The best defense against the hazards of low altitude windshear is:
1. Taking precautions before windshear is encountered.
2. Knowledge of standard operating techniques related to windshear.
3. The ability to perform specific recovery techniques in the event of an inadvertent windshear encounter.

Avoidance is emphasized as the best defense against the hazards of low altitude windshear.

Pilots should learn as much as they can about the effects of windshear and the practices that may be employed to recognize, avoid, and cope with inadvertent windshear encounters.

Hazardous wind variations at low altitudes can result from:
1. Temperature inversions.
2. Frontal systems.
3. Thunderstorms.

Wind variations at low altitude have long been recognized as a serious hazard to airplanes during takeoff and approach. These wind variations can result from a large variety of meteorological conditions such as:
1. Topographical conditions.
2. Temperature inversions.
3. Frontal systems.
4. Strong surface winds.
5. Thunderstorms and rain showers.

The most violent forms of wind change occur in the vicinity of thunderstorms and rain showers.

Pilots must learn to recognize conditions producing windshear in order to avoid windshear encounters.

Airmass thunderstorms appear to be randomly distributed in unstable air and develop from localized heating at the earth's surface.

Compared to airmass thunderstorms, frontal thunderstorms are more severe.

The downdraft of a typical thunderstorm is fairly large, about 1 to 5 miles in diameter. Resultant outflows may produce large changes in wind speed.

Though wind changes near the surface occur across an area sufficiently large to lessen the effect, thunderstorms always present a potential hazard to airplanes. Regardless of whether a thunderstorm contains windshear, however, the possibility of heavy rain, hail, extreme turbulence, and tornadoes make it critical that pilots avoid thunderstorms.

The majority of documented windshear-associated accidents and incidents have occurred in the United States.


Observations suggest that approximately five percent of all thunderstorms produce a microburst.

Downdrafts associated with microbursts are typically only a few hundred to 3000 feet across. When the downdraft reaches the ground, it spreads out horizontally and may form one or more horizontal vortex rings around the downdraft. The outflow region is typically 6000 to 12,000 feet across. The horizontal vortices may extend to over 2000 feet AGL.

Microburst outflows are sometimes symmetric, sometimes not.

More than one microburst can occur in the same weather system.

After they contact the ground, microbursts typically dissipate within 5 minutes after a initially contacting the ground.

An encounter during the initial stage of microburst development may not be considered significant, but an airplane following may experience an airspeed change two to three times greater.

Microbursts typically dissipate within 10 to 20 minutes after ground contact.


The wind speed change a pilot might expect when flying through the average microburst at its point of peak intensity is about 45 knots.


Some microbursts cannot be successfully escaped with known techniques.


Even windshears which were within the performance capability of the airplane have caused accidents.


If cumulus clouds are present in the sky, the greatest potential for microburst windshear exists. Even if there are only subtle signs of convective weather, such as weak cumulus cloud forms, suspect the possibility of microbursts, particularly if the air is hot and dry.

Microbursts can be associated with both heavy rain, as in thunderstorm conditions, and much lighter precipitation associated with convective clouds. Microbursts have occurred in relatively dry conditions of light rain or virga (moisture that evaporates before reaching the earth"s surface). Example: air below a cloud base (up to approximately 15,000 feet AGL) is very dry. Virga from convective clouds falls into low humidity air & evaporates. This cooling causes the air to plunge downward. As the evaporative cooling process continues, the downdraft accelerates. Pilots are cautioned not to fly beneath convective clouds producing virga conditions. They may encounter a dry microburst.


The primary lesson learned is that the best defense against windshear is to avoid it altogether. This is especially important because shears will exist which are beyond the capability of any pilot or airplane. In most windshear accidents, several clues -- LLWAS alerts, weather reports, visual signs -- were present that would have alerted the flight crew to the presence of a windshear threat. In all instances, however, these clues were either not recognized or not acted upon. Flight crews must seek and heed signs alerting them to the need for avoidance.

In a typical windshear encounter during takeoff - after liftoff, for the first 5 seconds after liftoff, the takeoff appeared normal.

Successful recovery from an inadvertent windshear encounter after liftoff requires
maintaining or increasing pitch attitude and accepting lower than usual airspeed.

To recognize and respond to a windshear encounter, you may have only 5 to 15 seconds.

Regarding windshear encounters on the runway, they may be difficult to recognize since the only indication may be a slower than normal airspeed increase.

One way to improve a flight crew's ability to recognize and respond quickly to a windshear encounter is to: Develop effective crew coordination, particularly standard callouts, for routine operational use.

Lack of timely and appropriate response, affected by weather conditions, inadequate crew coordination, and limited recognition time, was a significant factor in delaying recovery initiation in the typical accident involving a windshear encounter on approach.

Gradual application of thrust during approach may have masked the initial decreasing airspeed trend. Poor weather conditions caused increased workload and complicated the approach. Transition from instruments to exterior visual references may have detracted from instrument scan. Inadequate crew coordination may have resulted in failure to be aware of flight path degradation.

An increasing headwind (or decreasing tailwind) shear increases indicated airspeed and thus increases performance.


A rapid or large airspeed increase on approach should be viewed as a possible indication of a forthcoming airspeed decrease and thus may be reason for discontinuing the approach. Thus, a large airspeed increase may be reason for discontinuing the approach. However, since microbursts are often asymmetric and the headwind may not always be present, headwind shears must not be relied upon to provide early indications of subsequent tailwind shears. Be prepared!


An airplane encountering a decreasing headwind shear may tend to pitch down to regain trim speed


Vertical winds exist in every microburst and increase in intensity with altitude. Such winds usually reach peak intensity at heights greater than 500 feet above the ground.

Downdrafts with speeds greater than 3000 feet per minute can exist in the center of a strong microburst.

The severity of the downdraft the airplane encounters depends on both the altitude and lateral proximity to the center of the microburst.

An airplane flying through a series of horizontal vortices generated by microbursts experiences alternating updrafts and downdrafts causing pitch changes without pilot input.


The most significant impact of rapidly changing vertical winds in a microburst is to increase pilot workload during the recovery

Large crosswind shears tend to cause the airplane to roll and/or yaw. Large crosswind shears may require large or rapid control wheel inputs. These shears may result in significantly increased workload and distraction. In addition, if an aircraft encounters a horizontal vortex, severe roll forces may require up to full control wheel input to counteract the roll and maintain aircraft control.
 

 

The use of the stick shaker as a guide to airplane performance limits presumes that: Dual stick shaker systems are installed; The angle-of-attack vane is heated, the input from the stabilizer to the stick shaker computer is functioning correctly, and wing leading edges are clean and smooth and alpha vanes are undamaged.


During callouts and instrument scan in a windshear, use of radio and/or barometric altimeters must be tempered by the characteristics of each. Since radio altitude is subject to terrain contours, the indicator may show a climb or descent due to falling or rising terrain, respectively. The barometric altimeter may also provide distorted indications due to pressure variations within the microburst.


During a windshear encounter, the vertical speed indicator may significantly lag actual airplane rate of climb/descent.


When evaluating available weather data, clues to possible windshear might be found by checking terminal forecasts for potential thunderstorms, rainshowers, gusty winds, and low level windshear.


The following weather information should be examined for any potential windshear conditions affecting the flight:

Terminal forecasts.

Hourly sequence reports.

Severe weather watch reports.

LLWAS reports.

SIGMET’s & convective SIGMET’s.

Visual clues from the cockpit.

PIREPS.

Airborne weather radar.


With regard to a low level windshear alert system installed at an airport, detected windshear magnitudes may be underestimated

The value of recognizing microbursts by visual clues from the cockpit cannot be overemphasized. Pilots must remember that microbursts occur only in the presence of convective weather indicated cumulus type clouds, thunderstorms, rain showers, and virga. (Other types of windshear can occur in the absence of convective weather.)

Pilots must become aware that visual clues are often the only means to identify the presence of severe windshear.


The use of airborne weather radar to detect convective cells should be considered a matter of routine. Weather radar provides extremely useful information for the avoidance of thunderstorms in the airport terminal area, but cannot directly detect windshear.

Pilots have become adept at avoiding thunderstorms while enroute and at altitude. However, relatively little emphasis has been placed on their use near the terminal area. Most thunderstorms with heavy rain near the airport can be detected with conventional weather radar by a careful use of tilt control to scan above the intended flight altitude (15,000 to 20,000 feet).


One significant aspect of weather radar use is attenuation. Attenuation is caused by heavy rainfall reducing the ability of the radar signal to penetrate, causing the radar to present an incomplete picture of the weather area. In the terminal area, comparison of ground returns to weather echoes is a useful technique to identify when attenuation is occurring.

Tilt the antenna down and observe ground returns around the radar echo. With very heavy intervening rain, ground returns behind the echo will not be present. This area lacking ground returns is referred to as a shadow and may indicate a larger area of precipitation than is shown on the indicator. Areas of shadowing should be avoided.


Even though significant emphasis on simulator training is recommended in pilot training curriculums, avoidance must be the first line of defense.

Simulators are valuable for teaching windshear recognition and recovery. However, pilots are cautioned not to develop the impression that real-world windshear encounters can be successfully negotiated simply because they have received simulator training. In an airplane, complicating factors (i.e. turbulence, precipitation noise, instrument errors, etc.) may make shears much more difficult than in a simulator. In addition, simulator motion systems are limited in their capability to reproduce all the dynamics of an actual windshear encounter.

When implementing takeoff precautions due to possible windshear activity in the area, maximum rated takeoff thrust should be used for takeoff. This shortens the takeoff roll and reduces overrun exposure. Full thrust also provides the best rate of climb, thus increasing altitude available for recovery if required. Lastly, full thrust takeoffs may eliminate resetting thrust in a recovery, thereby maximizing acceleration capability and reducing crew workload.


To properly calculate increased rotation speed (VR) as a precautionary technique for takeoff in possible windshear conditions Increase the actual weight VR speed by 10% (not to exceed 20 knots over actual weight VR). Add 20 knots to the actual weight VR speed. Add the value of any reported windshear airspeed loss to your actual weight VR.
Use the field length limit maximum weight VR (up to 20 knots over actual weight VR).
If increased VR is to be used, the technique for scheduling & using it is: Determine V1, VR, & V2 speeds for actual airplane gross weight & flap setting. Set airspeed bugs to these values in the normal manner. Determine field length limit maximum weight and corresponding VR for selected runway. If field length limit VR is greater than actual GW VR, use the higher VR (up to 20 kts. in excess of actual GW VR) for takeoff. Airspeed bugs shouldn’t be reset to higher speeds. Rotate to normal initial climb attitude at the increased VR & maintain this attitude. This technique produces a higher initial climb speed which slowly bleeds off to the normal initial climb speed.


When commencing an approach into suspected windshear conditions and precautions are being taken, auto throttles: Should be monitored closely for inappropriate thrust reductions.


When calculating your approach speed for suspected windshear conditions which indicate precautions should be taken, you should remember that: You should not add any extra speed. An additional 20 knots at touchdown can increase your stopping distance by as much as 25 percent. Any increased approach speed must be bled off in the flare prior to touchdown. Approach speed may be increased up to the amount of any reported windshear. Increased airspeed on approach improves climb performance capability and reduces the potential for flight at stick shaker during recovery from an inadvertent windshear encounter. If available landing field length permits, airspeed may be increased up to a maximum of 20 knots. This increased speed should be maintained to flare. Touchdown must occur within the normal touchdown zone -- do not allow the airplane to float down the runway. Warning - Increased touchdown speeds increase stopping distance. An additional 20 knots at touchdown can increase stopping distance by as much as 25% and in some cases may exceed brake energy limits.


It is important for crews to remain alert for any change in conditions, remembering that windshear can be quick to form and to dissipate. The shears that proved to be most deadly are those which caught crews by surprise.

Crews should be aware of normal vertical flight path indications so that windshear induced deviations are more readily recognized. On takeoff, this would include attitude, climb rate, and airspeed buildup. On approach, airspeed, attitude, descent rate, and throttle position provide valuable information. Awareness of these indications assures that flight path degradation is recognized as soon as possible.


Crews should be prepared to execute the recommended recovery procedure immediately if deviations from target conditions in excess of the following occur during takeoff:

+/- 15 knots indicated airspeed.

+/- 500 feet per minute vertical speed.

+/- 5 degrees pitch attitude.

Crews should be prepared to execute the recommended recovery procedure immediately if deviations from target conditions in excess of the following occur during approach:

+/- 15 knots indicated airspeed.

+/- 500 feet per minute vertical speed.

+/- 5 degrees pitch attitude.

+/- 1 dot glideslope displacement.

Unusual throttle position for a significant period of time.


The most effective technique for recovering from a windshear encounter for turbojet aircraft when airborne is: Increasing thrust to maximum rated and increasing pitch attitude until the stick shaker is encountered. Maintaining the maximum lift/drag airspeed. Applying necessary thrust and rotating initially toward 15 degrees pitch up attitude. Reducing airplane drag by raising flaps, if possible.

Extensive analysis and pilot evaluations were conducted to establish a windshear recovery technique. Although a range of recovery attitudes (including 15 degrees and the range of all-engine initial climb attitudes) provides good recovery capability for a wide variety of windshears, 15 degrees was chosen as the initial target pitch attitude for both takeoff and approach. Additional advantages of 15 degrees initial target pitch attitude are that it is easily recalled in emergency situations and it is prominently displayed on attitude director indicators.

During recovery from an airborne windshear encounter, the correct use of power is to:

Use only maximum rated thrust.

Aggressively apply thrust as necessary to ensure adequate airplane performance.

Engage the auto throttles.

Automatically select full "overboost" engine thrust.

Use of thrust in recovery: Aggressively apply necessary thrust to ensure adequate airplane performance. Disengage the auto throttle if necessary. Avoid engine overboost unless required to avoid ground contact. When airplane safety has been ensured, adjust thrust to maintain engine parameters within specified limits.


To recover from a windshear encounter during takeoff after liftoff, what should be done with flaps and landing gear?
Configuration: maintain flap and gear position until terrain clearance is assured. Although a small performance increase is available after landing gear retraction, initial performance degradation may occur when landing gear doors open for retraction. While extending flaps during a recovery after liftoff may result in a performance benefit, it is not a recommended technique because:

Accidentally retracting flaps (the usual direction of movement) has a large adverse impact on performance;

If landing gear retraction had been initiated prior to recognition of the encounter, extending flaps beyond a takeoff flap setting might result in a continuous warning horn which distracts the crew.


A flight director and/or autoflight system which is not specifically designed for operation in windshear may command a pitch attitude change to follow target airspeeds or a fixed pitch attitude regardless of flight path degradation.

This guidance may be in conflict with the proper procedures for windshear recovery. Such systems must be disregarded if recovery is required and, time permitting, switched off by the pilot-not-flying.


During a windshear encounter recovery, always respect the stick shaker. Use intermittent stick shaker as the upper pitch limit. In a severe shear, stick shaker may occur below 15 degrees pitch attitude.


Immediately following a successful windshear recovery, a pilot should: As soon as possible, report the encounter to the tower. The airplane following may not have the performance required to recover from the same windshear encounter. The windshear also may be increasing in intensity making flight through it even more dangerous. Pilots & controllers must be aware that their timely actions may prevent a pending disaster -- seconds may save lives! Pilot reports for windshear encounters should contain the following: Maximum loss or gain of airspeed; altitude at which shear was encountered; location of shear with respect to runway in use; airplane type; use the term "PIREP" to encourage a rebroadcast of the report to other aircraft.

When a windshear is encountered on the runway after V1 but before VR,:
Overboost thrust will ensure that the airplane accelerates to liftoff speed.
Rotate toward 15 degrees pitch attitude by 2000 feet of runway remaining.
Delay rotation as long as possible in an attempt to attain VR.
Avoid aft body contact during rotation.
Controlling pitch when encountering windshear during takeoff on the runway after V1:

When VR is reached, rotate at normal rate toward 15 degrees pitch attitude. In severe windshear encounters, however, VR might not be reached and the option to reject the takeoff may not exist. If this is the case, rotation must be initiated no later than 2000 feet from the end of the usable surface.
 

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