitude does not vary with the angle of bank unless the bank is steep enough to affect the groundspeed. Distance from the pylon affects the angle of bank. At any altitude above that pivotal altitude, the projected reference line appears to move rearward in a circular path in relation to the pylon. Conversely, when the airplane is below the pivotal altitude, the projected reference line appears to move forward in a circular path. [Figure 6-13] To demonstrate this, the pilot will fly at maneuvering speed and at an altitude below the pivotal altitude, and then placed in a medium-banked turn. The projected visual reference line appears to move forward along the ground (pylon moves back) as the airplane turns. The pilot then executes a climb to an altitude well above the pivotal altitude. When the airplane is again at maneuvering speed, it is placed in a medium-banked turn. At the higher altitude, the projected visual reference line appears to move backward across the ground (pylon moves forward). After demonstrating the maneuver at a high altitude, the pilot should reduce power and begin a descent at maneuvering speed in a continuing medium bank turn around the pylon. The apparent backward movement of the projected visual reference line with respect to the pylon will slow down as altitude is lost and will eventually stop for an instant. If the pilot continues the descent below the pivotal altitude, the projected visual reference line with respect to the pylon will begin to move forward. The altitude at which the visual reference line ceases to move across the ground is the pivotal altitude. If the airplane descends below the pivotal altitude, the pilot should increase power to maintain airspeed while regaining altitude to the point at which the projected reference line moves neither backward nor forward but actually pivots on the pylon. In this way, the pilot can determine the pivotal altitude of the airplane. The pivotal altitude is critical and changes with variations in groundspeed. Since the headings throughout turns continuously vary from downwind to upwind, the groundspeed constantly changes. This results in the proper pivotal altitude varying slightly throughout the turn. The pilot should adjust for this by climbing or descending, as necessary, to hold the visual reference line on the pylons. This change in altitude is dependent on the groundspeed. Selecting proper pylon is an important factor of successfully performing eights-on-pylons. They should be sufficiently prominent so the pilot can view them when completing the turn around one pylon and heading for the next. They should also be adequately spaced to provide time for planning the turns but not spaced so far apart that they cause unnecessary straight-and-level flight between the pylons. The selected pylons should also be at the same elevation, since differences of over few feet necessitate climbing or descending between each turn. The pilot should select two pylons along a line that lies perpendicular to the direction of the wind. The distance between the pylons should allow for the straight-and-level flight segment to last from 3 to 5 seconds. The pilot should estimate the pivotal altitude during preflight planning. Weather reports and consultation with other pilots flying in the area may provide both the wind direction and velocity. If the references are previously known (many flight instructors already have these ground-based reference selected), the sectional chart will provide the MSL of the references, the Pilot’s Operating Handbook (POH) provides the range of maneuvering airspeeds (based on weight), and the wind direction and velocity can be estimated to calculate the appropriate pivotal altitudes. The pilot should calculate the pivotal altitude for each position: upwind, downwind, and crosswind. The pilot should begin the eight-on-pylons maneuver by flying diagonally crosswind between the pylons to a point downwind from the first pylon so that the first turn can be made into the wind. As the airplane approaches a position where the pylon appears to be just ahead of the wingtip, the
Figure 6-13. Effect of different altitudes on pivotal altitude. Too high Pivotal altitude Too low pilot should begin the turn by lowering the upwind wing to the point where the visual reference line aligns with the pylon. The reference line should appear to pivot on the pylon. As the airplane heads upwind, the groundspeed decreases, which lowers the pivotal altitude. As a result, the pilot must descend to hold the visual reference line on the pylon. As the turn progresses on the upwind side of the pylon, the wind becomes more of a crosswind. Since this maneuver does not require the turn to be completed at a constant radius, the pilot does not need to apply drift correction to complete the turn. If the visual reference line appears to move ahead of the pylon, the pilot should increase altitude. If the visual reference line appears to move behind the pylon, the pilot should decrease altitude. Deflecting the rudder to yaw the airplane and force the wing and reference line forward or backward to the pylon places the airplane in uncoordinated flight, at low altitude, with steep bank angles and must not be attempted. As the airplane turns toward a downwind heading, the pilot should rollout from the turn to allow the airplane to proceed diagonally to a point tangent on the downwind side of the second pylon. The pilot should complete the rollout with the proper wind correction angle to correct for wind drift, so that the airplane arrives at a point downwind from the second pylon that is equal in distance from the pylon as the corresponding point was from the first pylon at the beginning of the maneuver. At this point, the pilot should begin a turn in the opposite direction by lowering the upwind wing to the point where the visual reference line aligns with the pylon. The pilot should then continue the turn the same way the corresponding turn was performed around the first pylon but in the opposite direction. With prompt correction, and a very fine control pressures, it is possible to hold the visual reference line directly on the pylon even in strong winds. The pilot may make corrections for temporary variations, such as those caused by gusts or inattention by reducing the bank angle slightly to fly relatively straight to bring forward a lagging visual reference line or by increasing the bank angle temporarily to turn back a visual reference line that has moved ahead. With practice, these corrections may become slight enough to be barely noticeable. It is important to understand that variations in pylon position are according to the apparent movement of the visual reference line. Attempting to correct pivotal altitude by the use of the altimeter is ineffective.
