Waterfall Effect

South Wind

Every time pilots at Harris Hill fly with a south wind they must plan for some type of sink on landing. This does not mean there will always be a lot of sink. Sink with a south wind can develop in 3 general ways and each has resulted in broken gliders. Planning for sink and not being too high is covered elsewhere in the book but the key factors are a conservative turn to base leg, the pilot actively working the base leg, turning final in an approach wedge which is now adjusted to be steeper for headwind and expected sink.

The first situation is the classic waterfall effect. The south wind is so strong that gliders are pushed all the way back for a downwind takeoff north, or gliders are taking off to the south which is very rare at Harris Hill. In the case of a north takeoff, the glider and towplane barely climb or even sink initially as they launch into the valley. This is a no-brainer; there clearly is a broad area of sink as the air cascades off the front of the hill. Observe some takeoffs and landing before going flying on a day with south wind.

More subtly, on days when south winds are more moderate and the atmosphere is stable, the waterfall effect takes the form of narrow laminar band of sink dropping just outside of the observation area guard rail. This laminar waterfall has caught many Harris Hill flyers off guard because everything seems somewhat normal until the glider suddenly starts sinking away on final approach.

The third type of south wind sink happens on days with strong thermals in the pattern. Even a gentle south breeze can set up a path of least resistance, a low-level divergence, adjacent to a strong updraft in the pattern. The sink normally surrounding the thermal will coalesce into a strong area of sink falling away down the ridge. Whenever there are strong thermals in the pattern with a south wind the pilot should be looking for this phenomenon.

West wind

It is very common to see some westerly wind for takeoffs and landings at Harris Hill. There is not much extra to say about the interplay with terrain. The biggest danger is poor pattern planning where the glider is allowed to drift in toward the field on downwind resulting a button hook pattern with no base leg. Tailwind turns made with reference to the ground, such as a turn to final at Harris Hill with a west wind, require a steeper angle of bank. In strong winds a pilot can consider a pattern on the east side of the field to reduce the angle of bank needed. (This should always be a consideration in planning off-airport landings).

East wind

With an east wind expect some low-level turbulence induced by the buildings and trees near the museum. This really causes no problem but can startle the uninitiated. For pattern planning, pilots used to west winds may set up a downwind too far from the field. This means a base that is too long (too much time on base). The result can be a low turn to final.

Take note that terrain slopes away from the field to the west and southwest. This means with winds from the east and northwest a downslope or waterfall effect can set up here.

North wind and low-level windshear

A stronger wind from the north causes what is called the roll off the back side of the hill where the terrain dips sharply the rises again. It would be more accurate to describe this as a sharp dip as the airflow drops with the terrain and then is abruptly forced up by the knoll south of the airfield. As experienced by a landing pilot, the result is a strong positive gust followed immediately by a negative gust.

This generally is no problem for pilots flying a good pattern. Everyone should plan to cross this area established in a stable, wings-level, final. Many times, the entire phenomenon can be avoided by over-flying the area with a higher, steeper approach. Getting down is generally not a problem with a strong north wind.

Airspeed variations will be observed as the machine flies through positive and negative gusts. These are most noticeable in high performance gliders where the gusts are translated into larger airspeed variations. Large airspeed additives are not necessary nor are wild adjustments to chase airspeed variances. “Timely” and “controlled” best describe the technique. High performance aircraft should be careful to not carry extra energy into ground effect.

One important note on low level windshear: Normally on approach pilots raise the nose and perhaps open the dive brakes to decrease unwanted airspeed. We’ve observed pilots in this low-level regime raise the pitch attitude to level or even a climb and then enter the inevitable negative gust. Not good! Try this method instead while low to the ground and encountering a positive gust–judiciously open the dive brakes to counter the positive gust. Maintain a downward flight path and be ready closed the dive brakes in a measured response to a negative gust.

Flying at Harris Hill will sharpen one’s skills and better prepare a pilot for come-what-may in an off-field landing. The savvy pilot will make time to consider wind factors before every takeoff and landing.