Wind shear most frequently occurs along a cold front or pressure trough, in the vicinity of thunderstorms, and in areas of steep temperature inversions and tight pressure gradients. Mountain valley effects or land & sea breeze may be associated with wind shear at places during certain times of the day. Below about 3000 feet and down to about 300 feet, surface friction will cause the wind to “back” to a more counter-clockwise direction, the direction changing as much as 70 degrees. This would cause a descending aircraft to have a constantly changing crab angle to track the localizer. However, this higher altitude phenomenon occurs slowly in a less critical area than the final phase of an ILS approach. At the higher altitude, wind shear is not so critical because normally higher airspeeds and less precise navigation are involved. Remember that wind shear is an abrupt change in wind with small changes in altitude and horizontal movement of the aircraft. Using the displayed ground speed to compare with the IAS, and the airplane’s heading compared to the localizer course can provide advance and instant wind shear information. The inertia of a large aircraft will cause it to gain or lose airspeed and lift, whenever wind changes occur in less time than it takes for the speed of the aircraft to adjust to the new wind condition. An aircraft moving through the air at 140 knots IAS with a 20-knot headwind would have a ground speed of 120 knots. If due to wind shear the headwind suddenly dropped to 10 knots, the ground speed would remain at 120 knots until the aircraft overcame its inertia and the airspeed would drop to 130 knots. Immediate power application would be required to accelerate the aircraft to avoid high angle of attack approaching stall or loss of lift and a subsequent short or hard landing. In similar manner, wind shear causing an increase in airspeed in the final stages of approach would cause a landing well down the runway unless power is reduced and the aircraft flown onto the ground at the proper touchdown point where spoilers and brakes effective. One knot of airspeed amounts to about 100 feet of runway if the aircraft is allowed to float. In addition to airspeed variances, wind shear may considerably increase or decrease the crosswind component, requiring rapid crabbing or describing to maintain alignment. This will probably prove to be the greatest cause of missed approaches after visual contact is established. While appreciable wind shear under extreme low visibility conditions is rare, the pilot must be alert for its presence on every approach – instrument or visual. Pilots can become aware in advance that wind shear exists, and in which direction before starting the approach, by being aware of the forecast of wind aloft in the low altitude levels, compared to the reported surface winds. These should be used to estimate not only a change in crab on the final approach but, also for rate of ascent on the Go Around.
13.5 Operations on Contaminated Runways
Crew should be familiar with the operator’s policies and procedures concerning constraints applicable to Cat-II/IIIA/IIIB Ops and Low Visibility Take Off from contaminated runways. The limitations recommended in AFM for operation on contaminated and icy runways should be strictly observed. Crew should be familiar with the constraints related to braking friction reports and the method of providing these reports.
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