Low Level Wind Shear

Low Level Wind Shear

Definition

Wind shear is defined as a sudden change of wind velocity and/or direction.

Windshear may be vertical or horizontal, or a mixture of both types. ICAO defines the vertical and horizontal components of wind shear as follows:

  • Vertical wind shear is defined as change of horizontal wind direction and/or speed with height, as would be determined by means of two or more anemometers mounted at different heights on a single mast.
  • Horizontal wind shear is defined as change of horizontal wind direction and/or speed with horizontal distance, as would be determined by two or more anemometers mounted at the same height along a runway.

Description

Low Level Turbulence, which may be associated with a frontal surface, with thunderstorms or convective clouds, with microbursts, or with the surrounding terrain, is particularly hazardous to aircraft departing or arriving at an aerodrome. Wind shear is usually associated with one of the following weather phenomena:

Effects

The main effects of wind shear are:

  • Turbulence
  • Violent air movement (up- or down-draughts or swirling or rotating air patterns)
  • Sudden increase or reduction of airspeed
  • Sudden increase or decrease of groundspeed and/or drift.

Clear Air Turbulence (CAT) (CAT), which may be very severe, is often associated with jet streams.

Rotor action or down-draughts in the lee of mountain waves can create difficult flying conditions and may even lead to loss of control.

Defences

Effective defence against wind shear comprises the following components:

  • Forecasting, recognition and avoidance of wind shear (see below), aided by LLWAS (see below) and airborne avionics equipment; and,
  • Correct response to wind shear encountered during the takeoff, initial climb, approach and landing phases of flight.

Typical Scenarios

  • An aircraft on initial climb encounters a microburst with strong down-drafts, which prevent the aircraft from climbing away, even though the pilot immediately recognises the wind shear and takes correct action.
  • An aircraft on approach in tail-wind conditions encounters horizontal wind shear resulting in a change of wind component to a head-wind (increasing performance shear); the IAS increases and the aircraft climbs above the glideslope. The pilot attempts to salvage the landing and the aircraft touches down late and fast and overshoots the runway.

Solutions

  • Improved forecasting of wind shear;
  • Improved training in wind shear recognition, avoidance and recovery;
  • More widespread use of ground and airborne wind shear warning systems.

Low Level Wind Shear Alert System

Low Level Wind Shear Alert System (LLWAS) is a ground-based system for detecting the existence of wind shear close to an aerodrome.

The system comprises from 6 to 33 anemometers located at various points on the aerodrome surface. Data from the anemometers are fed into a computer which compares the wind speed and direction measured at the different points and provides a warning in the air traffic control tower if a hazardous wind shear is detected. Warnings issued by ATC can be general or runway specific, depending on the technology in use, and are broadcast immediately to pilots who may be affected.

LLWAS was first installed in the USA in the 1970's and is in widespread use in that country. Wind shear and microburst warnings from LLWAS can be enhanced by integrating with Terminal Doppler Weather Radar (TDWR) (TDWR); and in some locations TDWR is the sole means used for detecting low level wind shear.

Wind Shear Recognition and Avoidance

Flight Safety Foundation (FSF) Approach-and-landing Accident Reduction (ALAR) Briefing Note 5.4 — Wind Shear points out that "Flight crew awareness and alertness are key factors in the successful application of wind shear avoidance techniques and recovery techniques."

Whenever wind shear conditions are forecast, or reported by other aircraft, pilots should include discussion of wind shear recognition and response in the takeoff or approach brief.

Whether or not wind shear conditions are expected, the pilot must be able to recognise quickly when wind shear is affecting the aircraft. He/she may be aided in this by airport based warning systems (e.g. LLWAS and TDWR) or by onboard equipment, such as Ground Proximity Warning System or Airborne Wind Shear Warning Systems.

ALAR Briefing Note 5.4 lists the following indications of a suspected wind shear condition:

  • Indicated airspeed variations in excess of 15 kts
  • Groundspeed variations (decreasing head wind or increasing tail wind, or a shift from head wind to tail wind)
  • Vertical-speed excursions of 500 fpm or more
  • Pitch attitude excursions of five degrees or more
  • Glideslope deviation of one dot or more
  • Heading variations of 10 degrees or more and,
  • Unusual autothrottle activity or throttle lever position.

Wind Shear on Takeoff and Initial Climb

Horizontal and/or vertical Wind Shear on take off result in sudden loss of airspeed and/or reduction in climb rate, with potentially disastrous consequences. It is vital that such conditions should be quickly recognised if they are encountered, and that pilot response should be immediate and correct.

Before Departure

Flight Safety Foundation (FSF) Approach-and-landing Accident Reduction (ALAR) Briefing Note 5.4 recommends that whenever wind shear conditions are forecast or reported for take off, pilots "should include in their departure briefing the following wind shear awareness items:

  • Assessment of the conditions for a safe takeoff based on:
    • Most recent weather reports and forecasts;
    • Visual observations; and,
    • Crew experience with the airport environment and the prevailing weather conditions; and,
  • Consideration to delaying the takeoff until conditions improve."

"If wind shear conditions are expected," the Briefing Note continues, "the crew should:

  • Select the most favorable runway, considering the location of the likely wind shear/downburst condition;
  • Select the minimum flaps configuration compatible with takeoff requirements, to maximize climb-gradient capability;
  • Use the weather radar (or the predictive wind shear system, if available) before beginning the takeoff to ensure that the flight path is clear of hazards;
  • Select maximum takeoff thrust;
  • After selecting the takeoff/go-around (TOGA) mode, select the flight-path-vector display for the monitoring pilot (PM/PNF), as available, to obtain a visual reference of the climb flight path angle; and,
  • Closely monitor the airspeed and airspeed trend during the takeoff roll to detect any evidence of impending wind shear."

Wind Shear Recovery

The Briefing Note advises that "If wind shear is encountered during the takeoff roll or during initial climb, the following actions should be taken without delay:

  • Before V1:
    • The takeoff should be rejected if unacceptable airspeed variations occur (not exceeding the target V1) and if there is sufficient runway remaining to stop the airplane;
  • After V1:
    • Disconnect the autothrottles (A/THR), if available, and maintain or set the throttle levers to maximum takeoff thrust;
    • Rotate normally at Vr; and,
    • Follow the FD pitch command if the FD provides wind shear recovery guidance, or set the required pitch attitude (as recommended in the aircraft operating manual (AOM)/quick reference handbook (QRH));
  • During initial climb:
    • Disconnect the A/THR, if available, and maintain or set the throttle levers to maximum takeoff thrust;
    • If the autopilot (AP) is engaged and if the FD provides wind shear recovery guidance, keep the AP engaged; or,
      • Follow the FD pitch command, if the FD provides wind shear recovery guidance; or,
      • Set the required pitch attitude (as recommended in the AOM/QRH);
    • Level the wings to maximize the climb gradient, unless a turn is required for obstacle clearance;
    • Closely monitor the airspeed, airspeed trend and flight-path angle (as available);
    • Allow airspeed to decrease to stick shaker onset (intermittent stick shaker activation) while monitoring the airspeed trend;
    • Do not change the flaps or landing-gear configurations until out of the wind shear condition; and,
    • When out of the wind shear condition, increase airspeed when a positive climb is confirmed, retract the landing gear, flaps and slats, then establish a normal climb profile."

Wind Shear on the Approach and Landing

Horizontal and/or vertical wind shear during the approach can result in sudden loss of airspeed and apparent loss of power, with potentially disastrous consequences. A sudden change of wind component or drift prior to landing can make the approach unstable at a point where go-around is not possible or would be extremely hazardous. It is vital that such conditions should be quickly recognised if they are encountered, and that pilot response should be immediate and correct.

Descent Briefing

Flight Safety Foundation (FSF) Approach-and-landing Accident Reduction (ALAR) Briefing Note 5.4 recommends that whenever wind shear conditions are forecast or reported for approach and landing, the approach briefing should include the following:

  • "Based on the automatic terminal information service (Automatic Terminal Information Service (ATIS)) broadcast, review and discuss the following items:
    • Runway in use (type of approach)
    • Expected arrival route (standard terminal arrival (STAR) or radar vectors)
    • Prevailing weather and,
    • Reports of potential low-level wind shear (LLWAS warnings, Terminal Doppler Weather Radar data) and,
  • "Discuss the intended use of automation for vertical navigation and lateral navigation as a function of the suspected or forecasted wind shear conditions."

The briefing note contains some valuable recommendations for preparation and flight procedures. The section concerning Recovery during Approach and Landing is reproduced below.

Recovery During Approach and Landing

"If wind shear is encountered during the approach or landing, the following recovery actions should be taken without delay:

  • Select the takeoff/go-around (Take-off / Go-around (TO/GA) Mode) mode and set and maintain maximum go-around thrust
  • Follow the Flight Director pitch command (if the FD provides wind shear recovery guidance) or set the pitch-attitude target recommended in the AOM/QRH
  • If the AP is engaged and if the FD provides wind shear recovery guidance, keep the AP engaged; otherwise, disconnect the AP and set and maintain the recommended pitch attitude
  • Do not change the flap configuration or landing-gear configuration until out of the wind shear
  • Level the wings to maximize climb gradient, unless a turn is required for obstacle clearance
  • Allow airspeed to decrease to stick-shaker onset (intermittent stick-shaker activation) while monitoring airspeed trend
  • Closely monitor airspeed, airspeed trend and flight path angle (if flight-path vector is available and displayed for the PNF) and,
  • When out of the wind shear, retract the landing gear, flaps and slats, then increase the airspeed when a positive climb is confirmed and establish a normal climb profile.

Reporting Procedure

If significant wind shear is encountered during the takeoff and initial climb, or on approach and landing, it should be reported to air traffic control immediately. If the effects on aircraft control are exceptional and/or beyond the effects typically encountered, then an appropriate air safety report should be raised after flight completion.

Accident and Incident Reports

Events on the SKYbrary which involve turbulence and wind shear, include:

On 9 February 2020, a Boeing 737-800 rejected its takeoff from East Midlands from a speed above V1 after encountering windshear in limiting weather conditions and was brought to a stop with 600 metres of runway remaining. The Investigation found that the Captain had assigned the takeoff to his First Officer but had taken control after deciding that a rejected takeoff was appropriate even though unequivocal QRH guidance that high speed rejected takeoffs should not be made due to windshear existed. Boeing analysis found that successful outcomes during takeoff windshear events have historically been more likely when takeoff is continued.

On 15 August 2015, an Airbus A321 on approach to Charlotte commenced a go around but following a temporary loss of control as it did so then struck approach and runway lighting and the undershoot area sustaining a tail strike before climbing away. The Investigation noted that the 2.1g impact caused substantial structural damage to the aircraft and attributed the loss of control to a small microburst and the crew’s failure to follow appropriate and recommended risk mitigations despite clear evidence of risk given by the aircraft when it went around and available visually.

On 19 August 2017, an Airbus A340-300 encountered significant unforecast windshear on rotation for a maximum weight rated-thrust night takeoff from Bogotá and was unable to begin its climb for a further 800 metres during which angle of attack flight envelope protection was briefly activated. The Investigation noted the absence of a windshear detection system and any data on the prevalence of windshear at the airport as well as the failure of ATC to relay in English reports of conditions from departing aircraft received in Spanish. The aircraft operator subsequently elected to restrict maximum permitted takeoff weights from the airport.

On 4 August 2018, a Junkers Ju-52 making a low level sightseeing flight through the Swiss Alps crashed killing all 20 occupants after control was lost when it stalled after encountering unexceptional windshear. The Investigation found that the pilots had created the conditions which led to the stall and then been unable to recover from it and concluded that the accident was a direct consequence of their risky behaviour. It found that such behaviour was common at the operator, that the operator was being managed without any regard to operational risk and that safety regulatory oversight had been systemically deficient.

On 14 September 2010, the crew of a Sichuan Airlines Airbus A319 continued an ILS approach into Wuxi despite awareness of adverse convective weather conditions at the airport. Their inattention to automation management then led to a low energy warning and the inappropriate response to this led to the activation of flight envelope protection quickly followed by a stall warning. Inappropriate response to this was followed by loss of control and a full stall and high rate of descent from which recovery was finally achieved less than 900 feet agl.

On 29 October 2006, an ADC Airlines Boeing 737-200 encountered wind shear almost immediately taking off from Abuja into adverse weather associated with a very rapidly developing convective storm. Unseen from the apron or ATC TWR it stalled, crashed and burned after just over one minute airborne killing 96 of the 105 occupants. The Investigation concluded that loss of control during the wind shear encounter was not inevitable but was a consequence of inappropriate crew response. Concerns about the quality of crew training and competency validation were also raised.

On 23 December 2011, an Austrian Airlines Airbus A321 sustained a tail strike at Manchester as the main landing gear contacted the runway during a night go around initiated at a very low height after handling difficulties in the prevailing wind shear. The remainder of the go around and subsequent approach in similar conditions was uneventful and the earlier tail strike was considered to have been the inevitable consequence of initiating a go around so close to the ground after first reducing thrust to idle. Damage to the aircraft rendered it unfit for further flight until repaired but was relatively minor.

On 1 September 2018, a Boeing 737-800, making its second night approach to Sochi beneath a large convective storm with low level windshear reported, floated almost halfway along the wet runway before overrunning it by approximately 400 metres and breaching the perimeter fence before stopping. A small fire did not prevent all occupants from safely evacuating. The Investigation attributed the accident to crew disregard of a number of windshear warnings and a subsequent encounter with horizontal windshear resulting in a late touchdown and noted that the first approach had meant that the crew had been poorly prepared for the second.

On 27 May 2018, four losses of separation on final approach during use of dependent parallel landing runways occurred within 30 minutes at Madrid following a non-scheduled weather-induced runway configuration change. This continuing situation was then resolved by reverting to a single landing runway. The Investigation attributed these events to “the complex operational situation” which had prevailed following a delayed decision to change runway configuration after seven consecutive go-arounds in 10 minutes using the previous standard runway configuration. The absence of sufficient present weather information for the wider Madrid area to adequately inform ATC tactical strategy was assessed as contributory.

On 20 February 2018, a Boeing MD-83 attempting a night landing at Port Harcourt during a thunderstorm and heavy rain touched down well beyond the touchdown zone and departed the side of the runway near its end before continuing 300 metres beyond it. The Investigation found that a soft touchdown had occurred with 80% of the runway behind the aircraft and a communications failure on short final meant a wind velocity change just before landing leading to a tailwind component of almost 20 knots was unknown to the crew who had not recognised the need for a go around.

Airports where Low Level Turbulence applies

Related Articles

Further Reading

ICAO

Flight Safety Foundation

The Flight Safety Foundation ALAR Toolkit provides useful training information and guides to best practice. Copies of the FSF ALAR Toolkit may be ordered from the Flight Safety Foundation ALAR website UK CAA

FAA

  • "Lessons Learned from Transport Airplane Accidents": Windshear

Others

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