Weather Theory

near mountainous terrain, it is helpful for a pilot unfamiliar and headwind losses of 30–90 knots, seriously degrading with a mountainous area to get a checkout with a mountain performance. It can also produce strong turbulence and qualified flight instructor. hazardous wind direction changes. Consider Figure 12-17: During an inadvertent takeoff into a microburst, the plane Low-Level Wind Shear may first experience a performance-increasing headwind Wind shear is a sudden, drastic change in wind speed and/or (1), followed by performance-decreasing downdrafts (2), direction over a very small area. Wind shear can subject an followed by a rapidly increasing tailwind (3). This can result aircraft to violent updrafts and downdrafts, as well as abrupt in terrain impact or flight dangerously close to the ground (4). changes to the horizontal movement of the aircraft. While An encounter during approach involves the same sequence wind shear can occur at any altitude, low-level wind shear is of wind changes and could force the plane to the ground especially hazardous due to the proximity of an aircraft to the short of the runway. ground. Low-level wind shear is commonly associated with passing frontal systems, thunderstorms, temperature inversions, The FAA has made a substantial investment in microburst and strong upper level winds (greater than 25 knots). accident prevention. The totally redesigned LLWAS-NE, the TDWR, and the ASR-9 WSP are skillful microburst alerting Wind shear is dangerous to an aircraft. It can rapidly change systems installed at major airports. These three systems were the performance of the aircraft and disrupt the normal flight extensively evaluated over a 3-year period. Each was seen attitude. For example, a tailwind quickly changing to a to issue very few false alerts and to detect microbursts well headwind causes an increase in airspeed and performance. above the 90 percent detection requirement established by Conversely, a headwind changing to a tailwind causes a Congress. Many flights involve airports that lack microburst decrease in airspeed and performance. In either case, a pilot alert equipment, so the FAA has also prepared wind shear must be prepared to react immediately to these changes to training material: Advisory Circular (AC) 00-54, FAA maintain control of the aircraft. Pilot Wind Shear Guide. Included is information on how to recognize the risk of a microburst encounter, how to avoid an The most severe type of low-level wind shear, a microburst, encounter, and the best flight strategy for successful escape is associated with convective precipitation into dry air at should an encounter occur. cloud base. Microburst activity may be indicated by an intense rain shaft at the surface but virga at cloud base It is important to remember that wind shear can affect any and a ring of blowing dust is often the only visible clue. flight and any pilot at any altitude. While wind shear may be A typical microburst has a horizontal diameter of 1–2 reported, it often remains undetected and is a silent danger miles and a nominal depth of 1,000 feet. The lifespan of a to aviation. Always be alert to the possibility of wind shear, microburst is about 5–15 minutes during which time it can especially when flying in and around thunderstorms and produce downdrafts of up to 6,000 feet per minute (fpm) frontal systems. Strong downdraft Intended Path Increasing headwind 2 Increasing t O ai u lw tf i l n o d w Outflow 4 1 3 Figure 12-17. Effects of a microburst wind. 12-11