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The dangers of flying in or close to a thunderstorm are:
1. Turbulence. Turbulence, associated with thunderstorms, can be extremely hazardous, having the potential to cause overstressing of the aircraft or loss of control. Thunderstorm vertical currents may be strong enough to displace an aircraft up or down vertically as much as 2000 to 6000 feet. The greatest turbulence occurs in the vicinity of adjacent rising and descending drafts. Gust loads can be severe enough to stall an aircraft flying at rough air (maneuvering) speed or to cripple it at design cruising speed. Maximum turbulence usually occurs near the mid-level of the storm, between 12,000 and 20,000 feet and is most severe in clouds of the greatest vertical development.
Severe turbulence is present not just within the cloud. It can be expected up to 20 miles from severe thunderstorms and will be greater downwind than into wind. Severe turbulence and strong out-flowing winds may also be present beneath a thunderstorm. Microbursts can be especially hazardous because of the severe wind shear associated with them.
2. Lightning. Static electricity may build up in the airframe, interfering with operation of the radio and affecting the behaviour of the compass. Trailing antennas should be wound in. Lightning blindness. may affect the crew's vision for 30 to 50 seconds at a time, making instrument reading impossible during that brief period. Lightning strikes of aircraft are not uncommon. The probability of a lightning strike is greatest when the temperature is between -5ºC and 5°C. If the airplane is in close proximity to a thunderstorm, a lightning strike can happen even though the aircraft is flying in clear air. Lightning strikes pose special hazards. Structural damage is possible. The solid state circuitry of modem avionics is particularly vulnerable to lightning strikes. Electrical circuits may be disrupted. The possibility of lightning igniting the fuel vapor in the fuel cells is also considered a potential hazard.
3. Hail. Hailstones are capable of inflicting serious damage to an airplane. Hail is encountered at levels between 10 and 30 thousand feet. It is, on occasion, also encountered in clear air outside the cloud as it is thrown upward and outward by especially active cells.
4. Icing. Heaviest icing conditions occur above the freezing level where the water droplets are supercooled. Icing is most severe during the mature stage of the thunderstorm.
5. Pressure. Rapid changes in barometric pressure associated with the storm cause altimeter readings to become very unreliable.
6. Wind. Abrupt changes in wind speed and direction advance of a thunderstorm present a hazard during take-off and landing. Gusts in excess of 80 knots have been observed.
Very violent thunderstorms draw air into their cloud bases with great intensity. Sometimes the rising air forms an extremely concentrated vortex from the surface of the ground well into the cloud with vortex speeds of 200 knots or more and very low pressure in its center. Such a vortex is known as a tornado.
7. Rain. The thunderstorm contains vast amounts of liquid water droplets suspended or carried aloft by the updrafts. This water can be as damaging as hail to an aircraft penetrating the thunderstorm at high speed. The heavy rain showers associated with thunderstorms encountered during approach and landing can reduce visibility and cause retraction on the windscreen of the aircraft, producing an illusion that the runway threshold is lower than it actually is. Water lying on the runway can cause hydroplaning which destroys the braking action needed to bring the aircraft to a stop within the confines of the airport runway. Hydroplaning can also lead to loss of control during take-off.
ST. ELMO'S FIRE
If an airplane flies through clouds in which positive charges have been separated from negative charges, it may pick up some of the cloud's overload of positive charges. Weird flames may appear along the wings and around the propeller tips. These are called St. Elmo's Fire. They are awe-inspiring but harmless. It the airplane flies in the vicinity of a cloud where negative charges are concentrated, its positive overload may discharge into the cloud. In this case, it is the airplane which strikes the cloud with lightning! The electricity discharges cause a noisy disturbance in the lower frequency radio bands but do not interfere with the very high frequencies. This precipitation static, as it is called, tends to be most severe near the freezing level and where turbulence and up and down drafts occur.
Because of the severe hazards enumerated above, attempting to penetrate a thunderstorm is asking for trouble. In the case of flight, airplane pilots, the best advice on how to fly through a thunderstorm is summed up in one wordDON'T.
Detour around storms as early as possible when encountering them enroute. Stay at least 5 miles away from a thunderstorm with large overhanging areas because of the danger of encountering hail. Stay even farther away from a thunderstorm identified as very severe as turbulence may be encountered as much as 15 or more nautical miles away. Vivid and frequent lightning indicates the probability of a severe thunderstorm. Any thunderstorm with tops at 35,000 feet or higher should be regarded as extremely hazardous. Avoid landing or taking off at any airport in close proximity to an approaching thunderstorm or squall line.
Microbursts occur from cell activity and are especially hazardous if encountered during landing or take-off since severe wind shear is associated with microburst activity. Dry microbursts can sometimes be detected by a ring of dust on the surface. Virga falling and evaporating from high based storms can cause violent downdrafts.
The gust front, another zone of hazardous wind shear, can be identified by a line of dust and debris blowing along the earth's surface.
Swirls of dust or ragged clouds hanging from the base of the storm can indicate tornado activity. If one tornado is seen, expect others since they tend to occur in groups.
Do not fly under a thunderstorm even if you can see through to the other side, since turbulence may be severe. Especially, do not attempt to fly underneath a thunderstorm formed by orographic lift. The wind flow that is responsible for the formation of the thunderstorm is likely to create dangerous up and down drafts and turbulence between the mountain peaks.
Reduce airspeed to maneuvering speed when in the vicinity of a thunderstorm or at the first indication of turbulence.
Do not fly into a cloud mass containing scattered embedded thunderstorms unless you have airborne radar.
Do not attempt to go through a narrow clear space between two thunderstorms. The turbulence there may be more severe than through the storms themselves. If the clear space is several miles in width, however, it may be safe to attempt to fly through the center, but always go through at the highest possible altitude. When flying around a thunderstorm, it is better to fly around the right side of it. The wind circulates anti-clockwise and you will get more favorable winds. If circumstances are such that you must penetrate a thunderstorm, the following few simple rules may help you to survive the ordeal:
~ Go straight through a front, not across it, so that you will get through the storm in the minimum amount of time.
~ Hold a reasonably constant heading that will get you through the storm cell in the shortest possible time.
~ Before entering the storm, reduce the airspeed to the airplane's maneuvering airspeed to minimize structural stresses.
~ Turn the cockpit lights full bright. (This helps to minimize the risk of lightning blindness.) Check the pitot head. Fasten seat belts. Secure loose objects in the cabin.
~ Try to maintain a constant attitude and power setting. (Vertical drafts past the pitot head and clogging by rain cause erratic airspeed readings.)
~ Avoid unnecessary maneuvering (to prevent adding maneuver loads to those already imposed by turbulence).
~ Determine the freezing level and avoid the icing zone. Avoid dark areas of the cell and, at night, those areas of heavy lightning.
~ Do not use the autopilot. It is a constant altitude device and will dive the airplane to compensate for updrafts, causing excessive airspeed, or will cause the plane to climb in a downdraft creating the risk of a stall.
An instrument known as a stormscope, installed in the airplane, can help a pilot avoid thunderstorms. The stormscope detects the electromagnetic discharges associated with vertical air currents. All thunderstorms contain strong updrafts and downdrafts. These opposing ascending and descending air currents rub against each other, generating static electricity. The electrons tend to accumulate in positive and negative charges and when they have built up sufficiently, the potential difference will cause a current discharge. This discharge manifests itself not only as lightning but also in the radio spectrum. The stormscope picks up the radio frequencies from these discharges, a computer processes the signals, plots them by range and azimuth and presents them on a small, circular, radar like screen. The static electrical discharges picked up by the stormscope may or may not be associated with lightning. The stormscope receives these signals through 360 degrees around the airplane and from as far away as 200 nautical miles.
Each static discharge is represented by a bright green dot on the cathode ray tube display. Clusters of dots indicate areas of thunderstorm activity. The display can be programmed to 3 different range settings, 40, 100 and 200 miles. It is most accurate on the 40 mile range. On the 200 mile range, the stormscope sees everything but range is not so accurate. Generally, the display is more accurate and easier to read as the storm intensifies. In heavy electrical activity, the system has a problem called radial spread. The dots tend to spread over the display screening the areas between major clumps of storm caused dots.
The stormscope has some advantages over weather radar. Radar measures rainfall intensity. The stormscope is capable of detecting turbulence in clouds that have little or no precipitation. It is also able to see through areas of heavy precipitation to detect turbulent areas beyond. A stormscope does not, however, see rain. Recognizing the advantages of having both a stormscope and a weather radar, a recent model of stormscope interfaces with radar, displaying information from both systems on the same screen. The stormscope is not dependent on line of sight. It will see, for example, the weather behind mountains. The system can. Therefore, be used on the ground to determine weather for a 200 mile radius and is a useful flight planning tool.
Airborne weather radar is one of the best instrument aids that a pilot can have in locating and avoiding thunderstorms. It is able to detect and display on the cockpit radar screen any significant weather that lies ahead on the flight route. The radar equipment does this by measuring precisely rainfall density of targets under observation. The antenna of the weather radar radiates a very narrow and highly directional beam, in the X band of the radio spectrum, straight ahead of the aircraft. The beam is cone-shaped and from 3 to 10 degrees in diameter. (Beam width is a function of antenna size and type.) The antenna scans left and right to cover a sector of about 120 degrees.
Although, weather radar is not able to detect turbulence itself, the intensity of precipitation within a storm is a reliable indication of the amount of turbulence within a storm since strong drafts and gusts are necessary to produce water drops of significant size and quantity. Radar sees only water drops that are large enough to be affected by gravity and tall as rain. Because of the characteristics of X band radio waves and water, raindrops reflect the radiated beam back to the radar receiver. The sum of the reflections from all the raindrops appears on the screen as a target.
The computerized receiver measures the rainfall rate and grades the targets into levels which are represented on the screen by colors, green for level 1 which is light rain, yellow for level 2 which is medium rain and red for level 3 which is heavy rain. Areas of steep rain gradients are easy to see because of the color coding. A precipitation rate that changes from minimum to maximum over a short distance is known as a steep rain gradient and usually is associated with a shear zone. Any target that is showing red, said to be contouring, is considered to be a storm and must be avoided and detoured. Areas of no precipitation between targets remain black and are called corridors.
The airborne display is gradated into mileage rings. Distance to the storm as well as its bearing with respect to the airplane's heading are therefore displayed. As a result, the pilot is able to select a safe and smooth flight path through thunderstorm areas. It is wise to give all contouring targets at least a ten mile clearance. A corridor between 2 targets should be at least 20 miles wide before considering it a safe passage. Most weather radar systems manufactured today present a digital display that does not fade between sweeps. Some equipment incorporates automatic tilt to compensate for the altitude of the airplane. When an airplane, while flying at a level where the temperature is at or below freezing, strikes a supercooled water droplet, the droplet will freeze and adhere to the airplane. Dangerous icing can occur in clouds, freezing rain, or freezing drizzle.
The cloud in which icing most frequently occurs in winter is stratocumulus, but the heaviest deposits are encountered in cumulus and cumulonimbus. Clouds composed of ice crystals (such as cirrus) do not present an icing hazard. (The ice crystals do not adhere to the wing.) The more dangerous types of icing are encountered in dense clouds, composed of heavy accumulations of large supercooled drops, and in freezing rain. The seriousness of icing depends on the air temperature, the temperature of the aircraft skin and the amount of water striking the aircraft.
A supercooled water droplet freezes if disturbed. When struck by an aircraft, the drop begins to freeze immediately, but as it freezes, it releases heat to raise its temperature to 0°C. Freezing by impact then ceases and the remaining liquid in the drop begins to freeze more slowly as a result of cold surroundings. At very low temperatures, a large part of the drop freezes by impact. At higher temperatures, a smaller part of the drop freezes by impact leaving a greater amount to freeze more slowly. How fast this liquid part of the drop freezes depends on the temperature of the aircraft skin. The higher the temperature, the more the drop will spread from the point of impact before freezing is complete.
Whether or not a drop freezes completely before another drop strikes the same spot is another factor affecting the character of icing. The amount of water intercepted by an aircraft in a given time is called its rate of catch. This rate varies with the liquid water content of the cloud, the size of the water droplets, the airspeed and the type of wing of the aircraft. The liquid water content varies from level to level within the cloud. Generally, the amount of supercooled water in a cloud increases with height when the temperature is just a little below 0°C but decreases with height when the temperature is well below freezing, since at such low temperatures, more drops will freeze into ice crystals reducing the liquid content.
On some models, tilt is handled manually. Since the weather radar only can display targets illuminated by the radar beam, tilt management of the radar antenna is essential. The tilt feature controls the up and down angle of the antenna and consequently the plane of scan of the antenna. This feature is important in evaluating weather. The antenna beam does not see the whole storm, but only a 3 to 10 degree slice of it. By seeing the tilt higher, the beam scans the upper region of the cell. By setting it lower, the beam scans the lower region of the cell. Since rain is most concentrated in the middle and lower regions of a thunderstorm, the best part of a storm to scan is this middle/lower area that gives the best indication of size and intensity.
It does take skill and training to use airborne weather radar most effectively. Interpreting the display is not an exact science but depends on the pilot's general knowledge of thunderstorms, the quality of the pre-flight briefing that he has received, and his familiarity with the limitations of the radar equipment in his airplane. There are some limitations to weather radar. Moisture in relatively close proximity to the airplane can scatter the radar beams. This problem called attenuatlon, means that heavy rain areas can block out a radar return from significant weather that lies beyond. Moisture and ice on the radar dome (the radome) installation on the nose of the airplane can diminish the radar signal. Useful range of the weather radar is only about 90 to 100 miles.
The size of droplets also affects the rate of catch. Small drops tend to follow the airflow and are carried around the wing. Large, heavy drops tend to strike the wing. When a small drop does hit, it will spread back over the wing only a small distance. The large drop spreads farther. As for airspeed, the number of droplets struck by the aircraft in a certain time increases as the airspeed increases. The curvature of the leading edge of the wing also has an effect on the rate of catch. Thin wings catch more droplets than do thick wings. The rate of catch is, therefore, greatest for an aircraft with thin wings flying at high speed through a cloud with large droplets and a high liquid water content.
HOW ICING AFFECTS THE AIRPLANE
Ice collects on and seriously hampers the function of not only wings and control surfaces and propellers, but also windscreens and canopies, radio antennas, pilot tubes and static vents, carburetors and air intakes. Turbine engines are especially vulnerable. Ice forming on the intake cowling constricts the air intake. Ice on the rotor and starter blades affects their performance and efficiency and may result in flame out. Chunks of ice breaking off may be sucked into the engine and cause structural damage. The first structures to accumulate ice are the surfaces with thin leading edges: antennas, propeller blades, horizontal stabilizers, rudder, and landing gear struts. Usually the pencil-thin outside air temperature gauge is the first place where ice forms on an airplane. The wings are normally the last structural component to collect ice. Sometimes, a thin coating of ice will form on the windshield, preceded in some instances by frosting. This can occur on take-off and landing and with sufficient rapidity to obscure the runway and other landmarks during a critical time in flight.
Icing of the propeller generally makes itself known by a slow loss of power and a gradual onset of engine roughness. The ice first forms on the spinner or propeller dome and then spreads to the blades themselves. Ice customarily accumulates unevenly on the blades, throwing them out of balance. The resulting vibration places undue stress on the blades and on the engine mounts, leading to their possible failure. If the propeller is building up ice, it is almost certain that the same thing is happening on the wings, tail surfaces and other projections. The weight of the accumulated ice is less serious than the disruption of the airflow around the wings and tail surfaces. The ice changes the airfoil cross section and destroys lift, increases drag and raises the stalling speed. At the same time, thrust is degraded because of ice on the propeller blades and the pilot finds himself having to use full power and a high angle of attack just to maintain altitude. With the high angle of attack, ice will start to form on the underside of the wing adding still more weight and drag. Landing approaches and landing itself can be particularly hazardous under icing conditions. Pilots should use more power and speed than usual when landing an ice-laden airplane.
If ice builds up on the pilot tube and static pressure ports, flight instruments may cease operating. The altimeter, airspeed and rate of climb would be affected. Gyroscopic instruments powered by a venturi would be affected by ice building up on the venturi throat. Ice on radio antennas can impede VOR reception and destroy all communications with the ground. Whip antennas may break off under the weight of the accumulating ice.
TYPES OF ICING
The three main types of ice accretion, in order of their hazard to flying, are as follows:
A heavy coating of glassy ice which forms when flying in dense cloud or freezing rain is known as clear ice or glaze ice. It spreads, often unevenly, over wing and tail surfaces, propeller blades, antennas, etc. Clear ice forms when only a small part of the supercooled water droplet freezes on impact. The temperature of the aircraft skin rises to 0°C with the heat released during that initial freezing by impact of the part of the droplet. A large portion of the droplet is left to spread out, mingle with other droplets before slowly and finally freezing. A solid sheet of clear ice thus forms with no embedded air bubbles to weaken its structure. As more ice accumulates, the ice builds up into a single or double horn shape that projects ahead of the wing, tail surface, antenna, etc. on which it is collecting. This unique ice formation severely disrupts the airflow and is responsible for an increase in drag that may be as much as 300 to 500%.
The danger of clear ice is great owing to (1) the loss of lift, because of the altered wing camber and the disruption of the smooth flow of air over the wing and tail surfaces, (2) the increase in drag on account of the enlarged profile area of the wings. (3) the weight of the large mass of ice which may accumulate in a short time, and finally (4) the vibration caused by the unequal loading on the wings and on the blades of the propeller(s). When large blocks break off, the vibration may become severe enough to seriously impair the structure of the airplane. When mixed with snow or sleet, clear ice may have a whitish appearance. (This was once classified as rime-glazed but it is now considered to be a form of clear ice).
An opaque, or milky white, deposit of ice is known as rime. It accumulates on the leading edges of wings and on antennas, pilot heads, etc. For rime to form, the aircraft skin must be at a temperature below 0°C. The drop will then freeze completely and quickly without spreading from the point of impact. It is also dependent on a low rate of catch of small supercooled water droplets.
Rime forms when the airplane is flying though filmy clouds. The deposit has no great weight, but its danger lies in the aerodynamic alteration of the wing camber and in the choking of the orifices of the carburetor and instruments. Rime is usually brittle and can easily be dislodged by de-icing equipment. Occasionally, both rime and clear ice will form concurrently. This is called mixed icing and has the bad features of both types.
A white semi-crystalline frost which covers the surface of the airplane forms in clear air by the process of sublimation. This has little or no effect on flying but may obscure vision by coating the windshield. It may also interfere with radio by coating the antenna with ice. It generally forms in clear air when a cold aircraft enters warmer and damper air during a steep descent. . Aircraft parked outside on clear cold nights are likely to be coated with frost by morning. The upper surfaces of the aircraft cool by radiation to a temperature below that of the surrounding air.
Frost which forms on wings, tail and control surfaces must be removed before take-off. Frost alters the aerodynamic characteristics of the wing sufficiently to interfere with take-off by increasing stall speed and reducing lift. Frozen dew may also form on aircraft parked outside on a night when temperatures are just below freezing. Dew first condenses on the aircraft skin and then freezes as the surface of the aircraft cools. Frozen dew is usually clear and somewhat crystalline, whereas frost is white and feathery. Frozen dew, like frost, must be removed before take-off. In fact, any snow or moisture of any kind should be removed since these may freeze to the surface while the airplane is taxiing out for take-off. The heat loss due to the forward speed of the airplane may be sufficient to cause congelation.
INTENSITY OF ICING
Icing may be described as light, moderate, and severe (or heavy). In severe icing conditions, the rate of accretion is such that anti-icing and de-icing may fail to reduce or control the hazard. A change in heading and altitude is considered essential. In moderate icing, a diversion may be essential since the rate of accretion is such that there is potential for a hazardous situation. Light icing is usually not a problem unless the aircraft is exposed for a lengthy period. Clear ice is considered more serious than rime ice since the rate of catch must be high to precipitate the formation of clear ice. The seriousness of an icing situation is, of course, dependent on the type of aircraft and the type of de-icing or anti-icing equipment with which the aircraft is equipped or the lack of such equipment.
ICING IN CLOUDS AND PRECIPITATION
Cumulus. Severe icing is likely to occur in the upper half of heavy cumulus clouds approaching the mature cumulonimbus stage especially when the temperatures are between -25°C and 0°C. The horizontal extent of such cloud is, however, limited so that the aircraft is exposed for only a short time.
Stratus. Icing is usually less severe in layer cloud than in cumulus type clouds but it can be serious if the cloud has a high water content. Since stratus cloud is widespread in the horizontal, exposure to the icing condition can be prolonged. Icing is more severe if cumulus clouds are embedded in the stratus layer.
Freezing Rain is common ahead of warm fronts in winter.
Serious icing occurs when the aircraft is flying near the top of the cold air mass beneath a deep layer of warm air. Rain drops are much larger than cloud droplets and therefore give a very high rate of catch. In freezing temperatures, they form clear ice.
Freezing Drizzle. Drizzle falls from stratus clouds with a high water content. As the droplets fall through the clear air , prompt action on the radio is important when icing starts. Information about the latest weather for altitudes above and below will help the pilot to make the decision on what action to take. The final alternative would be to turn back, or, if the accumulation of ice has already become serious, to make a precautionary landing immediately. In any event, the decision must be made rapidly since once ice has started to form, the condition may become critical in a matter of approximately six minutes.
Pilots flying in light airplanes which are not fitted with an outside air temperature gauge will be well advised to have one installed as this instrument will warn of temperatures that are conducive to icing conditions. To avoid icing problems, here are a few rules to follow:
Avoid flight into an area where icing conditions are known to exist. Do not fly through rain showers or wet snow when the temperature is near 0°C. Do not fly into cumulus clouds when the temperature is low.
Always consult a weather office or flight service station to obtain a forecast about expected icing conditions before taking off on any flight in fall or winter. Icing in freezing drizzle is usually maximum just below the cloud base where the drops are largest. Icing is of the clear ice type.
Snow and Ice Crystals do not adhere to cold aircraft and do not usually constitute an icing problem. However, if the aircraft is warm, the snow may melt as it strikes the warm surface and ice accretion may result. If supercooled water droplets are also present with the snow, a rapid build up of rough ice can occur.
PROTECTION FROM ICING
Many modern airplanes that are designed for personal and corporate use, as well as the larger transport type airplanes, are fitted with various systems designed to prevent ice from forming (anti-icers) or to remove ice after it has formed (de-icers).
1. Fluids. There are fluids which are released through slinger rings or porous leading edge members to flow over the blades of the propellers and the surfaces of the wings. A fluid is an anti-icing device since it makes it difficult for ice to form.
2. Rubber Boots. Membranes of rubber are attached to the leading edges. They can be made to pulsate in such a way that ice is cracked and broken off after it has already formed. This is a de-icing device.
3. Heating Devices. Heating vulnerable areas is a method for preventing the buildup of ice. Hot air from the engine or special heaters is ducted to the leading edges of wings, empennages, etc. Electrically heated coils protect pilot tubes, propellers, etc.
Few single engine airplanes, or even light twin-engine types incorporate any means of ice prevention. A few tips for pilots flying airplanes in this category will therefore be in order.
When ice formation is observed in flight, there is only one certain method of avoiding its hazards and that is to get out of the ice-forming layer as quickly as possible. This may be done by climbing above the ice forming zone. This alternative would obviously require an airplane that has good performance and is fitted with radio and proper instruments for flying over the top. The next alternative would be to descend and fly contact below the ice forming zone. The advisability of this course would depend on the ceiling and visibility along the route at the lower level concerned.
Do not remain in icing conditions any longer than necessary. For that reason, during climbs or descents through a layer in which icing conditions exist, plan your ascent or descent to be in the layer for as short a time as possible. However, keep your speed as slow as possible consistent with safety. Speed of an airplane affects accretion of ice. The faster an airplane moves through an area of supercooled water drops, the more moisture it encounters and the faster will be the accumulation of ice.
If ice has started to build up on the airplane, do not make steep turns or climb too fast since stalling speed is affected by ice accumulation. Fuel consumption is greater due to increased drag and the additional power required. Land with more speed and power than usual. Do not land with power off. With the advent of the jet age, the problem of icing has taken on some surprising new aspects. At one time, the pilots of airplanes flying through high cirrus clouds did not worry about ice forming on the airplane as cirrus clouds are composed of ice crystals rather than water droplets. With the increased speeds of which jet airplanes are capable, the heat of friction is sufficient to turn the ice crystals in the cloud to liquid droplets which subsequently freeze to the airplane.
Turbulence is one of the most unpredictable of all the weather phenomena that are of significance to pilots. Turbulence is an irregular motion of the air resulting from eddies and vertical currents. It may be as insignificant as a few annoying bumps or severe enough to momentarily throw an airplane out of control or to cause structural damage. Most of the causes of turbulence have been mentioned in other sections of this chapter since turbulence is associated with fronts, wind shear, thunderstorms, etc.
In reporting turbulence, it is usually classed as light, moderate, severe or extreme. The degree is determined by the nature of the initiating agency and by the degree of stability of the air.
Light turbulence momentarily causes slight changes in altitude and/or attitude or a slight bumpiness. Occupants of the airplane may feel a slight strain against their seat belts.
Moderate turbulence is similar to light turbulence but somewhat more intense. There is, however, no loss of control of the airplane. Occupants will feel a definite strain against their seat belts and unsecured objects will be dislodged.
Severe turbulence causes large and abrupt changes in altitude and/or attitude and, usually, large variations in indicated airspeed. The airplane may momentarily be out of control. Occupants of the airplane will be forced violently against their seat belts. In extreme turbulence, the airplane is tossed violently about and is impossible to control. It may cause structural damage. Whether turbulence will be light or more severe is determined by the nature of the initiating agency and by the degree of stability of the air.
There are four causes of turbulence.
1. Mechanical Turbulence. Friction between the air and the ground, especially irregular terrain and man-made obstacles, causes eddies and therefore turbulence in the lower levels. The intensity of this eddy motion depends on the strength of the surface wind, the nature of the surface and the stability of the air. The stronger the wind speed, the rougher the terrain and the more unstable the air, the greater will be the turbulence. Of these factors that affect the formation of turbulence, stability is the most important. If the air is being heated from below, the vertical motion will be more vigorous and extensive and the choppiness more pronounced. In unstable air, eddies tend to grow in size; in stable air, they tend not to grow in size but do dissipate more slowly.
Turbulence can be expected on the windward side and over the crests of mountains and hills if the air is unstable. There is less turbulence on the leeward side since subsidence stabilizes the air. Mountain waves produce some of the most severe turbulence associated with mechanical agencies. In strong winds, even hangars and large buildings cause eddies that can be carried some distance downwind. Strong winds are usually quite gusty; that is, they fluctuate rapidly in speed. Sudden increases in speed that last several minutes are known as squalls and they are responsible for quite severe turbulence.
2. Thermal Turbulence. Turbulence can also be expected on warm summer days when the sun heats the earth's surface unevenly. Certain surfaces, such as barren ground, rocky and sandy areas, are heated more rapidly than are grass covered fields and much more rapidly than is water. Isolated convective currents are therefore set in motion which are responsible for bumpy conditions as an airplane flies in and out of them. This kind of turbulence is uncomfortable for pilot and passengers. In weather conditions when thermal activity can be expected, many pilots prefer to fly in the early morning or in the evening when the thermal activity is not as severe.
Convective currents are often strong enough to produce air mass thunderstorms with which severe turbulence is associated. Turbulence can also be expected in the lower levels of a cold air mass that is moving over a warm surface. Heating from below creates unstable conditions, gusty winds and bumpy flying conditions.
Thermal turbulence will have a pronounced-effect on the flight path of an airplane approaching a landing area. The airplane is subject to convective currents of varying intensity set in motion over the ground along the approach path. These thermals may displace the airplane from its normal glide path with the result that it will either overshoot or undershoot the runway.
3. Frontal Turbulence. The lifting of the warm air by the sloping frontal surface and friction between the two opposing air masses produce turbulence in the frontal zone. This turbulence is most marked when the warm air is moist and unstable and will be extremely severe if thunderstorms develop. Turbulence is more commonly associated with cold fronts but can be present, to a lesser degree, in a warm front as well.
4. Wind Shear. Any marked changes in wind with height produce local areas of turbulence. When the change in wind speed and direction is pronounced, quite severe turbulence can be expected. Clear air turbulence is associated at high altitudes with the jet stream.
Click here to go to the next section of this chapter: Hemispheric Prevailing Winds
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