Touring Machine Company

This post summarizes FAA Advisory Circular AC 00-54 PILOT WINDSHEAR GUIDE issued 11/25/88. I changed the punctuation a bit and left out a lot of the text. Additions are indicated by brackets [ ]. Bold indicates things I’d like to remember. This document uses windshear as a single word—other documents split it into two words—wind shear. The AC was developed to aid pilots of large transport-category airplanes made by Boeing, Douglas, and Lockheed so much of the AC deals with procedures for those aircraft. The techniques for recognition and avoidance are the same for small aircraft and are extracted in this post.

2.2 WINDSHEAR WEATHER

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: topographical conditions, temperature inversions, sea breezes, frontal systems, strong surface winds, and the most violent forms of wind change—the thunderstorm and rain shower.

Throughout this document several terms are used when discussing low-altitude wind variations. These terms are defined as follows:

Windshear—Any rapid direction or velocity. change in wind

Severe Windshear—A rapid change in wind direction or velocity causing airspeed changes greater than 15 knots or vertical speed changes greater than 500 feet per minute.

Increasing Headwind Shear—Windshear in which headwind increases causing an airspeed increase.

Decreasing Headwind Shear—Windshear in which headwind decreases causing an airspeed loss.

Decreasing Tailwind Shear—Windshear in which tailwind decreases causing an airspeed increase.

Increasing Tailwind Shear—Windshear in which tailwind increases causing an airspeed loss.

The Thunderstorm

There are two basic types of thunder storms: airmass and frontal. Airmass thunderstorms appear to be randomly distributed in‘unstable air and develop from localized heating at the earth’s surface (Figure 2). The heated air rises and cools to form cumulus clouds. As the cumulus stage continues to develop, precipitation forms in higher portions of the cloud and falls. Precipitation signals the beginning of the mature stage and presence of a downdraft. After approximately an hour, the heated up draft creating the thunderstorm is cut off by rainfall. Heat is removed and the thunderstorm dissipates. Many thunderstorms produce an associated cold air gust front as a result of the downflow and outrush of rain-cooled air. These gust fronts are usually very turbulent and can create a serious threat to airplanes during takeoff and approach.

Frontal thunderstorms are usually associated with weather systems like fronts, converging winds, and troughs aloft. Frontal thunderstorms form in squall lines, last several hours, generate heavy rain and possibly hail, and produce strong gusty winds and possibly tornadoes. The principal distinction in formation of these more severe thunderstorms is the presence of large horizontal wind changes (speed and direction) at different altitudes in the thunderstorm. This causes the severe thunderstorm to be vertically tilted (Figure 3). Precipitation falls away from the heated updraft permitting a much longer storm development period. Resulting airflows within the storm accelerate to much higher vertical velocities which ultimately result in higher horizontal wind velocities at the surface.

The downward moving column of air, or 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 Microburst as a Windshear Threat

Identification of concentrated, more powerful downdrafts—known as microbursts—has resulted from the investigation of windshear accidents and from meteorological research. Microbursts can occur anywhere convective weather conditions (thunderstorms, rain showers, virga) occur. Observations suggest that approximately five percent of all thunderstorms produce a microburst.

Downdrafts associated with microbursts are typically only a few hundred to 3,000 feet across. When the downdraft reaches the ground, it spreads out horizontally and may form one or more horizontal vortex rings around the downdraft (Figure 7). The outflow region is typically 6,000 to 12,000 feet across. The horizontal vortices may extend to over 2,000 feet AGL.

Microburst outflows are not always symmetric (Figure 8). Therefore, a significant airspeed increase may not occur upon entering the outflow, or may be much less than the subsequent airspeed loss experienced when exiting the microburst.

More than one microburst can occur in the same weather system. Pilots are therefore cautioned to be alert for additional microbursts if one has already been encountered or observed. If several microbursts are present, a series of horizontal vortices can form near the ground due to several microbursts being embedded in one another (Figure 9). Conditions associated with these vortices may produce very powerful updrafts and roll forces in addition to downdrafts.

Wind speeds intensify for about 5 minutes after a microburst initially contacts the ground (Figure 70). 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.

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 windspeed differences of almost 100 knots have been measured. In fact, a severe event at Andrews Air Force Base (Camp Spring, Maryland) on August 1, 1983 indicated headwind/tailwind differential velocities near 200 knots.

IT IS VITAL TO RECOGNIZE THAT SOME MICROBURSTS CANNOT BE SUCCESSFULLY ESCAPED WITH ANY KNOWN TECHNIQUES! Note that even windshears which were within the performance capability of the airplane have caused accidents.

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 (precipitation that evaporates before reaching the earth’s surface). The formation of a dry microburst is illustrated in Figure 12. In this example, air below a cloud base (up to approximately 15,000 feet AGL) is very dry. Precipitation from higher convective clouds falls into low humidity air and evaporates. This evaporative cooling causes the air to plunge downward. As the evaporative cooling process continues, the downdraft accelerates. Pilots are therefore cautioned not to fly beneath convective clouds producing virga conditions. .

2.3.1 ENCOUNTER DURING TAKEOFF AFTER LIFTOFF

In a typical accident studied, the airplane encountered an increasing tailwind shear shortly after lifting off the runway (Figure 13). For the first 5 seconds after liftoff the takeoff appeared normal, but the airplane crashed off the end of the runway about 20 seconds after liftoff.

In many events involving after-liftoff windshear encounters, early trends in airspeed, pitch attitude, vertical speed and altitude appeared normal. In this example, the airplane encountered windshear before stabilized climb was established which caused difficulty in detecting onset of shear. As the airspeed decreased, pitch attitude was reduced to regain trim airspeed (Figure 14). By reducing pitch attitude, available performance capability was not utilized and the airplane lost altitude. As terrain became a factor, recovery to initial pitch attitude was initiated. This required unusually high stick force (up to 30 pounds of pull may be required on some airplanes). Corrective action, however, was too late to prevent ground contact since the downward flight path was well established. Reducing pitch attitude to regain lost airspeed, or allowing attitude to decrease in response to lost airspeed, is the result of past training emphasis on airspeed control. Successful recovery from an inadvertent windshear encounter requires maintaining or increasing pitch attitude and accepting lower than usual airspeed. Unusual and unexpected stick forces may be required to counter pitching tendencies and lift loss.

To counter the loss of airspeed and flight path degradation resulting from windshear, pitch attitude must not be allowed to fall below the normal range. Only by properly controlling pitch attitude and accepting reduced airspeed can flight path degradation be prevented. Once the airplane begins to deviate from the intended flight path and high descent rates develop, it takes additional time and altitude to change flight path direction. [On takeoff the pilot must monitor airspeed, vertical speed, and altitude in order to detect windshear. In a normal situation, if airspeed is too low, then vertical speed is too high and the altimeter is rising too fast. In a windshear situation, airspeed and vertical speed are too low and the altimeter is showing a decrease. Strong air movement complicates this picture. See below.]

2.3.3 ENCOUNTER ON APPROACH

Analysis of a typical windshear encounter on approach provided evidence of an increasing downdraft and tailwind along the approach flight path (Figure 20). The airplane lost airspeed, dropped below the target glidepath, and contacted the ground short of the runway threshold.

Reduced airspeed, as the airplane encountered the windshear, resulted in decreased lift. This loss of lift increased the descent rate (Figure 21). The natural nose-down pitch response of the airplane to low airspeed caused additional altitude loss. Pitch attitude increase and recovery initiation were not used soon enough to prevent ground contact. 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. 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. A stabilized approach with clearly defined callouts is essential to aid in recognition of unacceptable flight path trends and the need to initiate recovery.

Windshear Effects on Airplanes

Headwind/Tailwind Shear Response

The various components of windshear have unique effects on airplane performance. In addition, the magnitude of the shear depends on the flight path through the microburst. An increasing headwind (or decreasing tailwind) shear increases indicated airspeed and thus increases performance. The airplane will tend to pitch up to regain trim airspeed. An additional consideration is that this type of shear may reduce normal deceleration during flare which could cause overrun. Any rapid or large airspeed increase, particularly near convective weather conditions, should be viewed as a possible indication of a forthcoming airspeed decrease. 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! In contrast to shears which increase airspeed, an increasing tailwind (or decreasing headwind) shear will decrease indicated airspeed and performance capability. Due to airspeed loss, the airplane may tend to pitch down to regain trim speed.

Vertical Windshear Response

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 3,000 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. Perhaps more critical than sustained downdrafts, short duration reversals in vertical winds can exist due to the horizontal vortices associated with microbursts. This is shown in Figure 22.

An airplane flying through horizontal vortices as shown in Figure 22 experiences alternating updrafts and downdrafts causing pitch changes without pilot input. These vertical winds result in airplane angle-of-attack fluctuations which, if severe enough, may result in momentary stick shaker actuation or airframe shudder at speeds well above normal, Vertical winds, like those associated with horizontal vortices, were considered in development of the recovery procedure. The most significant impact of rapidly changing vertical winds is to increase pilot workload during the recovery. The higher workload results from attention to momentary stick shaker actuation and uncommanded pitch attitude changes from rapid changes in vertical wind.

Crosswind Shear Response

A crosswind shear tends 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.

Turbulence Effects

Turbulence may be quite intense in weather conditions associated with windshear. Effects of turbulence can mask changing airspeed trends and delay recognition of severe windshear. Turbulence may also tend to discourage use of available airplane pitch attitude during a recovery by causing random stick shaker activity. These effects can significantly increase pilot workload and distraction.

Rain Effects

Accident investigations and the study of windshear have shown that some forms of windshear are accompanied by high rates of rainfall. NASA research is underway to determine if high rainfall rates contribute to a loss of airplane performance. The results available to date are inconclusive. However, because rain may serve as a warning of severe windshear, areas of heavy rain should be avoided. High rates of rainfall also cause significant increases in cockpit noise levels, making crew coordination and pilot concentration more difficult.

Captain Warren VanderBurgh talks about how windshear resulted in several airline crashes and how you should respond when encountering windshear.

This entry was posted on Sunday, September 21st, 2008 at 11:19 am and is filed under Weather. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.