A sudden change in wind direction or speed is called wind shear. It is most commonly found around thunderstorms, temperature inversions, and cold fronts (all of which we’ll learn more about later). Let’s look at some examples:
Picture an aircraft that is climbing with a headwind after take off. If it climbs into a higher layer of air with lighter winds, it will experience a decrease in headwind.
If the pilot does nothing, their airspeed will reduce, which reduces the lift created by the wings. The aircraft’s rate of climb will reduce and in extreme cases may begin to descend back towards the ground.
Let’s consider another, more critical, situation now. An aircraft is on approach to land and as the pilot descends closer to the ground the wind is blocked by the trees & hangars surrounding the runway. This causes the 25 knot headwind to reduce to 10 knots. At this point, while 50 feet or so above the runway, the airspeed suddenly decreases. Lift is reduced and the rate of descent suddenly increases. The pilot may not have enough time to react and a heavy landing or failure to reach the runway can result.
Wind shear is commonly found in the vicinity of thunderstorms and fast moving cold fronts. A developing storm can provide a good overall view of the different effects of wind shear, as they often have strong winds blowing down from the centre of the storm cell which blow outwards in all directions after reaching the ground – known as a microburst.
This image shows the situation a pilot can find themself in when approaching a runway with a growing Cumulonimbus cloud between the aircraft and the runway. Approaching the cloud, the wind will rapidly increase and the aircraft experiences a suddenly increased headwind – more lift is created and the aircraft appears to be overshooting the runway. The pilot reacts by reducing power to descend.
Underneath the cloud, the increased headwind disappears and the aircraft is subject to a downdraught. The rate of descent will rapidly increase and the pilot might start regretting their decision to reduce power earlier!
Passing through to the far side of the cloud, the aircraft is exposed to a sudden tailwind – further reducing airspeed, decreasing lift, and increasing the rate of descent.
Depending on the severity of the wind shear, the aircraft may not have sufficient power to overcome the rate of descent before impacting the ground.