Soaring – Chess in the Air

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

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:

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 soarbfss.org was just slightly more optimistic than topmeteo.com 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).

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.

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.

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.

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

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?

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).

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.

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.

 

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:

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.)

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%.

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.

Summary

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.

 

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.

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.

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.

Over the last two days I received my checkout in our club’s Discus CS.  It’s a nice plane with strong performance characteristics and easy to fly.  I am particularly impressed with the quick roll rate: you can go from a 45 degree bank angle in one direction to a 45 degree bank angle in the other direction in just about three seconds – very responsive! I also like how quick the plane picks up speed and how efficiently it reconverts it into height when pulling up into a thermal.  I look forward to flying it for longer distances when the weather permits.

The only real downside is that the plane is always stored disassembled in its trailer. That means I always have to have someone help me rig it before a flight and derig it afterwards. I was at first a bit intimidated by the 85-point rigging checklist but once I had done it a couple of times I realized that it actually goes pretty fast. It reminded me a bit of the process involved in rigging a sailboat – like the Hobie Cat that I sailed as a kid. The automatic hook-ups of the control surfaces are a big plus so there really isn’t all that much that can go wrong. Obviously, it’s still well worth double checking everything – after all, unlike with a sailboat where mistakes are usually pretty benign, the pilot’s life is on the line when you mess up the rigging of a plane…

The weather hasn’t been all that great for soaring. We’ve had about 10 cold days with clouds and rain,and the ground is still pretty wet (very unusual for Colorado). Temps were back around 20 degrees Celsius today but the air was stable and we stayed well short of the projected trigger temperature of 25 degrees.  There were a few weak thermals above the city of Boulder and the nearby plains but no lift at all over the foothills despite some decent looking cumuli. The cumuli had tempted me into a high tow to about 12,000 feet but 20 minutes later I was already back over town at about 8,000 feet.  There I stayed for more than an hour until I was done flying in circles that didn’t really lead anywhere.

The air was fairly clear so I still got some nice views of the Continental Divide, glistening white in its first fresh snow cover of the season.  The trees around Boulder have also started to take on nice color.  My other satisfaction was watching other gliders take off and land while I was able to hang on. Credit to the Discus, which thermals very nicely at a relatively low speed. I’m still hoping that we’ll get a few good thermal days before it’s winter wave or nothing.

 

Last Sunday didn’t look like a good day for soaring so I decided to use the time for a road trip along the Northern Front Range to research potential land-out fields – private airstrips, farmer’s fields, and other places where I might be able to land a glider safely if it became necessary. Knowing where such fields are allows me to safely venture beyond gliding distance of the main public airports and gives me more confidence to go on cross-country flights.

In preparation of my “field trip”, I created a set of criteria (see below) that would help me evaluate each landing area.  I also researched a number of potential fields upfront using satellite images on Google Maps.

In addition, I queried the Internet to see if other people had done such work before so I would have a good starting point to work from.  (In Europe, there are reasonably decent open-source databases of land-out fields available, e.g. as part of XC Soar.) Unfortunately, I wasn’t able to find anything comparable for Colorado. Also, my prior experience with such databases in Austria demonstrated that they cannot be relied upon as fields can subsequently become unusable (e.g. new construction, fencing, other obstacles, etc.).  In addition, I have found that just because someone else marked a field as useable, does not necessarily mean that I would be comfortable landing there myself.

My drive covered the area between Boulder, CO and Laramie, WY along the Northern Front Range. You can click here to view the complete results of my research. Please remember that I did this for my own purposes only and you must read this disclaimer before you use any of this information.

Please contact me if you are aware of similar efforts to research land-out fields for this or other areas in the Rocky Mountains.

In addition to learning about specific fields, I also gained a much better understanding of the topography along the Northern Front Range. And, as another side benefit, I saw some beautiful country side.

Finally, a note for non-glider pilots who may be unfamiliar with the concept of “landing out”: when soaring, a good principle is to always stay within glide range of an airport. However, airports aren’t everywhere so pilots try to stay at least within glide range of a field where they can land without breaking the plane, and, more importantly, themselves. Landing in a field is what’s called “landing out”.  It is certainly inconvenient (because someone will have to jump into a car to come get you and your glider (which will have to be disassembled and put into a trailer), but it is not unexpected. If done properly, it is definitely not a “crash” or even an “emergency.” Especially in gliding competitions where glider pilots cover long distances, often in marginal conditions, it happens all the time and is par for the course.

Yesterday, Captain Joe (not his real name) invited me to join him on a flight in the club’s ASK 21. It was supposed to be a strong thermal day, although overdevelopment, rain showers, and a potential for thunderstorms were also part of the forecast. As I left home on the way to the airport, I spotted what looked like a small lenticular cloud. That surprised me as the National Weather Service had predicted “poor” wave conditions.  Driving south along the foothills I noticed wind blowing from the West. Maybe there was wave after all? (I should have paid more attention to the wind speed aloft.) Westerly conditions at Boulder are notoriously challenging with the potential of strong downdrafts in the lee of the Rocky Mountains as well as turbulent rotors beneath any wave(s), and at this point I wasn’t so sure if we would go flying at all.

When I arrived at the airfield, three miles away from the foothills, the wind was only light and its direction variable. I ran into one of Boulder’s most experienced flight instructors and cross country pilots who confirmed that the winter wave season was indeed beginning and that we should be prepared for some rotor turbulence on tow.  He seemed confident, however, that our takeoff would not pose a problem. So Joe and I decided to go.

Takeoff was indeed uneventful and our tow pilot did a nice job staying out of the invisible rotors. Nevertheless, the air was clearly more turbulent than during my previous flights from Boulder, and soaring conditions seemed at first difficult: for long stretches of time we followed the tow plane in sink strong enough to have us lose altitude, and we did not hit our first real thermal until we reached almost 5,000 feet AGL (10,300 MSL) over the foothills.

Once off tow, the first climbs were a bit rough, the thermals narrow and ill defined. But only about 30 minutes later, cumulus clouds developed rapidly and the lift quickly became stronger and smoother.  Soon it was effortless to reach cloud base at just under 15,000 feet MSL. Not much later, the first sheets of rain began to fall. Dark clouds popped up and dissipated in what seemed to be no time at all. One minute we would fly in strong lift along a cloud street, and a few minutes later we found ourselves in heavy sink and surrounded by virga.

The rain curtains were very picturesque, and, since Joe did most of the flying, I had plenty of time to take pictures.

Two and a half hours into our flight back and forth along the foothills, a group of tall cumuli southwest of the flatirons drifted towards Boulder and appeared as if they might develop into cumulonimbi.  It was time to return to the airfield.

As we approached the airport on a straight glide from about ten miles to the south we encountered numerous patches of strong lift and heavy sink in quick succession.  About three miles south of the airport was another spot of strong lift.  Joe, obviously concerned that we might be arriving too high in the pattern, pulled out the spoilers until he had us back down at 6,300 feet (1,000 ft AGL) – the normal pattern entry altitude. I remember thinking that I would have kept the spoilers closed at this point because its much easier to lose height in the pattern than to gain it back. However, maybe out of respect for Joe, I did not say anything. We were, after all, still at the normal pattern altitude.

A glace down at the lakes and windsocks indicated calm winds on the ground. Even at our altitude we could not detect a noticeable wind drift in any direction. Joe said he would fly a normal approach to Glider Runway 8 (facing East).  I had no objection but suggested that he might want to add some extra airspeed, maybe fly at 65 kts instead of 55 kts, because we had just been through some significant turbulence.

We should have been prepared for what came next but it still took us by surprise. Just before we crossed the runway to enter the traffic pattern (see map above), we encountered by far the heaviest downdraft of our entire flight. The variometer needle hit the maximum sink indication (i.e., more than 1000 feet per minute), and the ground was visibly coming closer second by second. By the time we were across the runway we had already lost a few hundred feet and were now below 6,000 feet MSL (700 feet AGL).

I remember regretting at this point that I had not spoken up earlier about keeping the spoilers in. But I must say that Joe, the experienced airline captain, did everything right. He stayed (at least outwardly) calm, announced that he was going to fly a close abbreviated pattern, and began the downwind leg in close proximity to our runway. All the while the variometer needle remained stuck on maximum sink and it felt as if we dropped out of the sky. We were only a few seconds into our downwind leg when we had already lost so much altitude that Joe had to initiate our final turn. The club shed was directly under our left wing as we turned onto final. Joe kept the speed up throughout the turn, lined the plane up with the runway, and seconds later we were safely back on the ground. The map below shows our abbreviated pattern. Well done, Captain Joe!

As we climbed out of the cockpit, relieved and wondering what had just happened, we noticed that the wind was now blowing firmly from the West. It wasn’t obvious, at least to me, what had caused the massive downdraft and the rapid change in conditions on the ground. The nearest rain clouds were still far behind the flatirons, at least 10-15 miles to the southwest, and only approaching slowly. (Rain did not reach the airport until we had stowed the aircraft and packed everything away, more than 30 minutes after our landing.) The winds aloft had calmed during the day, so rotor turbulence, though possible, also seemed somewhat unlikely.  Two other gliders came in to land 10-15 minutes after us and neither seemed to have any troubles in the pattern (one landed to the East just like we did, the other one to the West).

Now, a day later, I am still questioning what caused the dramatic sink in the pattern. The only thing I’m sure about is that the air had been very unstable throughout the afternoon. This was evident by the short cycle time of the clouds, the frequent updrafts and downdrafts that only strengthened throughout the day, reaching lower and lower altitudes, and the many rain showers and lines of virga (albeit no thunderstorms) throughout the area. I also still consider rotor turbulence a possibility.

The truth is that I will never know for sure. Nevertheless, there were several key lessons to be learned:

1. Be prepared for the worst. Unless the air is stable and the wind is calm (i.e., conditions completely useless for soaring), massive downdrafts in the pattern are always a possibility. So don’t be taken by surprise.
2. Altitude is your friend. If there is any risk of strong downdrafts in the pattern, start higher than normal. At our rate of sink, 500 extra feet of altitude would have bought us 20-30 seconds of additional flying time in the pattern. That may not seem like a lot, but it would have made a big difference.
3. Don’t destroy excess altitude until you are in or at least very close to the pattern, especially if you don’t know what the conditions are likely to be.  In our case we approached on a straight glide from the south and had no idea what the conditions in the pattern would be until we got there and hit the big sink.
4. Always speak up when safety is concerned. It doesn’t help that I thought “I would keep the spoilers closed”, I should have said so.  The final approach would have been safer and less stressful and Captain Joe would have thanked me for it.
5. Just like altitude, speed is your friend, too.  Flying faster turned out to be important as well.  On our final turn we clearly turned into a tailwind and the extra knots helped ensure a safe flying speed as well as maintain control authority.

That brings me to the question in the title of this post: What’s a Safe Pattern Altitude?  The answer is: it depends, but the clear lesson is that higher is better.  Bob Whelan, an experienced Boulder pilot, wrote an article in the November 2007 edition of Soaring Magazine, entitled “Paranoia as a Virtue,” in which he eloquently addresses this subject. He details three close encounters, at least two of them in Boulder, where extra altitude was critical to his safe arrival back on earth. Yesterday’s experience was clearly nowhere near as dramatic as the examples he references. However, I’m glad I learned my own lesson. It will help me to always keep Bob’s advice in mind.

 

When I got to the airport just before 12pm yesterday, the ASK 21 I had reserved for the afternoon was just about to land. One of the local flight instructors used it to give an introductory ride to a gentleman who had earned his glider rating many decades ago but had not flown a sailplane since, and was now interested in getting back into gliding.

As they climbed out of the cockpit, the flight instructor asked me if I would mind taking his student, let’s call him Joe (not his real name) on a ride with me. I was happy to agree as it’s always fun to share a flight with someone else, discuss weather conditions, potential sources of lift, and perhaps share some of the workload. Also, the plane’s cruising performance is noticeably better with the additional weight of a second person on board.

Joe and I didn’t know much of each other until we were up in the air climbing behind the towplane.  That’s when I learned that he was a retired airplane captain who had flown Boing 747s for United for more than 35 years.

Prior to flying for United, Joe was a pilot in the US Air Force, mostly flying B52 bombers.  And yet, there he was, eager to be a passenger in a comparatively flimsy sailplane, and to learn about flying gliders from me!

Before we climbed through 12,000 feet I instructed Joe in the operations of the oxygen system, something he apparently never had to use during his career in pressurized cabins. He said he had still a lot to learn with respect to gliding. His instincts, honed over many decades of flying the largest multi-engine jets in the world, were to avoid any maneuver that would be noticeable to passengers.  I responded that I very much appreciated this on my hundreds (or thousands?) of commercial flights as a passenger where I either worked or tried  to get some sleep.  But it doesn’t quite work in a sailplane. After observing me pulling back on the stick and banking steeply when entering a thermal (something glider pilots have to do to avoid flying right through the area of lift) he told me that he had to overcome his instincts and become a lot more aggressive with the controls himself.

We had a great flight together.  Cloud base over the mountains was above 16,000 feet so we got a great view of the Continental Divide and the valley beyond.   When I told Joe that we had Granby airport (on the other side of the Divide) within glide range he said he had never imagined being able to get there in a sailplane.

We circled under dark clouds above 15,000 feet over Allenspark when light rain, and then light snow began to fall with some small graupel mixed in.  Joe, safety conscious by profession, was the first to notice that some of the water droplets froze to the wings and we quickly left that particular area.  It only took a few minutes of gliding under clear skies when Joe pointed out that the ice build up had already sublimated.

The area to the northwest of Boulder overdeveloped so we headed further south.  We found ourselves in sink over Golden Gate Canyon when we noticed rain showers moving towards Boulder. The automatic weather system reported winds from the West with gusts of 16 kts. I parked the glider in a weak thermal over the flatirons until the rain had moved through and the winds at the airfield had calmed down again.

We landed safely in light crosswind from the North after just over three hours in the air.  It was a great experience flying with a true veteran of the skies and I gained a better appreciation just how different gliding is from flying big aircraft. Thank you, Joe!

Boulder is still new to me.  In fact, whenever I fly at a new location there are usually a lot of things that are new and different. There are local weather and wind patterns to consider, as well as different procedures at the airfield I’m flying from, ranging from unfamiliar radio protocols to different landing pattern procedures.  I might also be flying unfamiliar equipment, in this case an ASK 21 with somewhat worse performance than the LS4b that I last flew in Austria.

One thing that’s always different in a new location is the terrain. Unfamiliarity can contribute to disorientation, not a great thing if you’re in a glider and hitting big sink.  So, where exactly was the wind coming from? And where is the nearest landable field?  These are not questions you want to be asking yourself if you don’t know where you are and you find yourself down low…

So I made it a rule for myself to to stay local during the first few flights at a new location, i.e., within glide range of the airport I’m flying from.  But how do I know that I’m still within glide range? It’s always a good idea to carry a map in the plane, but full-size maps tend to be big and cumbersome to work with, especially if you are already in a bit of a pinch.  And, most importantly, they actually don’t tell you whether or not you’re within glide range, especially in the particular glider you’re flying with.

So one thing I do as part of my preparations at a new location is to create my own one-page map with custom-drawn glide range circles.  To do this, I take a local flight map, in the case of Boulder that’s the sectional charts of Denver and Cheyenne (Boulder just happens to be on the edge of both of these), I put a transparent plastic sheet over the map and draw various distance rings that show the altitude above sea level (MSL) that I need to get back to the airport at pattern altitude (i.e. usually 1,000 ft AGL).

To be safe, I use a safety factor, essentially a degraded flight ratio, to account for less than perfect “still air” conditions.  I actually use two different safety factors to indicate a range of required altitudes for each of the distance rings: a “realistic” one that assumes no head or tail wind and a glider performance that’s about 30% degraded from the best L/D ratio.  And a “pessimistic” one that assumes a headwind of 10 kts and a glider performance that’s 50% degraded from best L/D.

I then take a picture of the map with the circle overlays and print it out on photo paper.  Actually, I make two printouts, one that covers the immediate vicinity of the airfield about 25-35 miles out and is easier to read (Fig 2), and one that covers a somewhat larger area and also shows when I get within glide range of various nearby airfields (Fig 1). I put these two sheets back to back into a plastic cover and take it with me into the glider. As long as I don’t leave the area shown on these print-outs I won’t even need to take my paper maps.

I also create a handy-dandy spreadsheet that shows how far I can glide from any given altitude in different wind conditions.  I print the spreadsheet on laminated paper (in a size that fits within my logbook) so I can take it with me and always have it available as a handy reference (see Fig 3).

I’d like to hear about some of the steps you take to familiarize yourself when flying in a new area.

 

Over the last month I added a US glider rating to my Austrian glider pilot’s license.  I was a bit surprised by the things I had to study, which, while important for power pilots to know, have little to do with flying sailplanes. E.g., I now know the meaning of airport signage and runway markings I have only ever seen on commercial flights and will probably never get to see from a glider cockpit. I also learned how to navigate with instruments that no glider is equipped with. On the other hand, the questions that pertained to soaring were rather basic and the required “correct” answers to some of them were actually wrong. I was also surprised that there are no medical checks whatsoever necessary to fly gliders in the US.  Anyway, it is curious that something like gliding, the natural conditions and physical laws of which are truly universal, is regulated so differently in various countries.

I passed my knowledge test and checkride without a hitch and got all the requisite checkouts to fly solo at my new club: the Soaring Society of Boulder. I’m impressed by their modern equipment (e.g., all planes have mode C transponders and oxygen), which stands in stark contrast to the airfield itself, which is rather basic (no hangar, no services, not even running water). The people at the club are friendly, smart, and eccentric – so at least that aspect appears to be globally consistent.

Yesterday, I finally took my first real soaring flight over the Colorado foothills in the club’s four-year old ASK 21. Dark clouds indicated and delivered strong thermal lift (with climb rates of more than 10kts = 5m/s) up to cloud base at 14,000 feet).

My first orientation to the foothills ended up being relatively short lived.  After an hour and a half the clouds started to get closer to the airport.  When the wind direction on the ground shifted from East to West this was a sure sign that it was time to descend and land.  I extended the spoilers and put the ASK 21 into a slip. Just three or four minutes later I had lost almost 8,000 feet and was down to pattern altitude and landed on Runway 26 into the now westerly wind.