16-7 particles. Sandstorms are best developed in desert regions where there is loose sand, often in dunes, without much admixture of dust.
16.1.8.1 Haboob. A haboob (see Figure 16-5) is a dust storm or sandstorm that forms as cold downdrafts from a thunderstorm turbulently lift dust and sand into the air. While haboobs are often short-lived, they can be quite intense. The dust wall may extend horizontally for more than 60 miles (100 kilometers) and rise vertically to the base of the thunderstorm. Spinning whirlwinds of dust frequently form along the turbulent cold air outflow, giving rise to huge dust/sand whirls.
Figure 16-5. Haboob
16.1.9 Volcanic Ash. Volcanic ash is made up of fine particles of rock powder that originate from a volcano and that may remain suspended in the atmosphere for long periods.
Severe volcanic eruptions that send ash into the upper atmosphere occur somewhere around the world several times per year. The ash plume may not be visible, especially at night or in instrument meteorological conditions (IMC). Even if visible, it is difficult to distinguish visually between an ash cloud and an ordinary cloud. Ash cannot be detected by air traffic control (ATC) radar. However, it may be detected by weather radar, particularly during the early stages of a volcanic eruption when the ash is more concentrated.
Flying into a volcanic ash cloud can be exceedingly dangerous. Volcanic ash is composed of silica (glass). When ash is ingested into a jet engine, it melts to produce a soft sticky molten product that adheres to the compressor turbine blades and fuel injectors/igniters.
With no air going into the engine, the fuel cannot ignite, the engine comes to a slow
16-8 spinning stop by spooling down, and a flameout occurs. As the aircraft exits the ash cloud and into colder temperatures, the cooled, hardened silicas on the turbine blades become dislodged, allowing the fan blades to start rotating, and allow for an engine relight as the air starts moving through the engine again. Piston-powered aircraft are less likely to lose power, but severe damage is almost certain to ensue after an encounter with a volcanic ash cloud that is only a few hours old.
Volcanic ash also causes abrasive damage to aircraft flying through it at hundreds of miles per hour. Particles impacting the windshield can sandblast the surface into a frosted finish that obscures the pilot’s view. The sandblasting can also remove paint and pit metal on the nose and leading edges of wings and navigation equipment.
Ash contaminates aircraft ventilation, hydraulic, instrument, electronic, and air data systems. Ash covering a runway can cover its markings and cause aircraft to lose traction during takeoffs and landings.
16.2 Low Ceiling and Mountain Obscuration.
16.2.1 Low Ceiling. Stratus is the most frequent cloud associated with low ceilings. Stratus clouds, like fog, are composed of extremely small water droplets or ice crystals suspended in air. An observer on a mountain in a stratus layer would call it fog. Stratus and fog frequently exist together. In many cases, there is no real line of distinction between the fog and stratus; rather, one gradually merges into the other. Flight visibility may approach zero in stratus clouds. Stratus over land tends to be lowest during night and early morning, lifting or dissipating due to solar heating by late morning or early afternoon. Low stratus clouds often occur when moist air mixes with a colder air mass, or in any situation where temperature-dewpoint spread is small.
Not all ceilings are equally hazardous to a pilot. An indefinite ceiling is more hazardous than an equal ceiling caused by a layer aloft. Once a pilot descends below a ceiling caused by a layer aloft, the pilot can see both the ground below and the runway ahead.
However, an indefinite ceiling restricts the pilot’s slant range (air-to-ground) visibility.
Thus, the pilot may not see the runway ahead after he descends below the indefinite ceiling (see Figure 16-6).
16-9 Figure 16-6. Layer Aloft Ceiling Versus Indefinite Ceiling
16.2.2 Mountain Obscuration. A mountain obscuration is a condition in which mountains or mountain ridges are obscured due to clouds, precipitation, smoke, or other obscurations.
Flight can be especially hazardous over mountain routes when the mountains are obscured. The large elevation variations around mountains can cause surface weather observations to mislead. For example, a weather station located in a valley could report a visual flight rules (VFR) cloud ceiling, while a hiker in the mountains sees fog.
17-1 CHAPTER 17. TURBULENCE
17.1 Introduction. Aircraft turbulence is irregular motion of an aircraft in flight, especially when characterized by rapid up-and-down motion caused by a rapid variation of atmospheric wind velocities. Turbulence varies from annoying bumpiness to severe jolts which cause structural damage to aircraft and/or injury to its passengers. Turbulence intensities and their associated aircraft reactions are described in the Aeronautical Information Manual (AIM).
17.2 Causes of Turbulence. Turbulence is caused by convective currents (called convective turbulence), obstructions in the wind flow (called mechanical turbulence), and wind shear.
17.2.1 Convective Turbulence. Convective turbulence is turbulent vertical motions that result from convective currents and the subsequent rising and sinking of air. For every rising current, there is a compensating downward current. The downward currents frequently occur over broader areas than do the upward currents; therefore, they have a slower vertical speed than do the rising currents.
Convective currents are most active on warm summer afternoons when winds are light.
Heated air at the surface creates a shallow, absolutely unstable layer within which bubbles of warm air rise upward. Convection increases in strength and to greater heights as surface heating increases. Barren surfaces such as sandy or rocky wastelands and plowed fields become hotter than open water or ground covered by vegetation. Thus, air at and near the surface heats unevenly. Because of uneven heating, the strength of convective currents can vary considerably within short distances.
As air moves upward, it cools by expansion. A convective current continues upward until it reaches a level where its temperature cools to the same as that of the surrounding air.
If it cools to saturation, a cumuliform cloud forms.
Billowy cumuliform clouds, usually seen over land during sunny afternoons, are signposts in the sky indicating convective turbulence. The cloud top usually marks the approximate upper limit of the convective current. A pilot can expect to encounter turbulence beneath or in the clouds, while above the clouds, air generally is smooth (see Figure 17-1). When convection extends to great heights, it develops larger towering cumulus clouds and cumulonimbus with anvil-like tops. The cumulonimbus gives visual warning of violent convective turbulence.
17-2 Figure 17-1. Convective Turbulence
When the air is too dry for cumuliform clouds to form, convective currents can still be active. This is called dry convection, or thermals (see Figure 17-2). A pilot has little or no indication of their presence until encountering the turbulence.
Figure 17-2. Thermals
17-3
17.2.2 Mechanical Turbulence. Mechanical turbulence is turbulence caused by obstructions to the wind flow, such as trees, buildings, mountains, and so on. Obstructions to the wind flow disrupt smooth wind flow into a complex snarl of eddies (see Figure 17-3).
An aircraft flying through these eddies experiences mechanical turbulence.
Figure 17-3. Mechanical Turbulence
The intensity of mechanical turbulence depends on wind speed and roughness of the obstructions. The higher the speed and/or the rougher the surface, the greater the turbulence.
The wind carries the turbulent eddies downstream. How far depends on wind speed and stability of the air. Unstable air allows larger eddies to form than those that form in stable air; but the instability breaks up the eddies quickly, while in stable air they dissipate slowly.
17.2.2.1 Mountain Waves. A mountain wave (see Figure 17-4) is an atmospheric wave disturbance formed when stable air flow passes over a mountain or mountain ridge. Mountain waves are a form of mechanical turbulence which develop above and downwind of mountains. The waves remain nearly stationary while the wind blows rapidly through them. The waves may extend 600 miles (1,000 kilometers) or more downwind from the mountain range.
Mountain waves frequently produce severe to extreme turbulence. Location and intensity varies with wave characteristics. Incredibly, vertically propagating mountain waves have been documented up to 200,000 feet (60,000 meters) and higher.
17-4 Figure 17-4. Mountain Waves
Mountain waves often produce violent downdrafts on the immediate leeward side of the mountain barrier. Sometimes the downward speed exceeds the maximum climb rate of an aircraft and may drive the aircraft into the mountainside.
A mountain wave cloud is a cloud that forms in the rising branches of mountain waves and occupies the crests of the waves. The most distinctive are the sharp-edged, lens-, or almond-shaped lenticular clouds. When sufficient moisture is present in the upstream flow, mountain waves produce interesting cloud formations (see Figure 17-5) including: cap clouds, cirrocumulus standing lenticular (CCSL), Altocumulus Standing Lenticular (ACSL), and rotor clouds. These clouds provide visual proof that mountain waves exist. However, these clouds may be absent if the air is too dry.
For additional information on hazardous mountain waves, refer to the current edition of AC 00-57, Hazardous Mountain Winds and their Visual Indicators.
17-5 Figure 17-5. Mountain Wave Clouds
17.2.3 Wind Shear Turbulence. Wind shear is the rate of change in wind direction and/or speed per unit distance. Wind shear generates turbulence between two wind currents of different directions and/or speeds (see Figure 17-6). Wind shear may be associated with either a wind shift or a wind speed gradient at any level in the atmosphere.
Figure 17-6. Wind Shear Turbulence
17-6
17.2.3.1 Temperature Inversion. A temperature inversion is a layer of the atmosphere in which temperature increases with altitude. Inversions commonly occur within the lowest few thousand feet above ground due to nighttime radiational cooling, along frontal zones, and when cold air is trapped in a valley. Strong wind shears often occur across temperature inversion layers, which can generate turbulence (see Figure 17-7).
Figure 17-7. Wind Shear Turbulence Associated with a Temperature Inversion
17.2.3.2 Clear Air Turbulence (CAT). Clear Air Turbulence (CAT) is a higher altitude (~20,000 to 50,000 feet) turbulence phenomenon occurring in cloud-free regions associated with wind shear, particularly between the core of a jet stream and the surrounding air. It can often affect an aircraft without warning. CAT frequency and intensity are maximized during winter when jet streams are strongest.
For additional information on CAT, refer to the current edition of AC 00-30, Clear Air Turbulence Avoidance.
17.3 Turbulence Factors. How an aircraft will respond to turbulence varies with: intensity of the turbulence, aircraft size, wing loading, airspeed, and aircraft altitude. When an aircraft travels rapidly from one current to another, it undergoes abrupt changes in acceleration.
18-1 CHAPTER 18. ICING
18.1 Introduction. In general, icing is any deposit of ice forming on an object. It is one of the major weather hazards to aviation. Icing is a cumulative hazard. The longer an aircraft collects icing, the worse the hazard becomes.
18.2 Supercooled Water. Freezing is a complex process. Pure water suspended in the air does not freeze until it reaches a temperature of -40 °C. This occurs because surface tension of the droplets inhibits freezing. The smaller and purer the water droplet, the more likely it is supercooled. Also, supercooled water can exist as large drops known as Supercooled Large Drops (SLD). SLDs are common in freezing rain and freezing drizzle situations (see paragraph 18.3.2.1).
Supercooled water content of clouds varies with temperature. Between 0 and -10 °C clouds consist mainly of supercooled water droplets. Between -10 and -20 °C, liquid droplets coexist with ice crystals. Below -20 °C, clouds are generally composed entirely of ice crystals. However, strong vertical currents (e.g., cumulonimbus) may carry supercooled water to great heights where temperatures are as low as -40 °C.
Supercooled water will readily freeze if sufficiently agitated. This explains why airplanes collect ice when they pass through a liquid cloud or precipitation composed of supercooled droplets.
18.3 Structural Icing. Structural icing is the stuff that sticks to the outside of the airplane.
It occurs when supercooled water droplets strike the airframe and freeze. Structural icing can be categorized into three types: rime, clear (or glaze), and mixed.
Icing intensities are described in the Aeronautical Information Manual (AIM).
18.3.1 Rime Icing. Rime ice is rough, milky, and opaque ice formed by the instantaneous freezing of small, supercooled water droplets after they strike the aircraft. It is the most frequently reported icing type. Rime ice can pose a hazard because its jagged texture can disrupt an aircraft’s aerodynamic integrity.
Rime icing formation favors colder temperatures, lower liquid water content, and small droplets. It grows when droplets rapidly freeze upon striking an aircraft. The rapid freezing traps air and forms a porous, brittle, opaque, and milky-colored ice. Rime ice grows into the air stream from the forward edges of wings and other exposed parts of the airframe.
18.3.2 Clear Icing. Clear ice (or glaze ice) is a glossy, clear, or translucent ice formed by the relatively slow freezing of large, supercooled water droplets. Clear icing conditions exist more often in an environment with warmer temperatures, higher liquid water contents, and larger droplets.
Clear ice forms when only a small portion of the drop freezes immediately while the remaining unfrozen portion flows or smears over the aircraft surface and gradually freezes. Few air bubbles are trapped during this gradual process. Thus, clear ice is less
18-2 opaque and denser than rime ice. It can appear either as a thin smooth surface, or as rivulets, streaks, or bumps on the aircraft.
Clear icing is a more hazardous ice type for many reasons. It tends to form horns near the top and bottom of the airfoils leading edge, which greatly affects airflow. This results in an area of disrupted and turbulent airflow that is considerably larger than that caused by rime ice. Since it is clear and difficult to see, the pilot may not be able to quickly recognize that it is occurring. It can be difficult to remove since it can spread beyond the deicing or anti-icing equipment, although in most cases it is removed nearly completely by deicing devices.
18.3.2.1 Supercooled Large Drops (SLD). A type of clear icing that is especially dangerous to flight operations is ice formed from SLDs. These are water droplets in a subfreezing environment with diameters larger than 40 microns, such as freezing drizzle (40 to 200 microns) and freezing rain (>200 microns).
These larger droplets can flow along the airfoil for some distance prior to freezing. SLDs tend to form a very lumpy, uneven, and textured ice similar to glass in a bathroom window.
SLD ice tends to form aft, beyond the reach of deicing equipment. Thus, ice remaining on the airfoil continues to disrupt the airflow and reduce the aircraft’s aerodynamic integrity. Even a small amount of ice on the lower and upper surfaces of the airfoil can seriously disrupt its aerodynamic properties.
The residual ice generates turbulence along a significant portion of the airfoil.
This residual ice can act as a spoiler, a device actually used to slow an aircraft in flight. In extreme cases, turbulence and flow separation bubbles can travel along the airfoil and inadvertently activate the ailerons, creating dangerously unstable flying conditions.
18.3.3 Mixed Icing. Mixed ice is a mixture of clear ice and rime ice. It forms as an airplane collects both rime and clear ice due to small-scale (tens of kilometers or less) variations in liquid water content, temperature, and droplet sizes. Mixed ice appears as layers of relatively clear and opaque ice when examined from the side.
Mixed icing poses a similar hazard to an aircraft as clear ice. It may form horns or other shapes that disrupt airflow and cause handling and performance problems. It can spread over more of the airframe’s surface and is more difficult to remove than rime ice. It can also spread over a portion of airfoil not protected by anti-icing or deicing equipment.
Ice forming farther aft causes flow separation and turbulence over a large area of the airfoil, which decreases the ability of the airfoil to keep the aircraft in flight.
18.3.4 Icing Factors. Structural icing is determined by many factors. The meteorological quantities most closely related to icing type and severity are, in order of importance:
Supercooled Liquid Water Content (SLWC), temperature (altitude), and droplet size.
However, aircraft type/design and airspeed are also important factors.
18-3 SLWC is important in determining how much water is available for icing. The highest quantities can be found in cumuliform clouds with the lowest quantities in stratiform clouds. However, in most icing cases, SLWC is low.
Icing potential is very temperature dependent. For icing to occur, the outside air temperature must be below 0 °C. As clouds get colder, SLWC decreases until only ice crystals remain. Thus, almost all icing tends to occur in the temperature interval between 0 °C and -20 °C, with about half of all reports occurring between -8 °C and -12 °C. In altitude terms, the peak of occurrence is near 10,000 feet, with approximately half of incidents occurring between 5,000 feet and 13,000 feet. The only physical cold limit to icing is at -40 °C because liquid droplets freeze without nuclei present.
In general, rime icing tends to occur at temperatures colder than -15 °C, clear when the temperature is warmer than -10 °C, and mixed ice at temperatures in between. This is only general guidance. The type of icing will vary depending on the liquid water content, droplet size, and aircraft-specific variables.
An airframe can remain cold (temperature below 0 °C) in a warm (temperature above 0 °C) atmosphere if it is cold soaked. For example, if an aircraft has been flying in a cold environment, but then descends into warmer temperatures, the airframe does not heat up immediately to the air temperature. For some aircraft, the airframe can remain colder than 0 °C for some time, even after landing. Aircraft with fuel tanks flush-mounted to the airframe are particularly susceptible to icing, even in an environment where the air temperature is slightly above 0 °C. Because these characteristics vary from airframe to airframe, it is important for pilots to be aware of the limitations of their aircraft.
Droplet size can influence icing, but is not as important as SLWC and temperature, unless the droplets are larger than cloud droplets in size (e.g., freezing drizzle and freezing rain).
Droplet size affects the collection of drops by the airframe. Small droplets tend to impact the airfoil near the plane’s leading edge. Larger drops, including freezing rain and freezing drizzle, can cross the streamlines and impact farther back.
Aircraft airspeed is an important nonmeteorological factor which determines icing type and severity. The rate of supercooled water droplet impact increases with airspeed, which acts to increase ice accumulation, but this is counteracted by the increase of airframe skin surface heating due to friction. Typically, airframe icing is negligible at speeds above 575 knots.
Aircraft type and design are also important factors. Because these characteristics vary, it is important for pilots to be aware of the limitations of their aircraft.
Commercial jet aircraft are generally less vulnerable to structural icing than light turboprop aircraft. This is due to their rapid airspeed, powerful deicing equipment, and tendency to cruise at higher altitudes where temperatures are typically too cold for icing.
Conversely, light turboprop aircraft are more susceptible to icing because they typically fly at lower altitudes where icing is more common and at slower speeds.