Mechanical equipment failure, environmental factors, and pilot error can cause abnormal aerotow occurrences during climb-out. Mechanical equipment failures can be caused by towline and towhook failures, towplane mechanical failures, and glider mechanical failures. Towline failure (one that breaks unexpectedly) can result from using a weak or worn towline. Towline failures can be avoided by using appropriately rated towline material, weak links when necessary, proper tow rings, and proper towline maintenance.
Towhook system failures include uncommanded towline releases or the inability to release. These failures can occur in either the towplane or the glider towhook system. Proper preflight and maintenance of these systems should help to avoid these types of failures. Towplane mechanical failures can involve the powerplant or airframe. When the tow pilot or glider pilot encounters a mechanical failure, he or she should signal the glider pilot to release immediately. [Figure 8-7] This is one of many situations that make it vitally important that both the tow pilot and glider pilot have a thorough knowledge of aerotow visual signals.
Glider mechanical failure can include towhook system malfunctions, flight control problems, and improper assembly or rigging. If a mechanical failure occurs, the glider pilot must assess the situation to determine the best course of action. In some situations, it may be beneficial to remain on the aerotow, while other situations may require immediate release.
If the glider release mechanism fails, the tow pilot should be notified either by radio or tow signal and the glider should maintain the high tow position. The tow pilot should tow the glider over the gliderport/airport and release the glider from the towplane. The towline should fall back and below the glider. The design of the Schweizer towhook mechanism is such that the line pulls free from the glider by its own weight. Since some gliders do not back release, the glider pilot should pull the release to ensure the towline is in fact released.
Towplanes are usually fitted with a variant of the Schweizer or Tost glider tow hitch. [Figures 8-8 and 8-9] The hitch is usually located at the extreme end of the rear fuselage below the rudder. The specific rings used to attach towlines to these hitch types can be seen in Figure 8-10. The wing runner must be familiar with the correct method of attachment. If the towplane has the Schweizer tow hitch, it is possible for the tow ring to rotate forward so that it traps the sleeve that locks the tow hitch in place. This may prevent the tow pilot from releasing the towline. [Figure 8-11]
Failure of both towplane and glider release mechanisms is extremely rare. If it occurs, however, radio or tow signals between glider pilot and tow pilot should verify this situation. The glider pilot should move down to the low tow position once the descent has started to the gliderport/airport. The glider pilot needs to use spoilers/dive brakes to maintain the low tow position and to avoid overtaking the towplane. The tow pilot should plan the approach to avoid obstacles. The approach should be shallow enough for the glider to touch down first. The glider pilot should use the spoilers/ dive brakes to stay on the runway, and use the wheel brake as necessary to avoid overtaking the towplane. Excessive use of the glider wheel brake may result in a hard landing for the towplane because the towline could slow the towplane below flying speed. Another valid method would be to stay on tow until well within gliding distance of the airport and then breaking the towline. The best experience to date suggests the procedure to break the towline would be by climbing above and diving down to develop slackline, then fully extending the dive brakes and/or spoilers to set up the overload condition as the towplane begins to accelerate without the load of the glider and the glider is decelerating due to the increase in drag. Once the towline breaks, the glider lands using the procedures of having the line attached, in case the line did not break at the gliders weak link.
Environmental factors for terminating the tow include encountering clouds, mountain rotors (area of turbulence created by wind and mountainous terrain), or restricted visibility. Any of these factors may require the glider pilot to release from the aerotow. During the aerotow, each pilot is responsible for avoiding situations that would place the other pilot at risk.
For the towplane pilot, examples of pilot error include deliberately starting the takeoff before the glider pilot has signaled the glider is ready for launch, using steep banks during the aerotow without prior consent of the glider pilot, or frivolous use of aerotow signals, such as “release immediately!” For the glider pilot, examples of pilot error include rising high above the towplane during takeoff and climb or leaving air brakes open during takeoff and climb.
The glider pilot may choose to deliberately terminate the tow/launch anytime it may appear to be a safer course of action. For example, the pilot discovers control binding once air pressure builds on the surfaces, releasing the towline is the better alternative than getting too high to stop and too low to bailout.
Premature terminations of the tow have been a leading cause of glider accidents and incidents according to the Soaring Safety Foundation. Towing failure incidents related to rope breaks are not as common as other distractions in the glider cockpit. Extension of spoilers/dive brakes, unlocked canopies, and other distractions are major causes of the tow failure incidents leading to a towline break. Prevention is achieved with the proper use checklists and proper prelaunch discipline. There are five planning situations regarding inmotion towline breaks, uncommanded release, or power loss of the towplane and are listed below. While the best course of action depends on many variables, such as runway length, airport environment, density altitude, and wind, all tow failures or emergency release have one thing in common: the need to maintain control of the glider. Two possibilities are stalling the glider or dragging a wingtip on the ground during a low altitude turn.
On takeoff, all towplanes may plan to drift downwind if there is a wind present. If there is no wind or traffic, then the tow pilot should select an area on the take path/profile that has the fewest obstacles for both the towplane and glider. The tow pilot should plan to drift in that direction. This downwind drift does a few things for the glider pilot, the most important of which is that it ensures the glider has maneuverability if a tow failure occurs. This downwind drift allows the glider pilot to complete the course reversal without the need of extensive lower altitude turns to line up on the runway, reducing the possibility of dragging a glider wingtip on the ground during a low altitude turn. The tow pilot must initiate this drift. Remember, tow pilots need to know wind direction and plan accordingly to allow the glider pilot maneuverability options.
If the towplane proceeds straight out, the glider pilot may need to make the last alignment turn at a low altitude. The first turn is the course reversal; a second turn aligns the glider with the landing area. As mentioned, the low altitude turn may be difficult or impossible to complete. Winds may compound the situation if the turn is not planned properly. Pilots under these high-stress conditions sometimes crosscontrol the glider, compounding the issue. Remember, keep the yaw string or ball centered.
Tow Failure With Runway To Land and Stop
If a tow failure occurs or is inadvertently or deliberately released prior to towplane liftoff, the standard procedure is for the towplane either to continue the takeoff and clear the runway or abort the takeoff and remain on the left side of the runway. If the towplane loses power during the takeoff, the tow pilot should maneuver the towplane to the left side of the runway. If the glider is still on the runway, the glider pilot should pull the release, decelerate using the wheel brake, and be prepared to maneuver to the right side of the runway. If the line breaks, is inadvertently released, or the towplane loses power after the glider is airborne, the glider pilot should pull the towline release, land ahead, and be prepared to maneuver to the right side of the runway. [Figure 8-12, panel 1] Pulling the towline release in either case ensures that the rope is clear of the glider. Since local procedures vary, both glider pilot and tow pilot must be familiar with the specific gliderport/ airport procedures.