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