When ground launching, it is essential to use a center of gravity (CG) towhook that has an automatic back release feature. This protects the glider if the pilot is unable to release the towline during the launch. The failure of the tow release could cause the glider to be pulled to the ground as it flies over the launching vehicle or winch. Since the back release feature of the towhook is so important, it should be tested prior to every flight. [Figure 7-13]
Some training and high-performance gliders have only a CG towhook. CG towhooks are necessary for ground launch operations so the glider is not pulled into the ground. Attachment at the center of gravity allows the glider pilot to have full control of the glider without undue influence from the ground pull. Attachment of a ground launch towline to the nose hook would tend to pull the glider into the ground and would overload the horizontal stabilizer and elevator. Conversely, depending on design, the CG hook may not have sufficient movement to fully release an aerotow line under pressure. If the hook only swings about 90° down, the towline may stay hung on the gliders hook until the pressure is released and slack allows the towline to simply fall off. It may be located either ahead of the landing gear, in the landing gear well, or the glider may utilize a bracket that attaches outside on the fuselage near the cockpit.
If the CG hook is in the landing gear well, retracting the gear on tow interferes with the towline. Even if the glider has a nose hook, retracting the gear on tow is not recommended until the aircraft is safely airborne and an immediate or emergency return is not necessary. Leaving the gear down allows the glider pilot more time to assess the best landing options.
A CG hook, as compared to a nose hook, makes a crosswind takeoff more difficult since the glider can weathervane into the wind more easily. In addition, a CG hook makes the glider more susceptible to kiting on takeoff, especially if the CG is near the aft limit. This can present a serious danger to the towplane during the aerotow.
Prelaunch Signals (Winch/Automobile)
Prelaunch visual signals for a ground launch operation allow the glider pilot, the wing runner, the safety officer, and the launch crew to communicate over considerable distances. When launching with an automobile, the glider and launch automobile may be 1,000 feet or more apart. When launching with a winch, the glider may start the launch 4,000 feet or more from the winch. Because of the great distances involved, members of the ground launch crew use colored flags or large paddles to enhance visibility, as shown in Figure 7-14. When complex information must be relayed over great distances, visual prelaunch signals can be augmented with direct voice communications between crewmember stations. Hard-wired ground telephones, two-way radios, or wireless telephones can be used to communicate between stations, adding protection against premature launch and facilitating an aborted launch if an unsafe condition arises. The towline should never be attached to the glider until the crew is onboard and ready to launch.
Since ground launches are of short duration, inflight signals for ground launches are limited to signals to the winch operator or ground vehicle driver to increase or decrease speed. [Figure 7-15]
Proper ground launch tow speed is critical for a safe launch. Figure 7-16 compares various takeoff profiles that result when tow speeds vary above or below the correct speed.
Each glider certificated for ground launch operations has a placarded maximum ground launch tow speed. This speed is normally the same for automobile or winch launches. The glider pilot should fly the launch staying at or below this speed to prevent structural damage to the glider during the ground tow.
Automobile launches today are very rare. During automobile ground launches, the glider pilot and driver should have a thorough understanding of the groundspeeds to be used prior to any launch. Before the first launch, the pilot and vehicle driver should determine the appropriate vehicle ground tow speeds, considering the surface wind velocity, the glider speed increase during launch, and the wind gradient encountered during the climb. They should include a safety factor to avoid exceeding this maximum vehicle ground tow speed.
If a crosswind condition is present, the glider should be positioned slightly downwind of the takeoff heading and angled into the wind to help eliminate control problems until sufficient airspeed is obtained. Due to the slow acceleration of the glider during an automobile ground launch, the towline should be laid out to allow the glider to obtain sufficient speed for control while still in a headwind. [Figure 7-17]
The tow speed can be determined by using the following calculations:
- Subtract the surface winds from the maximum placarded ground launch tow speed for the particular glider.
- Subtract an additional five miles per hour (mph) for the airspeed increase during the climb.
- Subtract the estimated wind gradient increase encountered during the climb.
- Subtract a 5 mph safety factor.
|Maximum ground launch tow speed||75 mph|
|1. Surface winds 10 mph||–10 mph|
|2. Airspeed increase during climb 5 mph||–5 mph|
|3. Estimated climb wind gradient 5 mph||–5 mph|
|4. Safety factor of 5 mph||–5 mph|
|Automobile tow speed||50 mph|
During winch launches, the winch operator applies full power smoothly and rapidly until the glider reaches an angle of 30° above the horizon. At this point, the operator should start to reduce the power until the glider is about 60° above the horizon where approximately 20 percent power is needed. As the glider reaches the 70° point above the horizon, power is reduced to idle. The winch operator monitors the glider continuously during the climb for any signals to increase or decrease speed from the glider pilot. [Figure 7-18]
Crosswind Takeoff and Climb
The following are the main differences between crosswind takeoffs and climb procedures and normal takeoff and climb procedures:
- During the takeoff roll, the glider tends to weathervane into the wind.
- After liftoff, the glider drifts toward the downwind side of the runway.
- In strong crosswinds there is a greater tendency for the glider to drift downwind.
- If space is available in the takeoff area, the towline or cable should be laid out in a manner that the initial takeoff roll is slightly into the wind to reduce the crosswind component of the glider. [Figure 7-19]
After lift-off, the glider pilot should establish a wind correction angle toward the upwind side of the runway to prevent drifting downwind. This prevents downwind drift and allows the glider to work upwind of the runway during the climb-out. When the towline is released at the top of the climb, it tends to drift back toward the centerline of the launch runway, as shown in Figure 7-20. This helps keep the towline from fouling nearby wires, poles, fences, aircraft, and other obstacles on the side of the launching runway. Should the glider drift to the downwind side of the runway, the towline could damage other aircraft, runway lights, nearby fences, structures, obstacles, etc.