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IMPACT OF SELECTED METEOROLOGICAL PHENOMENA ON FLIGHT OPERATIONS Sandra Krollová 1 Introduction Meteorology affects flight operations in significant ways. The impacts of atmospheric phenomena, contributing

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IMPACT OF SELECTED METEOROLOGICAL PHENOMENA ON FLIGHT OPERATIONS Sandra Krollová 1 Introduction Meteorology affects flight operations in significant ways. The impacts of atmospheric phenomena, contributing to accidents, range from dangerous low-level events (wind shear, gustfront, fog, freezing precipitation etc.), which are responsible for a series of commercial aircraft accidents, to en-route phenomena encounters that usually result in various onboard injuries. Primary hazards affecting flying aircraft are listed in Table 1. Combining the improved knowledge of physical processes of aviation hazards, modern prediction techniques using numerical and laboratory modelling, field experiments and widespread remote and in situ measuring helps to improve prediction, detection and warning of atmospheric flight hazards. Table 1: Meteorological phenomena affecting flight [Source: [1]] Atmospheric features Horizontal wind shear Vertical wind shear Mountain flows Valley flows Free atmospheric turbulence Thunderstorms Lightning Icing Dust devils, thermals Fog, Visibility Wake turbulence Volcanic ash Platforms aircraft Most aircraft Most aircraft Most aircraft Civil aviation, some commercial aviation, airports Most aircraft Most aircraft Most aircraft are not affected Most aircraft Small aircraft Most aircraft Light aircraft during approaches and take-offs behind heavy aircraft All aircraft at higher altitudes 1 Ing. Sandra Krollová, PhD., Fakulta prevádzky a ekonomiky dopravy a spojov, Katedra leteckej dopravy, 266 Figure 1: Summary of spatial and temporal scales of several atmospheric phenomena [Source: [2]] A plumes, roughness turbulence, dust devils, thermals, tornadoes, deep convection B - thunderstorms, microburst, low level jet, mountain flow C Squall lines, Clear air turbulence D - Fronts, cyclones, anticyclones, tropical cyclones E, F Planetary circulation, standing waves, ultralong waves The purpose of this paper is to describe the most important and hazardous weather phenomena affecting the flight and to indicate methodologies created for mitigation the hazards. Downburst Downburst is a strong, localized downdraft that often occurs along the leading edge of a severe thunderstorm and presages the arrival of thunderstorm precipitation. These cool, gusty winds result when downdrafts within the storm are strengthened by evaporative cooling as precipitation falls into the subsaturated air below the cloud base and evaporates. After the column of air hits the ground, it swirls up, creating a horizontal vortex which fans out radially in all directions (Figure 2). A downburst extending less than 4 km is called a microburst. The straight-line winds from microbursts have been known to exceed 75 m/s and thus can be as strong as rotating tornadic winds. The strongest microbursts can also generate a roaring sound that is usually associated with tornadic winds. The microburst intensifies for about 5 minutes then begins to dissipate and is gone in about minutes. It is estimated that only 5% of all thunderstorms produce microbursts, but it is nearly impossible to tell if a particular rain column contains one or not. In contrast, a macroburst is a large downburst with its outburst winds extending 4 km in horizontal dimension. Microburst is usually accompanied by strong wind shears, damaging winds on or near the ground. According to the NTSB 2 of the USA, between there were nine 2 NTSB - National Transportation Safety Board 267 accidents of commercial aircraft during take-off and landing. These resulted in 540 fatalities and 158 injuries. During the twenty years there were 50 accidents associated with microburst with 62 fatalities. Most of these accidents involved light aircraft. Between the years the EASA 3 documented 12 accidents involving microburst wind shear [1]. Figure 2: Vortex-ring circulation in a microburst [Source: [7]] Figure 3: Effects on flight through a microburst [Source: [2]] Downbursts are hazardous to aircraft, especially during take-off and landing. Turbulence can be violent in and all around the cloud. Under the cloud turbulence can be particularly dangerous during take off and landing. A plane encounters strong headwinds as it enters the downdraft and strong tailwinds as it exits. The effects on aircraft are displayed at figure 3. In position 1 the aircraft is experiencing increased headwind, airspeed is rising and the rate of descent is reduced. The aircraft has tendency to go high on glide path. In position 2 and 3 the headwind is reduced and the downdraught is increasing. Airspeed is falling, rate of descent is increasing and the aircraft has tendency to go low on glide path. In position 4 the tailwind increases, airspeed is falling and the rate of descent is checked by missed approach. Success of landing or take-off depends on height and speed plus reserves of power. Turbulence Turbulence is quasi-random movement of air parcels by small swirls (order of 2 mm to 2 km) of motion called eddies. The net effect of turbulence is to mix together air parcels with different temperature from different initial locations. Thus, turbulence tends to homogenize the air. During fair weather turbulence is strong during daytime over non-snowy ground and can cause significant heat transport within the atmospheric boundary layer. During stormy 3 EASA European Aviation Safety Agency 268 weather deep cumulus clouds and thunderstorms can cause vigorous turbulent mixing through the depth of the troposphere [5]. In the periods the NTSB identified 3599 cases of fatalities related to turbulence, of these 612 involved commercial aircraft. There are often more than 50 fatalities per year. Between 1999 and 2008 EASA documented 22 accidents involving turbulence. Lots of studies about the impacts of turbulence on airline crews have been realised. According to them the number and rate of turbulence accidents and injuries increased particularly after 1995, there were more than 50% serious or fatal injuries. Between years injuries were related to the phase of climbing (30 injuries), during the cruise (190 injuries) and during the descent (140 injuries) [1]. The sources of turbulence that usually affects flight are: terrain induced turbulence, thunderstorms, free atmospheric turbulence, wake vortices. Terrain induced turbulence The interaction of atmospheric flows with terrain can affect all flight levels. The stronger the wind and the rougher the obstruction, the more turbulence will be created. Strong wind also carries the turbulent eddies downstream, sometimes long distances from the source of the turbulence. The height and length of the terrain obstacle determines whether air goes over or around the barrier. The terrain induced turbulence implies: mountain and valley flows, mountain lee waves, low-level wind shear Mountain and valley flows These flows can modify the local meteorology and the boundary layer wind shears. They are generated within the mountain area through local temperature contrasts that form within the mountain region or between the mountains and the nearby plains. Terrain-forced 269 flows enter the mountain region from its surroundings, are modified or channelled by the mountains and create slope and valley winds. Such flow can provide flight challenges for mountain valley airports. The regular evolution of the winds, called diurnal cycle, consists of four phases: the evening-transition phase begins when slope winds reverse from upslope to downslope. The downslope winds drain cold air off the sidewalls into the valley resulting in the build-up of an inversion and causing along-valley winds to begin to reverse direction from up-valley to down-valley. This phase ends when down-valley flows prevail through the depth of the valley, during the night-time phase, downslope winds blow down the sidewalls and down-valley winds blow within the inversion layer, the morning-transition phase begins with the reversal of slope winds from downslope to upslope. Convective currents rising from the ground destroy the inversion from below, and along-valley winds reverse from down-valley to up-valley. This phase ends when the inversion is destroyed and winds blow up-valley through the depth of the valley, during the daytime phase, the upslope and up-valley winds prevail in an unstable convective boundary layer that extends from the valley floor and sidewalls into the above-valley atmosphere. Mountain waves As stable air flows over a mountain range, gravity waves or mountain waves can be generated either over the mountains or in the lee of the mountains. These standing waves can reach long distances and occur as vertical undulations or waves in the atmosphere. Mountain waves have a tendency to propagate vertically and can thus be found not only at low levels over hills and mountains, but throughout the troposphere and even in the stratosphere (Figure 5). High lenticular clouds are often indicators of these waves. Waves that form in the lee of the mountains are called lee waves. They are often confined or trapped in the lee of the barrier by a smooth, horizontal flow above (Figure 7). In general, mountain waves are found higher in the atmosphere and tend to have longer wavelengths and smaller amplitudes than lee waves. 270 Figure 4: Vertically propagated waves [Source: [8]] Figure 5: Trapped lee waves [Source: [8]] Mountain waves form most readily in the lee of steep, high barriers that are perpendicular to the approaching flow. In general, the higher the barrier, the greater the amplitude of the waves. The wavelength of a wave depends on variations in the speed and stability of the approaching flow. When stable air is carried over a mountain barrier, one of three flow patterns will result, depending on the characteristics of the wind profile. If the winds are weak and nearly constant with height, shallow waves form downwind of the barrier. When winds become stronger and increase with height, the air overturns on the lee side of the barrier, forming a standing lee eddy with horizontal axis parallel to the ridgeline called rotor. When winds become stronger and increase with height steeply, deeper waves form and propagate farther downwind of the barrier. If there is sufficient moisture in the atmosphere, clouds form in the crests of the waves as air is lifted through the wave: lenticular-shaped clouds, cap clouds and roll clouds in rotor region. The presence of ragged edges of lenticular clouds and the existence of roll clouds give warning of severe turbulence. When no clouds are present, the existence of waves can be assumed whenever a stable air mass approaches a significant barrier with a strong cross-barrier wind component [8]. Mountain waves and their turbulence encounters have caused several accidents. Many experiments have been done and increase knowledge of these complex flows. Doppler LIDAR 4 is a powerful fool for identifying and tracking the large increases in wind. Low level Wind shear 4 LIDAR - Light Detection And Ranging 271 The term wind shear supposes a sudden prolonged and large change in wind speed and/or direction which can occur in the horizontal or vertical direction. Low-level wind shear is a severe hazard in the most dangerous portions of a flight profile along the final approach path, along the runway during approach and take-off and during the initial climb-out. It can cause an aircraft to be displaced abruptly from the intended flight path. Vertical updraughts and/or downdraughts can also cause wind shear. When surface winds are strong, the vertical wind shear close to the ground is also strong. This shear hazard is compounded by the presence of the mechanical turbulence. The gustiness produced by the turbulent eddies will cause the shear that can produce significant airspeed fluctuations during take-off and landing. Wind shear below 2000 ft AGL along the final approach path or along the take-off and initial climb-out path is known as low-level wind shear. LLWS is often described in terms of the vertical wind shear over a 100-foot layer. If the pilot of an aircraft encounters wind shear on approach and fails to adjust for a rapidly decreasing headwind (or increasing tailwind), the airspeed will decrease and the aircraft will undershoot the landing. Similarly, a rapidly increasing headwind (or decreasing tailwind) can cause an overshoot. When wind shear is encountered on take-off and the headwind decreases (or tailwind increases), the climb-out will be slower. In critical situations, obstacles near the airport may not be cleared. During both take-off and landing, a strong wind shear with a crosswind component may cause the aircraft to deviate from the centreline of the runway. Thunderstorms There are many reasons for avoiding thunderstorms because of potential downdrafts, vertical and horizontal wind shears, turbulence, lightning, hail etc. Turbulence in thunderstorm can be violent in and all around the cloud. Under the cloud turbulence can be particularly dangerous during take off and landing. Loose particles in cabin can be a danger to passengers and crew. In thunderstorms substantial shafts of air can be encountered, with no warning, moving either vertically up or down. Entering the vertical updraught or downdraught from a horizontal airflow, the aeroplane s momentum will at first keep it on its original path relative to the new direction of flow. In addition to a loss of airspeed, it will also be realized that the shift of relative airflow will affect the angle of attack of the wing, which may result in either 272 an increase or decrease in angle. A slight increase of angle may not cause much concern. However, if the aircraft is already on the approach with a high angle of attack, an increase might put the wing near the stall and any decrease will bring a loss of lift. Neither result is desirable when the aircraft is near the ground. Normally the risk of a downdraught will be more likely than an updraught when below 300 m In the vicinity of the thunderstorm several turbulence hazardous conditions can be encountered: At lower altitudes up to 1 km gust front outflow, At levels, where the thunderstorm penetrates strong winds, the storm acts as a barrier and lee waves and downstream turbulence is produced, Vertically propagating waves can produce severe turbulence above the cloud top. Another important hazard is lightning. According to the NTSB, between years 1962 and 2006 there were 49 cases with direct lightning strike on aircraft. Lightning damage on aircraft can take a great variety of forms: communications interrupted, pilots blinded by the flash, cables burned through, windows shattered, damage of rudders, ailerons, wingtips, engines damaged and magnetized. Avoidance of thunderstorm is the best prevention of such encounters. Guidance can be provided by data from lightning detection network. Free atmospheric turbulence Free atmospheric turbulence refers to stochastic, random turbulence present in the atmosphere at all altitude. This turbulence is usually generated by dynamic processes and is called Clear Air Turbulence (CAT). CAT is commonly associated with jet streams; around the boundaries of a jet, vertically and horizontally, there are strong wind shears in terms of wind 273 speed. The turbulence is more severe on the top of the jet and particularly on the cold or polar side. It is also severe with stronger winds, with jets which are curved and with those which occur above and to the lee of mountain ranges. Frontal jets can produce more severe turbulence than the subtropical type because they move with the movement of the front. Figure 6: Schematic position of CAT in jet stream area [Source: At lower altitudes, CAT can be encountered when strong winds carry a volume of turbulent air away from its source. For example, mountain wave CAT might extend from the tops of the mountains to as high as 1500 m above the tropopause and can be as far as 150 km downwind from the mountains [8]. Wake vortices Between 1962 and 2006 there were 14 accidents involving wake vortices with 7 fatalities reported by the NTSB. Wake vortices are generated by a fixed-wing aircraft, where the pressure differential between the air flowing over the top and over the bottom of the wing creates a roll-up wake effect behind the wing tips. This wake consists of two counter-rotating vortices, the strength of which are determined by the weight, speed and wing shape of the aircraft. The greatest vortex strength is present when the aircraft is heavy, slow and has flaps and gear retracted. Figure 7: Wake turbulence behind the aircraft [Source: [7]] Figure 8: Hover wake and vortices behind the helicopter [Source: [2]] 274 The wake from a rotary-wing aircraft (helicopter) is a combination of a hover wake and a vortex pair. When the aircraft is hovering, the hover wake affects the column of air above and below the aircraft. The column of air influenced by the wake below the rotor expands laterally as it approaches the ground. A helicopter hovering above shakes the leaves, branches etc. Once, the helicopter begins forward flight, a pair of vortices forms behind the helicopter to the left and right of the flight path (Figure 11). As the helicopter speed increases, the wake vortex pair approximates more and more the vortex pair that forms behind a fixed-wing aircraft. Figure 9: Wake turbulence avoidance procedures [Source: [5]] The wake forms behind the aircraft and then descends and spreads out laterally, perpendicular to the flight path. The wake does not form until an airplane lifts off from the ground and it stops after the wheels touch the ground. In light winds (less than 5 kt), the wake can persist a long time after an airplane departs. Stronger winds will usually dissipate the wake vortices relatively fast. In a light crosswind conditions, one of the wakes can blow over the runway and stay there for many minutes [7]. Icing Modern transport aircraft have efficient anti-icing and de-icing systems. However, there can be occasions when the ice build-up is so severe that a system becomes less effective in keeping the skin clear of ice. Additionally there are instances where systems become inoperative. Knowledge of airframe icing characteristics can reduce possible hazards. In-flight icing occurs when an aircraft flies through visible moisture, such as rain or cloud droplets, and 275 the aircraft structure is below the freezing temperature of water. There are three types of inflight icing: clear, rime and mixed (Figure 13). Clear ice looks as clear as glass; it is transparent and has a smooth surface. It is very tough and adheres strongly to the aircraft skin. Rime ice is milky-white, almost opaque with a light texture; it rarely adds much weight to an aircraft, but it can also cause aerodynamic problems for the wings. Mixed ice is defined as a combination of rime and clear ice. The basic conditions for icing creation are: high water content, sub-zero temperature environment and supercooled liquid water (SLW). SLW occurs in convective clouds with strong updraft and also in clouds with weak updraft, but with tops warmer than -12 to -14 C [1]. Figure 10: Ice types [Source: [9]] Figure 11: Ice collection efficiency in dependence on droplet size, aircraft speed and aerofoil shape [Source: [9]] Icing intensity levels are relative because various aircraft accumulate and handle icing differently. Different types of aircraft accrete ice at different rates, depending on their airframe shapes and speed differences (Figure 15). For example, temperature rises with airspeed. High airspeeds create surface friction on the airfoils, thus preventing ice accumulation. Consequently, pilots of high-performance jet airplanes might not report icing, but slower-moving aircraft flying through the same area might experience heavy ice accumulation. The following forecast rules will help interpret the possibility of icing: In general, the colder the air temperature and wider the d
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