Windshear

• Windshear is a sudden change of wind velocity/direction.
• The following types of windshear exist:
• Vertical windshear (vertical variations of the horizontal wind component, resulting in turbulence and affecting aircraft airspeed when climbing or descending through the shear layer); and,
• Horizontal windshear (horizontal variations of the wind component (e.g., decreasing head wind or increasing tail wind, or a shift from a head wind to a tail wind), affecting the aircraft in level flight, climb or descent).
• Windshear is associated usually with the following weather conditions:
• Jet streams;
• Mountain waves;
• Frontal surfaces;
• Thunderstorms and convective clouds; and,
• Microbursts.
• Microbursts present two distinct threats to aviation safety:
• A downburst that results in strong downdrafts (reaching 40 knots vertical velocity); and,
• An outburst that results in strong horizontal windshear and wind-component reversal (with horizontal winds reaching 100 knots).

Source: Approach and Landing Accident Reduction Tool Kit - Windshear

Doppler radar wind measurements indicate that the wind speed change a pilot might expect when flying through the average microburst at its point of peak intensity is about 45 knots. However, microburst wind speed differences of almost 100 knots have been measured.

Source: Advisory Circular 00-54, Appendix 1, ¶2.2

Pilot reports (PIREPS) of windshear causing airspeed fluctuations in excess of 20 knots or vertical-speed changes in excess of 500 feet per minute (fpm) below 1,000 feet above airport elevation should be cause for caution.

Source: Approach and Landing Accident Reduction Tool Kit - Windshear

When describing conditions, use of the terms “negative” or “positive” wind shear should be avoided. PIREPs of “negative wind shear on final,” intended to describe loss of airspeed and lift, have been interpreted to mean that no wind shear was encountered. The recommended method for wind shear reporting is to state the loss or gain of airspeed and the altitudes at which it was encountered.

Source: Aeronautical Information Manual, ¶7-1-24.b.

The low-level windshear alert system (LLWAS) is used by controllers to warn pilots of existing or impending windshear conditions:

• LLWAS consists of a central wind sensor (sensing wind velocity and direction) and peripheral wind sensors located approximately two nautical miles (nm) from the center. Central wind sensor data are averaged over a rolling two-minute period and compared every 10 seconds with the data from the peripheral wind sensors.
• An alert is generated whenever a difference in excess of 15 knots is detected. The LLWAS may not detect down bursts with a diameter of two nm or less.

Source: Approach and Landing Accident Reduction Tool Kit - Windshear

• The LLWAS provides wind data and software processes to detect the presence of hazardous wind shear and microbursts in the vicinity of an airport. Wind sensors, mounted on poles sometimes as high as 150 feet, are (ideally) located 2,000,􏰀 3,500 feet, but not more than 5,000 feet, from the centerline of the runway.
• LLWAS was fielded in 1988 at 110 airports across the nation. Many of these systems have been replaced by new TDWR and WSP technology. Eventually all LLWAS systems will be phased out; however, 39 airports will be upgraded to the LLWAS-􏰀NE (Network Expansion) system, which employs the very latest software and sensor technology. The new LLWAS-􏰀NE systems will not only provide the controller with wind shear warnings and alerts, including wind shear/microburst detection at the airport wind sensor location, but will also provide the location of the hazards relative to the airport runway(s). It will also have the flexibility and capability to grow with the airport as new runways are built. As many as 32 sensors, strategically located around the airport and in relationship to its runway configuration, can be accommodated by the LLWAS-􏰀NE network.

Source: Aeronautical Information Manual, ¶7-1-26.f.2.

Terminal Doppler weather radar (TDWR) detects approaching windshear areas and, thus, provides pilots with an advance warning of windshear hazard;

Source: Approach and Landing Accident Reduction Tool Kit - Windshear

• TDWRs are being deployed at 45 locations across the U.S. Optimum locations for TDWRs are 8 to 12 miles off of the airport proper, and designed to look at the airspace around and over the airport to detect microbursts, gust fronts, wind shifts and precipitation intensities. TDWR products advise the controller of wind shear and microburst events impacting all runways and the areas 1/2 mile on either side of the extended centerline of the runways out to 3 miles on final approach and 2 miles out on departure.
• It is very important to understand what TDWR does NOT DO:
1. It DOES NOT warn of wind shear outside of the alert boxes (on the arrival and departure ends of the runways);
2. It DOES NOT detect windshear that is NOT a microburst or a gust front;
3. It DOES NOT detect gusty or cross wind conditions; and
4. It DOES NOT detect turbulence.
• However, research and development is continuing on these systems. Future improvements may include such areas as storm motion (movement), improved gust front detection, storm growth and decay, microburst prediction, and turbulence detection.
• TDWR also provides a geographical situation display (GSD) for supervisors and traffic management specialists for planning purposes. The GSD displays (in color) 6 levels of weather (precipitation), gust fronts and predicted storm movement(s). This data is used by the tower supervisor(s), traffic management specialists and controllers to plan for runway changes and arrival/departure route changes in order to both reduce aircraft delays and increase airport capacity.

Source: Aeronautical Information Manual, ¶7-1-26.f.3.

The LLWSAS has several limitations: winds above the sensors are not detected; wind shears beyond the peripheral sensors are not detected; updrafts and downdrafts are not detected; and if a shear boundary happens to pass a particular peripheral sensor and centerfield sensor simultaneously, an alarm will not occur. In addition, the dimensions of some meteorological phenomena — downbursts or microbursts — may be smaller than the spacing between the sensors and thus not be detected.

Source: NTSB AAR 83-02, ¶1.7

Onboard wind-component and groundspeed monitoring: On approach, a comparison of the head-wind component or tail-wind component aloft (as available) and the surface head-wind component or tail-wind component indicates the likely degree of vertical windshear;

Source: Approach and Landing Accident Reduction Tool Kit - Windshear

It is vital to recognize that some microbursts cannot be successfully escaped with any known techniques! Not that even windshears which were within the performance capability of the airplane have caused accidents.

Source: Advisory Circular 00-54, Appendix 1, ¶2.2

• The FMGS and managed speed is used to define the VAPP TARGET. It gives efficient speed guidance in approach with windy conditions, since it represents:
• VAPP TARGET = GS mini + actual headwind
• GS mini = VAPP – Tower wind
• Actual headwind is measured by ADIRS, and the tower wind is entered on the MCDU.

Source: Airbus Aircraft Performance, ¶3.1.2

• In addition to flight director (FD) windshear recovery guidance, some aircraft provide a "windshear" warning.
• The windshear warning and FD recovery guidance are referred to as a reactive windshear system, which does not incorporate any forward-looking (anticipation) capability.
• To complement the reactive windshear system and provide an early warning of windshear activity, some weather radars detect windshear areas ahead of the aircraft (typically providing a one-minute advance warning) and generate a windshear warning (red "windshear AHEAD"), caution (amber "windshear AHEAD") or advisory alert messages.
• This equipment is referred to as a predictive windshear system.

Source: Approach and Landing Accident Reduction Tool Kit - Windshear

• The windshear warning is based on the assessment of current aircraft performance (flight parameters and accelerations). The windshear warning is generated whenever the energy level of the aircraft falls below a predetermined threshold.
• The windshear warning system associated to the Speed Reference System (SRS) mode of the flight guidance constitute the Reactive Windshear Systems (RWS), since both components react instantaneously to the current variations of aircraft parameters.
• To complement the reactive windshear system and provide an early warning of potential windshear activity, some weather radars feature the capability to detect windshear areas ahead of the aircraft.
• This equipment is referred to as a Predictive Windshear System (PWS).
• PWS provides typically a one-minute advance warning.

Source: Airbus Flight Operations Briefing Notes, pg. 7

• Mode 1 provides alerts and warnings for excessive descent rates with respect to altitude above ground level (AGL) and is active for all phases of flight. This mode has an inner and outer alert / warning boundary. Penetration of the outer boundary causes an aural “SINK RATE” callout to be annunciated. Additional “SINK RATE” callouts will occur if penetration increases. Penetration of the inner boundary causes a red PULL UP icon to be displayed on the PFD and annunciates an aural “PULL UP, PULL UP” callout, repeated continuously until the inner boundary is exited.
• Mode 3 provides warnings for significant altitude loss after takeoff or low altitude go-around (less than 245 feet AGL), with landing gear or flaps not in the landing configuration. The amount of altitude loss permitted before an alert is given is a function of the height of the airplane above the terrain. Mode 3 is available until the EGPWS function determines that the airplane has climbed past the takeoff phase of flight. If there is a significant altitude loss after takeoff or during a low altitude go-around, EGPWS annunciates an aural “DON’T SINK” callout. The callout is only annunciated twice unless excessive altitude loss continues.
• Mode 5 provides two levels of alerting if the airplane flight path descends below the glideslope. The first alert occurs whenever the airplane is more than 1.3 dots below the centerline and is called a “soft” alert because the volume level is reduced. A second alert occurs below 300 feet radio altitude with greater than 2 dots deviation and is louder or “hard”. The glideslope upper altitude limit is a function of descent rate and varies from 500 feet AGL for level flight and climbs up to 1000 feet AGL for descent rates greater than 500 feet per minute (FPM). This allows glideslope capture from below the beam, between 500 and 1000 feet AGL, without unwanted alerts. Alerts for Mode 5 activate the ″BELOW G/S″ annunciator on both glareshield MASTER WARN PANELS and annunciate an aural ″GLIDESLOPE″ callout. Only two“ GLIDESLOPE” callouts are annunciated while in the Mode 5 outer“ soft” envelope. If the condition worsens, two more “GLIDESLOPE” callouts annunciate at a faster rate. This pattern continues until the inner“ louder” area is penetrated (2 dots below or greater deviation, below 300 feet AGL) at which time the “GLIDESLOPE” callouts become louder and continuous.

Source: G450 Aircraft Operating Manual, §2A-34-50, ¶3.

Enhanced Ground Proximity Warning System (EGPWS) / Description of Operating Modes / Detection of Windshear

• Mode 7 is designed to provide alerts and warnings if the aircraft encounters severe windshear. It is active between 10 and 1500 feet AGL and during takeoff, final approach and go-around.
• Windshear caution alerts are annunciated if the windshear consists of an increasing headwind (or decreasing tailwind) and/or a severe updraft, which may precede an encounter with a microburst. In the event of a windshear caution, an amber WINDSHEAR icon is displayed on the PFD and an aural "WINDSHEAR, WINDSHEAR, WINDSHEAR" callout is annunciated. The alert remains active for as long as the aircraft remains exposed to an increasing headwind and/or updraft condition in excess of the alert threshold.
• Windshear warnings are annunciated if the windshear consists of a decreasing headwind (or increasing tailwind) and/or a severe downdraft. In the event of a windshear warning, a red WINDSHEAR icon is displayed on the PFD and an aural "WINDSHEAR, WINDSHEAR, WINDSHEAR" callout is annunciated. The aural callout will not repeat unless another separate severe windshear event is encountered. The WINDSHEAR icon remains displayed for as long as the aircraft remains exposed to a decreasing headwind and/or downdraft condition in excess of the alert threshold. The alert threshold is adjusted as a function of available climb performance, flight path angle, airspeeds significantly different from normal approach speeds, and unusual fluctuations in static air temperature typically associated with the leading edge of microbursts.

Source: G450 Aircraft Operating Manual, §2A-34-50, ¶3.G.

• Timely recognition of windshear is vital for successful implementation of a windshear recovery procedure.
• Some flight guidance systems can detect a windshear condition during approach, and during go-around, based on analysis of aircraft flight parameters.
• The following are indications of a suspected windshear condition:
• Indicated airspeed variations in excess of 15 knots;
• 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.

Source: Approach and Landing Accident Reduction Tool Kit - Windshear

If windshear 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 windshear 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 windshear recovery guidance, keep the AP engaged; or,
  Follow the FD pitch command, if the FD provides windshear 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 windshear condition; and,
• When out of the windshear condition, increase airspeed when a positive climb is confirmed, retract the landing gear, flaps and slats, then establish a normal climb profile.
• During approach and landing
• Select the takeoff/go-around (TOGA) mode and set and maintain maximum go-around thrust;
• Follow the FD pitch command (if the FD provides windshear recovery guidance) or set the pitch-attitude target recommended in the AOM/QRH;
• If the AP is engaged and if the FD provides windshear 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 windshear;
• 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 windshear, retract the landing gear, flaps and slats, then increase the airspeed when a positive climb is confirmed and establish a normal climb profile.

Source: Approach and Landing Accident Reduction Tool Kit - Windshear

Following precautionary actions are recommended if windshear is suspected:

• Takeoff:
• Use maximum takeoff thrust
• Use the longest suitable runway
• Approach and Landing:
• Use flaps 30 for landing
• Establish a stabilized approach no lower than 1000 feet.
• Use most suitable runway to avoids the areas of suspected windshear.
• Use ILS G/S, VNAV path or VASI/PAPI indications to detect flight patch
• Add an appropriate airspeed correction (Max. 20 knots)

Source: Boeing 737-700/800 Flight Crew Operation Manual, pg. 86

• During Takeoff AT OR AFTER V_R:
1. Thrust levers..,Advance PF/PNF — Push thrust levers to maximum forward position.
2. Airplane . . . Rotate PF — Rotation should be promptly performed up to a maximum pitch attitude of 17.5 degrees. Anticipate a higher than normal stick effort necessary to lift the airplane off within the remaining runway.
• DURING INITIAL CLIMB:
1. Thrust levers and Take-Off/Go-Around (TOGA) switch . . . Advance/Press PF/PNF
2. Push thrust levers full forward (autothrottle OFF) or press either TOGA switch (autothrottle ON).
• FOR ALL CONDITIONS THEREAFTER, REGARDLESS OF WHETHER OR NOT CONDITIONS HAVE IMPROVED:
1. Flight instruments . . . Monitor PNF — The pilot-not-flying shall closely and continuously monitor the vertical flight path instruments and assure proper thrust setting. callout any deviation from the normal values of airspeed, airspeed trend, rate of climb, pitch, thrust and altitude. Make directive commentaries as appropriate, for example: "Altitude decreasing − pull the nose up".
2. Pitch attitude . . . Maintain PF — Accept a lower than normal indicated airspeed, up to intermittent stick shaker.
3. Configuration . . . Maintain— Changes in configuration are not recommended during a windshear encounter. Do NOT change flap, gear or trim position until positively out of the shear condition (not below 1,500 feet AGL).
• During the approach in suspected windshear conditions, the following procedures are recommended:
1. Flight instruments . . . Monitor PNF — The pilot-not-flying will closely and continuously monitor the vertical flight path instruments and callout any deviations in the normal indications of approach speed, airspeed trend, rate of descent, pitch, glide slope and thrust.
2. Thrust levers / Stabilizer trim . . . Adjust PF — Avoid large power adjustments or trim changes to correct large airspeed changes. Anticipate that a large airspeed increase is often followed by an equally large airspeed decrease.
• IF WINDSHEAR IS ENCOUNTERED AND FLIGHT PATH CONTROL BECOMES MARGINAL:
1. Accomplish windshear recovery procedures as outlined in "Windshear Warning During Initial Climb" in this section. — When clear of the windshear encounter, the windshear warnings will be canceled. The flight crew can then proceed with a normal climb-out once a positive climb gradient has been established. Report the encounter to ATC as soon as possible.
2. Thrust levers and Take-Off/Go-Around (TOGA) switch . . . Advance/Press PF/PNF — Push thrust levers forward to maximum take-off thrust while simultaneously pressing either TOGA switch.
3. Normal go-around . . . Initiate

Source: Bombardier BD-700 Flight Crew Operating Manual, Vol 1, §07-05-1, ¶4

Windshear escape guidance mode is activated under the following conditions:

• Windshear warning is present; and
• Either TOGA button is pressed; or
• Either throttle is moved above 37°

Source: Bombardier BD-700 Flight Crew Operating Manual, Vol 2, §04-10-36

Windshear Recovery

• GA pushbutton . . . Pressed
• Flight Director . . . Followed
• Autothrottle . . . Disconnected
• Power levers . . . TO
• AIRBRAKE handle . . . Position 0
• Slats-flaps handle . . . SF 2 maximum

At pilot’s discretion depending on airplane height and vertical speed:

• Landing gear handle . . . Up
• Disregard amber Low Speed Cue and TOO LOW FLAPS voice warning.

Source: Dassault Falcon 2000EX EASy CODDE 2, §04-05-10

• WindShear (WS) escape mode can be selected, each time a Windshear condition is detected by the Terrain Awareness and Warning System (TAWS) by pressing the GA pushbutton on the yoke. If GA is already activated at the moment of the windshear condition is detected, the windshear mode is automatically activated.
• This action disconnects the AutoPilot and changes the FD / TD commands. ROL and WSHR modes are displayed in the FMA.
• FD and TD will thus provide vertical and thrust guidance to achieve the best escape path. FD command, when followed, will maintain the IAS close to the stall speed. TD command, when followed, will maintain the maximum take-off power. The escape guidance aims to minimize altitude loss while maintaining a safe margin to stall.
• Windshear mode is de-selected upon selection of any other vertical mode.

Source: Dassault Falcon 2000EX EASy CODDE 1, §02-22-10

1. Takeoff:
   When the amber WINDSHEAR annunciation illuminates on the Primary Flight Display (PFD) during takeoff, or the crew recognizes the signs of increasing performance conditions, the following procedures should be followed:
1. The alert occurs during increasing performance conditions (i.e. increasing head wind/decreasing tail wind and/or updraft). The flight crew should be alerted to the possibility of subsequent decreasing performance (i.e., significant airspeed loss and down draft conditions).
2. Power levers should be advanced to achieve maximum rated thrust and takeoff/go-around target pitch attitude should be maintained until safe climb-out is assured, disengaging autopilot/autothrottle if necessary.
2. Final Approach:
   When the amber WINDSHEAR annunciation illuminates on the PFD during final approach, or the crew recognizes the signs of increasing performance conditions, the following procedures should be followed:
1. The alert occurs during increasing performance conditions (i.e. increasing head wind/decreasing tail wind and/or updraft). The flight crew should be alerted to the possibility of subsequent significant airspeed loss and down draft conditions.
2. Wind and gust allowances should be added to the approach speed, increasing thrust if necessary. It may be necessary to disengage autopilot/autothrottle.
3. Avoid sinking below the approach glide path or letting the power levers remain at flight idle for extended periods of time.
4. Coupled with other weather factors, the alert should be considered in determining the advisability of performing a go-around.
3. At Any Time:
   When the siren and ′′WIND SHEAR--WIND SHEAR--WIND SHEAR′′ WARNING occurs, and the red WINDSHEAR annunciation illuminates on the PFD, or an obvious wind shear condition is encountered, the following procedures should be followed:
1. Disconnect the autopilot and apply power to the mechanical forward limit.
2. Rotate at 3 to 4 degrees per seconds to increase pitch attitude to the highest possible value. (A pitch attitude of 25 degrees has been demonstrated on the G450 at maximum landing weight with flaps DOWN [39°]).
3. When stick shaker is encountered, or as VREF is approached, reduce pitch rate/angle of attack to intercept VREF -20 KCAS.
4. DO NOT retract flaps or landing gear until safe climb-out is assured.

Source: G450 Aircraft Operating Manual, §07-02-50, ¶2

1960's

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Source: Flight Safety Foundation - Aviation Safety Network Database, Windshear and Downdrafts