adjust the circle by either straightening or shallowing the turn toward the stronger lift. The pilot should anticipate the turn
and begin rolling out about 30° before the heading toward the strongest part. This allows rolling back toward the strongest
part of the thermal rather than flying through it and turning away from the thermal center. How long a glider remains in
a shallow bank or straight depends on the size of the thermal. Since gusts within the thermal can cause airspeed indicator
variations, the pilot should pay attention to the nose attitude.
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.
A pilot can use other techniques as depicted in the three scenes of Figure 10-11 and as described below. The pilot should:
1. Shallow the turn slightly (5° or 10°) when encountering weaker lift, then when encountering stronger lift again as
indicated by increase in positive G-force or the variometer, the pilot resumes the original bank angle. If shallowing
the turn too much, the glider may fly completely away from the lift.
2. Straighten or shallow the turn for a few seconds 60° after encountering the weakest lift or worst sink indicated by the
variometer, then resume the original bank angle. This accounts for the lag in the variometer since the actual worst
sink occurred a couple of seconds earlier than indicated.
3. Straighten or shallow the turn for a few seconds when the stronger seat-of-the-pants surge is felt. Then, the pilot
should resume the original bank and verify the result with the variometer trend.
Surge of lift felt here
Lowest variometer indication wait 60°
Do not shallow too much
Shallow, then steepen
Lowest variometer indication
Figure 10-11. Other centering corrections.
New glider pilots should develop proficiency centering using one of the above methods first, and then experiment with
other methods. As an additional note, thermals often deviate markedly from the conceptual model of concentric gradients
with lift increasing toward the center. For instance, it sometimes feels as if two (or more) nearby thermal centers exist,
which can make centering difficult. Glider pilots should continually adjust, and recenter to maintain the best rate of climb.
Other gliders can help pilots center a thermal as well. If a nearby glider seems to be climbing better, the pilot can adjust the
turn to fly within the same circle. Similarly, if noting a bird soaring close by, a turn toward the soaring bird may lead to a
better climb. Soaring birds have a much tighter turning radius than a glider, so they can stay in the strongest center of the
lift while the glider pilot circles around them.
Collision Avoidance
Collision avoidance takes priority over aerodynamic efficiency when sharing a thermal with other gliders. The first glider
in a thermal establishes the direction of turn, and all other gliders that join 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 visual separation. [Figure 10-12] A pilot should enter a thermal in a way that does not interfere with gliders
already in the thermal. Pulling up to bleed off excess speed in the middle of a crowded thermal would create a hazard to
other gliders. Safe technique involves bleeding off speed before reaching the thermal. Announcing the entry to the other
glider(s) on the radio (if equipped) may enhance collision avoidance.
Figure 10-12. Proper positioning with two gliders at the same altitude. Each number corresponds to the position of both gliders at a
given time.
Different types of gliders in the same thermal may have different minimum sink speeds, which makes it more difficult
for each glider to remain directly across from the other. Each pilot should remain in visual contact with the other glider.
Radio communication may help avoid a collision, but too much talking clogs the frequency and can impede the broadcast
of another pilot’s message. Pilots should not fly directly above or below another glider in a thermal since differences in
performance, or even minor changes in speed, can lead to unexpected altitude changes. If a pilot loses sight of another
glider in a thermal and cannot verify the other glider’s position via a radio call, the pilot should leave the thermal. After
10 or 20 seconds, the pilot can come back around to rejoin the thermal in a better position to see the other traffic. Unsafe
thermalling practices make a mid-air collision more likely, which could result in fatalities.
Exiting a Thermal
When exiting a thermal, appropriate methods can preserve some altitude. The pilot should increase speed as needed to
penetrate any sink often found on the edge of the thermal and leave the thermal in a manner that does not hinder or endanger
other gliders. While circling, the pilot scans the full 360° of sky. This allows the pilot to continually check for other traffic
in the vicinity and decide where to go for the next climb. Experienced pilots often decide where to go next while still in lift.
Managing Expectations
Glider pilots should adapt quickly to whatever the air has to offer at the time. While thermalling techniques become second
nature with practice, pilots should expect to land early if unable to find sufficient lift as part of normal flying experience.
Ridge/Slope Soaring
Efficient slope soaring (also called ridge soaring or ridge running) involves flying in the updraft along the upwind side of a
ridge. Although the concept seems simple, slope soaring presents significant hazards and places demands on the glider and
the pilot. Thorough preflight planning and route planning include ridge selection based on the current winds.
Surface winds of 15–20 knots perpendicular to the ridge optimize ridge soaring. Wind flow within 45° of the perpendicular
line also provides adequate lift. Winds less than 10 knots might produce adequate ridge soaring depending on the terrain,
but with 10 knots of wind or less, pilots should avoid flying low over any ridge due to the possibility of encountering sink.
Local ridge pilots know about of these conditions. [Figure 10-13]
Wind
Lift
Sailplane turns away from
ridge when turning around
Wind weaker near ground
Figure 10-13. Ridge wind flow.
Airflow follows the hill or ridge shape. The pilot can imagine a flow of water around the ridge instead of air. However, air
can compress and may develop local variations and eddies. [Figure 10-14]
Figure 10-14. Airflow generally follows the hill’ s shape.
The more complicated the ridge, the more erratic and hazardous the airflow may become. [Figure 10-15]
Figure 10-15. Irregular profiles may create a hazard.
Traps
Since traps or dangers exist during ridge soaring, glider pilots should obtain instruction when first learning to ridge soar or
slope soar. Pilots should approach the upwind side of the ridge at a 45° angle, so that a quick egress away from the ridge
can occur in the absence of lift.
NOTE: When approaching the ridge from downwind, approach the ridge at a diagonal. If excess sink is encountered, this
method allows a quick turn away from the ridge. [Figure 10-16]
Figure 10-16. Approach ridges diagonally.
If gliding above the ridge, an appropriate crab angle prevents drifting over the top into the lee-side downdraft. For the new
glider pilot, crabbing along the ridge may seem strange, and the pilot might resort to uncoordinated control input to point
the nose along the ridge. This could result in an inefficient and dangerous skid toward the ridge.
Thermal sink can turn the glider upside down, a phenomenon known as upset. A thermal may appear anywhere. When it
appears from the opposite side of the ridge, it has strong energy. When flying in complex conditions (winds and thermals),
fly with extra speed for positive control of the glider. DO NOT fly on the ridge crest or below the ridge on the downwind
side. [Figure 10-17]
Figure 10-17. Thermal sink can roll the craft toward the mountain.
Whenever flying downwind, groundspeed and the radius of any turn will increase. [Figure 10-18]
Ridge
Strong lee-side sink
Wind 15 knots
Wind 15 knots
Normal radius of turn—no wind
Increased radius of turn with wind
Safer, angled approach, then set crab angle
Figure 10-18. Flying with a wind increases the turn radius over the ground, so approach the ridge at a shallow angle.
In theory, to obtain the best climb, the pilot should fly at minimum sink speed. Since minimum sink speed gives less margin
above stall speed, flying at this speed near terrain may create danger. Maneuverability at minimum sink speed may not
provide for adequate control near terrain, especially if gusty wind or thermals complicate the ridge lift. When gliding at
or below ridge-top height, the pilot should fly faster than minimum sink speed—how much faster depends on the glider,
terrain, and turbulence. When the glider climbs to at least several hundred feet above the ridge and when shifting upwind
away from it within the best lift zone, the pilot can reduce speed.
NOTE: When flying close to the ridge, use extra speed for safety—extra speed gives the glider more positive flight control
input and enables the glider to fly through areas of sink quickly. [Figure 10-19]
Figure 10-19. When flying close to a ridge, pilots use extra speed for more control and to pass quickly through sink.
Procedures for Safe Flying
Several procedures enhance safe slope soaring and allow many gliders to use the same ridge simultaneously. The following
paragraphs and Figure 10-20 explain the procedures.
Wind
Wind
Wind
Wind
Figure 10-20. Ridge rules.