Why Thermal in a Slip? | And the Hazards of Skidding

Experienced soaring pilots usually advise thermalling in a slip. Dick Johnson in the July 1997 issue of Soaring magazine provides an excellent technical explanation for how and why to use this technique.

The bottom line is that a turn requires a combination of pitch, bank, and yaw to maintain a stable configuration. And it requires some compromises to achieve the most efficient combination.

If you thermal fully coordinated, you will need to use top aileron to avoid “falling into the turn”. If you slip using top rudder, you will minimize or even eliminate the top aileron. You are compromising by dragging the fuselage through the air stream, while making the wing cleaner.

It seems like some combination of top rudder and top aileron is often most efficient.

Conversely, turning with ANY amount of skid is VERY inefficient. The MORE you skid, the MORE top aileron you need to avoid falling into the turn.

In this case, you are BOTH dragging the fuselage through the air stream AND using quite a bit of aileron to keep the turn stable. This is very bad!

I notice that many pilots thermal with a heavy inside foot, resulting in a skidding turn. Aside from being more spin prone, even a minor skid will be very draggy!

Next, skidding into a turn (leading with the rudder) makes the glider especially spin prone. Initially you rudder into the turn, followed by feeding in back stick and opposite aileron to stabilize the turn. This serves to load up the glider while putting in control inputs that aggravate stalling/entering a spin.

This is especially dangerous in the pattern. Many pilots have a tendency to over-rudder their turns while on approach.

Here is a scenario that can easily result in a stall/spin. A low and slow base leg, followed by an over-ruddered, button-hooking turn.

As you start turning, you use too much rudder. This is followed by pulling back to get the turn established. And finally, feeding in opposite aileron to keep the glider from falling into the turn. This results in the glider departing into an unrecoverable spin.

Bringing It All Together | A Lap Along the Local Ridge

It’s a nice day with moderate wind, perpendicular wind direction, and decent thermals. We had towed over to the ridge, tested out the conditions and dropped down on the trees. We are finally ready to soar along the ridge, having trained all of the key ridge concepts and raring to go!

In this video, we do a simple lap from the Upper Reservoir, to the Delaware Water Gap, up to the Millbrook Powerline, and back to the Upper Reservoir. We discuss the local landmarks along the way and some of they key thoughts that come to mind in various stages of the flight.

If any of the concepts or landmarks are not familiar to you, refer back to their respective videos.

It’s worth noting that when the conditions are working well, ridge soaring *feels easy*. And in a way it is; when a pilot has the foresight and skill to choose the appropriate conditions and stays in the wonderful ridge band, there’s little that can go wrong.

But the training, coaching and the practice is to prepare you for that very reason. When you know the traps, have a plan and have the knowledge to extricate yourself from various situations, you will be able to fly the ridge safely.

And this is why there is about 3 hours worth of explanation and context that goes into a 20 minute video of showing how it works when everything goes right.

Introduction to Ridge Transitions | The Catfish Ridge Jump

The purple line is the flight path. We DO NOT fly in the “No fly zone” as this area is dangerous and the ridge works poorly. Instead, we make our transition from the local ridge to the catfish ridge. We often use the “gully line” to find a thermal and good air to make our crossing.

This segment is a crash course in how to do ridge transitions. We start out with the Catfish Ridge transition as it is the first one that many pilots will encounter in their ridge training. This little offset has all the features of a transition and gives our pilots a great way to practice their transition technique.

Ridge transitions often require a lot of attention in a ridge flight. These are areas where we have to cross from one ridge section to another, often due to the shape or current conditions making a section not work well enough to support the glider. These crossings are “hinge points”, as you are more likely to find sink, or a non-working section of ridge while completing a transition. Many ridge-related landouts are due to transitions going wrong.

When a transition goes wrong, you have to execute a landing very quickly in sporty conditions. A couple seconds here or there could make a big difference in the outcome.

Having extra altitude and margin goes a long way to mitigating the risk.

Situational Awareness

While flying the ridge, you should be completely aware of all the transitions on the ridge you intend to fly. You should recognize that you are approaching a transition point as you are flying the ridge. Next, you should have scouted the landing options at the base of the ridge, should the transition go wrong. You should find them and plan out your approach before you begin the jump.

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There are five steps a transition:

Gain altitude (Float up in the ridge band or thermal)

As you approach a transition, it usually pays to slow down early and gain altitude. By working your way up into the higher lift band, you can make the ridge do the work of giving you sufficient altitude, rather than struggling to climb later.

The amount of height you will need depends on the conditions, your experience, and your glider. We often give some numbers for pilots to start out with, such as ~2100ft MSL for the Catfish Ridge transition. When we provide these values, we are giving pilots “the highest common denominator” with respect to altitude. The height that you would make it across assuming you run a bad line with a less favorable wind direction. But with more experience, you can judge the conditions on a day-by-day basis.

Pick your line

A big decision is deciding where you are going to jump off from. The simplest way to do it is to simply find the shortest line; get as high and as close as you can to where you want to go and make a beeline across to the other side. However, this neglects the effect of the air in between. Often transitions will have thermals or energy lines that line up consistently due to local terrain features. It often pays to run “a good line” upwind and maintain higher altitude rather than simply taking the shortest path. Conversely, making a transition in strong sink into a strong headwind will make your altitude evaporate very quickly and you may not make it across.

Probe the air

When you start a transition, you are not immediately committed to going all the way across; you can test out the waters. If the transition is working out well and you are generally maintaining your altitude, that’s good. If you start hitting sink and dropping out, you can still turn around and try again.

Point of no return

There is a point when you can no longer get back to the ridge you started from. You must recognize and acknowledge this point as you do your transitions. Once you have gotten beyond the point of no return, you will either make it across, or land in a field that you have scouted out in case the transition does not work out.

If you have extra altitude and you are going upwind, your point of no return can be delayed quite a while. Downwind jumps will reach this point a lot earlier as you get blown too far downwind to make it back very quickly.

Commit to the transition

This is less the case in small jumps like to the Catfish ridge, but there are instances in large transitions that you get beyond the point of no return and may or may not be able to make it across. This is the case such as if you are trying to do big upwind transition and you are in the middle of the valley, considering whether you have enough energy to safely go over the top of the mountain. During this stage, you are considering your landing options along the way. As you get closer, you decide whether you will want to commit to the transition and continue over the top of the mountain.

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Doing Transitions Low

In the videos, we explored how to do transitions by the book and what happens when you do them marginally. As you get lower, factors such as the wind direction can play a significant role as to whether you will make it across or not. If the wind angle is off and you have a significant headwind component, you may have to go a long way into lee sink, headwind and around a corner to find a working section of ridge. You will lose a lot of altitude along the way.

Next, we see how the stages shift based on your margins. If you’re low, you will reach the point of no return sooner. This commits you much earlier and you are taking a greater chance of coming up short and landing in a field.

In sum, a couple hundred feet here or there makes a BIG difference in ridge transitions. Stay ahead of the steps, know your outs, and maintain your margins.

Off-Angle Wind

When the wind angle shifts more than 30 degrees off from perpendicular, the ridge lift becomes less reliable. As a result, we strongly recommend that beginners “stick to the formula”; wind speed 15-25 knots at ridge top, within 30 degrees of perpendicular for the section you are flying. In these conditions, you can generally expect the well-flown ridges to have “robust” ridge lift. This is to say that the ridge lift should be consistent and reliable; few factors can suppress or significantly affect the ridge band.

However, when the wind is outside the formula, this is not to say that the lift shuts off. Instead, there are many more complications that can affect how the ridge band works. Certain sections may work very nicely, while other sections are completely flat. The high part of the band may allow you to float at a moderate speed, whereas the low band may not be working.

With experience, it becomes possible to anticipate these factors. That said, it requires flying on many days, progressively exploring the ridge, and building up your margins. Intermediates may explore these days in the comfort and safety of the local ridge and experts may take advantage of these days going cross country.

While I do not advise beginners to fly on an off-angle ridge, beginners should nonetheless have a sense of what to expect if the wind angle shifts, for instance if the conditions unexpectedly change over the course of the day.

The more off-angle the wind is, the higher off the ridge you have to be.

As the angle shifts, the air becomes violent down on the treetops. There will be a turbulent wash as the wind angles over the mountain. By the time the angle is 45 degrees off, you typically have to be at least 300ft-400ft above the trees for the band to work well. Aside from being turbulent, the lift will typically be disorganized and weaker lower.

The more the angle shifts, the higher you must float along the ridge.

If the wind is on the weaker end of the spectrum (<20 knots) and/or the thermal activity is limited (think overcast above), the high band will get weaker.

If the wind is weak and/or the thermal activity is suppressed, watch out. This will weaken the high/float band. This is a really big problem! The lift band becomes compressed; it becomes only possible to float at best glide speed at 2000ft MSL or so and you cannot get higher or lower. It takes very little for this lift band to weaken to the point where it will no longer sustain you.

If the wind is weak/moderate and/or the thermals are not working well, do not count on an off-angle ridge.

Expect substantial sink when encountering thermals on the ridge.

Watch out for thermal suppression, especially when heading into a quartering headwind. Recognize that you will be approaching the thermals on a cross angle. This means you will need to traverse a long band of sink before you encounter the lift.

If the ridge band is not working well and the thermals are strong, you can easily get flushed down to ridge top. Moreover, this effect worsens as the wind angle increases. For example, if the wind angle on ridge top is 45 degrees off, but the thermal street above angles 55 degrees off at cloudbase, you may traverse 5-10 knot down sink over the course of a half mile! This amount of sink could be enough to knock a glider off a ridge, especially a low performance ship.

Watch your airspeed while down at ridge top.

When going upwind, your ground speed will be very low. However, when you turn downwind, your ground speed will be very high. When heading downwind, it is easy to let yourself get dangerously slow if you are not paying attention. The trees will be flashing by very quickly, giving you the illusion of flying a lot faster than you really are.

Certain ridges are tolerant of an off-angle wind; others are not.

Generally, ridges that are fairly high, straight, have few upwind obstructions, and few issues like gaps, bowls and spurs are more tolerant of an off-angle wind. Conversely, any of these issues will cause a ridge not to work in a given section if the angle is sufficiently off. A ridge with many issues may not work with a wind angle greater than 30 degrees off.


In summary, when the angle shifts beyond suggested tolerances (+- 30 degrees), the conditions become marginal. This makes flying in these conditions an expert endeavor. However, the lift does not necessarily go away. With practice and experience, you may still find a solid working ridge band. And if the conditions unexpectedly shift, you will know what to expect.

The Downhill Trap

As you fly the ridge, you must pay attention to how the mountain is changing ahead of you. One important change is if the elevation of the mountain is increasing or decreasing. If the mountain is getting lower ahead, be careful!

The solution is to maintain a constant altitude over a low section of ridge. If you’re able to maintain a selected altitude, then you’re doing well. If you start settling down, be ready to turn back on a moment’s notice.

The downhill trap occurs when you drive along the ridge while maintaining constant altitude in relation to the crest, but you are in fact sinking down along a descending mountain. The lift could be pretty weak, but yet you could maintain high speed down on the tree tops.

When you run out of mountain ahead of you and turn around, you will instantly find yourself low and slow. You will see the mountain extending way above you. At best, you will limp along in weak lift trying to climb along in the increasing elevation, scared out of your wits. At worst, you will promptly fall off the ridge.

The Blairstown ridge extending from Sunfish Pond to the Delaware Water Gap is an example of a section that has this trap. As you build experience and prepare to fly the local ridge, pay special attention to this area and don’t make this mistake!

Falling Off the Ridge

What happens when the wind is too weak to sustain a glider at ridge top? You will settle down lower and lower, until you almost get down to ridge top. Then, you will immediately have to leave the ridge and head toward a landing option.

This process happens very quickly. If you’re 400 ft above the mountain and the ridge is not working, you will be down at ridge top in as little as two minutes. The trees will be filling in your canopy very rapidly.

Note that you will NOT settle into an “equilibrium point”. No matter how low and slow you get, you will NOT sustain at ridge top.

Maintain your airspeed at least at Best Glide Speed with the ridge coming up at you, it will be tempting to pull back to keep the nose level with the mountain. Watch your airspeed like a hawk and don’t be tempted to pull that stick back and get any slower.

Plan on leaving the ridge at 150ft above the trees. Settling down lower and slower will rarely help you and yet the risk of doing so is very high.

Once you leave the ridge, transition immediately into a landing mindset. You will be down on the ground in the next 2-4 minutes. Take a deep breath. Do your landing checklist. Focus on making a nice landing.

Recovering From Below Ridge Top

You should not put yourself below ridge top. Should you attempt recovering below ridge top, you will be forced to fly low and slow in weak lift while in gusty conditions. This is dangerous. If you are a beginner, you must maintain good situational awareness and judgment to avoid these situations.

However, the Aero Club Albatross ridge training syllabus demonstrates a below ridge top recovery so that pilots: A) Understand how the ridge band works below ridge top, B) Are equipped with the knowledge to safely recover should they choose to.

1. How do we find ourselves below ridge top?

There are two ways to fall below ridge top. One way is because the ridge is too weak to sustain you above. In such a case, you should immediately leave the ridge and head for a field in the valley below. If the ridge is not working above ridge top, it will certainly be no better further below. A good minimum for a weak ridge is 150ft above the trees at Best Glide speed. Note that this is a minimum. Mentors and coaches would be wise to advise beginners to fly in considerably stronger conditions.

Also, a pilot can fly lower and faster if they would like to, but the ridge needs to be commensurately stronger allow them to go 10+ knots faster 50-100ft above ridge top.

If you settle down to ridge top, then expect to leave in short order.

The second reason we can find ourselves below ridge top is if we are transitioning low from elsewhere to a working ridge. For example, if we are crossing a gap and encounter sink in the middle of the transition.

Beginners should be doing transitions with ample energy so that they don’t end up below ridge top on the other side.

2. Does a ridge work below ridge top?

Sometimes yes, sometimes no. It depends on the wind speed, direction, and shape of the mountain. If the angle is off, you will often find the wind funnels parallel below ridge top and the lift works very poorly. If the angle is closer to perpendicular, there is a better chance that it will work. The next factor is wind speed. Sometimes the wind at ridge top is sufficient for the ridge to work well, but when you get 100ft or more below, the wind weakens to the point that the lift can no longer sustain a glider. The final factor is ridge shape. Usually a ridge flattens out toward its base. If the slope becomes sufficiently flat, even a strong wind will not deflect sufficiently upwards to sustain a glider.

However, if the slope is sufficiently steep, the angle sufficiently good and the wind sufficiently strong, it can be possible to recover 1/3 to 1/2 of the way below ridge top.

3. What is the technique for recovering below ridge top?

The first step is to select an appropriate airspeed. Usually best glide speed is a sufficient minimum airspeed. Next, slide into the ridge band at a flat angle. You will need to be right up against the trees as the lift band below ridge top is usually very narrow. Expect the lift to be weak; it can take a long time to work your way up. As you fly in the weak lift, avoid the temptation to turn in little gusts as you are more likely to lose more altitude than you will gain in the turn.

At any time, be prepared to turn away from the ridge. If you hit sink or turbulence, be on a hair trigger to turn away from the ridge and give yourself more room underneath you.

If you do decide to turn, recognize that you probably have only one shot. If you can complete the turn in lift, you will do well, but if you lose 50-100ft by misjudging the turn, you will likely fall into weaker lift below you.

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If this discussion sounds scary and risky, that’s because it is. Recovering from below ridge top is an expert maneuver and is not recommended for beginners. However, we emphasize these techniques for general knowledge and safety. At some point you may encounter a situation that requires applying these techniques. As a result, we train pilots such that they are familiar with what to do in these situations.

Ridge Testing!

After we release over the ridge, the next crucial step is to test out the soaring conditions. We are primarily interested that the thermals are working and that the wind strength and direction should make the ridge work nicely.

We often try to find a thermal right off of tow. This works well as it gives more time to assess the conditions and gives us confidence that we can find a thermal later when we want to get back home. If we find that the conditions don’t look all that solid below us, we will often take this thermal up to cloudbase and fly the rest of the day in the thermal band! As we thermal, we note the wind strength and wind drift as the thermal takes us a couple turns downwind of where we started.

Next, we compare our drift to wind markers at ridge top. At Blairstown, the Upper Reservoir is an excellent wind indicator. You could also look at the steam plume rising off of powerplants, flags, and trees dancing at ridge top.

With some experience, you will be able to look at these wind markers and use them to accurately estimate wind strength.

You could also pay attention to your crab angle on the straightaways; the more you must crab, the stronger the wind.

Ideally, you want to see the wind strength above ridge top be somewhat greater than at the surface. If you don’t have that wind gradient, then the top of the ridge band might get cut off lower than you would expect.

Normally, you want to see winds 15-25 knots at ridge top, within 30 degrees of perpendicular for the section you intend to fly.

Once you are satisfied that the thermals are working well and that the wind looks reasonably solid at ridge top, you test out the “high” band. This entails settling down to 2,400ft, checking your variometer and feeling if there are gusts coming up underneath the glider. A 1-26 will sink at ~200fpm at best glide speed. If you are in “good air”, you will be settling down only at 100fpm. Even though you might not be sustaining, this suggests that the ridge is working underneath you.

As you are settling down, you should be positioned to leave at one of the two exit points (Doc’s Thumb or Upper Reservoir). Once reach a decision point at 2,400ft MSL. If you feel confident that the ridge is working underneath you. Otherwise, you will strongly consider leaving back to Blairstown Airport. If you elect to settle down lower, you will becommitted to the field at the base of the ridge should the ridge not work.

As you settle down lower yet, you will probably feel the lift strengthing and you have to speed up to maintain the same height above the ridge. Speed up to 65-70 mph in a 1-26 and settle down as low as the ridge lift will let you go. If you get down to 100 ft or so above ridge top, you are now “down on the trees”. Sometimes, the ridge lift is so strong that you can’t get down on the trees and will still be several hundred feet above the mountain!

At this point, you have successfully tested the ridge! Now you could go and stay low and fast, or you could transition back into the higher “float” band if you prefer to avoid the beating.

The whole process can take you an hour or more to build your confidence in the conditions. Experienced pilots can judge the conditions considerably quicker and go through the process much more rapidly if they want to. However, they are still doing all of these steps; we don’t simply drive down on the ridge on a hope and a prayer.

Finding the Sweet Spot | Positioning Yourself in the Ridge Band

Ridge lift is awesome because it’s a nearly continuous lift source, so long as you have wind blowing and a mountain beneath you to push the wind up. When this happens, the lift sets up in a particular “lift band”. The best part of the band is called the “sweet spot”. This perfect position is very narrow; sometimes only one or two wingspans wide.

The sweet spot is simple to find down “on the crest”. The crest is defined abeam the top of the mountain and somewhat in front of the steep part. When you’re “down on the trees”, it’s fairly easy to find that sweet spot.

If you drift just a little bit downwind, the lift tapers off VERY rapidly. Drifting downwind is VERY bad.

Upwind of the ridge, the lift slowly weakens, but you still have good air quite a ways farther down the slope.

As a result, always err upwind rather than downwind. You can do little wrong being a little too far upwind. A little too far downwind, the lift goes away and you’re in a very bad place.

If you want to “float” higher up the ridge and get into the higher part of the lift band, you must push away from the mountain. This is because the sweet spot in the lift band moves further and further upwind as you get higher. If you get 1000ft above the mountain, the sweet spot may be as far as 1/2 mile upwind!

Common errors

  1. Failing to crab correctly- We always fly with a crosswind on ridge days. This requires crabbing upwind to maintain our position. As you slow down, swing the nose further upwind. Maintain wings perfectly level with the horizon.
  2. Holding steady pressure on rudder/ailerons- Pay attention to your controls. If you are consistently holding pressure to one side or another, the sweet spot is probably in that direction. Conversely, if you are unable to position the controls exactly neutral, you will have a much harder time finding the best part of the band.
  3. Aiming the nose down along the mountain- This is very common; pilots have a tendency to aim their eyes down along the mountain and try to align the glider accordingly. But as they do this, they end up flying at a minimum in a continuous slip. Other times, they end up drifting out of position relative to the mountain immediately beneath them.
  4. Drifting downwind- When you position yourself just a hair downwind of the lift band, the upwind wing gets pushed up because the lift is stronger upwind. This pushes you even further downwind. Pilots that get on the downwind side will then often “hunt” back and forth, tick tocking off the sweet spot and then bouncing back downwind. As we learned, the lift band is sharply defined on the downwind side and you really don’t want to be there. Instead, always err slightly to the upwind side.
  5. Gusts pushing upwind wing up- Never, ever let your wing get pushed above level with the horizon. If a gust pushes your upwind wing up, immediately correct for it and push back upwind. Don’t drift downwind!!

Returning From the Northwest Ridge Back to Blairstown Airport

Suppose you settled down lower and you felt that the ridge was not working. Or, it’s the end of the day and you want to go back home. At some point you will need to exit the ridge and make the 3.5 mile final glide back to Blairstown Airport.

The minimum altitude to leave is 2,400ft MSL at one of the ridge exit points (Upper Reservoir/Doc’s Thumb). There are several reasons the club selected this margin.

  1. In a low performance ship, you will definitely not be high enough to make a full pattern if you leave lower. Leaving at 2,400ft is marginal for a full pattern as it is and you should be fully prepared to make a non-standard approach.
  2. The lower you are, the stronger the sink. On the upwind side, the lower you get to the crest, the stronger the lift. The opposite is true on the lee side. So leaving 200ft lower may lead you to being 400 ft lower on the far side of the sink!
  3. It is hard to find the landmarks and the airport if you are lower than 2,400ft. This makes it easier for you to get lost along the way.

If you are unable to get to 2,400ft, simply land at the field at the base of the ridge.

If you’re able to thermal up higher than 2,400ft, that’s certainly better! Ideally, you would leave in a thermal at 2,900ft or higher, drifting downwind in it. This is called “coming home like a gentleman”. You will ride over the top of all the sink and turbulence and get back with plenty of energy to spare.

Situational awareness for the final glide

  1. Anticipate the sink. You may encounter severe sink on the back side of the ridge. Put the nose down and punch through it. It will eventually stop.
  2. Once you get out of the sink, trust the performance of the glider. You will have a 15-20 knot tailwind. You will be high enough to get back to the airport.
  3. Plan your approach early. Be willing to do a non-standard pattern if you’re a bit lower.

Execution

Once you decide to leave over the back of the northwest ridge, there is no turning back. You cannot change your mind 10 seconds later; you will very quickly not have enough energy to make it back to the ridge thanks to the strong tailwind pushing you along.

When you encounter the sink, put the nose down and ride it out. The sink is temporary and will go away. Even if it gets really bad, keep the nose down and drive through it.

When you get out of the sink, you will feel an acceleration as thought as you hit a thermal. This is from going in strong sink to no sink. Some folks confuse this for a thermal, but don’t circle in it. Even if you’re low, keep going. You will have the airport made.

Once you get to Rt.94, plan out your approach. Are you going to do a non-standard pattern? Be prepared for a lot of sink and turbulence. Keep extra speed in the approach and follow the guidance in the Ridge Landing video.

Critical Errors

  1. Leaving lower than 2,400ft- For reasons described above, this can get you into more severe sink and very low in a scary place.
  2. Believing the sink will last forever and crashing in the Lower Reservoir- Several pilots have crashed their gliders in the Lower Reservoir when they thought they could not make it home. Even if you get flushed in the sink, it will eventually end. Ride it out and cross the hill; don’t turn.
  3. Turning in the transition from sink to no-sink- Don’t get tempted by this transition. Keep the nose down and keep going; you will make it home.
  4. Not trusting the performance of their glider- A pilot crashed a two-place ship in a field short of the airport when they thought they could not make it. Once you get out of the sink, you will have a 15-20 knot tailwind. Remember that the strong sink is temporary. Once you’re out of it, you will have the airport made.
  5. Poor approach planning- Don’t aim for the middle, don’t make a low and slow pattern. Be prepared to make a non-standard approach.