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 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.
For ag pilots, that last part is especially hazardous because at low level with insufficient airspeed to climb over obstacles, it could be a bad day at the office.
More from the FAA’s WindShear Safety Publication: There are four common sources of low-level wind shear: frontal activity, thunderstorms, temperature inversions and surface obstructions.
Frontal Wind Shear
Not all fronts have associated wind shear. In fact, shear is normally a problem only in those fronts with steep wind gradients. As with so many things associated with weather, there is no absolute rule, but a couple of clues tell you that wind shear may occur: 1) The temperature difference across the front at the surface is 10 o F (5 o C) or more. 2) The front is moving at a speed of at least 30 knots.
Wind Shear From Thunderstorms
Wind shear is just one of the many unpleasant aspects of thunderstorms. The violence of these storms and their winds are well documented. The two worst problems outside actual storm penetration are shear related. These are the “first gust” and the “downburst.” The rapid shift and increase in wind just before a thunderstorm hits is the first gust. Gusty winds are associated with mature thunderstorms and are the result of large downdrafts striking the ground and spreading out horizontally. These winds can change direction by as much as 180 degrees and reach velocities of 100 knots as far as 10 miles ahead of the storm. The gust wind speed may increase by as much as 50 percent between the surface and 1,500 feet, with most of the increase occurring in the first 150 feet. The implications for a shear on aerial applications in such a case are obvious.The other wind problem mentioned previously, the downburst, is also downdraft related. It is an extremely intense, localized downdraft from a thunderstorm. This downdraft exceeds 720-feet-per-minute vertical velocity at 300 feet AGL. The power of the downburst can actually exceed aircraft climb capabilities, not only those of light aircraft, but, as is documented in one case, even a high-performance Air Force jet. The downburst is usually much closer to the thunderstorm than the first gust, but there is no absolutely reliable way to predict the occurrence. One clue is the presence of dust clouds, roll clouds, or intense rainfall. It would be best to avoid such areas.
Wind Shear From Temperature Inversions
Pilots who have flown in the Southwest, Southern California, or Colorado are familiar with this weather pattern. Overnight cooling creates a temperature inversion a few hundred feet above the ground. When coupled with high winds from what is known as the low-level jet stream, this inversion can produce significant wind shear close to the ground. One particularly bothersome aspect of temperature inversion shears is that as the inversion dissipates, the shear plane and gusty winds move closer to the ground. In some areas of the Southwest, a 90-degree change in direction and 20- to 30-knot increases in surface winds in a few minutes are not uncommon. Obviously, such a shift would make an application difficult at best (not to mention it would be during an inversion…)
Wind Shear From Surface Obstructions
Wind shear from surface obstruction is generally associated with hangars or other buildings near the runway. The sudden change in wind velocity can seriously affect a landing or other low level operations. Another type of surface obstruction—mountains—can also affect wind shear. Some fields are close to mountain ranges, and mountain passes are close to the final approach paths. Strong surface winds blowing through these passes can cause serious localized wind shear during the approach. The real problem with such shear is that it is almost totally unpredictable in terms of magnitude or severity. A pilot can expect such shear whenever strong surface winds are present.
Avoiding Wind Shear
How can the pitfalls of wind shear be avoided, in particular a tailwind component that rapidly increases with altitude? Because conditions at a field often are considerably different than at the home airstrip, enroute to the field you should be aware of give-away signs of potential steep wind gradient, mentioned above e.g. noticeable movement of large tree limbs, sharp change of speed or direction of smoke columns, nearby thunderstorm activity, passing frontal systems, temperature inversions, etc.
More to the point of my first “Holy Mackerel!” moment, don’t be tricked into thinking you have lots of “zoom power” just because your ground speed downwind gives you that impression. Looks are deceiving!
Editor’s Note: you can find an excellent FAA document on wind shear with some of the above information at: https://www.faasafety.gov/files/gslac/library/documents/2011/aug/56407/faa%20p-8740-40%20windshear%5Bhi-res%5D%20branded.pdf
Ted Delanghe is a former Canadian Air Force pilot turned ag pilot, having flown more than 50 aircraft types from ultralights to fighters, including a wide range of ag aircraft. He resides in Regina, Saskatchewan, where he is also a business consultant with a specific focus on aviation and aerospace.
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Low Level Wind Shear – The Invisible Enemy - AgAir Update
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