Turbulence - Aviation Basics Course PDF

Aviation Basics Course Course 5: Weather and Flight 12 Turbulence Key Points  Aviation Basics Course...

Aviation Basics Course Course 5: Weather and Flight 12 Turbulence Key Points  Aviation Basics Course Course 5: Weather and Flight Table of Contents Turbulence Overview........................................................................................... 2 Causes of Turbulence.......................................................................................... 2 Convective (or Thermal) Currents......................................................................................... 2 Mechanical Turbulence........................................................................................................... 3 Wind Shear............................................................................................................................... 4 Wake Turbulence..................................................................................................................... 4 Stable and Unstable Air....................................................................................... 5 Stable Air.................................................................................................................................. 6 Unstable Air............................................................................................................................. 6 Categories of Turbulence.................................................................................... 7 Friction Turbulence................................................................................................................. 7 Thermal Turbulence................................................................................................................ 9 Intensities of Turbulence................................................................................... 11 Downbursts......................................................................................................... 12 Wind Shear.......................................................................................................... 14 Types of Wind Shear............................................................................................................. 15 Where Wind Shear is Found................................................................................................. 15 High-Level Wind Shear......................................................................................................... 16 Low-Level Wind Shear.......................................................................................................... 17 Effects of LLWS on Aircraft............................................................................... 18 Arriving Aircraft..................................................................................................................... 18 Departing Aircraft.................................................................................................................. 21 Chop Versus Turbulence................................................................................... 22 Clear Air Turbulence.......................................................................................... 23 Pilot Procedures for Reporting Turbulence..................................................... 24 12: Turbulence—Key Points  Aviation Basics Course Course 5: Weather and Flight Wrap-Up............................................................................................................... 25 12: Turbulence—Key Points  Aviation Basics Course Course 5: Weather and Flight 12. Turbulence Turbulence can range from a few bumps to conditions severe enough to cause structural damage. It is important for you to be aware of the causes of turbulence and the effect it can have on aircraft performance. By the end of this section, you will be able to: List the causes of turbulence Describe stable and unstable air as it relates to the vertical motion of air and to flight conditions Describe the categories and intensities of turbulence Describe two types of downbursts Define wind shear and its effect on aircraft performance Define high-level wind shear, low-level wind shear, and how the latter affects arriving and departing aircraft Describe the difference between chop and turbulence Describe clear air turbulence and state where it is normally found Describe procedures for reporting turbulence to pilots 12: Turbulence—Key Points 1/25  Aviation Basics Course Course 5: Weather and Flight Turbulence Overview Turbulence is an irregular motion of the air resulting from the formation of eddies or vertical currents in the air. It can be a major hazard to aviation. The degree of turbulence often depends on the type of aircraft. For example, turbulence considered light for a Boeing 747 may be severe for a Cessna 172. Figure 1: Turbulence created by a jet aircraft Causes of Turbulence There are four main causes of turbulence. Convective (or thermal currents) Mechanical disturbances Wind shear Wake turbulence Convective (or Thermal) Currents Convective (or thermal) currents refer to disturbed airflow resulting from air moving vertically in convective currents. 12: Turbulence—Key Points 2/25  Aviation Basics Course Course 5: Weather and Flight Figure 2: Convective turbulence Mechanical Turbulence Mechanical turbulence refers to disturbed airflow resulting from air moving past or over obstructions, such as irregular terrain. Figure 3: Mechanical turbulence 12: Turbulence—Key Points 3/25  Aviation Basics Course Course 5: Weather and Flight Wind Shear Wind shear is a drastic change in wind speed or direction over a relatively short distance. Figure 4: Wind shear Wake Turbulence Wake turbulence is the turbulent air behind an aircraft that occurs due to various causes, including wingtip vortices. Remember that more than one cause of turbulence may be present at a time. Figure 5: Wake turbulence 12: Turbulence—Key Points 4/25  Aviation Basics Course Course 5: Weather and Flight Stable and Unstable Air The atmosphere can be either stable or unstable. Stable air generally means smooth and stable flying conditions since the air will resist upward or downward displacement. Stable air tends to return to its original horizontal level. Unstable air is lifted and continues to rise on its own, since the lifted air is buoyant and becomes warmer and lighter than the surrounding air. Warm air is less dense and tends to rise. Cold air is denser and tends to descend. The warmer the air, the more moisture it can hold. As it rises, warm air is cooled by the surrounding air. A rising water vapour molecule cools at a rate of approximately 10°C per kilometre. Rising air eventually reaches a level where its temperature is equal to its dew point and condensation occurs. This leads to the formation of fog and clouds. Recall that dew point is the temperature at which air cooled at constant pressure and without the addition or removal of water vapour becomes saturated, leading to condensation. Condensation is the process during which gas changes to liquid state. There are several indicators of stable and unstable air. Signs of stable air: Stratus-type cloud or fog Low visibility when dust, smoke, haze, or fog are present Steady precipitation Consistent, steady winds IFR conditions for ceiling and visibility Signs of unstable air: Cumuliform clouds Good visibility Gusty wind Showery precipitation Thunderstorms 12: Turbulence—Key Points 5/25  Aviation Basics Course Course 5: Weather and Flight Stable Air Since stable air resists vertical motion, stable air that is lifted will return to its original position. This is because the lifted air becomes colder and denser than the surrounding air and therefore sinks. For an aircraft flying in these conditions, the flight will be smooth and stable. Figure 6: Path of stable air Unstable Air Unstable air allows vertical motion. If warm air is lifted and becomes surrounded by colder air, it will continue to rise. The colder air aloft will then descend causing an up and down movement in the atmosphere. Because of these movements of air, flight conditions will be bumpy and unstable. Figure 7: Path of unstable air 12: Turbulence—Key Points 6/25  Aviation Basics Course Course 5: Weather and Flight Being able to identify what type of air mass is present will help you determine what type of weather to expect and, by extension, what flying conditions aircraft are likely to face. An unstable air mass will bring cumulus-type clouds, possible turbulence and heavy precipitation. As a result, flying conditions will be challenging. A stable air mass will bring stratiform-type clouds and no turbulence, though visibility may be reduced in fog. Flying conditions will be smooth, though aircraft may need to fly under instrument flight rules if fog is present. Categories of Turbulence There are two categories of turbulence. Friction Thermal Friction turbulence is caused by the Thermal turbulence is caused by a rapid restriction of the movement of the wind. change of temperature in the atmosphere. Friction Turbulence There are three types of turbulence caused by friction. Mechanical turbulence Shear turbulence Frontal turbulence Mechanical Turbulence Mechanical turbulence is the result of friction between the air and the ground (especially irregular terrain and other obstacles), which causes eddies. The intensity of the eddy motion depends on the strength of the surface wind, the nature of the surface, and the stability of the air. 12: Turbulence—Key Points 7/25  Aviation Basics Course Course 5: Weather and Flight Figure 8: A plane encountering mechanical turbulence Three factors will affect the strength and vertical extent (height) of mechanical turbulence. Stability of the air: The more unstable the air, the higher the turbulence will extend. Roughness of the ground: The rougher the surface, the greater the friction and the greater the degree of turbulence. Strength of the wind: The stronger the wind, the greater the friction and the greater the degree of turbulence. Loose material on the ground, such as sand or snow, may provide a visual indication of mechanical turbulence. If the air is unstable, this loose material may be blown around enough to restrict visibility. Examples include blowing snow and blowing sand. If the air is relatively stable, sand or snow may be blown about, but not lifted enough to restrict visibility. Examples include drifting snow and drifting sand. Shear Turbulence Shear turbulence is the result of friction between opposing air currents. This type of turbulence occurs when there is a strong wind shear, which is a drastic change in wind speed or direction over a relatively short distance. Frontal Turbulence Frontal turbulence is the result of friction between two opposing air masses near the frontal surface. This turbulence is most noticeable when the lifted warm air is moist and unstable. It becomes severe if thunderstorms develop. Recall that turbulence is more commonly associated with cold fronts, but it may be present to a lesser degree in warm fronts as well. 12: Turbulence—Key Points 8/25  Aviation Basics Course Course 5: Weather and Flight Figure 9: Frontal zones (here the directional shear is southwest to northwest and the speed shear is 10 knots to 20 knots) Thermal Turbulence There are two types of turbulence initiated by heat: Daytime convection turbulence Cold air advection turbulence Daytime Convection Turbulence Daytime convection turbulence is the result of vertical currents caused by unequal heating of the Earth’s surface. The strength, extent and distribution of convection currents are affected by: The stability of the air and the degree of difference in unequal heating Seasonal variations in heating throughout the day and across seasons Thermal turbulence is most pronounced in the summer and in the afternoon. Variations in the composition of the surface result in uneven heating of the air near the ground. For example, land heats up faster than water. Similarly, barren surfaces such as sandy or rocky wasteland and ploughed fields heat faster than ground covered by grass or other vegetation. Uneven heating of the air near the ground causes convection currents to vary in strength within short distances. 12: Turbulence—Key Points 9/25  Aviation Basics Course Course 5: Weather and Flight Figure 10: Thermal turbulence Cold Air Advection Turbulence When cold air moves over warmer water or land, heating from below creates unstable conditions that favour convection currents and turbulence. The stronger the contrast between the air and surface temperatures, the more heat will be generated and the stronger the turbulence. Figure 11: Cold air advection turbulence in winter 12: Turbulence—Key Points 10/25  Aviation Basics Course Course 5: Weather and Flight Intensities of Turbulence Turbulence is classified into three intensities: Light turbulence momentarily causes slight, erratic changes in the altitude and attitude of an aircraft. Occupants of an aircraft may feel a slight strain against their seat belts. Unsecured objects may be slightly displaced. Moderate turbulence is a little more intense than light turbulence. Changes in altitude and attitude occur, but the aircraft remains in control at all times. Moderate turbulence may cause variations in indicated airspeed. Occupants will feel a definite strain against their seatbelts and unsecured objects may be noticeably dislodged. Severe turbulence causes large and abrupt changes in aircraft altitude and attitude as well as large variations in airspeed. The aircraft may be momentarily out of control. Occupants of the aircraft will be forced violently against their seat belts. Unsecured objects will be thrown around. The aircraft will need to change its heading or its altitude, or both. Turbulence intensities are dependent on the initiating lifting agent and the degree of stability of the air. Unstable air will always be more turbulent than stable air. 12: Turbulence—Key Points 11/25  Aviation Basics Course Course 5: Weather and Flight Downbursts A downburst is a severe downward rush of air with outbursts of damaging winds. Downbursts are a major hazard in aviation and have led to many aircraft accidents, so it is important for you to be aware of the types of downbursts that exist along with vertical and horizontal speeds associated with them. A downburst can be classified as a macroburst or a microburst. A macroburst is a large downburst with a horizontal diameter of 4 km (2.2 NM) or more when it reaches the Earth’s surface. It can produce horizontal wind speeds up to 140 knots. Damaging winds can last from 5 to 20 minutes. A microburst is a downburst of less than 4 km (2.2 NM) in diameter. While microbursts are generally of short duration, typically less than five minutes, they can cause horizontal wind speeds of more than 140 knots at tree top level. Downdrafts in microbursts can have vertical speed approaching 6000 feet per minute (1 NM/min or 60 knots). The microburst downdraft shaft can have a diameter of up to 6000 feet (1 NM). Multiple microbursts are common. Microbursts can be classified as wet (with precipitation) or dry (without precipitation). A wet microburst occurs when dry air penetrates saturated air leading to rapid evaporation and cooling of the air. Cool air and precipitation descend. Figure 12: Wet microburst 12: Turbulence—Key Points 12/25  Aviation Basics Course Course 5: Weather and Flight A dry microburst occurs when precipitation evaporates and the air cools and descends. Figure 13: Dry microburst The presence of virga (precipitation falling but not reaching the ground) associated with TCUs and CBs is often a precursor of microbursts. Virga evaporates below the cloud and, as a result of the heat required for evaporation, the air in these regions becomes colder than its environment, sinking rapidly and accelerating downwards. Figure 14: The evolution of a microburst over time When the microburst hits the ground, the wind will go in all directions, just like if you take a garden hose and point it downward in your driveway. 12: Turbulence—Key Points 13/25  Aviation Basics Course Course 5: Weather and Flight The exact nature of the wind associated with a particular microburst is virtually impossible to predict. Figure 15: Air circulation of a microburst Wind Shear Recall that wind shear is one of the four main causes of turbulence and it has significant effects on the control of an aircraft. It is characterized as the sudden “tearing” or “shearing” effect encountered along the edge of a zone in which there is a significant change in wind speed or direction over a short distance. You must have a good understanding of wind shear because it has been a sole or contributing cause of many aircraft accidents. The effect of wind shear will vary depending on the direction and speed of the wind. The result will be either increased or decreased aircraft performance. Aircraft land and take off with their nose into the wind, so a change in the direction or speed of the wind could require an aircraft to take longer to depart or to land. An aircraft is affected by the change in wind direction and speed because the wind changes its motion relative to the ground. Wind shear exists in areas of temperature inversions where there is an increase in temperature with height or altitude. 12: Turbulence—Key Points 14/25  Aviation Basics Course Course 5: Weather and Flight It also exists along troughs and lows, and around jet streams. Recall from the Introduction to Air Traffic Services that jet streams are fast-flowing, narrow air currents found in the atmosphere. Wind shear can exist in a horizontal or vertical direction and occasionally in both. It can also occur at low or high altitudes. Types of Wind Shear There are two types of wind shear. Figure 16: Directional shear Figure 17: Speed shear Directional shear is when two Speed shear is when two winds are winds blowing from different close together and blow from the same directions meet direction but at different speeds Where Wind Shear is Found Wind shear may be found in flight and near the ground. In flight, wind shear is found… In the lower 3000 feet (due to changes that result from less friction) At frontal surfaces Around jet streams Near the ground, wind shear is found… At frontal shear With thunderstorms When there are temperature inversions Around physical obstructions like buildings and hills 12: Turbulence—Key Points 15/25  Aviation Basics Course Course 5: Weather and Flight High-Level Wind Shear Wind shear is classified as high-level or low-level. High-level wind shear (HLWS) turbulence is encountered at higher altitudes and is mainly associated with the jet stream. If the wind shear—when entering or leaving the jet stream—is quite violent, the turbulence caused by the wind shear is violent. This violent turbulence is known as clear air turbulence (CAT). CAT is sudden, severe turbulence that occurs without any visual warnings (such as clouds). It is caused by the meeting of air masses moving at very different speeds. The forces experienced in clear air turbulence can be intense enough to impose serious structural stress on an aircraft and physical injuries on its passengers, especially since it occurs without warning. You will learn more about CAT later. Figure 18: Pilots entering or leaving the jet stream can encounter high-level wind shear and turbulence 12: Turbulence—Key Points 16/25  Aviation Basics Course Course 5: Weather and Flight Low-Level Wind Shear Low-level wind shear (LLWS) is hazardous because of the proximity of the aircraft to the ground. LLWS is characterized by wind direction changes that could go as far as 180 degrees and speed changes up to 50 knots It is commonly associated with passing frontal systems, thunderstorms, and temperature inversions with strong upper-level winds (greater than 25 knots). LLWS is not a daily occurrence—in fact, it is unusual. That makes it more of a problem. In recent years, 60 percent of aircraft accidents that resulted in the destruction of the aircraft happened during the final approach and landing phase of flight. These losses were typically due to LLWS of one type or another. Figure 19: Although rare, aircraft could encounter low-level wind shear on final approach 12: Turbulence—Key Points 17/25  Aviation Basics Course Course 5: Weather and Flight As you have seen, extreme weather conditions can be very dangerous to aircraft. You can assist pilots by warning them in time to prevent surprise encounters with reported weather problems such as: Low-level wind shear Sudden deterioration of visibility Rapid change in precipitation You will learn more about how to do that a little later. Effects of LLWS on Aircraft You will now see examples of how wind shear can affect arriving and departing aircraft. Arriving Aircraft Example 1 In this example, a warm front is approaching the airport. The wind ahead of the front is from 040°, while behind the front the wind is from 220°. The lateral shear that occurs at the frontal surface causes the aircraft to drift rapidly to the left, requiring a large correction to the right near the runway. This is a dangerous situation and could be even more dangerous in conditions of poor visibility. Figure 20: A warm front approaches the airport 12: Turbulence—Key Points 18/25  Aviation Basics Course Course 5: Weather and Flight Example 2 In this example, an aircraft is on final approach to the runway. The aircraft is above the frontal surface in tailwind conditions and has developed a rapid ground speed equal to airspeed plus wind speed. When the aircraft passes through the frontal surface, it could encounter a headwind or calm conditions. Failure to sufficiently reduce power puts the plane at risk of overshooting the runway. The aircraft’s momentum will prevent it from immediately slowing to the new ground speed (airspeed minus wind speed). Airspeed will initially increase The aircraft will pitch up due to increased lift Altitude will increase, causing the aircraft to be above the glide path To correct, the pilot must drop the nose and reduce the thrust to regain the glide path and reduce airspeed. If the correction is not enough, the aircraft will be too high to land safely. Very soon after making the initial corrections, the momentum developed during the tailwind will fall off. Airspeed will decrease Lift will decrease An immediate increase in power will be required Failure to correct in time would result in a hard landing short of the runway or a crash. Figure 21: An aircraft on final approach encounters LLWS 12: Turbulence—Key Points 19/25  Aviation Basics Course Course 5: Weather and Flight Example 3 In this example, the final approach commences in headwind conditions, and a tailwind or calm is encountered when the aircraft passes through the frontal surface. The initial effects are: Airspeed decreases Aircraft nose pitches down Altitude decreases The aircraft is “low and slow” (a power-deficient state) and thrust must be increased initially to resume normal approach speed and regain the glide path. If the aircraft is too low or if the correction is not sufficient or soon enough, a hard short landing or crash could result. As the airspeed stabilizes, thrust must be reduced to maintain normal approach speed. Otherwise, the aircraft will soon be “high and fast” and may not be able to stop in the available runway length. Figure 22: An aircraft faces a headwind and then a tailwind on final approach 12: Turbulence—Key Points 20/25  Aviation Basics Course Course 5: Weather and Flight The examples you have seen give you an indication of what can happen when arriving aircraft encounter low-level wind shear. When a pilot is made aware of the wind shear, the pilot may choose to: Proceed to another destination Hold and wait for the front or CB to pass Request approach to another runway or adjacent airports Departing Aircraft Low-level wind shear may be dangerous to departing aircraft during climb-out. In this example, the turn after takeoff exposes the aircraft to a rapid change from headwind to tailwind conditions. The result is in an immediate loss of airspeed and altitude that could be fatal. Figure 23: A departing aircraft rapidly moves from headwind to tailwind conditions When wind shear is suspected during departure, the pilot should determine the direction for climb-out to avoid unfavourable conditions and request an appropriate ATC clearance (if required). If it is not possible to avoid an encounter with rapidly decreasing headwinds (or worse, a tailwind) the pilot may have to delay the flight. 12: Turbulence—Key Points 21/25  Aviation Basics Course Course 5: Weather and Flight Chop Versus Turbulence The terms chop and turbulence are often confused. Whereas turbulence causes changes in altitude or attitude, chop causes slight, rapid, rhythmic bumps and does not cause an appreciable change in altitude or attitude. Chop is similar to driving over a series of speed bumps in a parking lot. While both chop and turbulence may be reported as light or moderate, only turbulence is reported as severe. You have already seen descriptions of the different intensities of turbulence. This table compares the intensities of chop versus turbulence. Chop Turbulence Causes slight, rapid and somewhat Momentarily causes slight, erratic rhythmic bumpiness without changes in the altitude and attitude. Light appreciable changes in altitude or attitude. Causes rapid bumps or jolts without Causes changes in altitude and appreciable changes in aircraft attitude, but the aircraft remains in Moderate altitude or attitude. control at all times. Changes in indicated airspeed may occur. N/A Causes large and abrupt changes in aircraft altitude and attitude as well Severe as large variations in airspeed. The aircraft may be momentarily out of control. 12: Turbulence—Key Points 22/25  Aviation Basics Course Course 5: Weather and Flight Clear Air Turbulence We touched on clear air turbulence (CAT) earlier. Because CAT poses considerable danger to aircraft, it is important for you to understand it in more detail. Recall that CAT occurs when a fast-moving jet stream and a slow-moving jet stream pass alongside each other. The air between those streams is disturbed, and an aircraft flying through that area will experience very bumpy conditions. Figure 24: Clear air turbulence (CAT) When we use the term CAT we normally mean jet-stream wind-shear turbulence; however, it is possible for CAT to occur due to other causes like wake turbulence, convection currents, and irregular terrain. CAT usually occurs at altitudes of 20 000 feet to 40 000 feet. At lower altitudes it may occur near mountain ranges. It is most severe near or just above and below the jet stream core. CAT is especially hazardous because pilots cannot see it and radar cannot detect it. CAT occurs without warning in clear air that is closely associated with the jet stream in a mostly cloudless sky. CAT may be severe enough to be a hazard to high-performance aircraft. Its effect is often most pronounced on sharply curved jets. 12: Turbulence—Key Points 23/25  Aviation Basics Course Course 5: Weather and Flight What you should remember about CAT: It is characterized by a rapid change of wind direction over a short distance. It occurs in patches. Its area is elongated by the wind. It usually occurs at altitudes of 20 000 feet to 40 000 feet, but can be encountered at altitudes as low as 15 000 feet. It is associated with a marked change in wind speed over a short distance in the vertical (vertical shear) or in the horizontal (horizontal shear). There are no clouds present at the altitudes at which it occurs. It is transitory. It occurs most frequently during winter and least frequently during summer. Pilot Procedures for Reporting Turbulence Pilots have specific procedures for reporting turbulence. They must report: Position Time Altitude Type of aircraft Type of turbulence (turbulence, chop, CAT) and intensity (light, moderate or severe) You will learn more about procedures for reporting turbulence in subsequent lessons. 12: Turbulence—Key Points 24/25  Aviation Basics Course Course 5: Weather and Flight Wrap-Up In this section, you looked at turbulence and wind shear and how they affect aircraft. You should now be familiar with: Causes of turbulence Stable and unstable air Categories and intensities of turbulence Downbursts Wind shear (high-level wind shear and low-level wind shear) How low-level wind shear affects arriving and departing aircraft Chop and turbulence Clear air turbulence Procedures for reporting turbulence to pilots 12: Turbulence—Key Points 25/25