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9-33.
With no cloud present, a convergence zone is sometimes
marked by a difference in visibility across it,
which may be subtle or distinct. When there are no
clues in the sky itself, there may be some on the
ground. If lakes are nearby, look for wind differences
on lakes a few miles apart. A lake showing a wind
direction different than the ambient flow for the day
may be a clue. Wind direction shown by smoke can
also be an important indicator. A few dust devils, or
better, a short line of them, may indicate the presence
of ordinary thermals vs. those triggered by convergence.
Spotting the subtle clues takes practice and
good observational skills, and is often the reason a few
pilots are still soaring while others are already on the
ground.
The best soaring technique for this type of lift depends
on the nature of the convergence zone itself. For
instance, a sea-breeze front may be well-defined and
marked by “curtain” clouds, in which case the pilot can
fly straight along the line in fairly steady lift. A weaker
convergence line often produces more lift than sink, in
which case the pilot must fly slower in lift and faster in
sink. An even weaker convergence line may just act as
focus for more-frequent thermals, in which case normal
thermal techniques are used along the convergence line.
Some combination of straight legs along the line with
an occasional stop to thermal is often used.
Convergence zone lift can at times be somewhat turbulent,
especially if air from different sources is mixing,
such as along a sea-breeze front. The general roughness
may be the only clue of being along some sort of convergence
line. There can also be narrow and rough (but
strong) thermals within the convergence line. Work
these areas like any other difficult thermals—steeper
bank angles and more speed for maneuverability.
COMBINED SOURCES OF UPDRAFTS
Finally, lift sources have been categorized into four
types: thermal, slope, wave, and convergence. Often,
more than one type of lift exists at the same time. For
instance, thermals with slope lift, thermalling into a
wave, convergence zones enhancing thermals, thermal
waves, and wave soaring from slope lift were all considered.
In mountainous terrain, it is possible for all
four lift types to exist on a single day. The glider pilot
needs to remain mentally nimble to take advantage of
various pieces of rising air during the flight.
Nature does not know that it must only produce rising
air based on these four lift categories. Sources of lift
that do not fit one of the four lift types discussed probably
exist. For instance, there have been a few reports
of pilots soaring in travelling waves, the source of
which was not known. At some soaring sites it is sometimes
difficult to classify the type of lift. This should
not be a problem—simply work the mystery lift as
needed, then ponder its nature after the flight.
11-1
A cross-country flight is defined as one in which the
glider has flown beyond gliding distance from the local
soaring site. Cross-country soaring seems simple
enough in theory, but in reality, it requires a great deal
more preparation and decision making than local soaring
flights. Items that must be considered during crosscountry
flights are how good are the thermals ahead,
and will they remain active? What are the landing possibilities?
Which airport along the course has a runway
that is favorable for the prevailing wind conditions?
What effect will the headwind have on the glide? What
is the best speed to fly in sink between thermals?
Flying cross-country using thermals is the basis of this
chapter. A detailed description of cross-country using
ridge or wave lift is beyond the scope of this chapter.
FLIGHT PREPARATION AND PLANNING
Adequate soaring skills form the basis of the pilot’s
preparation for cross-country soaring. Until the pilot
has flown several flights in excess of two hours and can
locate and utilize thermals consistently, the pilot should
focus on improving those skills before attempting
cross-country flights.
Any cross-country flight may end in an off-field landing,
so short-field landing skills are essential. These
landings should be practiced on local flights by setting
up a simulated off-field landing area. Care is needed to
avoid interfering with the normal flow of traffic during
simulated off-field landings. The first few simulated
landings should be done with an instructor, and several
should be done without the use of the altimeter.
The landing area can be selected from the ground, but
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Glider Flying Handbook(148)
