Lee side flying
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When I learned to fly, my instructors taught me to avoid lees sides. And right they were. Lee sides could result in a rapid descent, often accompanied by being throwing around by turbulence. Not a pleasant spot to be in, and certainly not for a beginning pilot. After obtaining my paragliding licence, I really started learning to fly. Practice gradually completed my theoretical knowledge, and gradually I changed my opinion on lee sides. By error and trial (in that order), I found out that between the obvious black and white, the lee and windward sides of a mountain, there exists a large grey area that could be exploited to my advantage. As with many things in nature, flying a lee side is a relative affair that depends on altitude, wind direction and strength, as well as personal risk perspective.

Pilots flying in a lee side.The pilots in the image are flying in a lee side. Hopefully they are aware of that. If not, they will quickly find out when the breeze dies down or when they try getting closer to the tall mountains on the left. These mountains are part of a range that runs along a north-south axis for about thirty kilometres. We are looking in the northern direction here, thus the shape of the clouds tells us that a west wind blows over these mountains today. If you can't see that, then have a look at deriving wind direction from cloud shapes. The resulting lee is partially countered by the valley breeze on the east side. It allows us to soar on the lower ridge and exploit a few (calm) thermals here and there. But as soon as this breeze calms down, the pilots are likely to end up in a lee side.

Unfortunately that day, this happened to me at one of the two worst possible spots (from a cross-country perspective). The cloud cover had been increasing slowly during the day, the prelude of bad weather, but this accelerated in the beginning of the afternoon. A convincing stratus turned off the breeze a bit earlier than expected, just as I turned at the far end of the low ridge. Slowly sinking to the valley floor, I ran into a tiny bit of remaining breeze twenty metres from landing out. The breeze was weak indeed, as expected, but it still allowed me to scratch my way back to Lumbin. I enjoyed a close-up view of village and farm life as I hopped from one emergency landing spot on to the next, searching for the bus stops along the way in order to land close to them if needed. The stratus became thinner and conditions better as I closed in on the Saint-Hilaire take-off, but I couldn't find any altitude to reach it. Having enjoyed a pleasant and interesting flight, I did not want to push my luck any further and landed in Lumbin. A slight drizzle started an hour later as I drove away for a rendez-vous in the sunny south. Getting back to Lumbin had been a bit of hard work and luck, but it had also been more rewarding and certainly more interesting than an ordinary flight (if such a thing exists). I guess nature wanted me to pay for the easy flying I had earlier on, at the southern end of the mountain range. The range is less tall there (no mountains on/behind the low ridge), which allowed convergence of the west wind and east breeze. Getting to cloud base was a no-brainer there, as a click on the image will reveal. It seems more logical to fly on the west side under these conditions of course, except that I wanted to land close to my car when the weather would turn bad and not several hours of hiking in the rain.

Not understanding a situation carries a high a risk of incidents. Understanding the interaction between the various winds and breezes is the key factor in deciding whether a lee is flyable or not. Understanding increases your opportunities and safety, as well as your flying fun. Some trips or transitions for instance, are only possible with a bit of lee flying. And if you are used to flying lee sides, you won't panic that easily when you unexpectedly happen to end up in one. Understanding does not automatically imply you should fly lee sides, but it certainly helps in staying out of the wrong places.

Lee side flying in GréolièresThe bigger the lees side, the more chances of flying it safely and comfortably. The Dahu launch on the southern slope of the Cheiron is located 970 metres above sea level, about 160 metres above the landing field in Gréolières. It is an excellent spot for an early start, since the south facing slope mainly consist of bare rocks and stones that create thermals as soon as the sun rises. But despite the launch facing south, it can also be good spot to fly with north wind. With its summit at 1778 metres and its tall extending ridge, Cheiron is is significantly bigger than the surrounding mountains. It forms a wall that rises 800 to 1200 metres from the valley floor (as measured from the bottom of Gorge du Loup, ending at its feet). This wall shields the southern breeze from a (strong) north wind and allows lee side flights up and down the southern slope.

Getting above the ridge of Cheiron is still possible under these conditions, but tricky. The breeze lifts you up, but the north wind rolling over the mountain pushes you down as you get near the top of the ridge. Sometimes it is impossible or just too tricky, and I'll give it a miss. Once you get above the ridge however, you can let the north wind push you to the next and last mountain range in the southern direction (Vence, Tourettes, Gourdon). And if you notice that you are getting to low and won't make the transition, the southern breeze carries you back to Cheiron. Having the wind in the back on the outgoing as well as return leg, gives you far better flight performance than any hot wing can give you. In paragliding, understanding beats performance most of the times.

It is these kind of tricks that come in handy when playing chess with nature. As long as the risks are manageable, lee side flying is an interesting option that is hard to ignore in the cross-country game. But in order to do so safely, you need to be aware of the situation. Being constantly aware that the situation can quickly change from a pleasant to a (very) unpleasant one, is a key safety factor when flying lee sides. A substantial number of pilots don't realise they are flying the lee side of Cheiron, until the breeze gives way to the north wind. Always be prepared that you have got it wrong or will get it wrong soon. Prepare yourself for the scenario where the spot you are flying or will be flying in soon, turns into a nasty lee side with rotors, turbulence, and descending air. Have an answer ready in case trouble strikes, such as a safe landing spot or the minimum slope distance in order to get away safely.

Lee side flying on Cheiron.Let's have a look at a regular lee side flying day in Gréolières, in order to illustrates what has been said above. The Cheiron range (or rather half of it), is the mountain ridge that starts in the lower left corner of the image and disappears beyond the clouds to the east. The clouds above the ridge indicate a northerly wind creating a lee on the south side of Cheiron, but this did not prevent dozens of pilots from having a nice flying day there. We all started from the Dahu take-off and exploited the gentle thermals nearby, while some managed to climb out of the lee. The latter brought me in the convergence of north wind and south breeze, allowing an easy trip at cloud base to Bézaudun at the far end of Cheiron. Cloud cover considerable increased when I turned around there, giving me some hard work to get back. I was not really surprised by this, since I had a dinner rendez-vous later that day. Thermals often seem to shut down when I have an appointment or other obligation that requires my timely return. Being aware that I was now soaring in a lee, was crucial to my safety. It ensured for example that I kept more margin from the slope in order not to hit it when I would 'suddenly' starts sinking faster than usual. In addition to that, I kept an eye open for emergency landing options even more than usual.

Risk is a personal perception. What seems dangerous to me, might be considered safe by someone else. Or the other way around. When the usual route for heading north from Chalvet via the high mountains doesn't work that well (or not at all), there exists an alternative way via the low mountains in front. One of the options here consists of flying over to the first low mountain ridge by crossing a windy valley. I will lose quite a bit of altitude in the transition and arrive low on the east facing slope, but the strong valley wind will allow me to soar my way up again. I have to admit that an east facing slope is not the most logical place to be in the afternoon. In addition to that, the slope is in the lee of a much stronger breeze from the valley behind it. But this is precisely the reason why I am going there. It might be a bit turbulent below the ridge, but once I get above it, the corresponding convergence usually bring me in an orbit comparable to those generated by the much taller mountains to the east. Especially when the latter are sleeping, the low mountain still works. I have never been disappointed by it.

Once I showed this alternative route to an experienced pilot, who has been flying a lot more than me. He has flown the high route hundreds of times, often in record breaking time, but he doesn't like my alternative route via the low mountains. He spent a few seconds analysing it and concluded by answering: “Voila, c'est risqué!” (which roughly translates as “It works all right and certainly looks like an interesting challenge, but you better be careful I guess.”), followed by a plea on the virtues of the high route. Having pulled off this trick dozens of times and never having been let down by it (while the big mountains have let the us down both on various occasions), we disagree on its quality. We agree however, that there is certain risk to it. Stay away if you can't handle the situation. This area was designated as a no-go area by an even more experienced pilot during a briefing of the British open in 2010. I leave it up to you to whether this has something to do with the skills of the competing pilots or my death wish. As far as I know, I don't have any. Neither have those few other pilots that also appreciate this spot.

But if lee sides aren't that obvious as black and white, how can we actually tell whether we are flying in one? If observation and analytical skills can't help you making up your mind, then trust on feeling and intuition. If you spent lots of time in the air, you will get used to its behaviour under various conditions. Once you get used to it, you don't feel air movements anymore. At least not consciously. Similar to not feeling the waves of the sea until you walk on the shore again. You will notice that 'something is wrong' when flying through air that is different from normal. The air gives you lift for example, but feels 'strange' or 'not right'. In addition to that, the lift might be capped. Preventing you from gaining the altitude you expect, considering the current cloud base. You are climbing nicely, when all of a sudden your glider runs into a glass ceiling. When the lee is strong, this might feel like you are being hammered down by an invisible giant blacksmith over and over again. No matter how well you try centring, the thermal just won't allow you to climb any further. You spend your time impersonating a yo-yo, until a stronger thermal allows you to break through this zone or the wind from the other side weakens.

Overall, the air in a lee side is more lively and less structured than on a windward side. You could be going up fast and then going down just as fast a few seconds later, repeated ad nauseam. Depending on the stability of the breezes, lee side flying can be comfortable or sickening. When the breeze on the lee side suddenly weakens, you end up flying in a rotor and the descend could accelerate considerably. Expect this scenario to happen when flying a lee, not matter how reliable the latter seems. Keep more margin to the slope than you would in a similar situation on a windward side. Sometimes your glider is vibrating in a special manner when flying a lee side. Just before you get to a real turbulent spot, you might feel like flying a bumpy gravel gravel road instead of smooth air. This could either mean you are close to or in the wrong spot at the moment, or that you arrive near the right place. After a while, you will usually be able to tell which of those it is before trouble strikes.

The only way to tune this feeling, is to practice it. Preferably in a controlled and safe manner. Moving as far south as possible in order to escape the mistral, I have flown Gourdon with north wind on many days. The launches are facing south and fed by the southern sea breeze. A little higher however, the north wind takes over. The exact altitude of this wind shear depends on the relative strength of the south breeze and north wind, as well as other conditions such as humidity, air temperature and cloud base. It is interesting to feel what happens in air like this. It is a relatively safe affair, since there is plenty of space to land as soon as the breeze falters and you 'suddenly' find yourself flying in a rotor. Realising one day that I was flying in the lee here, I tried getting accustomed to these special conditions for an hour or two. Later on that season, I could suddenly feel these kind of conditions in other places as well. Those two hours have been one of the best I have ever spent learning to fly. They instantly taught me more than any theoretical lecture on lee side flying could ever have provided at that point.

Lee side flying on Chalvet.A similar situation occasionally occurs when flying the west slope of the Chalvet. Pilots launching from the west take-off in the (early) afternoon breeze, will run into good lift and thermals along the ridge. But sometimes it is hard to gain altitude, despite the breeze and a decent cloud base. The usual cause is the west slope being in the lee of a southerly to easterly wind. This wind is not strong enough to create a lee on launch or near the ridge. The breeze gets compressed near the slope, increasing its strength. Slightly above the ridge however, this compression has sufficiently diminished to let the wind get the upper hand again. The clouds in the image tell us that there are excellent thermals today, but we are flying downwind of them when launching from the west take-off. The back of a thermal is not a good thermalling spot, as you are hopefully aware.

The most logical way to join the thermals here is to start on the other side, on the south-east take-off. Thermals start in front of launch here, rather than behind it. You can easily find one and ride it all the way to cloud base. If you manage to stay there, the south wind helps you heading north instead of hindering you. As you look over your shoulder over La Bâtie a few minutes later, you are likely to see big dots still struggling in the distance to get away from the west launch. A click on the image suffices to enjoy the same view (taken on a different day, with west wind), as well as to see the major drawback of this flight path. The Issole valley is full of trees and there are very few, I repeat, very few landing options when running out of thermals. If you insist on seeing (low safe) thermals that are not there, you are likely to get locked into one of the gullies on the valley's sides. The best way of landing out here is to land on top of Maurel (the grassy fields on the mountain to the left), just before you get too low for doing that. A short walk north or south will get you to Petit Cordeil or the end of Maurel respectively. Relaunching from here is generally less risky than descending below Maurel and counting on a thermal to save you from a tree landing. Landing out in the valley is a tricky business. When it has become unavoidable, try aiming for bushes or trees (in that order). Don't be tempted to land on the banks of the stream or the iscles (the stony islands in the stream). Your glider is a lot larger than the banks are wide. The Issole is a small stream, but powerful enough to drown you. Just a wing tip getting caught by the water suffices. Only fly the Issole option when you are pretty sure that it works, i.e. when there are nice cumulus clouds indicating the way at a descent altitude. I have to admit though, that the most interesting days have been those where I had to scratch my way through the valley above the trees. It makes for some interesting flying and demands disciplined risk management. It is like a difficult exam that you are reluctant to pass, but feels great when you do.

Even though launching from the south-east take-off seems more logical in the presence of a southerly to easterly wind, this is usually not recommended in the afternoon. The launch is likely to be in the lee of the (compressed) breeze from the west. This is almost certainly to happen during the afternoon when the westerly breeze is strong. As long as the breezes are calm, pilots can be seen launching from both take-offs. The best way to get away with south to east wind, is to start early on the south-east take-off in the morning before the west breeze starts. But you risk a sledge ride if you start too early. This dilemma of missing a great flight when starting too soon or ending up in the lee when waiting too long, has caused quite a bit of frustration among pilots, as well as scary moments in the air. It certainly has enriched local flying history and will continue to do so in the future. I try not to become part of it.

Another way, and my preferred one under these conditions, is to start on the west launch and search for a slope that is well exposed to the south to east wind or a breeze that is (a lot) stronger than the wind. This could be the south side of Chalvet itself for example (going south, not shown in the image), or Meunier (going north, visible at the end of the yellow line). The former is probably the most comfortable option, but sets the Issole trap when thermals are hesitant. The other option requires a dedicated mind, but works well most of the time. It only takes fifteen minutes or so to get to (the foot of) Meunier on the western route under these kind of conditions, while pilots trying the southern route spend quite a bit of time gaining altitude. This altitude is usually lost as soon as they head north, forcing them to the west side of Chalvet in order to avoid the Issole trees. So why not fly the west slope straight away when the ceiling is low? The altitude loss when rounding Allier tends to be a bit discouraging in this case, but you are almost certain to land out when you get worried and turn around. When you make a well informed decision, stick to it. Nervously flying all over the place might increase your sense of actually doing something to control the situation, but usually does not increase your chances of success. Arriving at the foot of Meunier, you will often be able to soar your way up from desperately low altitudes. My current record is soaring my way up in front of the trees at the bottom of the valley, but I have also landed out without standing a chance. The nice part of landing out here is that there are paths to the summits of Allier and Meunier for re-launching. The hike up gives you an enjoyable two hour break from all that serious flying stuff. If you are lucky or skilled, you could run into the convergence from the west breeze (Tartonne) and the east breeze (Issole) during the transition. This convergence hides somewhere in the valley, but searching for it might take too much altitude and ground you. I usually don't bother to find it, but don't give it a miss either when I run into it.

Please note that the wind in the image is north-east rather than south-east. This means that Meunier is likely to be in the lee too when the breezes are weak. In this case your search for a well exposed slope continues, as does the lee side flying. The east slope of Sapée (the hill left from Meunier on the other side of the valley) might be a good option in this case. Unless it is in the lee of the Tartonne valley breeze of course. But then there should be a nice convergence somewhere between Meunier and Sapée, which you are likely to run into when crossing the Lambruisse valley. There are other options as well, but my purpose is to explain lee side flying, rather than writing a book on how to fly the Saint-André region.

Though lee side flying may be an interesting option when the windward side is out of reach, staying above a mountain (ridge) is generally the better choice. Flying high avoids lots of challenges, lee side flying among them. Arriving near Planfait one day, on my way back to St.-André during a vol bivouac trip, I used the convergence caused by the lake breeze and easterly wind to get above Dents du Lanfon. As I continued north at cloud base, I could see pilots near the Montmin take-off (altitude 1270 m) struggling to gain height, despite the take-off being shielded from the east wind by la Tournette behind it (summit at 2350 m). The convergence had allowed me to reach a surprisingly comfortable altitude, while below me pilots seemed to gain two hundred metres maximum. I was a bit worried about making it to Belledonne, my goal for the night, since I would be facing a strong wind when going east. But as I crossed the valley over to Bauges, I descended into the lake breeze from the west. It gently carried me to Dents des Cons, despite the stable conditions at low altitudes (a hot day at the end of August). Like a balloonist, choosing the right altitude got me in the right direction. Not that it was easy, but I managed to escape landing out in Doussard as all the other pilots were doing. Dreaming on Belledonne that night about the amazing things I could do with conflicting wind directions, thunder and lightning overhead brought me back to reality. Still half sleeping, I packed my make-shift camp in the rain, realising that luck had just as much to do with the flight as skill. Apparently my luck had finally ran out that day. The east wind had died and the thunderstorms I had seen towards the west at the end of the day had arrived a bit sooner than expected. Luckily, my skills were still working. I descended in the dark to the last shelter I had seen during the day. Twenty minutes later I was sleeping soundly again, but dreaming less.

As you may have noticed by now, wind blowing from the other side of the mountain does not necessarily mean you are in the wrong spot. But if you do not understand the above examples or similar situations on your own, then seriously ask yourself if you should be doing (challenging) cross-country flights, let alone venture into unknown places. Places that could turn out to be dangerous lee sides or rotors.

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2011-02-22