Before the arrival of the cold front: fantastic fall-thermal soaring conditions along the Front Range.
This past Wednesday was a great fall soaring day. Initially I struggled off tow for a while, but once I got up above the inversion I had a great and easy flight in strong convergence and thermal lift along the Front Range, covering 350km in about 3 hours. Cloud bases were around 20,000 feet and the best thermals produced climb rates of up to 10 kts average. Not bad for October!
This was before the cold front arrived. I had not expected it until late in the evening but luckily I could see it coming just as I was planning to return to Boulder: the wind had been blowing from the southwest all day when I noticed a wall of dust rapidly moving in the opposite direction. When I first spotted it, I was flying over the foothills west of the field, and the front was just north of Longmont heading south. I accelerated my descent and landed safely in completely calm conditions. 10 minutes later the wind kicked up sharply and the temperature began to plummet.
CX wasn’t so lucky. He arrived back in Boulder about 30 minutes after the front moved in. Strong gusty winds were blowing from the north. AWOS reported 20 kts on the ground, gusting to 31. Boulder only has an east-west runway so CX was faced with a cross-wind landing in very challenging conditions.
Here’s what it looked like from the ground:
This was perfectly executed. Well done! (The video quality is not great but it’s definitely worth watching.) Note how he pulled right to his normal parking position 🙂
I also downloaded CX’s flight trace and took a closer look at the landing pattern.
CX began the “downwind” leg at 8,500 feet. That is 3,200 AGL (!) (The typical altitude at this point is less than 1,000 AGL.) 3,000 AGL might seem excessively high but extreme sink in the pattern is always a possibility in these conditions and the extra height allows the pilot to fly a bigger pattern, align with the runway sooner, avoid turning close to the ground, and it provides more energy reserves to maintain a greater speed in order to deal with extreme turbulence and other potential hazards. CX chose to fly an approach to G26. This is the best option when the wind is from the north because it allows the pilot to avoid landing next to buildings and vegetation that are located along the western half of the runway and could cause additional turbulence. (Before entering the pattern, the pilot also pulled the straps as tightly as possible.)CX used the high altitude to fly a much wider pattern than usual and to maintain a much higher airspeed. The trace shows CX turning base to final about 2 miles east of the runway at an altitude of 6,100 feet (800 AGL). The ground speed is 120 kts (!), presumable reflecting an airspeed of about 80-90 kts and a tailwind of 30-40 kts. The turn looks much shallower than it actually was: the wind drift is very significant and a relatively steep bank angle was required during the final 90 degree turn.3,000 feet before the threshold, CX is aligned with runway at an altitude of 300 feet AGL and a groundspeed of 85 kts. The airspeed must be somewhat higher to compensate for the cross-wind. (This is about where the video begins.)CX reaches the runway threshold at 50-100 feet AGL. The ground speed is 70kts, the airspeed is still somewhat higher (presumably around 75-80 kts). The glider is still perfectly aligned with the runway. If you re-watch the video, you’ll also notice how CX is careful to always keep the right wing (on the windward side) slightly lower than the left wing. (This helps the pilot maintain direction and also prevents the wind from rolling the glider to the left.) The video also shows that the glider appears to be much more stable and easier to control as soon as it enters ground effect.CX flares perfectly and touches down just before the first buildings at a ground speed of 42 kts. The touch down location minimizes the risk of turbulence in the lee of buildings and vegetation along Independence Road. CX chose to land on the dirt runway to have more room to maneuver if necessary. Landing in the dirt may also reduce potential sideway forces at the point of touch-down.
PS: The pilot is one of the most experienced cross-country pilots in the United States. In the video, the landing looked almost like a non-event. This impression is amplified by the fact that he was able to roll right up to his normal parking spot, stopping precisely where he intended to (and where he always does). The flight trace, however, tells a very different story and illustrates very well how unusual and challenging the conditions were. Most importantly, it shows the mitigating actions that CX took to minimize the risks associated with these conditions (much higher pattern entry, much wider pattern, much higher airspeed, always keeping the upwind wing slightly lower, choice of runway and touchdown point).
Great write-up and analysis and of course superb pilotage by Bob. From the video he made it look like a walk in the park. From 8500′ over BDU I would have chickened out and got blown to Erie and landed on 33. The downside with that is you get it down but then no friendly hands to hold it down. That gust front went from about 0 to 25 in about 5 mins at Erie. Then the wind stayed up above 25 gusting much higher, trying to hold a sailplane down by yourself in that would not be much fine either.
In your excellent analysis you mention that a pilot ” flew a much wider pattern than usual”.
I can not agree with this and do not see the reasoning for this ! In strong wind conditions one shoud stay always within a safe reach of the rwy even if most extreme sink and wind was to be encountered. When I received my Commercial Glider rating at California City Airport where we flew on a regular basis in very strong winds and sink conditions I was thought never to fly too wide of a TP and the base leg was always flown within the “airport fence”. The same was thought by Karl Striedieck when we flew his winch course in extreme wind conditions at his Eagle Field. In strong wind condition excess altitude is hardly ever a problem and is easily managed by use of dive brakes. The problem is encountering strong sink, being short and therefore properly planning and managing this is a must !
Hi Milan, thanks for your comment. I thought a lot about this as well because I, too, was taught to fly a close-in pattern, especially if there is a chance of significant sink in the pattern.
And you make a very good point: we absolutely need to make sure that in extreme conditions we have enough altitude and energy to deal with whatever mother nature throws at us. If CX had started from a normal pattern altitude, the wider pattern would have indeed been quite dangerous for he might have run out of energy before getting to the runway.
But he entered the pattern at 3000 ft AGL, 3x higher than normal! Why would he do that? In addition to the strong wind, there was also considerable wind shear and turbulence. It would have been quite unsafe to fly the final turn close to the ground. Also, because of the strong wind drift, making the final turn further out than normal gave CX the opportunity to get aligned with the runway sooner and to have time and space to make corrections in case the wind would have blown him well past the centerline. But you are absolutely right that this is only safe to do if the pilot has a lot of energy and altitude to reach the runway and to deal with the possibility of extreme sink! That’s where the extra height was a huge asset because it ensured that CX had plenty of altitude and airspeed throughout the final approach and could fly the entire final approach fast and steep. (btw – CX is the first to tell you directly if he sees you fly a shallow final!)
Thus, the observation of a wider pattern must not be looked at in isolation; it only makes sense in conjunction with the much higher altitude in the pattern, and thus the extra energy reserves to safely complete the final approach even if there had been extreme sink.
Obviously, I’m not CX and I can only speculate on his thought process. But the entire landing approach made great sense to me and made me think that extreme conditions may require unconventional solutions.
Thanks again for your comment!
You left out that he rolled up to his parking space.
Indeed, he parked it perfectly just like on a calm day.
Great write-up and analysis and of course superb pilotage by Bob. From the video he made it look like a walk in the park. From 8500′ over BDU I would have chickened out and got blown to Erie and landed on 33. The downside with that is you get it down but then no friendly hands to hold it down. That gust front went from about 0 to 25 in about 5 mins at Erie. Then the wind stayed up above 25 gusting much higher, trying to hold a sailplane down by yourself in that would not be much fine either.
In your excellent analysis you mention that a pilot ” flew a much wider pattern than usual”.
I can not agree with this and do not see the reasoning for this ! In strong wind conditions one shoud stay always within a safe reach of the rwy even if most extreme sink and wind was to be encountered. When I received my Commercial Glider rating at California City Airport where we flew on a regular basis in very strong winds and sink conditions I was thought never to fly too wide of a TP and the base leg was always flown within the “airport fence”. The same was thought by Karl Striedieck when we flew his winch course in extreme wind conditions at his Eagle Field. In strong wind condition excess altitude is hardly ever a problem and is easily managed by use of dive brakes. The problem is encountering strong sink, being short and therefore properly planning and managing this is a must !
Hi Milan, thanks for your comment. I thought a lot about this as well because I, too, was taught to fly a close-in pattern, especially if there is a chance of significant sink in the pattern.
And you make a very good point: we absolutely need to make sure that in extreme conditions we have enough altitude and energy to deal with whatever mother nature throws at us. If CX had started from a normal pattern altitude, the wider pattern would have indeed been quite dangerous for he might have run out of energy before getting to the runway.
But he entered the pattern at 3000 ft AGL, 3x higher than normal! Why would he do that? In addition to the strong wind, there was also considerable wind shear and turbulence. It would have been quite unsafe to fly the final turn close to the ground. Also, because of the strong wind drift, making the final turn further out than normal gave CX the opportunity to get aligned with the runway sooner and to have time and space to make corrections in case the wind would have blown him well past the centerline. But you are absolutely right that this is only safe to do if the pilot has a lot of energy and altitude to reach the runway and to deal with the possibility of extreme sink! That’s where the extra height was a huge asset because it ensured that CX had plenty of altitude and airspeed throughout the final approach and could fly the entire final approach fast and steep. (btw – CX is the first to tell you directly if he sees you fly a shallow final!)
Thus, the observation of a wider pattern must not be looked at in isolation; it only makes sense in conjunction with the much higher altitude in the pattern, and thus the extra energy reserves to safely complete the final approach even if there had been extreme sink.
Obviously, I’m not CX and I can only speculate on his thought process. But the entire landing approach made great sense to me and made me think that extreme conditions may require unconventional solutions.
Thanks again for your comment!
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