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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:
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:
The main effects of wind shear are:
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.
Effective defence against wind shear comprises the following components:
A 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.
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:
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.
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:
"If wind shear conditions are expected," the Briefing Note continues, "the crew should:
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:
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.
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:
The briefing note contains some valuable recommendations for preparation and flight procedures. The section concerning Recovery during Approach and Landing is reproduced below.
"If wind shear is encountered during the approach or landing, the following recovery actions should be taken without delay:
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.
Events on the SKYbrary which involve turbulence and wind shear, include:
On 6 December 2018, a Boeing 737-700 overran the 1,770 metre-long landing runway at destination by 45 metres after entering the EMAS. Normal visibility prevailed but heavy rain was falling and a 10 knot tailwind component existed. The event was attributed to the pilots’ continuation bias in the face of deteriorating conditions and a late touchdown on the relatively short runway. A lack of guidance from the operator on the need for pilots to re-assess the validity of landing data routinely obtained at the top of descent was identified.
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.
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