CFI to CFI
Eights on pylons
Mastering flight by outside references
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Eights on pylons, also known as on-pylon eights, is a ground-reference maneuver that is required on the commercial (ASEL and ASES) and the flight instructor (ASEL and ASES) practical tests. It is one of the best exercises for teaching the pilot to fly the airplane automatically by reference to outside points and to control the altitude without referring to the horizon.
The pylons are picked so that straight-and-level flight between the halves is required. (A good estimate is for a four- or five-second straightaway.)
The pivotal altitude is that altitude at which the pylon will remain in a constant position relative to the pilot's line of sight (parallel to the lateral axis of the airplane). It is dependent on the relative speed of the airplane to the pylon and so will vary around the pylon except in a no-wind condition. (The path would be a circle in that case.)
The pivotal altitude may be found by the equation (V2 knots)/11.3.
The V is the relative velocity to the pylon (true airspeed in no-wind conditions, ground speed when the wind is blowing).
Figure 3 shows three airplanes starting to fly around a pylon in no-wind conditions and 60-degree bank at different altitudes at 106 knots true airspeed. Only one of the three is at the proper pivotal altitude, given as 1,000 feet (rounded off) for this problem. At a 60-degree bank only Airplane A fits the cone since the shaded area shows the turn radius of the airplanes at 106 kt. Airplane B is too high and will be turning into the pylon (the pylon will be moving forward of the line of reference), while Airplane C is too low and "gaining" on the pylon. Each airplane must move to the proper pivotal altitude of Airplane A in order to fit on the 60-degree cone. This procedure works for any angle of bank since the pivotal altitude depends only on the relative speed of the airplane to the pylon.
Figure 4 shows the path of a theoretical airplane around one pylon. The steepest bank and highest altitude are found when the airplane is flying directly downwind; the opposite occurs when flying directly upwind. The wing (more properly, your line of sight) is always pointed at the pylon and the path is a modified ellipse as shown. In a tandem airplane the occupants will have different "lines of sight."
Assuming a constant power setting, if the pylon is moving behind the reference line, up-elevators play a dual role by moving the airplane up toward the pivotal altitude (if you aren't flying the pylons on the back side of the power curve) as well as helping to slow the airplane as you attain the proper height/speed combination.
If the pylon is moving ahead of the reference, the elevators are used to ease the nose down to increase the airspeed and lower the altitude, again getting a double effect of "catch up."
Keep an eye out for any traffic that is crossing your path, or is performing on-pylon eights using the same pylons!
William K. Kershner has been a flight instructor since 1949. He is the author of several books, including Student Pilot Flight Manual and Advanced Pilot's Flight Manual. Visit his Web site.
By William K. Kershner