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As an example, assume two gliders are flying under a
cloud street with frequent thermals and only weak sink
between thermals. Glider #1 conserves altitude and
stays close to cloud base by flying best L/D through
weak sink. To stay under the clouds, he is forced to fly
Figure 11-11. Examples of glides achieved for different MacCready ring settings.
Figure 11-12. Thermal height and height band (shaded) vs.
time of day.
11-13
faster in areas of lift, exactly opposite of flying fast in
sink, slow in lift. Glider #2 uses the conditions more
efficiently by flying faster in the sink and slower in lift.
In a short time, Glider #2 has gained distance on Glider
#1. At the end of the cloud street, one good climb
quickly puts Glider #2 near cloud base and well ahead
of Glider #1. [Figure 11-13]
On an actual cross-country flight, a combination of dolphin
flight and classic climb and glide is frequently
needed. In a previous example, the two pilots who
decided not to stop and circle in the weaker thermals
would still benefit from dolphin flight techniques in the
lift and sink until stopping to climb in the strong lift.
SPECIAL SITUATIONS
COURSE DEVIATIONS
Diversion on a soaring cross-country flight is the norm
rather than the exception. Some soaring days supply
fair-weather cumulus evenly-spaced across all quadrants,
but even these days cycle and it is beneficial to
deviate toward stronger lift. Deviations of 10° or less
add little to the total distance and should be used without
hesitation to fly toward better lift. Even deviations
up to 30° are well worthwhile if they lead toward better
lift and/or avoid suspected sink ahead. The sooner the
deviation is started, the less total distance will be covered
during the deviation. [Figure 11-14]
Figure 11-13. Advantage of proper speed to fly under a cloud street.
Figure 11-14. Effect of starting course deviations at different times. White arrows show extra distance and indicate the benefit
of early course deviations.
11-14
Deviations of 45° or even 90° may be needed to avoid
poor conditions ahead. An example might be a large
cloudless area or a shaded area where cumulus have
spread out into stratus clouds. Sometimes deviations in
excess of 90° are needed to return to active thermals
after venturing into potentially dead air.
Deviations due to poor weather ahead should be undertaken
before the flight becomes unsafe. For instance, if
cloud bases are lowering and showers developing,
always have the option for a safe, clear landing area
before conditions deteriorate too far. It is very easy to
leave one safe landing site hoping to beat weather to
the next, only to have both spots go below VFR during
the glide. Thunderstorms along the course are a special
hazard, since storm outflow can affect surface winds
for many miles surrounding the storm. Do not count on
landing at a site within 10 miles of a strong thunderstorm—
sites further removed are safer. Thunderstorms
ahead often warrant large course deviations, even up to
180° (i.e., retreat to safety).
LOST PROCEDURES
Navigation has become far easier with the advent of
GPS. Since GPS systems are not 100 percent free from
failure, pilots must still be able use the sectional chart
and compass for navigation. It is important to have an
alternate plan in the event of becoming lost.
As discussed earlier, preflight preparation can help in
avoiding becoming lost. Spend some time studying the
sectional chart for airports or other notable landmarks
along the route.
If still lost after some initial searching, try to remain
calm. The first priority is to make sure there is a suitable
landing area within gliding distance. Then, if possible,
try to find lift, even if it is weak, and climb. This
buys time and gives a wider view of the area. Next, try
to estimate the last known position, the course flown,
and any possible differences in wind at altitude. For
instance, maybe the headwind is stronger then anticipated
and not as far along the course as expected. Try
to pin point the present position from an estimate of the
distance traveled for a given period of time and confirm
it with visible landmarks by reference to the sectional
chart. For instance, if at point X, averaging
about 50 knots, heading north for about 30 minutes,
should put you at point Y. Look again at the sectional
chart for landmarks that should be nearby point Y, then
search the ground for these landmarks. Thermalling
while searching has the added advantage of allowing a
　
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