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Preventing Encounters with Rotors and Mechanical Turbulence
Rotors and mechanical turbulence are created as the wind passes anything in its path that it cannot pass smoothly over. When the wind passes over these objects in its path, it will swirl violently. As the wind increases, so does the potential for dangerous rotors. The sharper the edges on the objects that the wind passes over, the stronger the mechanical turbulence will be. The following are all common rotor triggers:
- Horizontal Edges
- Vertical Edges
- Mountain Tops
- Trees
- Buildings
- Rocks
- Any object that blocks the wind’s path and has edges!!!
When flying coastal and mountain sites you need to know the area well and be familiar with the flow of air in different directions
- Understand and visualize rotor triggers and probable rotor areas.
- Be able to visualize how rotor location can change, based on the current wind direction.
- Continually monitor the wind direction and velocity to update these perceptions anytime the wind direction or velocity changes.
- Fly with your guard up to prevent direct encounters with mechanical turbulence.
Rotors are similar to eddies in the water. A good way to visualize where a rotor is, is to picture water flowing in the direction the wind is going and where the flow would carry the swirling water/representing air, created by the rotor trigger. Wherever a trigger is, picture how the terrain downwind of the trigger will affect where the swirling air will go. If the rotor runs into a slope behind the trigger, the airflow including the swirling rotor will be deflected upward. Rotors will funnel into low spots and thermals can even lift them high into the air.
On a cliff that has a flat or shallow slope behind its edge, the rotor will rise at an angle of about 20 degrees. If there is a ridge or slope behind a front rotor trigger, the rotor will rise at a steeper angle. Learning how to predict rotor triggers and where the rotor will be carried is a constant process while flying. If you stay upwind of any rotor triggers, you will not fly into rotors. When you top land on any mountain or cliff, learning to see where the rotors will be created and carried is necessary to avoid flying into the turbulence created by the triggers. Landing in stronger winds on top is extremely hazardous if you get your wing into a rotor. A hike up from a beach is a better option than top landing in a violent rotor.
If there is a steep cliff with a flat spot (like a road or flat trail) across its face, the airflow will be traveling up the cliff and the edge of the road will create a rotor. If the cliff above the road is sloped, the rotor will travel up the above cliff with the slope. Flying or scratching near the cliff above such a trigger is a common issue that catches new and sometimes even experienced pilots .
Click Image to view full size picture below …
The above image shows rotors from the horizontal edges from the flat area in front of the old Highway 1. The below image focuses on a spine and some topography that has caught a lot of pilots off guard. Cheetah Ridge is the bowl just South of Mussel Rock. There is a south facing cliff there that has a spine that angles down to the ocean. This spine is trecherous because with a north wind, the vertical face of this is very sharp. The top of the cliff is also a very sharp edge. What makes this location so precarious is that when a pilot flies from Mussel Rock and flies the cliffs in Pacifica, if the wind was WSW and switches to WNW or NW, getting back to Mussel Rock requires passing the below noted area. Pilots have tried to get around the north spine but sink on their way there as there is no lift with this direction of wind. Or, they try to fly above the top of the Death Bowl lip, this is possible, but you need more altitude as there will not be any lift from the north end of the Cheetah Bowl with this direction of wind.
If you are ever on the Pacifica side and the wind has switched to any north of west direction, you can always land south of Mussel Rock and walk/hike back!
The same spine I am talking about below could also produce a rotor in the Death Bowl when the wind is SSW or SW. The key to all rotor avoidance is to be aware of the direction and picture where the wind flow would carry a rotor. All vertical or horizontal edges create rotors.
Rotor Area if the wind is WNW or NW at Cheetah Ridge
(This spot has lots of complexity, be aware of wind direction and rotors)
Compression and lack of lift in SSW – SW winds at Death Bowl
(This spot has lots of complexity, be aware of wind direction and rotors)
The below picture shows the vertical nature of the Cheetah’s North Spine
The Cheetah Cliff/Bowl is just south of Mussel Rock and requires some altitude to cross either from the north or from the south. Crossing this area require more than basic ridge soaring skills
Rotors dissipate as they travel downwind. In lighter winds below 12 mph, about 200 – 300+ yards downwind of a steep cliff (this also depends on the sharpness of the edge and the vertical height of cliff and the relationship of height to the landing area), you might be far enough downwind for the rotor to be manageable. On a stronger day, the rotor can travel much further, as far as 10 times the height of the rotor trigger. Most ridge soaring sites have landing areas near the cliff, where the paraglider airfoil can be be kept above the swirling air during the approach and landing.
At some sites in more moderate or light conditions, you can land far enough downwind of the cliff edge that the rotor will have dissipated enough to allow for a safe landing. Local knowledge is a golden rule when flying new sites. Always find out from the locals where the best places to land are and how conditions change the options. The important point to keep in mind is to not be let the airfoil encounter the rotor at any point close enough to the trigger point to cause any collapses or loss of control for the landing approach.
Pilots can sometimes get into trouble with vertical edges of a cliff. If the wind in cross and pilots will try to stay very close to the cliff, this is called scratching. But, if there are vertical or even horizontal edges to the cliffs or anything directly upwind of where you are flying, rotors can occur.
So, on a cross day, you need to see such a rotor trigger and give yourself padding where such a rotor trigger might spit its mechanical turbulence.
I have an entire article written about wind management and avoiding getting blown back. Getting blown back behind a ridge will put you into a rotor, so please read this article to help you learn how to avoid it.
To summarize:
- Check the Wind Speeds before you fly.
- If you are new to the site, get a full introduction. Read the below Checklist before flying the site.
- Have a speed system ready on your glider. This means connected, adjusted, and ready for use.
- Know that the winds can be dramatically stronger and dangerous at th top of the Westlake cliffs.
- Continuously monitor the wind speeds as you fly. (Do this via your crab angle and lateral groundspeed along the ridge).
- Avoid flying higher on windy days.
- If you do fly higher in stronger winds, fly well upwind of the cliffs as you ascend and traverse the cliff.
- Know how and when to penetrate out in front and how to descend to lower winds.
- Penetrate and get down as soon as you notice that the wind is getting strong. When you get on your speed bar, your purpose should be getting down, not staying up in the strong winds.
- Avoid the area above and behind the top of the cliff at all times.
- Below is a Checklist for other safety issues related to flying at Mussel Rocks / The Dumps.
- Never fly alone.
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