Mention wind shear to most pilots and they'll think of some of the worst accidents in airline history: Delta 191 at Dallas-Fort Worth, Eastern 66 at John F. Kennedy, or USAir 1016 at Charlotte, to name but a few. These accidents all involved transport-category jets flying approaches or takeoffs in or near thunderstorm activity. The thunderstorms spawned microbursts — intense, localized, downward-directed columns of air caused by rapidly sinking cold air issuing from a mature thunderstorm's downdrafts. The common threads linking all of these accidents — thunderstorms, low-altitude operations, and jet airplanes — serve as strong reminders to all pilots to avoid this combination of conditions. That goes double for pilots flying turbofan-powered airplanes. Why? Because jet engines take several critical seconds to spool up from low- to maximum-power values. Even if a captain is quick to notice a dangerous drop in airspeed or altitude, it will be four or more very long seconds before the engines can produce enough thrust for a lifesaving climb.
Propeller-driven airplanes have an advantage over jets in this regard. Jam the throttle forward in a piston-powered airplane and power response is almost instantaneous, owing to the immediate propeller blast of lift-producing air over the wings. Same thing with Garrett-powered turboprop engines: their single-shaft, gear-driven power trains provide quick response to thrust lever commands.
Does this mean that pilots of propeller airplanes needn't worry about combating wind shear-induced airspeed and altitude excursions when taking off or landing? No way. While quick response to power commands is nice, it's no inoculation against wind shear's ill effects. Any number of piston singles, piston twins, and turboprop airplanes have come to grief after bouts with wind shear. The National Transportation Safety Board and the FAA can provide thousands of examples of wind shear-related accidents involving propeller airplanes. The trouble is, wind shear is seldom singled out as a cause or factor. Instead, the broad category of "adverse winds" is usually trotted out. This means that wind shear events are mixed in with downwind landings, botched crosswind landings and takeoffs, and other adverse wind situations. Therefore, we'll probably never know the real level of wind shear involvement in general aviation accidents.
However, the AOPA Air Safety Foundation's Safety Review of General Aviation Weather Accidents, published in 1996, did identify 46 accidents in the 1982-to-1993 time frame with wind shear as a contributing factor. Out of a total of 2,835 wind-related accidents, that's not a whole lot. The top prize went to the "loss of control landing in crosswind/gusts/tailwind" category, with 804 accidents. "Loss of control during crosswind/gusting conditions" was a factor in another 183 accidents.
But I'll wager that many of those 804 and 183 also involved wind shear to one extent or another, because if there are gusts, and if the wind is blowing hard enough, a certain amount of shear is always present.
The real key to avoiding bouts with wind shear is to build awareness. First of all, when can you expect wind shear? Here are a few of the most common scenarios, all of which dwell on low-level wind shear:
- Strong and/or gusty surface winds. As mentioned earlier, any time there are strong surface winds, expect turbulence and the shear-induced airspeed fluctuations they bring. This is especially true when winds are shifting in direction.
- Frontal passages. Many times, frontal passages bring dramatic shifts in surface wind strength and direction. The more vigorous the front, the wilder the shifts and the stronger the chance of low-level wind shear.
- Thunderstorms and gust fronts. It almost goes without saying that these phenomena can send out downbursts, microbursts, and just plain strong straight-line winds at all altitudes. Try to tackle these while on approach or departure and you're asking for the worst in altitude and airspeed excursions.
- Orographic effects. Winds flowing up and down nearby high terrain can cause localized bursts of wind that can send airspeeds up and down. The same is true of winds blowing through mountain or valley passes. In this case, obtaining information about local wind and shear behavior from pilots familiar with airports affected by proximate terrain should be a high priority.
- Airport buildings and other man-made features near an airport. Air blowing past buildings, hangars, and other structures often sets up downwind eddies of disturbed air. Fly through them and expect a bumpy ride. The same is true of parking lots and large, empty fields located along approach and departure paths. Rising thermals from these sources are other causes of low-level turbulence and shear.
- Surface boundary layer effect, or wind gradient effect. Below approximately 5,000 feet, wind flows are strongly influenced by friction with the earth's surface. Actually, frictional effects can be felt as high as 10,000 feet — or even higher in mountainous areas. But below 5,000 feet, and increasing to a maximum in the air just above ground, wind speeds can slowly drop and fluctuate in direction. This is one reason why sailplane pilots are encouraged to keep their approach airspeeds slightly above their best lift-over-drag-ratio values. A decreasing-headwind situation is something that no pilot needs on short approach — especially in an aircraft without an engine.
- Tree lines. Like buildings, tree lines can block wind flows. The result can be wind shadow — a term used in micro-meteorology (the study of extremely small-scale weather events) that's very familiar to hang glider and ultralight pilots. Flying into a wind shadow can deprive you of any headwind component you may have had, and produce enough shear to cause airspeed and altitude to trend downward.
Visit any airport long enough and you'll hear lore of notorious "sinkholes" or other destabilizing wind effects that always seem to affect one particular runway or another. At AOPA's headquarters at the Frederick (Maryland) Municipal Airport, every pilot soon learns about the sinking air that's just off the approach end of Runway 23. You'll be flying on a perfect day, have the final approach all set up and stabilized, then cross a tree line and watch your airspeed fluctuate and your altitude drop — really drop, if you've extended full flaps. Although it has never been scientifically examined, this sinking air is no doubt the product of friction effects, mechanical effects caused by nearby buildings (probably including AOPA's building!) and a Maryland State Police medevac helicopter hangar, and wind shadows from the tree lines that partially surround the airport. The point is, every airport is affected by some unique degree of wind shear — even on non-gusty, non-convective days.
The low-level wind shear alert systems (LLWASs) installed at some of the nation's busiest airports can give important clues about the presence of wind shear. LLWASs work by comparing wind speeds and directions at anemometer sites located at the center and the periphery of an airport. This is fine for reporting conditions at the surface, but of no use whatsoever in analyzing any wind shear that might be present along approach and departure paths. That's the job of Terminal Doppler Weather Radar (TDWR). TDWR, now being installed at various large airports, can be set up to scan specific runway extended centerlines for signs of shear. That's the kind of advanced warning that's really helpful. The pilot who knows that wind shear awaits him on final is obviously at a distinct advantage over one who doesn't. Too bad more general aviation airports aren't included in the FAA's TDWR upgrade plans.
The procedures for dealing with a wind shear encounter center on three basic goals: preventing altitude loss; preventing a stall; and climbing. Different airplanes use different procedures to accomplish these goals, but for propeller-driven piston airplanes the drill is usually to add power at the first signs of a dangerous plunge in airspeed or altitude; obtain V X (best angle of climb speed) if obstacles are near, or V Y (best rate of climb speed) if obstructions or terrain are not factors; then establish a climb attitude and configuration in order to reenter the traffic pattern or divert to an alternate airport with more favorable wind conditions.
The best antidote to wind shear, however, is a good forewarning. If forecasts mention low-level wind shear, be prepared for a go-around and carry extra airspeed on final approach. The usual recommendation is to add one-half the gust factor to your usual, 1.3 V SO approach speed. So if the wind is 10 gusting to 20 knots, add 5 knots to your pilot operating handbook's target airspeed for final approach. Consider using partial flaps for your landing in gusty or shear conditions; the extra lift and drag caused by full flaps may hinder your ability to preserve any semblance of a stabilized approach in gusts or wind shear and may even lead to control problems or an inadvertent liftoff once on the runway.
Finally, remember that you do have an out: You can abandon the approach at the first signs of trouble. Is the airplane aligned with the extended runway centerline? Can you safely control any crosswind drift? Are the airspeed and descent rate reasonably stable? Is the turbulence manageable? If your answer to one or more of those questions is "no," then a go-around or missed approach procedure should be performed.
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