What’s a Safe Pattern Altitude?

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.

Circling over the foothills northwest of Boulder

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.

Cruising narrow bands of lift adjacent to rain and virga

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.

Downtown Boulder and the adjacent foothills. Longs Peak is visible at the horizon.

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.

Looking south across the flatirons.

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

Pretty rainbow over the city of Boulder

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.

Strong lift under dark clouds adjacent to a rain curtain. Cloud base was at 15,000 feet.

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.

The wing points straight at the Boulder Municipal Airport. You can see the runway adjacent to a small lake.

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.

The blue arrow shows the normal landing pattern for gliders at BDU (Boulder Municipal Airport) using Runway Glider 08. (G8, as its known, is a narrow asphalt strip parallel to the main runway). The field altitude is 5288 ft MSL (shown in yellow). The blue numbers indicated the approx. altitude MSL in a normal pattern.

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!

The red arrow is an approximation (from memory) of the abbreviated pattern flown in massive sink. The red numbers indicate the approx. altitude at various points. (Note that the spoilers were completely closed until we were lined up with the runway.)

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.

 

A Special Passenger

Beautiful cumulus clouds over the foothills indicate a great day for flying. The glider in the foreground is the club’s older trainer, a Twin Grob (which is currently for sale). The (much newer) ASK 21 I was about to fly was still in the air when I took this picture just before noon.

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.

Great view of the Continental Divide

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.

Pretty cumulus

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.

A Boing 747 operated by United out of San Francisco – that’s where Joe was flying out of. For all I know he might have been on the controls when this picture was taken.

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!

B52 – a little different from an ASK 21 (Photo: U.S. Air Force)

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.

A cloud street (presumably aided by convergence lift) paralleled the Divide. Longs Peak is just to the left of the nose of the plane.

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.

Dark clouds over Allenspark

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.

Cumuli over the foothills

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!

Getting My Bearings

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.

Fig 1: Excerpt from the Denver and Cheyenne Sectional Charts. The red hand-drawn circles indicate the minimum altitude necessary to safely return to the Boulder airport (BDU). The green circles show the altitude required to reach a few select airports nearby.

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.

Fig 2: close-up of the immediate vicinity of the Boulder airport. The red distance rings are at about 15, 25, and 35 statute miles from the Boulder airport.

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.

Fig 3: Spreadsheet showing the altitudes MSL (above sea level) needed to safely return to the Boulder airport in an ASK 21 with two people on board. On the left are also the optimum speeds to fly at various wind conditions and the assumed effective glide ratio over ground (based on the selected safety margin). BDU is at 5,288 ft MSL and I want to be sure to arrive at least 1,000 foot above the airfield for a safe landing.

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.

 

Intro to Soaring at Boulder (BDU)

View of the city of Boulder from above. The airport is just to the left of the center of the picture – a bit difficult to spot from up here.

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.

Cool light effects as the sun broke through the clouds, illuminating the precipitation falling onto the foothills.

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.

Sheets of rain mixed with sleet and graupel fell from the sky, obscuring the view of the Continental Divide right behind in the soft afternoon glow.

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).

Circling in strong lift under dark rain clouds.

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.

Soaring is not a fashion show: in addition to the dorky hat, I now also get to wear an even more dorky cannula. Flying in the American West regularly takes you to high altitudes where supplemental oxygen is just a smart thing to have (it’s also required at any time above 14,000 feet).

How It All Began

Flying has fascinated me for as long as I remember.

I must have been about seven or eight years old when I took a piece of string to strap an old ironing board onto the back of a kid’s tricycle. With this contraption I raced down a small hill in our neighborhood in an attempt to take to the air. Today, I am thankful that this endeavor didn’t work; but also, that I wasn’t discouraged.

I recall folding paper gliders, experimenting with different designs to suit different purposes: gliders that could fly the fastest, gliders that could do aerobatics, gliders that would fly slowly and and stay up the longest. I even remember organizing a paper glider competition in school: our classroom was on the top floor and it was winter.  In a break we would run down the stairs and open the windows in the classrooms below.  Then we would run back up and launch our paper gliders out the window and see whose gliders could soar the longest. And it worked: the warm air, escaping the classrooms below, rose in the winter chill, and with it the gliders that we had carefully designed to fly in circles.  I remember our excitement when several paper gliders rose high above the school building and eventually disappeared over the roof. Although we could not identify a winner of the competition, the experiment itself was a huge success.

I must have been eleven or twelve when I began to build radio controlled gliders.  There was a nice steep ridge not too far from where I lived.  My friend and I strapped our gliders onto our bicycles and pushed up the hill to reach our slope.  The ridge faced in a westerly direction – perfectly suited for the prevailing winds.  We had many long flights on that slope – sometimes several hours until our fingers would get stiff from the cold wind.  We also learned some important lessons: about the difference between air speed and ground speed when the winds were so strong that a glider heading away from the ridge could actually move backwards towards it; about the dangers of ridge flying when one of our gliders was pushed into the trees in heavy turbulence; or about excessive load factors when a glider broke apart in midair during an overly aggressive aerobatic maneuver.

And then, in 1983, just a few days after I turned sixteen, I began my flight training to become a “real” glider pilot.

Why Start a Glider Blog?

Launch grid at the airport LOGO in Niederöblarn, Austria in June 2017

Well, there aren’t that many around. I encourage you to look at the blog roll and see what else is out there.  And if you find another interesting blog or vlog (video blog), please go to the contacts page and let me know about it.

Anyway, here are three key reasons why I decided to give it a go:

  1. To learn: the basics of gliding aren’t that difficult. If you can drive a car or ride a bicycle, you can also learn to pilot an airplane. However, when it comes to soaring I can’t think of any sport that is more difficult to truly master. I want to document what I learned so I will remember it for the future. And maybe it will help you too.
  2. To be safe: I’ll be honest: soaring is a relatively dangerous sport. Perhaps on par with riding motorcycles. But the risks can be managed and controlled by being aware of them and always staying one step ahead of potential disaster.
  3. To inspire: Who has never dreamed of flying like a bird? Who has not stared in fascination at hawks, eagles, or condors rising high up into the sky without ever flapping their wings? Soaring is not just chess in the air. Soaring is also poetry. Don’t you want to write yours?

Whether you currently are a glider pilot or are just fascinated by the idea of flight without an engine, I hope you will find something of interest.  I also appreciate your thoughts, feedback, and ideas.  There is always more we can learn, more we can do to be safe, and more to be inspired.

Glück ab, gut land!