Wind shear is defined as a change in wind speed and/or direction over a small area. Wind shear can be vertical or horizontal and can occur at any altitude.

While wind shear can cause uncomfortable turbulence and aircraft controllability problems at any altitude, it is of most concern at lower altitudes, simply because of the risk that an altitude loss could result in the airplane impacting terrain. Wind shear at low altitudes is referred to, logically, as low level wind shear, LLWS.

Hazardous wind shear is common with passing frontal systems and thunderstorms. It may also exist as an unseen hazard, producing clear air turbulence. In low level temperature inversions, pilots can expect to encounter wind shear when the wind speed at 2,000 to 4,000 feet above the surface is at least 25 knots. Pilots must be aware of the effects wind shear can have on airplane performance. A few scenarios involving wind shear are described below to aid in this understanding.

Tailwind Becomes A Headwind

With a tailwind, the airplane is flying with the wind, like a boat moving with the water's current. If that tailwind shears to a headwind, the airplane all of a sudden finds itself blasted in the face with wind. The airspeed increases. With this additional airspeed, the wings produce more lift and the flight controls become more effective. The pilot can see the airspeed jump up on the airspeed indicator and feel the airplane's performance increase.

Headwind Becomes A Tailwind

In this case, the airplane is flying into the wind. When the headwind shears to a tailwind, the airplane experiences a loss of airspeed. This loss of airspeed can be seen on the airspeed indicator, and the loss of performance can also be seen and felt by the pilot.


Microbursts are incredibly hazardous. Microbursts are an intense downdraft of air. When an airplane flies through a microburst, this downward moving air tries to push the airplane into the ground. Mircobursts have overcome even large jet aircraft, and can easily force a piston powered general aviation aircraft into terrain.

A microburst is typically less than one mile wide and occurs within 1000 feet vertically, lasting about 15 minutes before it dissipates. A typical general aviation airplane is capable of climbing at a rate of 700 to 1500 feet per minute, while a microburst can produce downdrafts of up to 6000 feet per minute. Even as the pilot is climbing through the air at the airplane's maximum rate of climb, that air is descending at a significantly greater speed, so the airplane descends uncontrollably.

It gets worse than that, however. When the air from the microburst strikes the ground, it spreads out in all directions. As a result, microbursts often produce wind speed changes of 45 knots or more.

As an airplane flies into a microburst, it will initially encounter the outflow as an increasing headwind. Next, the headwind will fall off as the airplane enters the downdraft. As the airplane progresses, the downdraft becomes a tailwind. So, airplane performance increases initially, then the microburst produces decreasing airplane performance from that point.

Microbursts are hard to detect and usually remain in a somewhat confined area.

LLWS Alerting Systems

In an attempt to warn pilots when LLWS exists at a particular airport, LLWS alerting systems have been installed at many airports. These systems consist of wind measuring devices placed at different locations on the airfield. These wind measurements are compared in order to detect the presence of wind shear. These systems are not common at smaller airports, and are unable to detect LLWS in the area surrounding the airport.