Ask Captain Lim ~ All about Aviation

There were many speculations that wind shear was one of the cause of the Singapore Airlines Boeing 747-400 crash at the Taipei's Chiang Kai-Shek International Airport (Tuesday evening, 31st October 2000). However, preliminary evidence of the investigation revealed that it was not so. In fact, it was reported that the pilot took off from the wrong runway that was closed for repairs and it was littered with digging equipment.

Nevertheless, it would interest air travelers to know more of what wind shear is all about.

Wind shear is a sudden and drastic change in wind direction or speed over a short distance along the flight path, usually associated with a microburst that often occurs in the vicinity of thunderstorms or typhoons.

Generally, most wind will travel horizontally, but under certain conditions in thunderstorms and frontal system, wind shear will travel in a vertical direction, causing up and downdrafts. Microburst wind shear is an extremely violent downward blast of air that hits the ground and radiates outward with its sharp shifts in wind speed. It can cause an aircraft to lose lift and crash, especially during take off or landing when slower speeds and its proximity to the ground make altitude correction very difficult.

Wind shear can be either an overshoot shear where there is an increase in headwind or decreasing tailwind resulting in an increasing speed of the aircraft. It can also be an undershoot shear where there will be an increasing tailwind or decreasing headwind with consequent loss of speed of the aircraft causing it to sink to the ground if no immediate action is taken by the pilot.

Since 1996, all US airliners have been required to be equipped with instrument that provides the pilot with advance warning of wind shear.

On the Boeing 777, the aircraft is equipped with a device known as the predictive wind shear warning system. This system uses wind velocity data gathered by the weather radar system to identify the existence of wind shear. The system is activated automatically on the ground when the thrust levers are set to take off position. It has an effective range of 3 nautical miles ahead of the aircraft.

There are two alerts, a CAUTION and a WARNING. When a caution alert is detected during take off or approach (between half and three nautical miles ahead and 25 degrees left or right of the aircraft heading), an aural alert, "MONITOR RADAR DISPLAY !" is announced. When the warning alert is detected on the ground before take off, the aural alert "WIND SHEAR AHEAD !" is sounded. Pilots have been trained to abort the take off when this warning comes on.

Pilots have learned to detect the presence of wind shear through the knowledge that they are indicated by thunderstorms, micro bursts, typhoons, virga activities (rain that evaporates before reaching the ground) and by other fellow pilots reports. Certain airports are also equipped with low level wind shear alerting system (LLWAS) warning. Such information are transmitted to all pilots through the air traffic information service (ATIS) at regular intervals.

Pilots have also been trained to stay clear of thunderstorms, heavy rain and areas of known wind shear. If severe wind shear is suspected, they are advised to delay the take off or approach.