Windshear and microburst

What is windshear?

Windshear is any variation of wind speed and/or direction from place to place, including updrafts and downdrafts. The stronger the change and/or the shorter the distance within which it occurs, the greater is the windshear. However, in practical terms, only a wind change of a magnitude that causes turbulence or a loss of energy disturbance to an aircraft’s flight path is generally considered to be windshear. It should be remembered that windshear is a complex subject and is still not fully understood.

Note: Windshear can affect the flight path and airspeed of an aircraft and can be a hazard to aviation.

How is windshear detected?

Windshear is detected by identifying a difference in wind (direction and/or speed) and/or temperature between two places or identifying certain weather phenomenon, e.g., cumulonimbus clouds, that give rise to possible windshear conditions from weather reports. This can be done in the following ways:

1. Pilot appreciation of differences in reported wind and/or temperature between two places and the location of certain weather and terrain phenomenon. For example, the following phenomena should alert the pilot instinctively of the possibility of windshear along his or her’s flight path:

a. Cumulonimbus clouds in the general area (microbursts and/or thunderstorm gusts)

b. Heavy rain or even thunderstorms in the vicinity

c. Fronts (change in wind direction)

d. Virga rain (temperature inversion), e.g., rain that evaporates before it reaches the ground, absorbs latent heat out of the surrounding air that creates denser/heavier air with an associated higher pressure causing a downdraft windshear.

e. Land/sea breezes at coastal airports, especially at dawn and dusk periods f. Terrain, i.e., trees, mountains, or even surface obstructions.

Note: Pilots are required to report windshear whenever they encounter it.

2. Aerodrome equipment/reporting. Many of the major aerodromes around the world have windshear measuring equipment.

3. Aircraft warning equipment. Modern aircraft have windshear warning systems, which usually are incorporated into the ground proximity warning system (GPWS).

How does windshear affect an aircraft?

Windshear is a change of wind speed and/or direction (including vertical updrafts/downdrafts) that affects the lift capability of an aircraft. The two main aerodynamic principles of this are as follows:

1. The aircraft’s dynamic speed is reduced as a result of the reduction in wind speed.

2. The effective aircraft weight is increased as a result of experiencing a downdraft, which requires the pilot to recover the flight path by pitching the aircraft up. This is accomplished by deflecting the stabilizer/elevator upward (to get the aircraft to pitch up). However, deflecting the elevator/stabilizer upward creates a downward air force on the control surface, which also has the effect of increasing the aircraft’s effective weight.

These two properties both subject the aircraft to turbulence and affect the aircraft’s lift potential, which brings the aircraft closer to its stall.  A change in wind speed and/or direction (windshear) is in general terms manifested as flight path overshoot and undershoot effects.

The actual windshear overshoot or undershoot effect depends on the following:

1. The nature of the windshear

2. Whether the airplane is climbing or descending through that particular windshear in which direction the aircraft is proceeding An overshoot windshear effect may result from flying into

1. An increasing headwind

2. A decreasing tailwind

3. An updraft An undershoot windshear effect may result from flying into

1. A decreasing headwind

2. An increasing tailwind

3. A downdraft

What is a microburst?

A microburst is a severe downdraft, i.e., vertical wind, emanating from the base of a cumulonimbus cloud during a thunderstorm.

Note: A microburst is a severe form of windshear.

Where do you find microbursts?

Microbursts are found close to, normally underneath, mature cumulonimbus clouds and are associated with thunderstorms. The microburst downdraft is highly concentrated, typically only 5 km across, and often centered in the middle of a thunderstorm, surrounded on all sides by strong updrafts.

Microbursts are severe vertical downdrafts associated with mature cumulonimbus clouds. They are usually highly concentrated, only about 5 km across, and are found in the middle of a thunderstorm under a mature cumulonimbus cloud. The mechanics that produce a microburst are found at the height of a mature cumulonimbus cloud’s cycle; e.g., they only last for a few minutes (up to 10 minutes) because they are centered in the mature stage of a cumulonimbus cloud (where updrafts are produced that fuel the downdrafts), which itself only last for about 40 minutes.

A mature cumulonimbus cloud produces strong continuous updrafts around its outer edges (and sometimes outside the cloud itself) that build up to fuel the downdrafts in the center of the cloud. The microburst is a result of the downdrafts breaking out of the base of the cloud being colder than the surrounding air because it has only been warmed at the saturated adiabatic lapse rate (SALR) during the descent within the cloud. Therefore, since it is colder, it is also denser/heavier than the surrounding air, and thus it continues down to the surface, where it often can be felt as the first gust.

A microburst can be associated with precipitation or dry downdrafts. When they are at their most severe, microburst downdrafts can reach up to 3000 ft/min and have a wind speed of approximately 100 knots. Therefore, it is always best to wait for it to pass because the downdrafts and the rapid reversals of wind direction of a microburst can be fatal to an aircraft.

What identifies (are the indications of) a microburst?

The typical indications of a microburst are

1. Mature cumulonimbus clouds with thunderstorm activity, especially rain

2. Roll cloud formation around a cumulonimbus cloud

Note: Roll clouds are formed by the turbulent outflow, associated with microburst downdrafts, from underneath a cumulonimbus cloud.

3. Virga rain, especially beneath or near to a cumulonimbus cloud.

Note: Rain that evaporates before it reaches the ground absorbs latent heat out of the surrounding air and creates denser air with an associated higher pressure, causing the downdrafts to fall to the ground at an even greater rate. Therefore, virga rain indicates a possibly severe microburst.

4. Flight path and indicated airspeed (IAS) fluctuations, especially on the approach path

5. Wind direction and speed changes, which can be either reported or measured by aircraft systems, if fitted.

How does a microburst affect an aircraft?

With a microburst out of the base of a cumulonimbus clouds, the downburst will spread out as it nears the ground and then rebound off the ground as an updraft. If an aircraft’s flight path (approach) is beneath such a cloud, then the aircraft will experience the following characteristics:

1. Initially, when entering a zone underneath a cumulonimbus cloud with a microburst present, an aircraft will experience an updraft (as a result of the downburst spreading outward and rebounding off the ground as an updraft). This has the following effects on an aircraft’s flight path:

a. Energy gain from an increasing headwind will cause the aircraft’s nose to rise.

Note: In an extreme case, the updraft may be severe enough to cause the aircraft to exceed its stalling angle.

b. Airspeed (IAS) will rise.

c. Rate of descent will be reduced. The overall effect in this portion of flight is to cause an overshoot effect as the aircraft tends to fly above the desired flight path.

Note: When such an updraft is encountered at the start of an approach underneath a cumulonimbus cloud, a pilot should be aware of the potential for the initial overshoot effect of the aircraft to be reversed as the aircraft proceeds along the flight path. This is known as windshear reversal effect, i.e., an overshoot followed by an undershoot.

2. Approaching the central area underneath a cumulonimbus cloud with a microburst present, an aircraft would start to experience a strong downdraft, which would be a severe change, over a short distance, having previously experienced an updraft. This has the following effects on an aircraft’s flight path:

a. There will be an energy loss from a reducing headwind, which will cause the aircraft’s nose to fall.

b. Airspeed (IAS) starts to fall (due to the reducing headwind).

c. Rate of descent starts to increase, with a tendency to go below the glide path.

3. As the aircraft’s flight path progresses to the far side of the cloud with a microburst, the aircraft will then experience a severe downdraft and a tailwind directly from the cloud base. This has the following effects on the aircraft’s flight path:

a. There will be energy loss from an increasing tailwind.

b. Airspeed (IAS) continues to fall sharply.

c. Rate of descent continues to increase with a tendency to go further below the glide path, with even the possibility of ground contact if not checked by initiating a missed approach, and even then, success depends on the power height and speed reserves available being sufficient enough to overcome the downdraft. The overall effect in this portion of flight is to cause an undershoot effect as the aircraft tends to fly below the desired flight path. It should be understood clearly that microbursts can be of such a strength that they can down an aircraft no matter how much power, time, and height the aircraft has available. Therefore, always, always avoid possible microburst areas.

Where do you find windshear?

Most forms of windshear are found at low levels, i.e., below 3000 ft. Therefore, the term low-level windshear is used to specify the windshear along a final approach path prior to landing, along a runway, and along a takeoff and initial climb-out flight path. Low-level windshear, i.e., near the ground, generally is present to some extent as an aircraft approaches the ground because of the difference of speed and/or direction of the surface wind compared with the wind at altitude. This is critical in terms of aircraft safety because the loss of energy caused by these changes can easily lead to the aircraft losing altitude and/or stalling.

Low-level windshear includes

1. Clear air turbulence (CAT).

2. Frontal passage

3. Microburst and thunderstorm gusts.

Medium- and high-level windshear also can be experienced at high or medium altitudes and include

1. CAT in the form of jetstreams.

2. Frontal passage (e.g., a low-level frontal passage)

What effect does windshear have during an approach for landing?

A change in wind direction and/or speed windshear has the following effects on an aircraft during the landing approach:

1. If the headwind component increases, then the aircraft will experience a transient increase in performance. This will lead to an overshoot tendency.

2. If the headwind component decreases, then the aircraft will experience a transient decrease in performance.

Note: This is a typical scenario as you get closer to the ground, causing an increase in the rate of descent if allowed to go unchecked. However, normally, this effect is only slight and doesn’t cause too much of a concern, although any significant windshear drop should be treated with respect. Loss of performance in this scenario will cause the aircraft to sink and possibly undershoot the aiming point.

What indications should you look for if windshear is expected?

You should look for indicated airspeed, temperature, and lift trends. That is, an increasing headwind would increase the indicated airspeed and lift performance for no power or attitude changes. And a decrease in the headwind would decrease the indicated airspeed and lift performance for no power or attitude changes. The rate at which these changes occur indicates the severity of the windshear ahead.

How would you fly an approach if you suspect windshear?

You would increase the approach speed to compensate for the loss of energy that is common with low-level windshear. This action will help to guard against a stall when you attempt to maintain the flight path by increasing the aircraft’s attitude. You would monitor the conditions ahead in case the conditions worsened from mild to severe windshear (microbursts).

What is the recovery technique for windshear?

For mild or even moderate windshear that causes a manageable flight path deviation, i.e., less than a single dot deviation on the approach, then it is acceptable to use normal power (for speed) and pitch (for the descent profile) corrections to regain the desired flight path.

Note: Unless it is believed that this condition is a warning of more severe disturbance ahead. If this is the case, then you should adopt a more aggressive attitude and initiate an immediate go-around. For severe windshear (which can include microbursts), a TOGA fullpower and maximum pitch-up go-around should be initiated immediately.

How would you fly an approach if you suspected a microburst?

You should not attempt an approach into an area where a microburst is reported or likely. Hold for 10 to 15 minutes to allow the storm to move away from the approach path before attempting an approach. Continue to monitor for indications of possible microburst activity.

Note: The early identification of a possible microburst or severe windshear downdrafts on an approach is vital to ensure that you have sufficient power, height, and speed reserves available to successfully initiate a go-around.

Therefore, it is important not to overcompensate for an overshoot tendency by reducing power early in an approach because this will incur a delay in the delivery of full power when you pass into the downdraft area later on the approach and when the go-around has to be initiated. This time delay could be crucial.

If, however, you do encounter an unexpected microburst on an approach, you should initiate an immediate full-power (TOGA) maximum pitch-up go-around. If you are close to the ground and the landing gear is already down, then you should delay retracting the gear until ground contact is definitely no longer a possibility.

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