Wind Shear
Until now, we have only considered steady winds that have no noticable change. Under consistent conditions, the only effect the wind has on the aircraft is to alter its path over the ground (angle of climb, gliding range etc). However, changes in wind strength and direction are common place and when an aircraft experiences a sudden change in wind direction or speed, the different effects are noticable.
This sudden change in wind direction or speed is called wind shear.
Wind Shear in a Climb
If an aircraft is climbing at 80 knots with a 20 knot headwind, the aircraft’s airspeed is 80 knots and the ground speed is 60 knots. The ground speed can be thought of as the speed at which the aircraft’s shadow is moving across the land.
The aircraft then climbs into a higher layer of air where an abrupt change in the wind speed occurs and the headwind decreases to 10 knots. The pilot maintains the same nose attitude throughout the climb but the aircraft cannot have an instant change in its speed due to inertia. With the suddenly reduced headwind, the airspeed will reduce to 70 knots. With the sudden loss of airspeed, lift will reduce and the aircraft’s rate of climb will reduce.
If the pilot continues to maintain the same nose attitude, the aircraft will eventually accelerate again to 80 knots – but this takes time. It is during this time that the aircraft will experience the effects of wind shear.
Wind Shear on Approach
This loss of 10 knots may not seem like a major issue in a climb, but let’s consider another case at low level.
An aircraft is on approach on a windy day to a runway with large hangars and trees off to both sides of the runway. The pilot is flying at an airspeed of 80 knots into a 25 knot headwind. As the pilot descends closer to the ground the wind is blocked by the trees & hangars, causing the 25 knot headwind to reduce to 10 knots. At this point, while 50 feet or so above the runway, the airspeed suddenly decreases from 80 to 65 knots. 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.
Similarly, if wind shear leading to a loss of airspeed is encountered in the climb immediately after take off, the aircraft may descend rapidly towards obstacles and the ground.
Different Effects of Windshear
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 sudden 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 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.