What Causes Windshear and How Can It Be Avoided?

Deployment of Melbourne windshear prevention, escape, and recovery tactics successfully depend on flight crew awareness and attention.

This safety article summarizes operational suggestions and instruction recommendations for pilots operating aircraft in predicted or probable downburst or windshear circumstances.

What is windshear?

The term “windshear” refers to an abrupt change in wind speed or direction.

Although Melbourne windshear can occur in any direction, it is more practical to quantify it along a horizontal and vertical axis, resulting in horizontal and vertical windshear.

Vertical Windshear:

Changes in the horizontal wind component along the vertical axis can cause turbulence that can slow down or speed up an airplane as it climbs or descends through a Windshear Melbourne layer.

The wind component typically varies between 20 kt and 30 kt per 1000 ft, although a vertical windshear can be as high as 10 kt per 100 ft.

 Horizontal windshear:

Changes in the wind component all along the horizontal axis (e.g., a change from a headwind to a tailwind, decreasing headwind to a growing tailwind, etc.) – Changes in the wind component may reach up to 100 kt per nautical mile.

Jet streams, thunderstorms, frontal surfaces, mountain waves, convective clouds, and microbursts are frequently linked to Melbourne windshear conditions.

Two separate dangers to aircraft safety merge in microbursts:

  • The downburst part, which produces powerful downdrafts (up to 6000 ft/mn of vertical velocity)
  •  The outburst produces significant horizontal windshear and a change in the wind direction from a headwind to a tailwind (horizontal winds might take up to 45 kt)

What effect does windshear have on an aircraft’s performance?

Effects on flying performance include:

– A downdraft influences both the aircraft route and the aircraft’s angle of attack (AOA), which increases as the aircraft sinks. – A headwind gust tends to cause the aircraft to fly above its intended course and accelerate.

– A tailwind gust immediately reduces the airplane’s speed, which causes it to fly off course or decelerate.

In contrast to microbursts, which happen closer to the ground, windshears linked with jet streams, mountain waves, and frontal surfaces typically occur at elevations that do not pose the same risk.

Depending on where the aircraft is about the microburst, four situations can be seen:

  1. Before the aircraft, a microburst

A rise in a headwind is only sometimes seen as a danger by the crews. However, if the pilot does not respond quickly enough, the aircraft will prefer to fly above the course and/or accelerate as it approaches the headwind.

The airplane’s performance will improve if the headwind shear happens at takeoff. Once out of the shear, the apparent airspeed drops, which causes an increase in AOA and may cause the stick shaker and alpha-floor protection to activate.

  • A microburst downdraft is encountered by the airplane.

Usually, a rise in the headwind component comes before a vertical downdraft. If the pilot is unaware of the circumstances, he or she may react to the headwind gust effects by cutting power and pressing the stick to return to the intended course. The aircraft will then drop more quickly, bringing it below the targeted route due to a vertical downdraft.

  • Microburst behind the airplane can be seen.

The aircraft’s airspeed immediately drops in the event of a sudden rise in the tailwind. As the lift declines, the airplane starts to fly below the targeted approach route.

Without applying enough force, the AOA will increase dramatically, and the aircraft will sink if the pilot pushes on the stick to regain the route.

The aircraft will soar above the path and accelerate if enough thrust is set to reclaim the planned path, but the pilot reacts slowly to lessen the thrust once back on the path.

  • A microburst passes through the aircraft.

The worst scenario is the combination of the previously mentioned scenarios: a headwind gust, a downdraft, and a tailwind gust.

How to Avert WindShear?

1.       Windshear Avoidance

To prevent being in regions where windshear Melbourne is likely or already present, use the following information:

Weather reports and forecasts: Some airports include a Terminal Doppler Weather Radar (TDWR) or a Low-Level Windshear Alert System (LLWAS) (TDWR).

  • A central wind sensor that detects wind direction and speed is part of the LLWAS, along with peripheral wind sensors. It makes it possible for controllers to inform pilots of current or to approach Melbourne windshear conditions. An alert is generated whenever a difference of more than 15 kt is found. LLWAS might not detect downbursts of a diameter of 2 nm or less.
  • To provide pilots more time to prepare for windshear hazards, TDWR can detect impending Melbourne windshear zones.

 Pilot reports: Crews should pay close attention to PIREPS of windshear above 20 kt or downdraft/updraft of 500 ft/mn below 1000 ft above ground level.

Visual observation: Melbourne Windshear is frequently indicated by blowing dust, rings of dust, dust devils (whirlwinds containing dust and debris), or any other strong local air outflow near the surface.

2.       Windshear Recognition

For the windshear recovery/escape operation to be carried out successfully, early detection of a windshear Melbourne condition is essential.

A possible windshear condition may be indicated by any of the following deviations:

  •  Indicated airspeed variations of more than 15 kt
  •  Ground speed variations
  •  Analog wind indication variations. Orientation and speed
  • 500 feet per minute deviations in vertical speed;
  • 5 degrees deviations in pitch attitude
  • Heading variations of 10 degrees
  • Glide slope deviation of 1 dot
  • Unusual throttle lever position or auto thrust activity.

Windshear operational standards

Generally speaking, you should postpone takeoff if windshear is forecast.

1.       Preparation of the cockpit and departure briefing

The flight crew should take into account all available windshear-awareness items and:

  •  Determine the likelihood of a safe takeoff based on the most recent weather reports and forecast, visual observations, crew familiarity with the airport environment, and the current weather conditions; or
  • Postpone the takeoff as necessary until the conditions improve.

2.       Takeoff and Initial Climb

Suspected Windshear

The flight crew should consider delaying takeoff if windshear Melbourne conditions are suspected during takeoff, choose the best runway based on where the windshear/downburst is most likely to occur, and use weather radar (and a predictive windshear system, if available) to check the flight path for hazards before starting the takeoff roll.

Choose the highest takeoff thrust possible. Pay close attention to the airspeed and speed trend throughout the takeoff roll to look for any signs of Melbourne windshear.

Windshear Recovery Technique: If windshear Melbourne is experienced during the takeoff roll, immediately use the following recovery techniques:

  •  Before V1: Only reject the takeoff if there are unacceptable airspeed variations and the pilot determines there is still enough runway to halt the aircraft.
  •  After V1:  maintain or adjust the thrust levers to the maximum takeoff thrust (TOGA), follow the Flight Director’s pitch instructions, or establish the necessary pitch attitude if FD is not present.

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