INTERNATIONAL
Low Level Wind Shear - The Invisible Enemy by Ted Delanghe
It was one of my first “Holy Mackerel!” moments in ag aviation and I remember like it happened yesterday, almost 30 years later. A huge infestation of Bertha armyworms were gobbling up the canola so fast you could almost hear them eating. It was my first year spraying and I was flying a 230 HP AgWagon with a 120-gallon load on board, just about max for the day, as the thermometer was edging past 85°F. The windsock at the home strip showed around 10 mph and a bit, but I knew that if I didn’t push things a bit, the farmers would be out of luck big time come harvest. So, there I was trucking down the field with a full load and a very obvious tailwind. It looked like I was whipping over the ground at Warp Factor 8 compared to the into-wind swath. At the end of the run stood a foreboding 150-foot high tension powerline tower. But no worries I reasoned, I’ve got lots of ‘zoom power’ because I was really clicking along! At any rate, on the previous trip just a few hours before, when nearly empty after finishing another field, I had trimmed three swaths parallel to the powerline to provide an extra margin of safety and noticed nothing unusual weather-wise. I continued with the spray run until it was time to pull up, but as I pulled back on the stick, the only thing that was zooming was my heart rate. The airspeed was headed in the wrong direction fast, the controls were starting to feel mushy and that darn tower was getting bigger and bigger. I do remember reaching for the dump lever as the situation played out, but luckily I made it over the tower, toes curled tight as can be as if that was going to help anything. Once my heart rate calmed down a bit, I realized how
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close it had been to mixing airplane aluminum with big tower aluminum; never a good idea. What had happened? It seemed I had lots of energy approaching the tower, but obviously that was seriously in error. The answer? Off to one side I saw a line of trees about 100 feet in height with the top branches whipping back and forth like in a gale, not at all like the conditions at spray height. And right then the term “wind shear” popped into my mind, also known as “wind gradient”. Because those were the days before GPS, I couldn’t really state with any degree of accuracy the precise wind speed at treetop level. Given the hectic motion of the large branches, the wind at 100 feet was at least 30 mph and even more at the height of the tower. That translates into a rapid 20 mph plus decay in airspeed from spray height, all things being equal. It also meant I had gained a few gray hairs courtesy of wind shear, that invisible but deadly weather phenomena that makes for the big surprise. Let’s get into the books to check this thing out. From the FAA Pilot’s Handbook of Aeronautical Knowledge: “Wind shear is a sudden, drastic change in wind speed and/or direction over a very small area. Wind shear can subject an aircraft to violent updrafts and downdrafts, as well as abrupt changes to the horizontal movement of the aircraft… a tailwind quickly changing to a headwind causes an increase in airspeed and performance. Conversely, a headwind changing to a tailwind causes a decrease in airspeed and performance.” In my situation, climbing with a rapidly increasing tailwind component causes a rapid decrease in airspeed and performance. ➤
