Collision avoidance is of primary importance when thermaling with other gliders. The first rule calls for all gliders in a particular thermal to circle in the same direction. The first glider in a thermal establishes the direction of turn and all other gliders joining the thermal should turn in the same direction. Ideally, two gliders in a thermal at the same height or nearly so should position themselves across from each other so they can maintain best visual contact. [Figure 10-12] When entering a thermal, strive to do so in a way that does not interfere with gliders already in the thermal, and above all, in a manner that does not cause a hazard to other gliders. An example of a dangerous entry is pulling up to bleed off excess speed in the middle of a crowded thermal. A far safer technique is to bleed off speed before reaching the thermal and joining the thermal at a “normal” thermaling speed. Collision avoidance, not optimum aerodynamic efficiency, is the priority when thermaling with other gliders. Announcing to the other glider(s) on the radio that you are entering the thermal enhances collision avoidance. [Figure 10-12]
Different types of gliders in the same thermal may have different minimum sink speeds, and it may be difficult to remain directly across from another glider in a thermal. Avoid a situation where the other glider cannot be seen or the other glider cannot see you. Radio communication is helpful. Too much talking clogs the frequency, and may make it impossible for a pilot to broadcast an important message. Do not fly directly above or below another glider in a thermal since differences in performance, or even minor changes in speed can lead to larger than expected altitude changes. If sight of another glider is lost in a thermal and position cannot be established via a radio call, leave the thermal. After 10 or 20 seconds, come back around to rejoin the thermal, hopefully with better traffic positioning. It cannot be stressed enough that collision avoidance when thermaling is a priority! Midair collisions can sometimes be survived, but only with a great deal of luck. Unsafe thermaling practices endanger everyone. [Figure 10-13]
Exiting a Thermal
Leaving a thermal properly can also save some altitude. While circling, scan the full 360° of sky with each thermaling turn. This first allows the pilot to continually check for other traffic in the vicinity. Second, it helps the pilot analyze the sky in all directions to decide where to go for the next climb. It is better to decide where to go next while still in lift rather than losing altitude in sink after leaving a thermal. Exactly when to leave depends on the goals for the climb—whether the desire is to maximize altitude for a long glide or leave when lift weakens in order to maximize time on a cross-country flight. In either case, be ready to increase speed to penetrate the sink often found on the edge of the thermal, and leave the thermal in a manner that does not hinder or endanger other gliders.
Exceptions to normal or typical thermals are numerous. For instance, instead of stronger sink at the edge of a thermal, weak lift sometimes continues for a distance after leaving a thermal. Glider pilots should be quick to adapt to whatever the air has to offer at the time. The mechanics of simply flying the glider become second nature with practice, as do thermaling techniques. Expect to land early because anticipated lift was not there on occasion—it is part of the learning curve.
If thermal waves are suspected, climb in the thermal near cloud base, then head toward the upwind side of the Cu. Often, only very weak lift, barely enough to climb at all, is found in smooth air upwind of the cloud. Once above cloud base and upwind of the Cu, climb rates of a few hundred fpm can be found. Climbs can be made by flying back and forth upwind of an individual Cu, or by flying along cloud streets if they exist. If no clouds are present, but waves are suspected, climb to the top of the thermal and penetrate upwind in search of smooth, weak lift. Without visual clues, thermal waves are more difficult to work. Thermal waves are most often stumbled upon as a pleasant surprise.