Understanding Windshear
Definition
Windshear can be defined as a sudden change in wind velocity and/or direction over a short distance. It can occur in all directions, but for convenience, it is considered along vertical and horizontal axis, thus introducing the concepts of vertical and horizontal windshear:
- Vertical windshear consists of wind variations along the vertical axis of typically 20 to 30 knots per 1000 ft. The change in velocity or direction can drastically alter the aircraft lift, indicated airspeed, and thrust requirements when climbing or descending through the wind shear layers.
- Horizontal windshear consists of variations in the wind component along the horizontal axis – e.g. decreasing headwind or increasing tailwind, or a shift from a headwind to a tailwind – of up to 100 knots per nautical mile – shows how a penetration would appear as an aircraft crosses a cold front.
This weather phenomenon can occur at many different levels of the atmosphere; however it is most dangerous at the lower levels, as a sudden loss of airspeed and altitude can occur.
It is usually associated with the following weather conditions: jet streams, mountain waves or temperature inversion layers, frontal surfaces, thunderstorms and convective clouds or microbursts, occurring close to the ground.
Focus on Microbursts
A microburst creates the most dangerous form of windshear. It consists of a small column of exceptionally intense and localized sinking air, which descends to the ground (called “the downdraft”) and upon contact with the earth’s surface, diverges outwards in all directions, thus forming a ring vortex. It is capable of producing powerful winds near ground level.
Microbursts are either dry (i.e. little or no rain reaches the ground) or wet (usually within a downpour). They typically form under or close to thunderstorms and cumulonimbus clouds in particular.
From a safety perspective, microbursts bring a threat to aircraft due to the scale and suddenness of this phenomenon. To put it briefly, microbursts combine two distinct threats to aviation safety:
- The downburst part, resulting in strong downdrafts that rapidly push the aircraft downward. The power of the downburst can actually exceed aircraft climb capabilities.
- The outburst part, resulting in large horizontal wind shear and wind component shift from headwind to tailwind. This sudden change from headwind to tailwind reduces the lift of the aircraft, which may force the aircraft down, typically during take-off or landing.
Anticipating Windshear
Windshear awareness and detection means
The best ways a pilot can prevent an encounter with windshear is to know windshear is there and to avoid it where possible. However, should an encounter be unavoidable, it is important to know the likely magnitude of the change, and be prepared to react immediately. Although there is no absolutely reliable way to predict the occurrence, different tools and information can be used to detect areas of potential or observed windshear, and thus be able to develop efficient avoidance strategies.
Flight crew should consider all available windshear awareness means and assess the conditions for a safe take-off or safe descent, approach and landing based on:
- Most recent weather reports and forecast. Pay a careful attention to ATC indications in particular.
- Visual observations.
- Crew experience with the airport environment and the prevailing weather conditions.
- Weather radar implemented at airports. These systems serve to detect microbursts in close proximity to the airport and send out alerts to both pilots and ATC alike.
- On-board weather radar to ensure that the flight path is clear of hazard areas.
- On-board Predictive Wind shear System (PWS).
Focus on Predictive Windshear System
Today, most aircraft models have predictive windshear equipment to warn pilots of possible threats via aural and visual means.
To provide an early warning of potential windshear activity, some on-board weather radars feature the capability to detect windshear areas ahead of the aircraft, based on a measure of wind velocities ahead of the aircraft both vertically and horizontally.
This equipment is referred to as a Predictive Wind shear System (PWS). This system is active and provides reliable indications between 50 and approximately 1000 feet above the ground surface.
The PWS provides typically a one-minute advance warning by showing first an amber “W/S AHEAD” message on the PFD.
If conditions worsen and the wind shear location gets closer to the aircraft, the “W/S AHEAD” amber caution turns into a red warning and is associated with an aural synthetic voice “WINDSHEAR AHEAD, WINDSHEAR AHEAD” during take-off, or “GO AROUND, WINDSHEAR AHEAD” at landing. This is a possible indication that the aircraft is approaching a microburst.
Recognizing and Handling Windshear
Recognition
As rare as an actual encounter with severe windshear may be, timely recognition of this condition is key for the successful implementation of windshear recovery / escape procedures.
The following deviations should be considered as indications of a possible windshear condition:
- Indicated airspeed variations in excess of 15 knots
- Ground speed variations
- Analog wind indication variations: direction and velocity
- Vertical speed excursions of 500 ft/minute
- Pitch attitude excursions of 5 degrees
- Glide slope deviation of 1 dot
- Heading variations of 10 degrees
- Unusual autothrust or auto throttle activity.
On-board functions
A reactive windshear warning system is available on most aircraft models.
This system is capable to detect a windshear encounter based on a measure of wind velocities, both vertically and horizontally. When it activates, the audio “WINDSHEAR” is repeated 3 times, and a red “WINDSHEAR” warning appears on the PFD.
The windshear warning system associated to the Speed reference System (SRS) mode of the flight guidance constitute the Reactive Windshear System (RWS), since both components react instantaneously to the current variations of aircraft parameters.
For more information on windshear, refer to the Safety First article dedicated to this subject.
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