Performance shear


Wind shear is meteorologically defined as a change in wind speed and/or direction over a short distance. It can occur at any altitude and can be manifested in both the vertical and horizontal planes. Wind shear has the potential to dramatically impact aircraft performance, affecting both speed and trajectory, and can be particularly hazardous during the departure and approach phases of flight.

For optimum performance during departure and on approach, aircraft are operated at reduced speeds that result in narrow stall margins. The close proximity to the ground leaves little room to sacrifice altitude to gain speed. A sudden change in wind velocity and/or direction can lead to an immediate corresponding change (positive or negative) in both the indicated airspeed (IAS) and the amount of lift generated by the aerofoils by affecting the air flow over the wings. If these changes are negative, ground contact becomes a significant possibility. Successful recovery from a wind shear encounter will depend upon the type and severity of the shear, the altitude at which the encounter occurs, pilot reaction time and technique, and the aircraft response capability.

Wind Effect

In most circumstances of stabilised flight, an aircraft moves through the surrounding airmass at a (pilot selected) indicated airspeed (IAS) that is within its defined flight envelope. Correcting that IAS for instrument error, static system pressure error and altitude, renders true airspeed (TAS) which is equivalent to the speed of the aircraft across the earth's surface (ground speed) if there is no wind. Under normal in-flight circumstances, movement of the air, or wind, will have little effect on the aircraft other than to potentially subject it to turbulence and to increase or decrease its ground speed. This is not the case in a wind shear event.

During wind shear, the speed and/or direction of the wind changes over a very short distance; that is, very quickly along the aircraft path of flight. The time factor and the effect of a wind shear event can be illustrated by example. Suppose an aircraft is in level flight at 180 knots TAS flying into a 30 knot headwind. Its ground speed will be 150 knots. Over the next hour, the wind gradually changes to become a 30 knot tailwind. TAS has remained constant throughout so the only effect on the aircraft will be a ground speed change from 150 to 210 knots. The aircraft has effectively accelerated across the ground without a power adjustment by the pilot. However, if the same change in wind was to occur over a very short distance such as that flown in five to ten seconds, aircraft inertia would not allow an immediate change in the aircraft's ground speed (Newton's first law). The air would effectively accelerate away from the aircraft causing both a rapid reduction of the IAS to a value well below 180 knots and a corresponding loss of lift due to the decreased speed of the airflow across the aerofoils.

Performance Shear

From an aircraft performance perspective, there are two types of shear, those that enhance performance and those that diminish it. The term “performance shear” is used to describe the wind shear conditions as they relate to a specific aircraft’s performance ability. During an increasing performance shear, both IAS and lift production will increase whilst encountering a decreasing performance shear will result in a loss of both lift and airspeed. The potentially most dangerous scenario occurs when there is an increasing performance shear followed closely by a decreasing performance shear as could be the case in a Microburst encounter, when the wind direction can change 180o within seconds.

Increasing Performance Shear

Increasing performance shears are interchangeably referred to as Positive Shears, Performance Increasing Shears, Decreasing Tailwind Shears, and Increasing Headwind Shears. Increasing performance shears can be manifested as any one of a rapidly increasing headwind component, a rapidly decreasing tailwind component, or a tailwind component which rapidly changes to a headwind component as the aircraft progresses along its flightpath. An increasing performance shear causes an immediate increase in the velocity of the airflow across the aircraft surfaces resulting both in a corresponding increase in IAS, and an increase in the amount of lift generated by the aerofoils. This, in turn, causes the aircraft to climb above the intended glidepath. If the shear is encountered on short final and the approach is not abandoned, the landing is likely to be long and fast and could lead to a Runway Excursion.

Decreasing Performance Shear

Decreasing performance shears are interchangeably referred to as Negative Shears, Performance Decreasing Shears, Decreasing Headwind Shears, and Increasing Tailwind Shears. Decreasing performance shears are characterised by any of a rapidly increasing tailwind component, a rapidly decreasing headwind component, or a situation where a headwind rapidly becomes a tailwind. A decreasing performance shear is manifested by both an immediate loss of IAS and an immediate loss of lift which result in descent below the intended glidepath. A decreasing performance shear encounter at very low level can result in landing short of the runway during approach, or ground contact immediately after takeoff.

Pilot Reaction

Pilot reaction time and technique are critical factors in a wind shear recovery. A mishandled recovery can lead to an undesired aircraft state (UAS) and a potential accident. In many circumstances, trying to "salvage" an approach after a wind shear encounter can diminish the likelihood of a successful recovery. For example, in an decreasing performance shear situation situation, the loss of speed and descent below the glidepath would necessitate that the pilot increase both thrust and pitch to regain the desired glidepath. However, should the pilot overcorrect or maintain the recovery actions for too long, the outcome could potentially culminate in the aircraft being both fast and above the glidepath - a situation that could result in a long, fast landing and potential runway excursion. Conversely, overly aggressive reductions in thrust and pitch in an attempt to reduce speed and simultaneously regain the desired flightpath during an increasing performance shear have the potential to leave the aircraft low and slow.

In a microburst encounter, normal pilot reaction can negate any potential for recovery. As the aircraft enters the microburst, it first encounters an increasing performance shear in the form of a strong headwind. Normal pilot reaction would be to reduce power and pitch to recover to the original flightpath. As those actions are occurring, the aircraft then loses the headwind and encounters a strong downdraft which pushes the aircraft down whilst the thrust is at a reduced setting. The pilot then increases thrust but the situation is further exacerbated as the wind rapidly becomes a tailwind, a decreasing performance shear, as the aircraft exits the microburst. This shear then causes a further loss of speed and lift, leaving the aircraft dangerously low and slow and, potentially, unrecoverable. For these reasons, most NAA's, manufacturers and operators have specific wind shear recovery profiles which should be flown if significant shear is encountered. Refer to the article Low Level Wind Shear for information on wind shear recognition, avoidance, best practices, and recovery.

Accidents and Incidents

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 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 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 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.

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