Juhani Polvinen / Application Manager / Vaisala / Helsinki, Finland Are you prepared for safety hazards caused by wind shear? Weather plays a significant role in aviation safety. Some 30% of all fatal accidents are caused by or related to weather (ICAO). Wind shear is one of the most dangerous - and least known - weather phenomena in aviation.

conditions - for example, sea and land breeze, jet streams (fast flowing, narrow air currents), weather fronts, showers or thunderstorms. It has also been noted to commonly occur near mountains and coastlines. The most dangerous type of wind shear is caused by convective weather. It is very difficult to forecast due to its local nature. Wind shear poses the greatest danger to aircraft during takeoff and landing. Airplane pilots generally regard significant wind shear to be a horizontal change in airspeed of 30 knots (15 m/s) for light aircraft, and near 45 knots (22 m/s) for airliners (FAA). Although wind shear as a meteo- rological phenomenon has been Air traffic controllers usually have US Aviation Safety Network (ASN), recognized in aviation from the late no means of directly detecting a low- at least two major accidents were 60s, it is still not fully understood level wind shear hazard. It may take caused by wind shear between 1990- today. Many airports suffer the even the most experienced pilots by 2000, resulting in over 90 fatalities. effects of wind shear, but airport surprise, and put them in a situation Also in many recent accidents, wind authorities have little information where the wrong decisions can have shear has been suspected to be a about the phenomenon and how to disastrous effects. 831 fatalities were strong contributing factor - such as address it. recorded to have been caused by in the case of the TANS Airlines crash One of the first great eye-openers wind shear between 1956-1994 (FAA, in Peru (2005), or FedEx cargo plane was the Boeing 727 accident at the NTSB Records, & Fujita), or 700 crash in Japan (2009). JFK Airport in 1975, which led to fatalities between 1970-1985 (ICAO). Wind shear is a term referring to systematic studies on wind shear. More recent statistics are harder to rapidly changing winds. It is a small Tetsuya Theodore Fujita pioneered find, but the phenomenon has not scale meteorological phenomenon, the study of wind shear and its disappeared. Wind shear continues which occurs over a very small effects. However, it wasn’t until 1997 to pose a threat to aviation safety all distance. It is usually connected to that ICAO formally established a Low- around the world. According to the rapid changes in specific weather Level Wind Shear and Turbulence

4 180/2009 Group to promote global awareness How to address wind areas can be easily identified thanks about the phenomenon. shear safety risks? to the system. Access to wind shear data eases the air traffic controller’s Challenge to pilots When wind shear occurs below 2,000 burden, increases the pilots’ confi- and aircraft safety ft altitude, it is called low-level wind dence at a particular airport, and shear. Many airports prone to micro- improves the overall aviation safety. Microbursts and wind shear go burst and wind shear are still lacking All required services should also hand in hand. Microbursts are small adequate solutions to mitigate this be mapped in close cooperation with scale intense downdrafts which, on threat. the airport authorities, in order to reaching the surface, spread outward The first step in addressing safety ensure the optimal performance of in all directions from the downdraft hazards caused by wind shear is the system throughout its lifecycle. center. This causes the presence of to investigate the likelihood of the Planning ahead pays dividends in the both vertical and horizontal wind occurrence of the phenomenon at long run, as system maintenance and shears. Microbursts spread radially the airport in question. If a problem operations become proactive and on the ground, causing rapid changes is recognized, different options for organized, and the need for ad-hoc in wind direction and speed. They solving it need to be investigated in fixes is reduced. Good data avail- are associated with cumulonimbus order to find the optimal solution. ability can be maximized through clouds, as well as line squalls (severe Each airport is unique. This work various well-planned services, such thunderstorms). A distinction can is best carried out in cooperation as preventive maintenance, software be made between a wet microburst with the airport authorities and an upgrades, and regular solution which consists of precipitation and expert organization with deep under- performance verifications. a dry microburst which consists standing of the phenomenon. All the studies and investigations of virga - that is, precipitation that Once the existence of low-level materialize in an implementation evaporates before reaching the wind shear has been verified through project plan. Once the low-level wind ground. Dry microbursts present a studying the weather conditions at shear system has been installed, it more difficult problem because pilots the airport, the next step is to specify is carefully tested to ensure that it have no visual clue of their occur- the optimal wind measurement site meets all the requirements. Profes- rence, and weather radars cannot see locations and measurement mast sional user-training as well as period- them either. heights by studying the topology and ical training updates are an important Wind shear and microbursts are obstructions in the area. After this, part of the project. Tailored lifecycle among the most dangerous of all the required system and interfaces services, designed before system weather-related threats to flying. The can be specified by investigating the implementation and according to unpredictable changes in wind speed existing infrastructure. the specific requirements, support and direction make it difficult to A Low-Level Wind Shear Alert smooth and safe operations. A profes- control the aircraft, with headwinds, System (LLWAS) comprises wind sionally run wind shear project is a tailwinds and up and down drafts speed and direction sensors sited huge improvement in airport safety. all in quick succession. At worst, it around the runway, and connected to If you would like to discuss the can cause a sudden and dramatic a data collection package at the site. implications of wind shear for airport loss in height, and result in a serious Wind shear alerts are presented both operations and aviation safety, please accident. visually and audibly, and the affected contact [email protected].

Further information: Downdraft www.vaisala.com/weather/ products/avi-llwas

While the pilot compensates References: for the headwind by dipping Fujita, T.T.; The Downburst, the nose, the aircraft enters a microburst and macroburst A headwind slows and downdraft. lifts the aircraft above FAA; Advisory Circular Pilot its normal flight path. A tailwind dangerously Wind Shear Guide reduces the aircraft’s speed. Guan, Wen-Lin & Yong Kay; Review of Aviation Accidents Caused by Wind Shear and The glide path of a normal landing. Identification Methods Juhani Polvinen; Wind shear: predicting the unpredictable

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