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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
best maintain visual contact. [Figure 10-12] When
entering a thermal, strive to do so in a way that will not
interfere with gliders already in the thermal, and above
all, in a manner that will 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” thermalling speed. Collision avoidance, not
optimum aerodynamic efficiency, is the priority when
thermalling with other gliders. Announcing to the other
Lake
Shift Circle
Toward Lake
Lift Strongest When
High Wing Points
Toward Lake
Figure 10-10. Centering by shifting the circle turn toward
stronger lift.
Shallow
then Steepen
Do not
Shallow
Too Much
Lowest
Variometer
Indication-
Wait 60¡
Lowest
Variometer
Indication Surge of Lift
Felt Here
1. 2. 3.
Figure 10-11. Other centering corrections.
10-8
glider(s) on the radio when 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 putting yourself in a situation where you
cannot see the other glider, 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 you lose sight of another glider in a
thermal and cannot establish position 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 thermalling is a priority! Mid-air
collisions can sometimes be survived but only with a
great deal of luck. Unsafe thermalling practices not
only endangers your own safety but that of your fellow
glider pilots. [Figure 10-13]
Leaving a thermal properly can also save you some
altitude. While circling, scan the full 360° of sky with
each thermalling 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 in order
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 will not hinder or endanger other gliders.
The preceding pages describe techniques for locating
thermals, as well as entering, centering, and leaving
thermals. 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
A
B
1
1
2
2
3
3
5
4
5
4
Figure 10-12. Proper positioning with two gliders at the same
altitude. Numbers represent each glider’s position at that
time.
Figure 10-13. When thermalling, avoid flying in another
glider’s blind spot, or directly above or below another glider.
10-9
time. Just as the mechanics of simply flying the glider
become second nature with practice, so do thermalling
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
　
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