Windshear can be caused by several situations, is fairly common, and is frequently more of an annoyance than a danger. But given the right conditions and/or improper pilot technique, it can be a real hazard. Numerous accidents include the term "windshear" in the probable cause finding so let's take a look at this phenomenon.

This is thunderstorm season and thunderstorms can produce microbursts. A microburst is simply a strong, concentrated downdraft. When this strong downdraft reaches the ground it has to go somewhere. The somewhere is actually everywhere. Outflows of horizontal wind move in all directions in proximity to the surface. An airplane may encounter a headwind, tailwind, or crosswind depending on its location relative to the microburst.

Microburst induced horizontal wind (FAA diagram)

A microburst induced crosswind can wreak havoc if it happens as the airplane is on or close to the runway while either taking off or landing. Without the full weight of the airplane on the wheels (and even sometimes with the full weight on the wheels) the airplane is vulnerable to being pushed off the runway or flipped over. As disconcerting, embarrassing, and expensive as that may be, it is rarely fatal.

The unexpected sudden headwind or tailwind can be quite another matter. The headwind or tailwind may not be as obvious as the crosswind when it begins because we aren't careening sideways toward the weeds. But the airspeed indicator will give us a clue providing the attitude of the airplane is stabilized with a constant power setting. This means either a constant pitch attitude with full or climb power on departure or a constant pitch attitude with power set to deliver the desired approach angle. A momentary increase in airspeed indicates a headwind while a momentary decrease in airspeed gives evidence of a tailwind. The greater the mass of the airplane the longer the duration of the airspeed change and the greater the potential for problems. That's right; here's a case where heavy airplanes don't have the advantage in the wind. A light airplane will momentarily gain or lose airspeed because of the windshear but will rapidly accelerate or decelerate to achieve equilibrium. The heavy (or high mass) airplane will take longer to accelerate or decelerate after the windshear and will be more susceptible to a stall or at least greatly reduced lift.

Effect of 10 know headwind reduction (Graphic by GB)

A sudden tailwind or a sudden reduction of the headwind will have the same result: loss of airspeed. The sudden reversal of a headwind into a tailwind is probably the most dangerous situation because it will result in the greatest loss of airspeed. Remember that the loss of airspeed is only momentary, but even a momentary stall or serious loss of lift can make for a spectacular arrival. It's always nice to be greeted when we land, but not by first responders in turnout gear.

Effect of a 15 knot headwind reversal (Graphic by GB)

Windshear can also be caused by the orographic effect of wind flowing over mountainous terrain, frontal activity, or even temperature inversions. Regardless of its cause, we need to be vigilant for windshear. Weather reports and forecasts must be checked and we should be aware of the conditions conducive to windshear. Any unexplainable variation in airspeed while on approach or departure should be taken as a warning flag.

How to best deal with windshear of course becomes a judgment call. I would rather avoid the embarrassment of breaking an airplane so I am probably overcautious. My personal minimum is to execute an immediate go-around if I encounter an airspeed variation of +/- 7 knots while on final in an airplane weighing less than 12,500 pounds. Individual pilots may choose to be more or less conservative than that; it's up to the individual but it's best to have a number in mind and be prepared.

Windshear on takeoff presents even a bigger problem. A go-around is a relatively easy fix if windshear is encountered on final approach. But on takeoff the problem will probably not be evident until airborne and past the departure end of the runway. This presents a "nowhere-to-go" scenario and may not end well. The best way to avoid that problem is to study the wind direction relative to the terrain and form a mental picture of what the air might be doing. Of course, thunderstorms in the area should raise warning flags about possible microbursts.

The bottom line in minimizing the risk of windshear accidents is to exercise vigilance and be conservative.

This accident occurred due to windshear that was not a result of a thunderstorm, but most likely due to the terrain surrounding the airport. The Boeing Stearman was substantially damaged and the pilot and passenger received minor injuries. The accident happened in October of 2004 at the Silver Ranch Airport near Jaffrey, New Hampshire.

Photo courtesy of Doug Fortnam

According to the pilot, he taxied to runway 16 and attempted to takeoff, but had trouble keeping the right wing down. He aborted the takeoff and decided from the position of the windsock that it would be better to takeoff from runway 34. As the airplane neared the departure end of the runway, he observed another airplane approaching to land on runway 16, which subsequently initiated a go-around. The pilot then positioned the airplane onto the runway, and began his takeoff roll. At an altitude of approximately 300 to 350 feet, after passing the departure end of the runway, the airplane started to descend. The pilot recognized this as windshear and prepared for a forced landing. In an effort to avoid trees and a residential area ahead, the pilot turned the airplane to the right, and performed a forced landing to a field. Upon touching down hard in the field, the airplane nosed over and came to rest inverted. The pilot added that he observed the windsock as indicating a northwest wind at 15 knots, and that the engine was developing full power during the flight.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows: The pilot's inadvertent encounter with a downdraft wind condition and the airplane's inability to climb, which resulted in a forced landing. A factor related to the accident was the downdraft wind conditions.

This was not an amateur operation. The pilot, an ATP, reported having 23,000 total flight hours with about 1000 hours in make and model.

Click here to read the full accident report.

This Cessna 340 accident was blamed on a number of factors including windshear. It happened in February of 2007 and resulted in three deaths. The airplane, piloted by an airline transport pilot, was destroyed during an in-flight collision with terrain and trees 3 nautical miles south-southeast of Council Bluffs Municipal Airport (CBF), Council Bluffs, Iowa.

At 2051:23, the flight was cleared for the VOR-A approach into CBF. The controller instructed the pilot to maintain 3,000 feet until established on the approach. The aircraft was approximately 26 miles south of CBF at that time and descending through 7,000 feet. About 2055:34, the pilot leveled at 3,000 feet, at which time the aircraft was approximately 15 miles south of CBF. The controller informed the pilot of wind gusts to 39 knots at Omaha Eppley Airfield. The controller released the pilot to the CBF Common Traffic Advisory Frequency (CTAF) at 2058:20. The flight was approximately 10 miles south of CBF at that time. No further communications were received from the accident airplane.

In-flight weather advisories had been issued for the route of flight. Airman's Meteorological Information (AIRMET) Tango, warned of moderate turbulence below 18,000 feet, with conditions ending in the vicinity of the accident site after the anticipated arrival time. The AIRMET also warned of the potential of low-level wind shear. The next issuance of AIRMET Zulu, noted the possibility of moderate icing below 16,000 feet. Several pilot reports were on file that reported in-flight icing encounters in the vicinity of CBF.

About 9 minutes prior to the accident, the automated observation at CBF reported wind from 300 degrees at 18 knots, gusting to 23 knots; visibility 1-3/4 miles in unknown precipitation; overcast cloud ceiling at 1,000 feet above ground level (agl); temperature 0 degrees Celsius; dew point -1 degree Celsius. About 11 minutes after the accident, the automated observation at CBF reported: wind from 330 degrees at 25 knots, gusting to 36 knots, visibility 3/4 mile in unknown precipitation; overcast cloud ceiling at 1,000 feet agl; temperature -1 degree Celsius; dew point -3 degrees Celsius.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows: The pilot's continued flight into adverse weather, and his failure to maintain altitude during the instrument approach. Contributing factors were the presence of severe icing, moderate turbulence, and low-level wind shear.

We know that accidents frequently have more than one cause. This was probably a case where the airplane was carrying some ice. It was equipped with deice boots but the system may not have been able to keep up with the ice accretion rate. Turbulence was also present and that can produce fatigue. The pilot had already flown about 7 hours on the day of the accident. So most likely there was a tired pilot, flying a non-precision approach, picking up ice, and fighting turbulence. The sudden addition of low level windshear might have exceeded the pilot's ability to handle the situation. Or, the windshear might have been severe enough, given ice on the airplane, to cause a stall or at least substantial loss of lift.

Regardless of which of the factors played the major roll, we can learn from this that fatigue should be anticipated. If the day has been long and the weather conditions look problematic, it's probably best to stay on the ground. If the decision to go is made, extra speed on the approach to compensate for ice and possible windshear when forecast or anticipated is probably also wise.

Click here to read the full accident report.

An online dictionary provides several definitions of professional. One is, " a person who is expert at his or her work." I would like to define a professional pilot as one who is expert, diligent, and striving to learn more. Note that I didn't specify the size of the aircraft flown, the ratings held, or whether the pilot was monetarily compensated. I know pilots that I would classify as professionals even though they do not sit behind more than 100 horsepower and are certainly not paid. We have also seen a couple of recent, unfortunate situations in which pilots wearing uniforms, powering up turbine engines, and carrying paying passengers have been anything but professionals.

I began this newsletter with an update of my safety work. In that section I mentioned that I had delivered a presentation to a group of pilots who mostly are flying light sport airplanes. The RUFF members who attended were genuinely interested, asked good questions, and wanted to keep going even after we had met the criteria for issuance of Wings credit. I came away believing that I had just spent a morning with a group of professional pilots.

I have also had quite the opposite experience, but with pilots of all certificate levels, including some who are paid to fly much larger airplanes.

Flying can be very safe. But it is not inherently safe. It is up to us to make it safe. This is where the old term, "preaching to the choir" comes to mind. Those of you who are reading this are most likely professionals, regardless of your ratings or what you fly. If you weren't, you would be doing something else right now.

So I am encouraging each and every one of my readers to become evangelists for safety and join me in my Safety Outreach. You are probably already setting a good example by conducting yourselves as professionals. That is a very important element. Let's take it one step further. Let's not remain silent when we hear pilots talk of taking shortcuts or doing anything less than safe. Let's politely interject that we would not do it that way and why. We can be assertive without being aggressive. Most pilots know in their hearts what is right. Maybe that little nudge that we provide just might save somebody's life.