Efficient slope soaring (also called ridge soaring or ridge running) is fairly easy; simply fly in the updraft along the upwind side of the ridge. Although the appearance may seem simple, it is very complicated and can be very hazardous for the untrained glider pilot. Ridge soaring can also be very demanding on the glider and the pilot. Even though it is easy to fly, there are many situations in which a glider pilot can be exposed to hazards if proper training has not been received. A thorough preflight and route planning needs to be accomplished. This planning also includes ridge selection based on the current winds. The horizontal distance from the ridge varies with height above the ridge, since the best lift zone, or optimum lift zones (OLZ) tilts upwind with height above the ridge. These zones, or OLZ, vary but usually are slightly off the top of the ridge, with a slight angle into the prevailing wind. The bottom of the OLZ may be slightly down from the top line under normal conditions. These OLZ vary with the size and terrain makeup of the ridge. [Figure 10-14]
Surface winds of 15–20 knots that are perpendicular to the ridge are ideal. Wind flow within 45° of the perpendicular line also provides adequate lift. Winds less than 10 knots have also produced adequate ridge soaring dependent 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 and the need for good preflight planning and training is required. [Figure 10-15]
- Airflow mirrors a hill or ridge shape. Imagine a flow of water around the ridge instead of air. However, air is thinner and can be compressed as in a “venturi effect” and can be “squeezed” and accelerated, especially along the ridge. [Figure 10-16]
- Ridges that have an irregular profile are hazardous. The more complicated the ridge is, the more erratic the airflow may become. [Figure 10-17]
Even though the idea is simple, traps exist for both new and expert glider pilots. Obtain instruction when first learning to ridge soar/slope soar. Avoid approaching from the upwind side perpendicularly to the ridge. Instead, approach the ridge at a 45° angle, so that a quick egress away from the ridge is possible should lift not be contacted.
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 10-18]
While flying along the ridge, a crab angle is necessary to avoid drifting too close to the ridge or, if gliding above the ridge, to avoid drifting over the top into the leeside downdraft. 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 strong 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 10-19]
For the new glider pilot, crabbing along the ridge may be a strange sensation, and it is easy to become uncoordinated while trying to point the nose along the ridge. This is both inefficient and dangerous, since it leads to a skid toward the ridge. [Figure 10-20]
In theory, to obtain the best climb, it is best to slope soar at minimum sink speed. However, flying that slowly may be unwise for two reasons. First, minimum sink speed is relatively close to stall speed, and flying close to stall speed near terrain has obvious dangers. Second, maneuverability at minimum sink speed may be inadequate for proper control near terrain, especially if the wind is gusty and/or thermals are present. When gliding at or below ridge top height, fly faster than minimum sink speed—how much faster depends on the glider, terrain, and turbulence. When the glider is at least several hundred feet above the ridge and shifting upwind away from it in the best lift zone, reduce speed. If in doubt, fly faster.
NOTE: When flying close to the ridge, use extra speed for safety—extra speed gives the glider more positive flight control input and also enables the glider to fly through areas of sink quickly. Ensure that seat and lap belts are tightened. [Figure 10-21]
Procedures for Safe Flying
Slope soaring comes with several procedures to enable safe flying and to allow many gliders on the same ridge. The rules are explained in the following paragraphs and illustrated in Figure 10-22.
Make all turns away from the ridge. [Figure 10-22A] A turn toward the ridge is dangerous, even if gliding seemingly well away from the ridge. The groundspeed on the downwind portion of the turn is difficult to judge properly, and striking the ridge is a serious threat. Even if above the ridge, it is easy to finish the turn downwind which may take the glider over the ridge crest; this puts the glider into heavy sink.
Do not fly directly above or below another glider. [Figure 10-22B] Gliders spaced closely together in the vertical are in each other’s blind spots. A slight change in climb rate between the gliders can lead to a collision.
Pass another glider on the ridge side, anticipating that the other pilot will make a turn away from the ridge. [Figure 10-22C] Sometimes the glider to be passed is so close to the ridge that there is inadequate space to pass between the glider and the ridge. In that case, either turn back in the other direction (away from the ridge) if traffic permits or fly upwind away from the ridge and rejoin the slope lift as traffic allows. If using a radio, try to contact the glider by the completion number and then coordinate the passing. When soaring outside of the United States, be aware that this rule may differ.
The glider with its right side to the ridge has the right of way. [Figure 10-22D] Title 14 of the Code of Federal Regulations (14 CFR) requires both aircraft approaching head-on to give way to the right. A glider with the ridge to the right may not have room to move in that direction. The glider with its left side to the ridge should give way. Additionally, when overtaking a slower glider along the ridge, always pass on the ridge side. If the overtaking glider encounters sink, turbulence, etc., it must maneuver away from the ridge. This is acceptable. When piloting the glider with its right side to the ridge, ensure the approaching glider sees you and is yielding in plenty of time. In general, gliders approaching head-on are difficult to see; therefore, extra vigilance is needed to avoid collisions while slope soaring. The use of a radio during ridge soaring is recommended. Pilots must be familiar with 14 CFR part 91, section 91.113, Right-of-way rules: Except water operations, and section 91.111, Operating near other aircraft.