Fly long enough, and you’ll encounter the phenomenon of wind shear. Sometimes known as air pockets, sinkers, bumps, CAT (clear air turbulence) or simply updrafts and downdrafts, the common ingredient is a change in wind speed or direction over a relatively short horizontal or vertical distance. While aircraft tend to fly at a constant speed relative to the air supporting them, when the air itself changes energy properties by altering the speed of its flow, there is an initial effect on the aircraft occupying that air parcel.
Encountering changing wind speed requires a corresponding change in the aircraft’s speed, in order to restore the airspeed previously held. Given no change in thrust, the airspeed will slow down with a reduction in the relative wind speed, or speed up as relative wind increases. These speed changes cause altitude excursions if not managed by thrust adjustment, and any directional changes in the wind causes sudden turbulence in an otherwise smooth flight.
Wind shear, therefore, can cause injuries to unsecured cabin occupants, loss of aircraft control, and disorientation in low visibility. With plenty of altitude at our disposal, the inconvenience of jolts and vertical displacement can be only a nuisance, as long as everyone’s belted in. Some advanced airborne radars can provide warning of wind speed changes ahead, but they are not infallible.
High altitude wind shear largely originates with crossing the edges of a jet stream, which are unseen rivers of flowing air in the upper atmosphere. The associated turbulence may be in the form of buffeting as one enters or leaves the jet, vertically or horizontally, while the ride is relatively smooth in the core of the jet, where the wind speed is constant. Less well understood are low-level jets blowing at 40 to 50 knots above a temperature inversion, typically seen during a clear, cool evening at 1,000 to 3,000 feet AGL. While surface winds are calm in the cold air below, warmer air atop the inversion level can be moving rapidly, ready to disrupt the energy state of the climbing aircraft. Turbulence will be felt as the aircraft or descends through the inversion boundary, but the ride will be smooth above or below it.
Wind shear encounters may also be experienced in the standing waves downwind of a mountain ridge, produced by terrain-level winds blowing perpendicular to the ridgeline. Multiple waves can exist, disrupting the air at altitudes well above and below the peaks. Aircraft control can be compromised and even structural damage can occur, particularly if the pilot attempts to hold altitude with strong control inputs. Expect mountain waves to develop when speed exceeds 15 knots, even when the precipitous ridges are only 500 feet or so in height.
Wind shear met in the upper flight levels during cruise may require a request (or demand) for a block of altitudes until the disturbance subsides. Automatic flight controls may disconnect, requiring basic manual attitude flying and thrust changes to mitigate the altitude excursion. Similar to riding out the up and down drafts in a thunderstorm, it’s best to hold the nose and wings level and allow the airplane to ride out the turbulent air.
With limited clearance from terrain, particularly when the airplane is in a low energy state during takeoff and landing, wind shear becomes a much greater hazard. Obviously, hitting the ground is to be avoided at all costs. If the descending airplane encounters a reduction in headwind, sink rate will increase because the aircraft suffers a loss of airspeed. At a constant thrust setting, the only way to regain lost energy is to convert altitude into speed, precisely what the aircraft will attempt to do. Conversely, encountering a stronger headwind component causes the airplane to rise above the previously-stable glidepath, tempting the pilot to reduce thrust and lower pitch attitude, perhaps creating an excessive sink rate.
Wind shear on approach is a poisonous brew requiring prompt, strenuous correction. If sink rate increases near the ground, the pilot should immediately go to TOGA (takeoff/go-around) power and increase pitch attitude until the stick-shaker activates, easing off only after achieving a positive rate of climb and an altitude that will clear all looming obstacles. This is no place for hesitation.
Should the approach become unstabilized, either from airspeed excursions or piloting inputs against the wind shear, it’s better to go around and make another attempt, rather than cross the threshold with 20-knots extra airspeed or, worse yet, 20 knots loss of speed and a heavy rate of sink toward the approach lights. Again, prompt piloting action is required. Any second attempt should be flown with regard to the wind shear action seen with changing altitude on the first approach.
One technique that can be used to maintain a safe energy state in turbulent conditions is to fly to maintain a constant groundspeed readout. Varying thrust aggressively to keep groundspeed steady keeps the aircraft moving down the glideslope in a relatively stabilized condition, even as it encounters wind shear during descent.
Taking off with thunderstorms in the vicinity of the airport is asking for a wind shear encounter, reached either horizontally, as one approaches outflowing surface winds from the storm, or vertically, as wind speed changes during climbout. The aircraft is typically heavy, already using max power, and in an energy state leaving little reserve to sacrifice. Flying into an increasing headwind is beneficial, as climb gradient will steepen, while losing speed during the climb will make it more difficult to meet minimum gradient profile.
Be alert for wind shear advisories during weather briefings, shown as “WS” notes in terminal forecast remarks and pilot reports. The “WS” label is followed by an altitude and the wind’s speed and direction, which can be compared to the reported surface wind to see the apparent change to be expected during departures and approaches.
Supplying pilot reports of speed loss during climbout or descent is most helpful, more so than just asking “is everyone making it in?” A preceding aircraft may have escaped the full brunt of the windshear, while your approach can be in peril. Leaving a report of “20 knots airspeed loss at 2,000 feet” warns the next crew of what to expect on their approach, and to be ready.
The worst-case scenarios are those with convective weather in the area, particular with mature storms in their dissipating stage producing strong outflow winds. These situations are variable, minute by minute, as individual storms move relative to the runway in use. It helps to stay in visual conditions, both to see the rain shafts and observe blowing wind indicators and tossing trees, and to maintain attitude control when fighting turbulence. Wind shear alert is available at some airports, with perimeter wind detectors able to show varying winds at opposite sides of the field. If the tower controller broadcasts a wind shear alert from this information, take it as a sign that things are getting dicey.
Wind shear, whether encountered up high in jet stream country or near the ground during terminal area operations, requires caution. This is serious flying, not systems monitoring, so be prepared to assume full PIC duties. n