There are several ways that weather might endanger an aircraft. One of the toughest situations is windshear, in which highly concentrated winds occur close to the ground. What is windshear, how can it affect an aircraft and how can it be prevented are all questions to be answered in this article, let’s dig in!
What is Windshear?
When the wind abruptly changes direction or speed over a small distance, then this scenario is referred to as windshear. It can be vertical or horizontal, impacting an aircraft’s horizontal flight path or its takeoff or descent motion. In contrast to the prevailing wind, windshear occurs across a smaller area. It frequently occurs close to microbursts or downbursts that are generated by a thunderstorm or weather front.
If you don’t know what is microbursts and downbursts; a downburst is a wind system that develops from a single point source, blows down toward the ground until it collides with it, then spreads outward. When this occurs over a small area, it is referred to as a microburst.
Wind component variations along the horizontal axis (e.g., decreasing headwind or increasing tailwind, or a shift from a headwind to a tailwind) Wind component variations can reach 100 knots per nautical mile.
Variations in the horizontal wind component along the vertical axis, result in turbulence that may affect aircraft airspeed when climbing or descending through the windshear layer. Wind components ranging from 20 knots per 1000 ft to 30 knots per 1000 ft are typical, but vertical windshear can reach 10 knots per 100 ft.
How can Windshear affect an aircraft?
Windshear can occur at both high and low elevations; vertical windshear can cause changes in airspeed and rate of descent at high altitudes. This is commonly felt as turbulence, however, windshear is most dangerous at low altitudes, close to the ground. Downbursts near an airport can disrupt aircraft takeoff and landing. The horizontal windshear, which acts as a headwind as the aircraft approaches the downdraft, will increase airspeed, as the aircraft exits the downdraft, the wind changes to a tailwind, which reduces the amount of lift produced by the wings. This can be summarized to:
- A headwind gust instantly increases the aircraft’s speed, causing it to fly above its intended path and/or accelerate.
- A downdraft affects both the aircraft’s Angle-Of-Attack (AOA), which increases, and the aircraft path, as it causes the aircraft to sink
- Tailwind gusts reduce aircraft speed instantly, causing the aircraft to fly below its intended path and/or decelerate.
Pilots do not always see an increase in a headwind as a risk. However, such a headwind gust destabilises the aircraft’s approach, causing it to fly above the path and/or accelerate if the pilot does not react appropriately. If headwind shear occurs during takeoff, aircraft performance will improve. Once out of the shear, the indicated airspeed decreases, resulting in an increase in AOA, which may activate the alpha-floor protection (an Airbus low-speed protection available in Normal Law which will command full power from the engines if the system detects the aircraft is dangerously close to a stall) on Airbus airliners and stick shaker (a mechanical device designed to rapidly and noisily vibrate the control yoke of an aircraft, warning the flight crew that an imminent aerodynamic stall has been detected).
The headwind component typically increases before vertical downdrafts, if the pilot does not observe the situation, he or she will react to the headwind gust effects by reducing power and pushing on the stick to return to the intended path; a vertical downdraft will then accelerate the aircraft’s sink rate, bringing it below the intended path.
When the tailwind suddenly increases, the aircraft’s airspeed drops instantly. As the lift diminishes, the aircraft tends to fly below the intended approach path. If the pilot pulls on the stick to retrace the path without adding enough thrust, the AOA will certainly increase and the aircraft will sink. If enough thrust is set to regain the intended path, but the pilot’s reaction time to reduce thrust once back on the path is slow, the aircraft will fly above the path and/or accelerate.
How to prevent Windshear?
Since the 1980s, there has been a significant improvement in the ability to detect and avoid windshear. Windshear is recognised by flight crews and avoided whenever possible.
Onboard Aircraft Radar
Radar technology advancements have aided in the advanced detection of windshear. Windshear and changes in wind speed close to the ground can now be detected by onboard aircraft radar (by detecting the frequency shift of the microwave pulses caused by the microburst). Earlier radar systems, in comparison, detected moisture, which is not a good indicator of windshear. Potential windshear conditions will be indicated on the onboard radar, and severe conditions will also trigger an audible alert. Since 1993, the FAA has required this for all commercial jet aircraft.
Rather than the radar onboard aircraft weather reports and forecasts may be used to to avoid areas of potential or observed windshear. For this, A Low-Level Windshear Alert System (LLWAS) and/or a Terminal Doppler Weather Radar (TDWR) are available at some airports.
LLWAS & TDWR
The LLWAS system is made up of a central wind sensor (which measures wind velocity and direction) and peripheral wind sensors. It allows controllers to alert pilots to current or impending windshear conditions. When a difference of more than 15 kt is detected, an alert is generated. LLWAS may not detect downbursts with a diameter of less than 2 nm, whereas TDWR can detect approaching windshear areas and thus provide pilots with more advanced warning of windshear hazards.
With these improvements in detection, windshear-related accidents are much less common today. A Bhoja Air flight in Pakistan crashed in windshear on the approach to Islamabad airport in recent years, killing all of the passengers and crew. And in mid-2019, an Aeroflot SJ100 that had been flying through a thunderstorm despite a windshear warning from onboard radar crashed in Moscow, killing 41 people.
Related: Why did Emirates Flight EK521 Crash?
In general, if windshear is suspected, takeoff must be postponed.