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.

 

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