Windshear

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An extremely dangerous form of windshear is created by a microburst. As very wel explained in Wikipedia, a microburst is an intense small-scale downdraft produced by a thunderstorm or rain shower. As can be seen very well on the video, they go through three stages in their cycle: the downburst, outburst, and cushion stages also called "Suriano's Stroke".

A microburst can be particularly dangerous to aircraft, especially during landing, due to the wind shear caused by its gust front. Several fatal and historic crashes have been attributed to the phenomenon over the past several decades, and flight crew training goes to great lengths on how to properly recover from a microburst/wind shear event.

Windshear

An aircraft entering the area of a microburst within 1,000–3,000 feet AGL will first encounter an increasing headwind (1). The aircraft will initially maintain its inertial speed over the ground (its groundspeed) and the increased headwind will cause it to have a higher airspeed, therefore increased performance. It will tend to fly above the original flight path (2). Then the aircraft will enter the downburst shaft and will be carried earthward in the strong downward air current with a dramatic loss of performance (3).

As the aircraft flies out of the downburst shaft (hopefully), the situation is not greatly improved. It will fly into an area of increasing tailwind (4). As the aircraft will tend to maintain its inertial groundspeed initially, the increasing tailwind will cause the airspeed to decay, with the reduced airspeed resulting in reduced aircraft performance.

Even with the addition of full power and suitable adjustments to pitch attitude by the pilot, the airplane may struggle to maintain a safe airspeed and flight path. Traversing some small, strong microbursts safely may be beyond the performance capabilities of any aircraft.

Windshear in the A320 Simulator

Using the Instructor Station Website, the flight instructor can set the windshear via the Weather - Wind page. The width of the downdraft shaft can be set: Micro (1 NM), Small (1.5 NM), Medium (2 NM), Large (2.5 NM). And also the strength can be set: Low, Medium, High, Extreme. The windshear effect can be set to occur at a specific Distance and Bearing from the current aircraft position, or alternatively can be set to occur instantaniously directly in front of the aircraft.

Windshear

The below Instructor Station Approach webpage shows the violent and dangerous effect a microburst has during a landing. The A320 Simulator pilot has set the Autopilot to perform an ILS autolanding. The flight instructor triggers an extreme microburst that the aircraft flies into at a distance of approx 3 NM from the runway, being at that moment on the Glideslope at an altitude of 1100 Feet.

Windshear

Although the Autopilot actively tries to maintain the glideslope, the aircraft starts flying above the glideslope. When it hits the downdraft shaft it suddenly starts loosing altidute and falls to well below the glideslope. At this point the pilot should have already aborted the landing and initiated the Windshear recovery procedure, applying full TOGA. In the testflight example below, we however show the result when the pilot does not take action and leaves the Autopilot to perform the ILS autoland. The Autopilot over stears the aircraft resulting in a violent increase in altitude at around 0.6 NM from the runway, and the aircraft at the same time comes in the tailwind phase of the windshear whereby its performance significantly decreases. The aircraft overshoots the runway threshold, and shortly after slams into the ground.

Luckily in our A320 Simulator Software we have implemented both Predictive (via the Weather Radar) as well as Reactive (via the FAC and EGPWS) windshear detection and warning. In the described example, the pilots in the A320 Simulator received both visual warnings on the ND as well as aural warnings from the EGPWS, well in advance and with taking the appropriate action can safely fly out of the Microburst.

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