Wind Shear

Defense against this force of nature has several routes, most notably predictive and reactive

Aviation, while not inherently unsafe is unforgiving.” This is a statement of life in our world with the aircraft of today designed to improve the man/machine interface while reducing the impact of potential hazards. The majority of reported mishaps involves weather as either the stand-alone factor or compounded with other issues.

One of the most notorious weather phenomenons is the micro-bust or wind shear. While a noted adversary to flight crews in all aspects of flight, it causes catastrophic results during critical phases such as takeoff and landing. A sudden updraft or downdraft routinely results in a rapid pitch change combined with a variation in airspeed. Strong outflow from thunderstorms causes rapid changes in the three-dimensional wind velocity just above ground level.

Initially, this outflow causes a headwind that increases airspeed, which normally causes a pilot to reduce engine power if they are unaware of the wind shear. As the aircraft passes into the region of the downdraft, the localized headwind diminishes, reducing the aircraft’s airspeed, and increasing its sink rate. Then, when the aircraft passes through the other side of the downdraft, the headwind becomes a tailwind, reducing lift generated by the wings, and leaving the aircraft in a low-power, low-speed descent. This can lead to an accident if the aircraft is too low to recover before ground contact.

The frequency of scheduled airline flights provides greater opportunity for wind shear encounters. Commercial flight data illustrates pilots are faced with wind shears on average of once every 2,200 flights, and that certain wind shear “hot spots” exist.

Columbia, SC: 1 every 150 landings

Denver, CO: 1 every 2,300 landings

Dallas, TX: 1 every 7,100 landings

The question of defense against this force of nature has taken several routes. Terms such as predictive and reactive describe the different approaches to the dilemma.

As the result of the accidents in the 1970s and 1980s, most notably following the crash of Delta Air Lines Flight 191, the Federal Aviation Administration mandated that all commercial aircraft have onboard wind shear detection systems by 1993. Between 1964 and 1985, wind shear directly caused or contributed to 26 major civil transport aircraft accidents in the U.S. that led to 620 deaths and 200 injuries.

Since 1995, the number of major civil aircraft accidents caused by wind shear has dropped to approximately one every 10 years, due to the mandated onboard detection as well as the addition of Next Generation Doppler weather radar units on the ground (NEXRAD).

Predictive solutions

Predictive wind shear has taken on several forms. NEXRAD (Next-Generation Radar) is a network of 159 high-resolution Doppler weather radars operated by the National Oceanic and Atmospheric Administration (NOAA). Its technical name is WSR-88D, which stands for Weather Surveillance Radar, 1988, Doppler. The technology detects precipitation and atmospheric movement or wind. The principle uses a phase shift in transmitted versus returned signal.

This is like standing by the side of a road and when a car approaches, the sound waves compress the closer the vehicle comes and start to expand once it passes by. The returned data when processed is displayed in a mosaic map which shows patterns and movement. The radar system operates in two basic modes, selectable by the operator. The slow-scanning clear-air mode is for analyzing air movements when there is little or no activity in the area, and a precipitation mode, with a faster scan, is utilized when tracking active weather.

NEXRAD has an increased emphasis on automation, including the use of algorithms and automated volume scans to enhance the accuracy of predictions. It has incorporated a number of improvements over systems previously used. The new Doppler velocity technology improves rapid air movement detection. It provides improved resolution and sensitivity, allowing detection of cold fronts, thunderstorm gust fronts, and micro-bursts. These characteristics had never been visible on previous radars. The NEXRAD also provides volumetric scans of the atmosphere allowing operators to interrogate the vertical structure of storms and provide detailed wind profiles above the radar site. The radars also had a much increased range allowing detection of weather features at much greater distances from the radar site.

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