Windshear occurs in all directions, but for convenience it is measured along the vertical and horizontal axis, thus becoming vertical and horizontal windshear:
- Vertical windshear: Variations of the horizontal wind component along the vertical axis, resulting in turbulence that may affect the aircraft airspeed when climbing or descending through the windshear layer. Variations of the wind componenet of 20 kt per 1000ft to 30 kt per 1000ft are typical values, but a vertical windshear may reach up to 10 kt for 100ft
- Horizontal windshear: Variation of the wind component along the horizontal axis (decreasing headwind or increasing tailwind, or a shift from a headwind to a tailwind.) Variations of wind component may reach up to 100 ft per nautical mile.
Windshear conditions usually are associated with the following weather situations: Jet streams, mountain waves, frontal surfaces, thunderstorms and convective clouds and microbursts.
There are two distinct threats to aviation safety which are the downburst part and the outburst part. The downburst results in strong downdrafts reaching up to 6000ft/min of vertical velocity, while the outburst results in large horizontal windshear and wind component shifts from headwind to tailwind up to 45 knots.
Influence of Windshear on Aircraft
A headwind gust instantaneously increases the aircraft speed and thus tends to make the aircraft fly above the intended path and/ora accelerate. The downdraft affect both the aircraft angle of attack (AOA), the increases and the aircraft path since it makes the aircraft sink. The tailwind gust instantaneously decreases the aircraft speed and thus tends to make the aircraft fly below the intended path and or decelerate.
The crews do not always perceive an increase of the headwind as a risk, but such a headwind gust de-stabilizes the approach of the aircraft which will tend to fly above and/or accelerate if the pilot does not react. If the headwind shear occurs at takeoff, the resulting aircraft performance will increase. Once out of the shear, the indicated airspeed decreases thus leading to an AOA increase which might trigger stick shake activation.
Vertical downdrafts are usually preceded by an increase of the headwind component. If the pilot does not fully appreciate the situation, he/she will react to the headwind gust effects to regain the intended path by reducing the power and pushing on the stick. At that point, a vertical downdraft will increase the aircraft sink rate, which will bring the aircraft below the intended path.
In case of a sudden increase of the tailwind, the aircraft airspeed decreases instantaneously. The lift decreases and the aircraft tends to fly below the intended approach path. If the pilot pulls on the stick to recapture the path without adding sufficient thrust, the AOA will increase significantly and the aircraft will sink down. If sufficient thrust is set to regain the intended path, but the pilot's reaction is then slow to reduce the thrust once back on path, the aircraft will fly above the path and/or accelerate.
Windshear Awareness and Avoidance
Airport weather reports and forecast
LLWAS (low level windshear alert system) consists of a central wind sensor and peripheral wind sensors. It enables controlles to warn pilots of existing or impending windshear conditions. An alert is generated whenever a difference in excess of 15kts is detected. LLWAS may not detect downburst with a diameter of 2nm or less.
PIREPS of windshear in excess of 20 knots or downdraft/updraft of 500ft/min below 1000ft above ground level should draw the attention of the crews.
Blowing dust, ring of dust, dust devils or any other evidence of strong local air outflow near the surface often are indication of potential or existing windshear.
On-board Wind Component and GS Monitoring
On approach, a comparison of the headwind or tailwind component and the surface headwind or tailwind component indicates the potential and likely degree of vertical windshear. This monitoring increases situational awareness.
The weather radar indicates the hazardous regions, those regions are at a high risk of windshear.
Timely recognition of windshear condition is vital for the successful implementation of the windshear recover/escape procedure. The following deviations should be considered as indications of a possible windshear condition:
- Indicated airspeed variations in excess of 15 kts
- Ground speed variations
- Analog wind indication variation; Direction and speed
- Vertical speed excursions of 500ft/min
- Pitch attitude excursions of 5 degrees
- Glide slope deviation of 1 dot
- Heading variations of 10 degrees
- Unusual power lever position
Cockpit Preparation - Departure Briefing
Flight crew should consider all available windshear-awareness items and assess the conditions for a safe take-of based on the most recent weather reports and forecast, visual observations and crew experience with the airport environment and the prevailing weather conditions. A delayed takeoff should be considered as warranted until conditions improve.
If you suspect windshear conditions, the flight crew should:
- Consider delaying the takeoff.
- Select the most favourable runway considering the location of the likely windshear downburst
- Use the weather radar before commencing the takeoff roll to ensure that the flight path is clear of hazard areas.
- Select the normal takeoff power
- Closely monitor the airspeed and speed trend during the takeoff roll to detect any evidence of windshear.
Before conducting an approach in forecast or suspected windshear conditions, the flight crew should:
- Consider delaying the approach and landing until conditions improve or divert to a suitable airport. Especially if reported via PIREPS or LLWAS
- Assess the conditions for a safe approach and landing based on the points listed above.
- select the most favourable runway considering the location of possible windshear and runway approach aids
- Use the WX Radar (or predictive windshear system as available) to ensure the flight path is clear of hazard areas
- Select less than full flaps for landing and adjust the final approach speed accordingly
- If an ILS is available, engage the autopilot for a more accurate approach tracking
- If gusty conditions are expected, increase Vref.
- Compare headwind and tailwind components aloft and the surface wind to assess the potential and likely degree of vertical windshear.
- Closely monitor the airspeed, trend and ground speed during the approach to detect any evidence of imminent windshear. Minimum ground speed and level of energy should be maintained to ensure proper thrust management.
Factors Affecting Windshear Awareness
- Aircraft Equipment: Absence of reactive or predictive systems
- Airport equipment: Absence of LWAS or Terminal Doppler Weather Radar (TDWR)
- Training: Absence of windshear awareness program or simulator training for windshear recovery
- SOPs: Inadequate briefings, inadequate monitoring of flight parameter and incorrect use of automation
- Human Factors and CRM: Absence of cross check, inadequate back-up and callouts, and fatigue
- SOPs Should emphasize the following windshear awareness items.
- Windshear awareness and avoidance
- Takeoff/departure and approach/go around briefings
- Approach hazards awareness
- Task sharing for effective cross-check and back-up, particularly for excessive parameter deviations
- Energy management during approach
- Elements of a stabilized approach
Windshear recovery and escape procedures
- Readiness and commitment to respond to a reactive or predictive windshear advisory or warning as available
2. Pilot Reports
- In all cases after an encounter, it is important you make a detailed report to ATC in increase other aircraft's crew awareness. The report should contain the words "pilot report," intensity and vertical and horizontal position
- The windshear recovery/escape procedure should be trained in a full-flight simulator using realistic windshear profiles recorded during actual encounters.