It Doesn’t Take Much Sun

Thermal flying under overcast skies

Yesterday was supposed to be a really good day for my first thermal flight of the year.  I now use several soaring forecasts to see which ones are most reliable in Colorado. However, this tends to be more pain than boon and can be quite confusing because these forecasts rarely agree with one another.

To my amazement, yesterday they all pretty much lined up, forecasting moderately strong thermals between 11:30AM and 5PM MDT, attractive attainable flight distances, no risk of overdevelopment, and an amazing 100+ mile convergence line all along the foothills.

Thermal height around 14,000 feet over the foothills.
Possible flight distance in a standard class glider (like the Discus) around 400-500 km.
Virtually no risk of overdevelopment.
Amazing convergence line all along the foothills of the front range – from south of Denver, CO all the way to north of Cheyenne, WI.

The thin cirrus shield above the plains soon dissolved after sunrise and the day started out under cloudless blue skies.

The thin cirrus clouds visible at sunrise soon disappeared and the day started out blue.

I planned on a takeoff around 12:30PM MDT.  When I arrived at the airport around 11 AM the sky looked just like the forecast.  A long line of cumulus clouds had already formed to the west of the Boulder airport, stretching from Golden, CO to the north as far as the eye could see.

I hadn’t rigged a plane in more than four months and things went slower than I had expected.  There was also a lot of activity at the airfield as many pilots had turned out for their spring checks and by the time I was ready to get in the air it was already past 1PM.

In the meantime the clouds had developed much faster than projected and when I finally took off around 1:20PM, the sky had become completely overcast.

The cumulus clouds during the second half of my flight were fairly easy to read and provided good indications of lift. During the first hour of the flight however (sorry no pictures), the sky was an amorphous grey.

There wasn’t much lift as I followed the towplane into the foothills where I hoped to find rising air from the convergence.  The clouds were dense and grey and I could not discern where the best lift was likely to be.  With my hand on the release I followed the tuck for a long time and finally set myself free in weak lift over the mountain hamlet of Ward at an altitude of 11,000 feet.

I looked all over the sky around me and still detected very little movement in the clouds.  I was able to hold my altitude in the narrow, broken lift and was basically just buying time to see if the conditions would change.  I was also at the bottom of a wind shear layer and had to pay attention not to stall each time when I turned into the direction of the wind.

Some streeting along the conversion line over the foothills.

After more than 10 minutes of parking in the sky I saw than the sun had broken through the clouds  a few miles further to the east, directly warming a south facing slope.  I held my position for another five minutes to give the slope some time to warm the air before making my move.

By the time I got there the slope was already in the shade again and I was doubtful that five minutes of sunshine could have made much of a difference.  However, to my surprise the slope actually worked.  The lift wasn’t strong but I managed to climb about 1,000 feet in six minutes before the energy was exhausted.  In the absence of clear indications in the clouds, the same strategy helped me locate my next lift as well.

Pretty view of the Continental Divide from a position along the Peak-to-Peak Highway south of Ward. The Eldora ski resort is just atop the wing and James Peak is in the background.

Then the weather changed.  The clouds over the plains started to dissolve and once again I headed for the area that was in the sun.  I followed the first row of foothills where the canyons open up towards the plains.  There are several bowls into which an easterly wind from the plains gets funneled. Yesterday there was a light wind from the southeast at the lower levels.  Combined with the afternoon sun (shining from the southwest) I was hoping to find lift above one of these bowls.

Lift along the bottom of the foothills with winds from the southeast: the wind is funneled into the canyons and up along the south-east facing slopes that are warmed by the sun. When the air reaches the top of the slope it can no longer cling to the ground and instead rises above.

I crossed over these bowls flying from north to south (from right to left on the map above) and found my best climb of the day in the Seven Hills area (the second such indicated bowl from the left) allowing me to climb from 7,500 to 10,500 feet in about 12 minutes.

As I was climbing, dark clouds began to rapidly form once again.  Unlike earlier in the day they were much better organized and provided much clearer indications of the areas of lift.  I followed a cloud street a few miles north where I effortlessly climbed to cloud-base.  Then I pushed south-west under another row of dark clouds where I finally topped out at 12,000 feet, my highest altitude on this flight.

Once again, the sky had completely overdeveloped. Heavy snow showers had engulfed the peaks around Rocky Mountains National Park, about 25 miles north of my position.  With the sun completely shielded off, the thermals rapidly weakened once again.  Even under the completely closed ceiling there was still enough lift to stay in the air.  However, climbing was slow and after a while I lost interest in flying holding circles and decided to return to the airfield.

Here’s a link to the flight track.

Lessons Learned

  • Five minutes of sunshine can be enough for thermals to form.  If the temperature profile is right (i.e. the air is sufficiently unstable), it doesn’t take long for the sun to heat the ground for thermals to develop. The sun at the end of March is already quite powerful.
  • When the clouds don’t tell you much, the ground can. For the first 90 minutes of today’s flight I was unable to read the clouds for indications of lift. Observing areas where the sun broke through the clouds to warm the slopes, and overlaying my understanding of the direction of the wind helped me identify areas of lift.
  • Weak thermals can even persist in a completely overcast sky.  Thermals definitely weakened when the sky was completely grey and overcast but there was still enough lift to stay airborne.
  • Parking in weak lift can pay off. When the sky over-develops and the thermals dramatically weaken it may pay off to hold on to a spot with weak lift where you can hold your position and wait for conditions to change.  Had I not done this, I would have landed within 40 minutes of releasing. I just waited long enough for the sun to break through the clouds and warm certain areas long enough for new thermals to form.
  • Stalls can happen very quickly when thermaling in wind-shear conditions. Finding the best speed to fly when circling in narrow, broken lift can be quite tricky. At my first climb of the day staying in lift required steep circles flown just above minimum speed.  I was coming up to a wind shear layer and once in three circles or so I was hit by a gust from behind that was sufficiently strong for my flying speed to drop below stall.  Small, hard-fought altitude gains can quickly be lost in a stall and then the slow and steep thermaling technique becomes quite inefficient. At one point my inside wing dropped and I had to quickly correct with opposite rudder. Flying so slow is definitely a no-no when close to the ground (I was at least 2,000 feet AGL and there were no other gliders around so safety was not an issue.)
  • The weather forecast can be wrong even when all forecasts agree. I learned that I cannot rely on the forecast even when all forecasting tools say the same thing.  None of the forecasts for yesterday predicted any over-development. Forecasts are still far from perfect even when based on data collected just a few hours before the flight.


Hypoxia Simulation – Get To Know YOUR Symptoms

On August 14, 2005, 121 people died in the crash of Helios Airways Flight 522 after the aircrew became hypoxic due to the air pressurization system being incorrectly set to manual.

On April 1, 2011 a glider flight from Boulder, CO ended in a fatal accident after the pilot had spent 14 minutes above 22,000 feet. From there the sailplane spiraled to the ground. The accident report found hypoxia of the pilot to be the most likely cause.

These accidents were on my mind when I attended yesterday’s Hypoxia Simulation Training session, provided by AirCare Facts at Independence Aviation in Centennial, CO.

After an hour of classroom training covering the causes as well as the potential signs and symptoms of Hypoxia, I had the opportunity to participate in a simulation of low pressure conditions at up to 28,000 feet.  This was accomplished by breathing through a mask feeding reduced levels of oxygen into the respiratory system.

Hypoxia is an insidious killer because it is often very difficult to recognize any symptoms before it is too late.  The potential symptoms even include feelings of wellbeing and euphoria, which may make it even less likely that a pilot would take corrective action before passing out (and eventually dying – either due to oxygen deprivation or due to the plane crashing in uncontrolled flight).

The only good news is that the symptoms of hypoxia tend to be specific to each individual and relatively constant over time.  Hence, it is possible for everyone to experience and “get to know” their early indications that something may be amiss.  Recognizing these indications early is likely one’s best (and maybe only) chance to take the necessary actions.

At the earliest onset of hypoxia symptoms at altitude it is vitally important to act immediately (while still being “usefully conscious”). Normally this means beginning a rapid descent to lower altitudes where the air pressure is higher and normal oxygen saturation levels are restored relatively quickly (normally within a few minutes).

I took the following video during my own training session so that I would be able to see my own reaction and be able to remember my specific symptoms.

I can recommend to any pilot to participate in such a simulation. Knowing your individual symptoms may one day safe your life.

The Dangers of Sailplane Racing – What Condor Taught Me

I recently demonstrated that soaring is an objectively dangerous pastime.  On a per-activity-hour basis it is approx. 35 times as dangerous as driving, 70 times as dangerous as bicycling, and still about 3 times as dangerous as riding motorcycles.

One contributing factor has to do with the high number of fatalities during soaring competitions. (This article shows that during global soaring contests, the number of fatalities per number of flights has been more than 10 times higher than during flights outside of competitions.) Even though there is no (relevant) price money on the line, contests tend to tempt pilots into lowering or suspending their normal safety standards. To have a chance of winning or placing well, pilots are often inclined to take higher risks than they normally would accept – consciously or subconsciously. E.g., they will fly closer to terrain than they would on a normal cross-country day; they will fly in bigger gaggles, thermal closer to stalling speed, get closer to Vne – even in turbulent air, attempt safes lower to the ground, scrape across ridges or mountain passes, fly low over unlandable terrain, calculate their final glides with a narrower margin, etc.

It’s the same behavior I observe (and – to be honest – participate in myself) during races on the Condor competition soaring simulator.  Fortunately it’s a simulator so if you crash you still get to live another day. But I’ve found that the dynamics in human behavior are very similar to real life competitions. No one wants (or expects) to crash but at the same time, most everyone flies in ways that they would consider irresponsible outside a contest environment. Sure, taking great risks won’t guarantee a good placement, but a good placement almost inevitably means that the pilot assumed a high degree of risk.

Here’s an example: yesterday I flew a Condor race set in the foothills of the French Alps as part of a competition called Regatta Cup. It was a short 188 km Club-Class task along and across several low mountain ridges. Thermals were moderate but a steady 12-14 kt wind from west-north-west made for optimal ridge flying conditions along the steep slopes of the area. As the name Regatta Cup implies, the race was to start at a set time for everyone with all 26 gliders trying to cross the start line below 1,800m simultaneously and as close to Vne as possible. From there they would all fly along the same course, round eight tightly-spaced turn points, then dash for the finish line.

Task set in Provence, France. With wind from WNW, the entire race could be flown in ridge lift.

The start was set away from any of the ridges so big gaggles formed underneath one of the few cumuli west of the Romans Saint Pau airfield.  Cloud base was around 2,200 meters so everyone circled up to the base of the clouds and tried to stay there in order to maximize the potential energy when the start gate would open. 26 gliders were sharing two thermals, all flying within an altitude band of approx. 50 meters. I was not surprised when I witnessed two pairs of gliders colliding with one another. In fact, I had several close calls myself – and all of that before the race even got underway.

Trying to stay aware of everyone around me I began to dive about 20 seconds before the start of the race. Burning excess energy using the spoilers and watching the altimeter, the speedometer, the GPS, and the traffic around me all at the same time, I managed to cross the start line about 5 seconds after it opened, 20 meters below the ceiling, flying just below Vne with a ground speed of 272 kph.  I considered this a very good start even though about half of the competitors were already ahead of me and the other half only seconds (or fractions thereof) behind.  And, luckily, I was still alive.

With the wind at the tail and flying at a “conservative” air speed of 160-170 kph the altitude at the start was just sufficient to get the unballasted LS4 to TP1.  I was in the bottom third of the pack but the leaders were less than a minute ahead.  From there the ridge race began. The strongest lift tends to be near the top of the ridges and not more than one or two wingspans away from the terrain.  That means everyone will attempt to fly in that narrow zone between winning and dying, and with so many gliders packed into the same tight spot at the same time, surviving is not much more than a game of Russian roulette.

Added to this is the complexity of different climb rates based on the angle of the ridge line with respect to the wind.  You look ahead trying to anticipate where the best climbs are likely to be.  Just before you get there you pull up sharply in order to fly two or three seconds longer through the best climb zone, then you push the nose down again, even more so if you anticipate an area of sink. Everyone else tries to do the same thing: flying as close to the ridge as possible, pulling up right before the best climbs, pushing down right before any anticipated sink. In doing so the speed of the glider might vary anywhere between minimum sink speed and three times as fast. Each time the altitude fluctuates by 200-300 meters as speed gets converted into height, or height gets converted into speed.  Pulling up or pushing down too early or too late, or incorrectly judging the strength of the lift, costs precious seconds that add up and ultimately decide about your placement. Flying like this is more than risky enough if you are the only one around but doing so in the midst of a pack of more than 20 gliders is simply an enormous gamble.

After several close calls between TP1 and TP4 and being about two minutes behind the leaders at this time, I decided not to follow the gaggle in front of me on the direct route to TP 5 but to take a slightly longer route along the higher and steeper ridges further to the east, hoping not only for better climb rates along this route but for some stress relieve as well.

Soon I discovered that I faced another challenge: there was a mountain pass in front of me.  I didn’t want to waste any time turning so I hugged the mountainside and flew at minimum sink speed hoping that the lift would be strong enough to carry me over the pass before I got there.  I was aware that this was a hugely risky maneuver: if a wind gust would force the glider to stall I would not have enough altitude to recover before hitting the ground. And if a thermal would break off on the valley side and turn the glider towards the slope, I could easily get pushed into the trees. “There is no way I would fly like this in real life,” I thought.

The gamble paid off and I made it across the pass and dove for TP 5.  As I got there I noticed that I had caught up with the pack.  I could still see several gliders ahead of me but they seemed a few hundred meters lower.  With a valley to cross ahead, they would likely have to stop to climb somewhere while I could cruise along the top of the ridge at a much higher speed. I dove across the valley at over 250kph and still reached the ridge on the other side at a good altitude.  Again, I hugged the higher ridge to the east while the handful of gliders ahead of me were lower and further west.

There was another valley to cross between TP 6 and TP7.  Again I put the nose down only to realize shortly thereafter that I would arrive too low on the other side. I dialed the speed back to 140kph – my slowest cruising speed of the entire race – to conserve what altitude I had left.  As I headed straight towards TP 7 the trees on the slope ahead of me were getting closer and closer.  Would I be able to get to the turn cylinder before the trees would get me? I wasn’t sure but just as I was forced to initiate a turn right over the tree tops the GPS confirmed that I had rounded the TP.  If I had only been 20 meters lower I would have had to turn away from the slope and find a spot to climb.

Having turned TP7 I could now hug the ridge again and fly toward TP8, the final turn point.  I could only see one glider ahead of me.  My final glide calculator indicated that I would arrive a few hundred meters too low but I was confident that I could easily make that up by hugging the ridge between TP 8 and the finish line. So I decided to dive, flying a direct route towards TP8. I kept watching the other glider to my right and though that I might have a chance to beat him.  I reached the next ridge closer to the valley with only 5 kilometers to go to TP8 and 20 kilometers to the finish when … I suddenly died.

Another glider that I had not noticed must have been slightly above or below me.  I had not seen him at all and I must believe that he had not seen me either. It was quite a shock and a revelation.  Obviously, had this been in real life I would not be here to write about this experience.  Instead, my wife and children would stand by my graveside and wonder, with tears in their eyes, how this could have happened.

Yes, I know, Condor is just a game.  I must and do believe that I would not have taken many of the risks described, if I had been in a real glider. Also, some of the race settings described here are not realistic. For starters, contest directors are unlikely to plan a task that is as dangerous as this one.

However, there are many things that are not so different from real life. The pressure of the competition, the desire to win, the fear of embarrassment.  Also the fact that taking high risks does have the potential of giving you an advantage in the race: circling in gaggles under the cloud base to conserve energy for the start, flying close to ridges without adequate safety margin, hugging the tree tops, scraping over mountain passes, circling at minimum speed close to the ground, flying low over unlandable terrain, aggressively calculating the final glide – all these are risks that have killed many real life pilots. I believe that this is especially true in race settings when usual risk-mitigation strategies get too often ignored or even willfully suspended.

BTW – in case you’re wondering: I did analyze my flight track and those of the eventual race winners.  I believe that I would have come second or third in that race – it would have been my best result against some of the world’s best Condor pilots.  You can see the race results here. However, it was definitely not worth dying for.


Boulder Wave Routes

The best soaring routes almost always correspond in one way or another to the terrain below, no matter what lift you use.

E.g., you would expect thermal lift over terrain that is most exposed to the sun (e.g. slopes that are most directly warmed by the sun based on the time of day); you would expect convergence lift where terrain features redirect the wind such that air masses collide with one another and are forced upwards; and you would expect ridge lift along long and steep slopes that are more or less perpendicular to the direction of the wind.  (It’s no surprise that pilots love to fly along the top of ridge lines where thermal, ridge, and convergence lift often come together.)

It’s no different with wave lift.  Wave lift forms when the wind pushes (relatively stable) air downward along the lee slope of a mountain, thereby warming it at the dry adiabatic lapse rate (such that it becomes warmer than the surrounding air near the ground). It will then rise again because it became lighter than the surrounding air mass, thereby starting a wave motion that oscillates on the back side of the mountain.  (You can find more details about wave lift here.)

Wave lift will form only if the wind is relatively strong.  In most locations, such strong winds usually come from the same direction.  In Boulder that is from the west – especially in the wintertime when the jet stream blows at our latitudes. What makes Boulder a particularly great wave location is the fact that a tall, nearby, mountain range – the Colorado Front Range – is conveniently laid out in north-south direction (hence the prevailing wind has to cross it at a perpendicular angle) and the Boulder airport is just to the east in the lee of the mountains.

With all that said, it should be no surprise to see that wave flights from Boulder tend to follow the same routes: parallel to the mountains on the lee side. In fact, the following chart depicts 40 wave flights from Boulder from 2010 and 2017 that were longer than 2 hours in duration and extended above 17,000 feet.

Source: OLC.  Wave flights from Boulder. Depicted are 40 flights from 2010-17, each more than two hours long and with a maximum altitude of at least 17,000 feet. All depicted flights were in December, January, or February, and all were flown in wave and rotor lift. (Note that good wave flights can also be flown in other months, especially November, March, and April.)

If you study the flight logs a bit, you quickly notice that the traces tend to be parallel to the curving ridge line.  The distance of each trace to the mountains depends on two things:  (1) the wave length on the particular day (it can be longer or shorter depending on the strength of the wind and the stability profile of the air); and (2) in which wave bar the pilot was flying (e.g. the primary, secondary, or tertiary wave).  The primary wave is the one closest to the mountains; it usually (though not always) provides the strongest and highest lift. As the name implies, the secondary is the second wave bar behind the mountains, the tertiary the third, and so forth.

Take a look at the red trace that extends furthest to the west – it is the only one in this set that crosses the Continental Divide.  This flight was flown by Al Ossorio on Dec 29, 2010 in the club’s DG505 and reached more than 27,000 feet within the designated wave window (Arapaho Peaks Soaring Area).  However, note that the high point was not over the Continental Divide; it was several miles further east, just where the red trace blends with all the other traces – the typical location of the primary wave.

Also quite interesting are the two greenish traces that extend furthest to the north. Both were flown much more recently by Bob Faris on two subsequent days in December 2017 (Dec 1 and and Dec 2) in his DG800.  Both flights reached altitudes of just under 18,000 feet. During the more yellowish of the two, Bob got above 17,000 feet only on the outbound leg (following a fairly straight line parallel to the mountains).  He then lost the wave near the Wyoming border and had to fly the return leg at much lower altitudes between 9,000 and 12,000 feet mostly in thermal lift (a very warm day in December!). During the more greenish of the two traces, Bob stayed in wave above 16,000 feet almost the entire time and actually flew back and forth along the mountains three times, covering 617 kilometers at the remarkable average speed of 174 kph (108 mph).

Where Are the Ghost Gliders? or: How Dangerous is Soaring Really?

Preface: I wrote the article below in January of 2018 and I have since done more detailed research about the same question.  My most recent effort uses slightly different (and probably more reliable data) but the overall conclusion remains valid.   If you’re interested in seeing the results of my most recent research that also considers the relative risks of other air sports such as General Aviation, Hang Gliding, Paragliding, and Skydiving, you can find it here.

The tragic death of Tomas Reich during the last day of the most recent Sailplane Grand Prix final in Santiago de Chile and the ensuing debate about the safety in soaring competitions brought – once again – a key question to the forefront of my mind: How Dangerous is Soaring Really?

When I started soaring in 1983 at the age of 16, I often heard people say that “the most dangerous aspect of gliding is the drive to the airport”. Intuitively this never felt right to me and several people have since pointed out that it is indeed far from the truth. (See, e.g., the speech Safety Comes First, delivered by Bruno Gantenbrink).

But just how dangerous is it? To get a better sense we need a reference point. I believe the best way to think about the dangers of soaring is to compare it to the dangers of other relatively dangerous activities we might indulge in: e.g. we could go on a road trip, ride a bike, or ride a motorcycle.  And I think the best way to make such a comparison is on the basis of participation hours (rather than on the basis of miles traveled for example). E.g., when we have an afternoon to spend we may want to know: is it more dangerous to spend that time riding our bike or to go fly our glider? We have all seen the white-painted “ghost bikes” on the side of the road marking the spots where a cyclist was killed but we haven’t seen any “ghost gliders”.  However, we would be kidding ourselves if we thought that gliding was somehow less dangerous. (Spoiler Alert: the comparison is not even close.)

Unfortunately, good, global statistics about the dangers of soaring are hard to come by. In most countries, a comprehensive and reliable database of gliding accidents does not exist. Nor is there a reliable global record of the number of flights or hours flown that would provide a good reference point.

However, while the available data is not globally comprehensive, there is enough out there to draw these comparisons – at least directionally.

My analysis of gliding accidents is based on data from Germany: the German government keeps meticulous track of all flights and even separates out glider flights and flights in motor gliders. It also maintains a database of all flight accidents and reports on an annual basis the the number of fatalities, and the number of persons injured.  Now, one might think that using German data is rather limiting.  But that is not quite true because gliding is much more popular in Germany than elsewhere.  In fact, according to a report presented to the International Gliding Commission in 2010, Germany accounts for approx. one third of all glider flights worldwide.  If there is a limitation to using German data, it might be that it actually underestimates the dangers of soaring elsewhere simply because Germany has such a particularly well developed soaring and safety culture. But, since I can’t prove that, let’s assume the German stats do a fair job of representing the dangers of soaring in general.

So here is what I found.  The result is – unfortunately – rather sobering.

On average, soaring pilots have an accident every 10,000 flights (this is based on all flights in Germany from 2002 through 2016 – the exact number is 10,070). Fortunately some of these accidents only damage the glider or some other property. But once every 60,000 flights someone (usually the pilot and/or passenger) is seriously injured, and once in 83,000 flights the pilot and/or passenger dies.

If you consider that the average glider flight takes about 38 minutes (arguably my least generalizable assumptions since it is simply based on the flightlog of all club flights of members at the Soaring Club in Boulder between 2002 and 2017) this means that soaring pilots can expect to get seriously injured every 40,000 hours and die every 50,000 hours.

Wow! Fortunately we do also other things in life because these stats mean that we would die every 6 years if we did nothing else but fly gliders!

So how does this compare to other activities? Well, not favorably to say the least.  On a “per hour” basis, gliding is about 35x more dangerous than driving; 70x more dangerous than riding a bike, and still 3x more dangerous than riding a motorcycle.

Risk of dying per hours of engaging in a particular activity. Note that the comparison is directional because the data for the various activities are from governments in different parts of the world. (Gliding is based on German data, bicycling and motorcycling are based on UK data, and driving is based on data from the AAA in the United States).

Another way to look at this is to say that 1 hour of gliding is about as dangerous as going on a 35 hour road trip in a car, e.g. from Denver to San Francisco and back again. Or as dangerous as riding a bicycle from Denver all the way to Minneapolis (70 hours). Or as dangerous as riding a motorcycle from Boulder to Salida (3 hours).

Is this an acceptable risk to take? I think that is a question we all have to answer for ourselves. But the important thing is that we should all ask that question and think hard about what we can do to minimize the risk in our own flying decisions.  And no one should kid themselves into believing that those stats don’t apply to them because they are simply a better pilot.  (Instead, they should remind themselves that it’s often the best pilots, like Tomas Reich, who make up the sad statistic.)

With sincere condolences to the family and friends of Tomas Reich.

Rotor Fun – First Flight of 2018

Unseasonal warmth greeted me this morning as I stepped out onto our porch to film the clouds in the rising sun. Wearing only shorts and a t-shirt I felt as comfortable as I would on a mild summer’s day. A gentle breeze whisked around the corner as I mounted my camera onto the tripod, pointing it east towards the horizon.

The clouds told a story of winds aloft, but where I stood, in the lower foothills, 400 vertical feet above the valley, and 5,800 feet above sea level, the movement of the air was gentle and kind.

The night before, the outlook had already looked promising for my first soaring flight in the New Year: TopMeteo projected westerly winds of 30 kts at 12,000 feet, increasing to 40 kts at 18,000 feet.  Meteoblue projected a stable layer between 11,000 and 15,000 feet – right around the tops of the mountains. Dr. Jack’s cross-section chart for Boulder indicated multiple wave bars with modest climb rates even though it projected the wind to have a pronounced southerly component. Based on past experience, I decided to – once again – dismiss the Soaring Forecast from the National Weather Service, which predicted good thermals (very unlikely in the flat January sun despite the unusually high temperatures) and poor wave conditions.

But the best indicator for good soaring conditions was right in front of me: beautifully turning rotor clouds – as always an unmistakable indicator of mountain wave.

On my way to the airport I reflected upon my most recent wave flight, which was characterized by extreme turbulence below 13,000 feet. I braced myself for the possibility of earning another set of bruised shins even though I was hopeful that the comparatively modest wind speed might be a mitigating factor.

One decision was made for me already: I had learned at my club’s monthly meeting that a recent attempt to open the Arapahoe Wave Soaring Area (which allows flights above 18,000 feet within a pre-defined area) had failed because Air Traffic Control was completely unaware of its existence. This would be clarified in an upcoming meeting with ATC but until then it would be better not to put in further requests. This meant that I would have to stay below 18,000 feet and not have a chance to earn Diamond Altitude (which requires a 5,000 meter (16,400 feet) altitude gain in soaring flight after release from tow).  It also meant that I would not need to bring the more sophisticated oxygen equipment required for flights further aloft; and there was one more benefit: the risk of freezing my toes off would be much reduced 😉

At 11:00am local time I was the first pilot of our club ready to launch.  There were two beautiful lines of rotor clouds in the sky, indicating the positions of the primary and the secondary wave. There were also some isolated rotor clouds from the tertiary just to the north of the airfield.  I asked the tow pilot to take me to the upwind side of the secondary, which seemed to promise the opportunity for a longer flight along the wave bar.

Lines of rotor clouds indicate the position of wave lift. The lift is always on the upwind side of the rotor clouds, i.e. on the side facing the mountains.

After two initial turns near the airfield to gain altitude I followed the towplane toward the northwest. Soon after we had passed underneath a small rotor cloud from the tertiary we encountered the first pockets of strong lift.

When the third pocket of lift had lasted more than a few seconds I felt comfortable to release from the tow. I would try to climb in the tertiary and then attempt to push forward into the secondary without the help of a tow plane.

Release from tow in rotor lift from the tertiary wave at 7,900 feet (2,600 AGL). (You can see the tow plane turning left underneath.)

After releasing my first focus was to stay in the area of lift to reach a more comfortable altitude. (2,600 ft AGL may sound unproblematic but where there is strong lift there is also strong sink, and 2,600 feet may only equate to two minutes of remaining flying time if I were to encounter a major downdraft.)

I looked at my GPS (mounted to my right and not visible in the pictures) and quickly worked out the the crab angle necessary not to drift further away from the mountains. The wind speed was considerably weaker than during my previous wave flight. There was some turbulence but nowhere near as pronounced as during my prior wave flights in Colorado.

The area of lift in the tertiary was not very large. However, it was surprisingly calm even though I never reached a truly laminar air flow. The lift was moderately strong, varying from 5 to 10kts. Within 20 minutes after takeoff I climbed through 17,000 feet.

At 16,600 feet I was still in rotor lift from the tertiary wave. The airflow became gradually smoother the higher I climbed but it never turned completely laminar.

After some time in the tertiary I decided to try to move forward into the secondary. I looked for a gap in the clouds along the secondary rotor line and pushed forward into the wind. Vividly remembering my prior wave flight where I lost more than 6,000 feet during a wave bar transition I prepared for the potential of a similar loss in altitude.

This time however, the transition turned out to be easy and smooth.  There was some modest sink along the way but the entire push into the wind did not take more than three minutes during which I only lost 1,500 feet.

Transition from the tertiary into the secondary: my flight path took me around the end of the secondary wave bar. This had multiple advantages: less turbulence, less sink, and a flight path not obstructed by clouds.

Having arrived in the secondary, the lift was clearly stronger, and within two minutes I was back at just under 18,000 feet. Although the line of clouds was interspersed with blue skies it was fairly easy to locate the area of lift. I increased the airspeed of the Schweitzer 1-34 to 80mph and flew south where I could see the next clouds to the west of the Flatirons. I looked at the shape of the Continental Divide to my right and sought to maintain a more or less constant distance to the mountains, accounting for the direction of the wind, blowing from WSW.

Following the line of lift through a “blue” stretch. I had just left a line of rotor clouds from the secondary wave behind and followed a line towards the next rotor clouds in the south. To gauge the best flight path, I tried to maintain the same distance to the mountains on the right while staying to the right of an imaginary line that connected the visible rotor clouds. I also watched the vario for changes in the vertical air movement and corrected the flight path by adjusting the crab angle as necessary.

West of the city of Golden I turned around and retraced my route to the north, again following the line of lift. Without a single turn I continued to fly straight for over 40 miles until I was just west of Carter Reservoir. The lift in this area (north of Lyons) was the strongest of my entire flight: I had to fly at 90 mph with the air brakes fully extended in order to neutralize the lift and keep the plane below 18,000 feet. Next to me was an imposing rotor/lenticular cloud, its western side almost vertical, extending many thousand feet above and below my flight level. Based on my location and the direction of the wind, I assumed that the airflow forming this massive cloud was likely triggered by the steep downslopes of Mount Meeker and Longs Peak.  (I noticed that this area lies outside the boundaries of the Arapahoe Wave Soaring Area so I could not have used this location to climb above 18,000 feet even if the wave window had been active.)

From there I flew back towards the south. I briefly contemplated a push forward into the primary but at this point my feet had become quite cold and I decided to call it a day and return to the airport.

The frozen lakes surrounding the Boulder airport reminded me that it was the middle of winter.  Obviously, they were of no help detecting the wind direction on the ground.  However, the windsock, once in sight, was easy to read, showing a stiff breeze straight from the west.

I entered the landing pattern at 1,500 feet AGL and turned onto final at the end of the runway, still almost 1,000 feet above ground.  I pushed into the wind, flying the final approach at an airspeed of 80mph.  Seconds later, I touched down gently at a very low ground speed, just fast enough to roll the remaining 100 feet right up to the parking position.

Here is a link to the flight track.

For those interested, I have also compiled a lot of information about wave flying that you can find here.

Lessons Learned

  • A high tow may not be necessary to reach wave lift. I released at 7,900 feet and had no problem at all to climb into the tertiary. Today, the first good climb on tow was at 7,300 feet. It would have probably been sufficient. To practice, I need to be willing to release early and risk having to take a second tow. This is especially important with respect to reaching Diamond altitude. With today’s release altitude I would have had to climb to 24,400 feet to accomplish a gain of 5,000m (16,404 feet).  If I could release even earlier, I would not have to fly all that high.
  • Rotor turbulence can be gentle. Today’s rotors were very different from those I encountered during my most recent wave flight. I attribute today’s conditions to the much lower wind speed at altitude (about 25 kts versus 50 kts). On Nov 16 the winds were so strong that I struggled to make progress along the wave bar because most of my air speed was needed to push into the wind, whereas today the necessary crab angles were fairly modest. The flight on Nov 16 offered a bigger challenge. Today’s offered more pleasure.
  • Laminar air flow may only start above 18,000 feet. During today’s flight I never encountered a fully laminar air flow. That tells me that the rotors extended well above 18,000 feet. There was only little moisture at altitude; however, I did see a few lenticular clouds high above the rotors  (my guess is above 30,000 feet). Today would have likely been a great day to reach Diamond altitude.
  • Good climb rates are possible even when wind speeds are moderate. It does not take a howler to produce good climb rates in wave conditions. Today’s climb rates were between 5-10kts in the tertiary and reached well above 10kts in the secondary. At one point the average netto climb rate was 14kts at an altitude of just under 18,000 feet (demonstrating the great potential of today’s lift.)
  • Wave transitions don’t have to cost a fortune (in altitude). Rotor clouds can be super helpful in identifying the best locations for a (forward) transition from one wave bar to another. Today I deliberately picked a spot “in the blue” to push forward into the wind. This did not only reduce the risk of getting sucked into a cloud, it also dramatically reduced the sink rates encountered and therefore the amount of altitude lost during the transition.
  • New rotor clouds develop within seconds.  While I experienced no close encounters with developing clouds I observed numerous times how new clouds can form within seconds. It is critical to always be aware of your location relative to the line where new clouds could possibly form. Especially when a strong crab angle is required it may be difficult to spot that you are about to be engulfed in (newly developing) clouds from behind.



Rotor Bruises

Soaring is not exactly a contact sport.  I always thought the only time you could get hurt is when making contact with the ground (or, very rarely, another object in the sky). Well, today I learned there is also another way.

But first things first: my last flight on Monday taught me not to trust the wave forecast but instead to rely on observing the sky.  When I woke up this morning, this is what I saw: a whole sky full of wave.

Beautiful sunrise from our porch.

There was even this little, frazzled-looking, rotor cloud right above our house in the foothills:

The obscurely shaped gray cloud is the rotor. It is much lower than the cirrus clouds far above.

This made it easy for me to ignore the National Weather Service, which, once again predicted “poor wave”, and “good thermal” conditions.  A glance at the sky at 6:15am, and I already knew better than that. (Of course that’s not quite true: as always, I did look at a sounding, the winds aloft, the thermal projections from and, and the distance of the next front that was projected for Friday.)

Line of small rotor clouds far out into the plains in the rising sun.

So off to the airport I went. And I wasn’t the only one. Other pilots had put their own reading of the sky ahead of the forecast as well.

Once again, I got the Tin Can ready. As I filled the oxygen tank I talked to the tow pilot who had just come back from his third tow of the day. He gave me a taste of what to expect: rotor turbulence “bordering on violent”. He said this with a big grin on his face, so apparently it was also going to be fun. He advised on where he suggested to tow, and explained that he would speed up to dive through an area of heavy sink. He would slow down before we would hit the heaviest turbulence.  Or, rather, he said he would try: for neither of us could be sure that it would still be at the same place as before.

I climbed into the cockpit, secured all loose items, fastened the straps as tight as they would go, looked through the checklist again, and off we went. (You can see the flight track here.) Takeoff was relatively smooth although we didn’t climb much until the end of the runway. Then came the first bump. Suddenly it went up at 8-10kts but it was still surprisingly smooth.  At about 1,000 feet above ground we entered the wind shear zone. The wind at the ground had been 5-8 kts from the northeast but now the wind shifted to the strong westerly flow above.

Appropriately dressed for a wave-flight today. The chemical foot warmers in my boots were perfect.

The towplane in front of me started to jolt around: sometimes it would drop all of a sudden, sometimes it would bank to one side or the other, sometimes it would rise straight up.  Any of these erratic motions were also an indication as to what would happen to my glider about two seconds later, for that’s about how long it took for the glider to reach the air that the tow plane had just passed through. “Compared to the tow pilot I’m really lucky”, I thought, “Unlike him, I know exactly what to expect.”

The tow pilot turned west and dove through the sink just as per our briefing.  I followed right behind, mentally preparing for the heaviest jolts that were yet to come when we would hit the next rotor. Glancing back at the airport I felt reassured by our altitude: if the tow-rope would snap or if I was forced to release, I felt certain that I could make it back on my own. Just after I had finished that thought, my glider was tossed down in a sudden down-draft.  The tight straps kept me in my seat but my legs were out of control: inertia wanted them to be 20 feet higher but they only had a few inches to move up until they hit the instrument panel. Bang! Then, a split second later, I was firmly pushed down into my seat as the plane was lifted up again and my feet regained contact with the rudder pedals.

This up and down, left and right, had lasted for maybe 20-30 seconds when the vario indicated strong lift. Just as I moved my hands towards the release knob, the tow pilot came on the radio to say that this is where the other pilots had released as well. A quick pull and off I was.

At just under 18,000 feet over the foothills. Spoilers are open to prevent an inadvertent climb above 18,000 while taking pictures…

From there I worked the front side of the rotor to about 13,000 feet when I pushed into the laminar flow of the secondary wave. The wind was so strong, blowing at about 50-60 mph, all I really had to do was point the nose into the wind and rise, stationary above the ground.

The strength of the wind made it difficult to fly sideways along the wave bar.  To maintain the same velocity into the westerly wind while also moving north or south, I had to speed up, which resulted in a greater sink rate.  Also, I noticed that the lift was less consistent than during my flight this past Monday. Several times I returned to an area where there had been strong lift only to find myself in sink.

I was just a few miles northwest of the airport when I decided to attempt a push into the primary. I started at well above 17,000 feet knowing that I would have to fly very fast and loose a lot of altitude while penetrating through an area of heavy sink. Determined to keep the airport within reach at all times, I resolved to turn around if I would not get to the primary at an altitude of at least 12,000 feet.

I put the nose down, increased my indicated airspeed to 110 mph, and flew straight into the wind. As expected, the needle of the altimeter began to spin backwards and the surface got visibly closer. When I got down to 13,000 feet I began to wonder it it would work. Just as I prepared to turn and make a quick escape towards the airport, I entered the rotor zone behind the primary.  I quickly reduced my air speed to 80mph (the maximum for rough air in this glider) and continued to push into the wind.  The sink rate slowed but I wasn’t out of the woods just yet.

Estes Park right in front below. Stormy Peaks, appropriately named, behind.

As before, the plane got tossed around by heavy turbulence. My legs were loose sticks again, and I couldn’t keep my feet on the rudder pedals even though I tried. A few more bangs against the instrument panel and finally: I started to climb again.  At the low point I was down to 11,500 feet, a bit lower than I wanted to be, although still high enough to make it back to the airport. (I lost almost 6,000 feet during the transition into a 50-60 mph headwind. I estimate that a backward transition with the wind at my tail would have cost at most 2,000 feet in altitude, probably less. That would have put me at 9,500 feet into the rotor zone of the secondary – roughly at the same spot where I released from the tow plane and definitely within reach of the airport.)

After a short climb in the rotor I was back in laminar flow: I had made it into the primary! I climbed back up to 17,000 feet and began to explore along the wave bar flying between Longs Peak and just west-southwest of Boulder.  Just as I had experienced in the secondary, the strength and location of the lift was inconsistent.  Within 20 minutes, regions with strong lift turned into regions with modest sink.  E.g., in an area to the west of Lee Hill I had found strong lift on my first leg to the south.  On my second leg, I only found moderate sink at the same spot.  I explored back and forth along a few streamlines but wasn’t able to find any lift that would carry me back up.

Continental Divide. Beaver Reservoir in the foreground. The illuminated peak center-right is Mt. Audubon.

From there I retreated closer to the airport, all the while expecting to get into massive rotor turbulence again. However, the air stayed surprisingly smooth as I gradually drifted back towards town.  Whenever I noticed some lift, I would turn into the wind and remain stationary over the ground, trying to climb. But in all cases the lift evaporated after a minute or two, and I finally decided to return to the airport to land.

Then, just as I arrived directly south of the airport, I found strong and unexpectedly smooth lift right next to the runway.  I pointed the nose into the wind and, without doing anything, climbed back up from 8,000 feet to over 13,000 feet within about 13 minutes.

Observing the curls of water on the surface of the nearby lakes, I noticed the wind on the ground now also blowing straight from the west, and it appeared to be getting stronger.  So after leveling off at 13,200 feet, I took advantage of the Tin Can’s terminal velocity dive breaks to begin a rapid descent to 7,500 feet.  Now, just 2,200 feet above ground, the wind was still blowing at almost 50mph.

Still signs of wave at sunset tonight.

I crossed the runway at 2,000 feet above ground and flew a close pattern to Runway G26 with a very steep and fast descent against the strong headwind.  Once in ground effect, calmness enveloped the plane and I touched down smoothly at a very low ground speed.

Lessons Learned

  • You can get bruised while in flight.  Even very tightly worn straps cannot prevent your legs and feet from flying around the cockpit and hitting the instrument panel. (It’s not as bad as it sounds, though. The fun factor was definitely greater than the pain from the small bruises. Playing soccer is definitely more hazardous for your shins.)
  • Slack-line training is not for naught. It’s impossible to prevent slack-line while towing through rotor turbulence, all you can do is correct it when it happens.
  • Forward wave transitions cost a lot of altitude. 6,000 feet in my case today.  Always keep a safe escape route – ideally to the airport.
  • Wave lift is not always stationary to the ground. During my flight on Monday it stayed reliably in place.  Today, I frequently encountered situations where strong lift was replaced by moderate sink within minutes.
  • Wave lift can be where you don’t expect it. The smooth climb right next to the Boulder airfield today is a good example.  (I’m not sure if it was from the secondary or the tertiary.)
  • Rotor turbulence can happen at very high altitudes. As I flew in the secondary today above 17,500 feet I ran into rotor turbulence that I had not expected at that height.  This is a safety consideration as one might be flying well above rough-air speed at this level.
  • Progress along a wave bar can get really difficult in very strong winds. Today, most of the plane’s forward motion was needed to not drift backwards. To fly along the wave bar required high air speeds corresponding to sink rates that at times consumed more than the available lift.
  • More moderate wind speeds are preferable to very high wind speeds: they are better for XC flying (smaller crab angle required), and the rotor turbulence will be less severe.




Do I Stay or Do I Go?

When you see this you no longer ask: do I stay or do I go?

Yesterday, as so often, I began the day by looking at the weather forecast.  This is what I saw:

NWS Soaring Forecast for Nov 13, 2017 for Denver/Boulder

Really?  Good thermal soaring with 4 m/s lift up to 14k feet in the middle of November? Sure, it was going to be an unseasonably warm day with highs around 70 degrees F.  But 4 m/s seemed way too good to be true.  So I took a look at some other sources:

Source: Location forecast for Boulder Nov 13, 2017

Now that seemed more likely: thermal climb rates of 1.8 kts (0.9 m/s) up to 9,500 feet – more realistic given the season but barely enough to stay up in a glider with a minimum sink rate of ~1.5 kts.

The Thermal Updraft Velocity chart provided by was just slightly more optimistic than about the thermal projections: max. climb rates between 200 and 300 feet/minute (1 – 1.5 m/s) with the strongest updrafts just south of Boulder (over the Flatirons).

Source:, Thermal Updraft Velocity for the Colorado Front Range; the circle highlights the area around Boulder

Which of these thermal forecasts should I believe?

And if thermals wouldn’t work, would there be wave?  The wind forecast looked fairly favorable: 29 kts from WNW at 13,000 feet increasing in strength to 38 kts at 18,000 feet with no change in direction.

Source: Location forecast for Boulder Nov 13, 2017

The cross-section chart for Boulder suggested a strong primary (climb rates of 5 m/s and more) and a weak secondary (climb rates around 1 m/s).  The main problem with that outlook is that the secondary would be too weak to climb in, and getting into the primary would require a very long and high tow deep into the mountains crossing through the area of sink between the secondary and the primary.

Source:, 270 cross-section for Boulder

The sounding for Boulder confirmed the wind forecast but did not show the presence of a stable layer at the relevant altitude (between 11k and 15k feet – the height of the Continental Divide that would trigger the wave).  The theory says that wave will not form without a stable layer around the tops of the mountains because only a stable airmass will have the tendency to bounce back after it is forced to descend and warm up on the lee side.

The wide gap between the temperature line (red) and the dew point line (blue) suggested blue skies (no clouds) at all altitudes.

Source:, Sounding for Boulder

The National Weather Service (NWS) was even more pessimistic than these charts suggested:

Source: NWS. Soaring Forecast for Denver/Boulder Nov 13 2017

So the usual question arose: do I go, or do I stay? The NWS said there would be great thermals but I did not believe their projections. And despite favorable winds aloft, none of the wave forecasts looked particularly promising.

So, what did I do?

Blue Wave over the Colorado Foothills. The yaw string points straight at Longs Peak. The flight track, however, is parallel to the mountains in front. Climbing through 15,000 feet at 8kts (and the lift kept improving).

I went.  Why? Not because I suddenly thought the NWS’s amazing thermal forecast of 4 m/s might be true after all but because I looked at the sky: there was a small, but beautifully formed, lenticular cloud standing right above Boulder.  There were also some small rotor clouds. These were clear signs of wave.

I prepared the “Tin Can” (aka the Schweitzer 1-34), installed my new toy (an Oudie IGC flight computer), checked the oxygen level in the tank and off I went.

Takeoff was easy with a few knots of wind from the east on the ground.  That quickly changed at about 1,000 feet AGL when the wind direction switched to the West and the ride through the rotor began. The tow was very bumpy, frequently requiring full control deflections, but I didn’t find it too hard to follow right behind the towplane.  Only a few times did I have to correct for a developing slack line.

At just around 9,000 feet MSL we entered the first strong rotor climb just at the entrance to the Left Hand Canyon.  After the lift held out for several seconds I released without hesitation (that’s good because at times I waver and stay on for much longer than really necessary).

Yesterday’s flight track was directly along the line where the foothills end and the plain begins. The flatirons are right in front below the plane. The city of Boulder is in the center. A thin lenticular cloud is above. A small lonesome rotor cloud – likely fueled by moisture from Gross Reservoir (which is in front and slightly to the left of the plane) – is below to the right.

The wind was quite strong and I knew I had to stay in the area of lift, otherwise I might drift back into sink and end up on the ground again in no time. That’s were the moving map from the Oudie came in extremely handy. The flight trace showed where I was climbing and where I was sinking and all I really had to do was stay more or less stationary to the ground to remain in an area of overall lift.  It was rough with short upward bursts being followed by short downward bursts, but overall it went up at a good clip.  Within a few minutes I climbed through 10,000 feet, 11,000, then 12,000.  Suddenly the air went still and I had reached the laminar flow. The wild high and low beeps from the acoustic vario were now replaced by a happy sound with a constant pitch.  Initially the climb rate was not particularly strong but it was consistent and smooth.  I moved the trim back to reduce the speed to just over 40 mph and flew in shallow S-turns into the wind, maintaining my position over the ground.

Whenever the climb rate decreased I would first probe into the wind to see if the lift would strengthen and if that did not work I would just let the plane drift back and invariable the climb rate improved again.  It was actually quite simple and I just did what the theory of wave flying had taught me to do.  Once I had climbed above 14,000 feet I began to explore along the wave bar and just as I had expected, I was able to continue to climb as I began to fly north, parallel to the mountain range which was about 16-18 miles to the west.

I looked into the direction of the wind to identify as well as possible where along the mountain range the particular streamline I was flying in had been triggered so that I could follow the topography and anticipate potential shifts in the location of the best areas of lift as I moved north or south.

I also noticed that I had to adjust the crab angle based on the speed I was flying at: the faster I would fly the less of a crab angle was needed to stay in the best zone of lift and when I slowed down I had to move the nose towards the wind again.  It was actually all surprisingly easy and I even understood why some pilots think that wave flying can be a bit boring.

Within no time at all I was at 17,000 feet and the climb rate actually kept improving.  I had not called the Denver Center to request the opening of the wave window so I had to stay below 18,000 feet.  I increased the airspeed to 100 mph (when the Schweitzer’s sink rate is 3 meters per second) and I was still climbing at 2 m/s.  Also, the faster I flew the colder it got.  The cockpit of the Tin Can is not exactly well insulated from the outside and while the sun was shining the outside temperature was well below zero.

So I pulled the airbrakes and slowed down.  Now I had a better way to manage my altitude without freezing my toes off. At slow speeds the plane climbed even with the airbrakes fully extended.  But I just had to speed up a little bit to force to plane to descend.

Once I had figured it out I kept yo-yoing along the foothills between White Ranch Park to the south and Lyons to the north.  On my second leg flying south over the Flatirons I looked out to the left and saw a Boing 737 about 2-3 miles ahead to the southeast and about 2000 feet below. It was climbing in westerly direction and definitely getting closer.  I checked that my transponder was still on (which it was) and wondered why ATC had not kept us further apart.  While we were certainly not in danger of colliding I still felt this was too close for comfort, so I held my position for 30 seconds or so until the jet had passed before I continued my flight to the south.

After about an hour above 17,000 feet I was getting uncomfortably cold despite flying a good amount with open spoilers, so I decided it was better to return to the airfield.  I flew into the wind until I was right in the middle of the sink between the primary and the secondary wave and used it as a downward elevator.  It was fun watching the altimeter quickly turn backwards and the ground coming closer while still flying in perfectly smooth air.

Continental Divide from 17,600 feet. I was flying with the spoilers completely open so I could take pictures without inadvertently climbing into Class A airspace (above 18,000 feet).

Remembering that I would have to return to a more turbulent zone, I was about to pack away my camera when – at about 13,000 feet – I got still surprised by the sudden violent jolt upon reentering the rotor.  Despite being strapped in fairly tightly, my head hit the top of the canopy; my Oudie’s suction cup gave way and the Oudie as well as my camera flew through the cockpit. I felt thankful for the sturdiness of the sailplane and that I didn’t get hit by anything.

The turbulence stayed with me all the way to the ground.  Remembering my prior experience with massive sink in the landing pattern, I made sure to arrive over the airport with ample height. I flew a few circles to get rid of excessive altitude and took note of the distribution of lift and sink near the airfield. I entered the pattern at about 1,600 feet AGL and stayed high along the downwind leg before flying a steep and fast final approach.  As expected, conditions smoothened considerably at about 30 feet above the ground and the landing was gentle and right on target.

During my flight I had stayed in the secondary wave the entire time and it provided great and consistent lift of up to 10kts (5m/s).  Two other Boulder pilots penetrated into the primary where the lift was probably even stronger.  Their flights are here and here.  A third pilot tried to get into the primary but reverted back to the secondary when his height evaporated during the attempt. His flight is here. My northern and southern turn points were at locations where I felt the lift getting weaker and I wasn’t confident about continuing given the increasing distance to the airfield. The other pilots proved that the wave lift extended much further north and south but you had to adjust the flight path.

Lessons Learned

  1. Read the weather forecast but don’t trust it.  It is no substitute for looking out the window and forming your own judgement.  [Especially the NWS forecast was completely off: there were no thermals to speak of (NWS had predicted thermal lift of 4 m/s); however, wave conditions turned out to be excellent (NWS had predicted “poor”).]
  2. The wave flying theory really works in practice.  Yesterday was actually very easy, I’m wondering if it was unusually easy.
  3. Seeing my flight trace on the moving map is invaluable. My new toy (Oudie) worked great but it needs a better mount (which I ordered already). The suction cup does not hold up to turbulence (and it probably isn’t great for the canopy either).
  4. Pack your stuff away before beginning to descend.  I was already half-way down and got surprised by the violent re-entry into the rotor zone.
  5. Dress even more warmly.  Warmer gloves and chemical foot warmers in my hiking boots would have been great. It was 22 degrees C in Boulder, 0 degrees C at 12,000 feet, and -15 degrees C at 18,000 feet.  It could have been much colder. Also: the faster you fly the colder it gets.
  6. Keep a good lookout, even with a transponder.  Commercial jets taking off from Denver towards the West will still be significantly lower than 18,000 feet over the foothills.
  7. Where do the (few) clouds come from? If you see a few rotor clouds even though the sounding suggests there should not be any because the air is so dry, they are likely fueled by the moisture of one of the lakes in the foothills.
  8. Experiment more when you think you’re at the end of a wave bar.  A slight change in course direction would have allowed greater distances.


Boulder Soaring Season(s)

After nine months living in Boulder I have learned that the weather in Colorado is generally nice, but also fickle and variable: one day you experience 80 degree heat and brilliant sunshine and the next morning you wake up to a foot of snow on the ground … which melts at an astonishing rate such that you might return to the tennis court in shorts on the same afternoon. Nobody seems to store their winter or their summer wardrobe for it’s not unusual to need t-shirts and snowshoes in the same week.

However, although warm and cold days can happen in any season, the differences between summer and winter are still profound: the length of daylight, the level of humidity, the location of the jet stream (and hence the direction and strength of the prevailing winds) are highly seasonal.

Local soaring pilots will tell you: summer is monsoon season, winter is wave season. You can fly all year round.  The best soaring is often in late Spring or early Fall.

But what exactly does this mean? I wanted to take a look at some data. How many days per month can you go soaring?  When are the longest distances flown? When can you go cross-country? Fortunately, Boulder pilots have been pretty good about uploading their flights to the OLC website. There is a treasure trove of information: more than 10 years of data, in fact. That’s almost 3,500 flights that were uploaded to OLC.

So here is what I learned:

Source: OLC; all flights from Boulder from 2007 to 2016. Soaring Days are defined as days when at least one flight was over one hour in length; XC days are defined as days when at least one flight was longer than 200 kilometers.

So, it’s easy to see that, yes, one can fly in every month of the year.  From May to September roughly every other day is soarable.  However, in the winter months this is true for only about one day in six.

(Important caveat: I suspect that there were good soaring days when nobody had time to go soaring.  It’s also possible that on some soaring days no-one uploaded their flights to the OLC.  If either or both of that is true, the implication is that many more days may be soarable throughout the year.)

Source: OLC; all flights from Boulder from 2007 to 2016.

From March through November it is usually not a problem to stay up: the vast majority of flights  in these months (ranging from 81%-85%) exceeded one hour in length (defined in the chart as “Soaring Flights”).  Not so from December through February: not only are far fewer flights attempted in these months, one third of the time the flight duration ends up being less than one hour (and that is only for flights that were uploaded to OLC).

The contrast is even starker if you look at the percentage of cross country flights.  From April through September about 50% of flights are longer than 200 kilometers, whereas in December and January that percentage drops to well below 10%.

Source: OLC; all flights from Boulder from 2007 to 2016.

This is also reflected in the attainable flight distance: flights exceeding 1,000 kilometers have been achieved from May through August with April and September not far behind. The average flight distance of all uploaded flights exceeded 200 kilometers in each month from April through September.

Not surprisingly, the attainable flight distance is shortest from December through February with the average being below 100 kilometer.

Source: OLC; all flights from Boulder from 2007 to 2016.


Based on this analysis the soaring year in Boulder can be grouped into three seasons:

  • The “Peak Soaring Season” from May through the end of September (five months)

– About 50% of all days are soarable

– Staying up is usually not a problem (more than 80% of flights exceeded one hour)

– The average flight distance was well above 200km and 50% of flights were longer than 200km

– More than two thirds of all soaring flights and almost 80% of all XC flights were in these five months

  • The “Low Season” from November through March (five months).  Flying is possible in every month. However, November through March (and especially December and January) tend to be the most difficult.

– Only 4-8 days per month were soarable (i.e. flights of more than one hour were attained)

– Approx. 30% of flights uploaded to the OLC lasted less than one hour.

– While a few XC flights were achieved, the average flight distance was below 100 kilometers.

– 16% of all soaring flights and only 7% of all XC flights were in these five months.

  • The “Shoulder Season” comprised of the two months April and October in-between the Peak Season and the Low Season

– Approx. one in three days is soarable

– Staying up on these days is usually not a problem (more than 80% of flights were longer than one hour)

– Going cross-country is definitely possible on good days.  The average flight distance was over 200km for April and 150km for October.

– 16% of all soaring flights and 14% of all XC flights were in these two months.


Ready to Wave

My club, the Soaring Society of Boulder, has a designated plane for wave flights above 18,000 feet: an old Schweitzer SGS 1-34.  Yesterday I got checked out in it. The plane looks old because it is:  built in 1978 it has already experienced a lot.

Schweitzer SGS 1-34 of the Soaring Society of Boulder

A few things make it particularly well-suited for wave flights:

First, it is made of aluminum. There is no sensitive gel-coat that could crack when you’re descending from 35,000 feet where air temperatures might be 50 or 60 degrees Celsius below zero. A side benefit is that the plane can park assembled outside all year long and doesn’t even need covers (except for the canopy). Just get rid of any snow and fly!

Second, it has terminal velocity dive breaks: that means if you need to come down fast (e.g. if the oxygen system should malfunction), you can.  Just point the nose straight to the ground, pull the dive breaks out and you won’t exceed the maximum allowed airspeed. That sounds wild but it will get the job done if you need to breath.

Third, it is very well equipped for wave flights: not only does it have a transponder that will make it visible to Air Traffic Control (after all you might be flying at altitudes normally reserved for commercial jet traffic), it even has two oxygen systems including a pressure demand system certified for flights up to 45,000 feet.  Wow!  I wouldn’t go nearly that high even if I could.  I don’t know if there’s anything on my body that wouldn’t freeze off at that altitude! Also, it really is seriously dangerous to do so in a non-pressurized cabin.

  • West Wind Takeoff

Flying in wave at Boulder likely means contending with west wind takeoffs.  A few weeks ago I did a separate check out for those because they can be quite tricky.

The airfield in Boulder is less than 3 miles away from the Foothills. The westerly winds that trigger the wave flow down the slope of the mountains. This means that just to the West of the Boulder airport their could be nothing but massive sink. This could be made worse by potentially severe rotor turbulence, which can quickly put an end to an aero-tow: tow plane and glider could easily get so out of position that either is forced to release, or the tow rope may simply break.  (A local tale tells of a towplane getting inverted in rotor turbulence where the pilot was able to roll back while the glider was hanging on…  Another tells of a glider releasing in massive turbulence a few hundred feet above the ground and being able to circle away in rotor lift…)  Needless to say that if any of these things happen right after takeoff, you can quickly find yourself in an emergency where you have to pick the next field and land because returning to the airport might be impossible.

Adding to the challenge is the lack of good fields should you find yourself in this situation. Just to be clear: there are fields around and it is very likely that you will be able to reach one of them but you may have to decide immediately what to do (within a second or two) and most of them are not great for landing. I want to be prepared if such a situation ever arises and I have therefore created the following map with potential out-landing fields and key decision points.

West wind takeoff at Boulder Municipal airport.

The potential landing fields are marked A through F.  Key decision points are marked 1 through 6.

At Boulder, the default runway is 08 – i.e. takeoff to the East.  West wind takeoff (i.e. runway 26) is normally only used if there is considerable wind from the West (min 5-8 knots or more, which would make a tail-wind takeoff to the East too risky or impossible).

For a West wind take-off, gliders are moved all the way to the East, one tow-rope length (200 feet) beyond the end of the asphalt strip. The graph above shows a stylized image of a tow-plane and glider in staging position.  Club policy requires the use of a powerful tow-plane for West wind takeoffs.

Once the towplane starts moving, the first decision point comes up very quickly:  if the tow-plane is not air-born just after the middle of the runway, it is time to release and abort:  there is still enough runway ahead to land the glider safely while the tow-plane takes off on its own and flies a pattern.

After decision point 1 the next landing possibility is Field A.  It is located behind a row of trees at the West side of the little lake. The trees can create significant turbulence in their wake.  If the glider is forced to release at an altitude insufficient to clear the trees (usually somewhere between point 1 and point 2), the best bet is still to land straight ahead, even if it might mean running out of runway and ending up in the lake.

At point 2 it is very likely that the glider can clear the trees ahead and at this point the best option is to land in Field A.  The field is about 1,200 feet long, which is not a lot because you have to first fly over the trees, but it should be sufficient, especially if the headwind is fairly strong. The surface is fairly rough with a lot of holes from prairie dogs and there is a small tree to avoid.

At point 3, the default option should be Field B. It is equally rough as Field A but it is long enough (1,500 feet) and the obstacles in it should be avoidable. Field C (Pleasant View Soccer Fields) may or may not be an option: it is obviously flat and in great condition but there may be people in it or the movable goal posts may be arranged in a way that prevents a save landing.  Only chose it over Field B if you’re certain that you can land safely without endangering anyone.  (There may be no time to decide, which is why the default option is Field B.)

At point 4, the best plan depends on your altitude:  if you’re very low and descending fast, Field B may still be an option.  If you’re already fairly high, it might even be possible to return to the airport.  If it’s somewhere in between, then Field D may be the best option.  It’s 1,100 feet long and you have to clear bushes and trees but it should be doable.

At point 5, you should already have multiple options, depending on your altitude.  A downwind landing on runway 8 may be possible.  And if not, Fields D, E, and F should be within reach.   At point 6, a safe return to the airport (and landing to the West on runway 26) should be possible.

As always, it is good practice during takeoff, to call out the field(s) that you would be landing at should the tow be terminated at that point for any reason.  This way you already know what the decision is should anything happen and can concentrate on executing your plan rather than waste precious seconds (and altitude) in formulating one.

  • Opening the Wave Window

The airspace above 17,999 feet is designated Class A airspace in the US.  Flying above this level requires special permission from air traffic control.  Therefore, the final piece to flying high in wave at Boulder is the procedure to open the Arapahoe Peaks Soaring Area wave window.  It is as follows:

Before the flight, call the Denver Flight Desk at 303-651-4247.  You will talk to the “Mission Desk at the Air Traffic Control Center”. They will ask for the following information:

– Your name
– Name of the Airspace: “Arapahoe Peak Soaring Area”
– The requested altitude expressed as “Flight Level”, e.g. 30,000 feet is FL300
– The time frame (in UTC, i.e. “Zulu Time”) for when you would like the window to be opened.
– The aircraft registration (N number)
– Tell them that the aircraft is equipped with a transponder.  They will tell you a squawk code to use instead of 1202.

Calling them is just a pre-notification!  Once air-born you will still have to call the Denver Center on the radio at frequency 128.65 MHz (or another frequency that may be assigned to you) for your aircraft to be cleared into the Airspace.

Also, remember that above 18,000 feet = FL180 (the “transition altitude”) you must set your altimeter to 1013.25 hectopascals (millibars) or 29.92 inches of mercury.

While flying above FL 180 you must remain in radio contact with the Denver Center and follow all instructions.

Once below 18,000 feet you must contact Denver Center at 128.65 MHz (or on the phone) that the Airspace is no longer needed.  Also, after exiting the wave window and calling the Denver Center, re-adjust your transponder back to squawk code 1202.