Once airborne and climbing, the glider can fly one of two tow positions. High tow is aerotow flight with the glider positioned slightly above the wake of the towplane. Low tow is aerotow flight with the glider positioned just below the wake of the towplane. [Figure 7-6] Climbing turns are made with shallow bank angles and the glider in the high tow position. Pilots are trained using these positions to learn coordinated towing procedures and understanding the dynamics of the aerotow. In training, glider pilots are advised to control vertical position relative to the towplane using the horizon.
The glider pilot’s sight picture depends on the type of towplane being used for the launch. The instructor, through flight experience, can determine the particular towplane’s vertical wake boundaries and describe the positions. Sometimes, the glider may use the picture of the towplane’s wings on the horizon. On another type of towplane, maintaining the towplane’s rudder centered over the fuselage of the towplane ensures the glide is directly behind the towplane in straight flight. Any excessive deviation from the low or high tow position by the glider requires abnormal control inputs by the tow pilot, which always generates more drag and degrades climb performance during the tow.
The towplane’s wake drifts down behind the towing aircraft. Straight ahead climbs are made with the glider in the level or high tow position. The tow pilot strives to maintain a steady pitch attitude and a constant power setting for the desired climb airspeed. The glider pilot uses visual references on the towplane to maintain a proper lateral and vertical position.
In a level flight tow, the pilot should position the glider above the wake of the towplane. Low tow offers the glider pilot a better view of the towplane and provides for a more aerodynamically efficient tow, especially during climb, as the towplane requires less upward elevator deflection due to the downward pull of the glider. However, because of the risk of towline fouling if the towline breaks or is released by the towplane, low tow should be mainly used for level flight aero-tows such as for a cross-country flight.
Climbing turns are made with shallow bank angles in the high tow position. During turns, the glider pilot observes and matches the bank angle of the towplane. In order to stay in the same flightpath of the towplane, the glider pilot must aim the nose of the glider at the outside wingtip of the towplane. This allows the glider’s flightpath to coincide with the flightpath of the towplane. [Figure 7-7]
If the glider’s bank is steeper than the towplanes bank, the glider’s turn radius is smaller than the towplane turn radius. [Figure 7-8] If this occurs, the reduced tension on the towline causes it to bow and slack, allowing the glider’s airspeed to slow. As a result, the glider begins to sink relative to the towplane. The correct course of action is to reduce the glider’s bank angle so the glider flies the same radius of turn as the towplane. If timely corrective action is not taken and the glider slows and sinks below the towplane, the towplane may rapidly pull the towline taut and possibly cause it to fail and/or cause structural damage to both aircraft.
If the glider’s bank is shallower than the towplane, the glider’s turn radius is larger than the towplane’s turn radius. [Figure 7-9] If this occurs, the increased tension on the towline causes the glider to accelerate and climb. The correct course of pilot action when the glider is turning outside the towplane radius of turn is to increase glider bank angle. As the glider moves back into position behind the towplane, the glider corrects to the same radius of turn as the towplane. If timely corrective action is not taken and the glider accelerates and climbs above the towplane, the towplane may lose rudder and elevator control. In this situation, the glider pilot should release the towline and turn to avoid the towplane.
Common errors in aerotow climb-out include:
- Faulty procedures by not maintaining proper vertical and lateral position during high or low tow.
- Inadvertent entry into towplane wake.
- Failure to maintain glider alignment during turns on aerotow causing towplane upset.