Bowls and Spurs
If the wind is at an angle to the ridge, bowls or spurs (i.e., recessed or protruding rock formations) extending from the main ridge can create better lift on the upwind side and sink on the downwind side. If at or near the height of the ridge, it may be necessary to detour around the spur to avoid the sink, then drift back into the bowl to take advantage of the better lift. After passing such a spur, do not make abrupt turns toward the ridge. Always consider what the general flow of traffic is doing. If soaring hundreds of feet above a spur, it may be possible to fly over it and increase speed in any sink. This requires caution since a thermal in the upwind bowl, or even an imperceptible increase in the wind, can cause greater than anticipated sink on the downwind side. Always have an escape route or, if in any doubt, detour around. [Figure 10-23]
It is not uncommon for thermals to exist with slope lift. Indeed, slope soaring can often be used as a “save” when thermals have temporarily shut down. Working thermals from slope lift requires special techniques. When a thermal is encountered along the ridge, a series of S-turns can be made into the wind. Drift back to the thermal after each turn if needed and, of course, never continue the turn to the point that the glider is turning toward the ridge. Speed is also important, since it is easy to encounter strong sink on the sides of the thermal. It is very likely that staying in thermal lift through the entire S-turn is not possible. The maneuver takes practice, but when done properly, a rapid climb in the thermal can be made well above the ridge crest, where thermaling turns can begin. Even when well above the ridge, caution is needed to ensure the climb is not too slow as to drift into the lee-side sink. Before trying an S-turn, make sure it would not interfere with other traffic along the ridge. [Figure 10-24]
A second technique for catching thermals when slope soaring is to head upwind away from the ridge. This works best when Cu mark potential thermals, and aids timing. If no thermal is found, the pilot should cut the search short while still high enough to dash back downwind to the safety of the slope lift. [Figure 10-25]
As a final note, caution is also needed to avoid obstructions when slope soaring. Obstructions include wires, cables, and power lines, all of which are very difficult to see. When flying at extremely low altitudes along the ridge (tree top level), the glider and pilot may be placed at a high risk of collision with wires. Ensure an adequate reconnaissance has been completed when flying at these altitudes. Aeronautical charts show high-tension towers that have many wires between them. Soaring pilots familiar with the area should be able to provide useful information on any problems with the local ridge.
Tips and Techniques
Observe the ridges slope for collectors or dividers of the wind flow. Plan to determine which of the slopes is gathering wind flow. [Figure 10-26] Due to the changes in wind directions and or sun angles, any portion on the ridge can change in only a few minutes.
- Collectors are mountain/ridge bowls and canyon ends that can offer extreme areas of lift when the wind is blowing into them. Remember to have a way out.
- Dividers are ridges parallel with the wind and tend to have airflow separation. A collector that may be downwind from a divider may receive more airflow, making better lift possible.
The downwind side of any ridge or hill produces turbulence and sink. The larger or higher the ridge and the greater the wind velocity, the wider the turbulence may be. During these conditions, remember to ensure that seat and shoulder harnesses are tight. Sink calls for speed. Turbulence and speed are very hard on the glider airframe and pilot comfort. Pilots must obey glider limitations set forth in the GFM/ POH. Also, do not exceed the design speed for maximum gust intensity (Vb). [Figure 10-27]
The ridge crest is where all airflow starts down. Steep ridges with narrow ridge tops can collect thermal action from both sides of the crest. The best lift could be directly above the crest. However, this terrain can be very hazardous as a pilot cannot see it or may not have visual cues. If wind is the only source of lift, the crest can be very dangerous. Always stay upwind of the crest. [Figure 10-28]
Areas along the ridge that are in deep shade is where the air goes down and sink can be found. If there is strong lift on the sunny side of the ridge, then chances are that strong sink can be found on the shady or dark side of the ridge. This is true when the sun angle is low or late in the afternoon. [Figure 10-29]
Thermal source can be found where drainages area along the ridge meet. Where the ridge or peaks are numerous and slope down into a valley, thermal sources may be found. Canyons or large bowls hold areas of warm air.
Ridges may form cloud streets above the ridge. A glider pilot should try to climb in a thermal to reach these streets. Fast cruising speeds can be found under these streets. The glider pilot must stay upwind to stay in this type of lift near the ridge.
Since gliders tend to seek the same conditions for lift, thermals and areas of ridge lift are areas for special diligence for avoiding other aircraft. To some extent, for the same reasons that gliders seek rising air, other low flying airplanes and helicopters will plan to use that area for flights, so be aware for all aircraft. A portable VHF radio and sharing a common channel for traffic calls is an enhancement for safety, but nothing replaces a good visual scan at all times looking for other aircraft.
Density altitude is increasing as the glider climbs. At 10,000 feet mean sea level (MSL), a pilot is required to have approximately 40 to 45 percent more room to maneuver a glider. The air is less dense by 2 percent per one thousand feet of altitude gained. [Figure 10-30]