"Shear" Madness

Mobility Forum, Jul/Aug 2004 by Meintel, Julie

Flying jets for a living has got to be one of the best jobs in the world. Most aviators are lucky enough to really love what they do, and let's face it, where else can you actually earn a living doing what you love, while getting to see the entire world? Yes, there's a lot to love about this aviation career.

Before we get too carried away, though, we should remember that there are plenty of hazards associated with flying, too. Many of the dangers fall within the scope of things we can control, such as our own situational awareness, alertness, paying attention to what is going on around us, gaining skill and expertise through experience, and not exposing ourselves to unnecessary risks. There are some hazards that are completely out of our control, though, and the best things we can do with these risks are first of all, to be aware of them, be aware of our limitations, and avoid these risks as best we can.

Weather is one of those things we just can't control, as much as we might like to. Mother Nature is most definitely in charge here, and we do well to respect her whims and peculiarities.

Wind shear is one of those very dangerous whims. We've all read and heard about accidents that happen as a result of wind shear, and we thank our lucky stars that it wasn't us that time. But it could happen to any of us, at any time, since wind shear is often unpredictable and tricky. You could say that wind shear is kind of like Mother Nature throwing a tantrum. Wind shear is defined by regulation as "a change in wind speed and/or direction over a short distance." This change can and often does happen very suddenly and without a lot of warning. This is not to say, however, that it just happens out of a clear blue sky. Conditions resulting in wind shear can be forecasted, and there are certain weather phenomena that are known to cause wind shear. It's just that it is virtually impossible to pinpoint the exact location and strength of an instance of wind shear.

Some of the conditions that can cause wind shear to crop up include thunderstorms, microbursts, frontal systems and fronts, low-level jets at the top of a radiation inversion, funneling winds, and mountain waves. Again, these conditions can be and are forecast through weather radar, PIREPS, visual reports and other means of weather prediction, but it cannot always be ascertained just exactly where and how violently wind patterns will change.

According to parameters outlined in AFI 11-203, Vol I, Weather for Aircrews, severe wind shear is defined as a rapid change in wind direction or speed causing airspeed changes greater than 15 knots or vertical speed changes greater than 500 feet per minute. Wind shear can occur at any altitude, but it is especially dangerous within 2000 feet of the ground. This is most often not enough space for a crew to successfully outmaneuver the wind.

Microbursts and thunderstorms go hand-in-hand; thunderstorms are the key factor in two out of three incidences of wind shear. Microbursts, which arise from harmless-looking showers, create extremely hazardous wind shears with little or no warning. They are so dangerous because of their small size and how quickly the wind patterns change; they give a pilot no time to respond. When an aircraft goes through a microburst, it runs into a headwind, followed immediately by a powerful downdraft, and then a tailwind, all in very rapid succession. You can see why this is an incredibly dangerous weather condition, and why it should be avoided to the greatest extent possible.

Cold fronts and warm fronts create conditions conducive to wind shear also. The window of opportunity for cold fronts to produce wind shear is smaller - typically less than two hours - due to the fact that cold fronts move faster than warm fronts. Wind shear from a warm front is more hazardous to aircraft operations at a given station since strong winds associated with the warm front may cause a rapid change in wind direction and speed where the warm air overrides the cooler denser air near the surface. And remember how we defined wind shear? A rapid change in wind direction and/or speed!

Touching on some of the other weather conditions that would indicate the probability of a wind shear event, funneling winds and mountain waves result from the way certain types of terrain can drive the prevailing winds though a smaller space such as a canyon or a gorge. From there, it can flow out of the gorge and spill into the flight path of aircraft operating in the area. This is another way in which wind patterns become unpredictable and dangerously unstable.

Low-level jet streams, which are simply areas of concentrated high winds, at the top of an inversion layer, which is an area where the air temperature gets warmer instead of cooler with altitude, reach maximum strength just before sunrise. Wind speeds in excess of 60 knots have been reported in relation to low-level jet streams on top of an inversion layer, and any time that this condition exists, there also exists the possibility for wind shear.