2-1. AIRCRAFT ICING CONDITIONS.
a. Supercooled Clouds. Nearly all aircraft icing occurs in supercooled clouds. Liquid drops are present at outside air temperatures (OAT) below 0 °C (32 °F) in these clouds. At OAT close to 0 °C (32 °F), the cloud may consist entirely of such drops, with few or no ice particles present. At decreasing temperatures, the probability increases that ice particles will exist in significant numbers along with the liquid drops. In fact, as the ice water content increases, the Liquid Water Content (LWC) tends to decrease since the ice particles grow at the expense of the water particles. At temperatures below about -20 °C (-4 °F), most clouds are made up entirely of ice particles.
b. Ice Accumulation. The general rule is that the more ice particles and the fewer liquid drops that are present, the less ice accumulation on the airframe. This is because the ice particles tend to bounce off an aircraft surface, while the supercooled drops freeze and adhere. As a result, ice accumulation is often greatest at temperatures not too far below 0 °C (32 °F), where LWC can be abundant. LWC is usually negligible at temperatures below about -20 °C (-4 °F).
c. Thermal Ice Protection. An exception to the general rule just stated may be made for surfaces heated by a thermal ice protection system (or from compressibility at higher speeds).
d. Runback Ice Accretion. Tests have shown that when outside air temperatures (OAT) are near freezing, the result is no ice accretion near the stagnation point, but the freezing or refreezing of water running back on the airfoil, causing runback ice accretions, possibly behind the protected areas. The formation of a ridge is possible. Pilots should be vigilant at OAT between -5 °C (23 °F) and +2 °C (35 °F).
e. Drop Adherence. The greater the LWC of the cloud, the more rapidly ice accumulates on aircraft surfaces. The size of the drops also is important. Larger drops have greater inertia and are less influenced by the airflow around the aircraft than smaller drops. The result is that larger drops will adhere to more of the aircraft surface than smaller drops.
f. Median Volume Diameter (MVD). Every supercooled cloud contains a broad range of drops, starting from between 1 and 10 micrometers (millionth of a meter) and usually not exceeding 50 micrometers (by comparison, the thickness of the average human hair is approximately 100 micrometers). A single drop size must be chosen as representative, and in icing terminology this is the MVD, the diameter such that half the liquid water is in smaller drops, and half in larger drops.
NOTE: Icing conditions can occur during operations in clouds with a significant amount of liquid water in drops with diameters larger than 100 micrometers. These conditions are referred to as freezing drizzle aloft in cloud or Supercooled Large Drop (SLD) in cloud. Paragraph 3-16 discusses some cues developed for aircraft with unpowered controls and pneumatic deicing boots, mainly relating to the location of the airframe ice, which the flightcrew can use in attempting to determine if such drops may be present in a cloud.
g. SLD Conditions. An aircraft does not always have to be in a cloud to encounter SLD as conditions can exist in freezing precipitation below a cloud deck.
2-2. CLOUD TYPES AND AIRCRAFT ICING.
a. Formation. Air can rise because of many factors, including convection, orographic lifting (i.e., air forced up a mountain), or lifting at a weather front. As the air rises, it expands and cools adiabatically. If a parcel of air reaches its saturation point, the moisture within the parcel will condense and the resulting drops form a cloud. Cloud water drops are generally very small, averaging 20 micrometers in diameter, and are of such small mass that they can be held aloft by small air currents within clouds.
b. Extent of Icing. If rising air is moist (i.e., water vapor is plentiful) and lifting is vigorous, the result can be clouds with substantial LWC and, sometimes, large drops. The greater the LWC, the more rapid the icing; and the larger the drops, the greater the extent of icing. Tops of clouds often contain the most liquid water and largest drops, because the drops that reach the tops have undergone the most lifting. If the temperatures are cold enough at the tops (below or around -15 °C (5 °F)), ice particles will usually start to form that tend to deplete the liquid water.
c. Hazardous Conditions. Several types of clouds and the hazardous aircraft icing conditions that may be associated with them are discussed below.
(1) Stratus Clouds.
(a) Stratus clouds, sometimes called layer clouds, form a stratified layer that may cover a wide area. The lifting processes that form them are usually gradual, and so they rarely have exceptionally high liquid water contents. Icing layers in stratus clouds with a vertical thickness in excess of 3,000 feet are rare, so a change of altitude of a few thousand feet may take the aircraft out of icing.
(b) Lake-effect stratus clouds are exceptional in that they may have very high LWC because of the moisture available when they form over lakes. In the continental United States, lake-effect stratus clouds are most common in the Great Lakes region, particularly in early winter when cold northwesterly winds blow over the unfrozen lakes.
(c) Drizzle-size drops occasionally occur in stratus clouds, and pilots should always be on the lookout for cues that might indicate the presence of these drops (see paragraph 3-16 for a list of cues developed for aircraft with unpowered controls and pneumatic deicing boots).
(2) Cumulus Clouds.
(a) Cumulus clouds, which often form because of vigorous convection, can have high LWC. If an aircraft traverses them, the icing can be rapid. Because they tend to be of limited horizontal extent, it may be possible to avoid many of them. Because of the vertical development of cumulus clouds, icing conditions can be found in layers thousands of feet in depth, but with much less horizontal development than in stratus clouds.
(b) This class of clouds includes the cumulonimbus, or thunderstorm, clouds. Updrafts in such clouds can be great and result in very large LWCs. Thus, a large icing threat can be added to the other excellent reasons to stay out of such clouds. The thunderhead anvil can spread out from the core for several miles and is composed mainly of ice crystals. These crystals will not adhere to unheated surfaces when they hit, but they may melt on a heated surface, run back, and refreeze. The ice content in the anvils can be high, and ingestion of the ice crystals has resulted in uncommanded thrust reductions.
(3) Orographic Clouds, Wave Clouds, and Cirrus Clouds.
(a) Orographic clouds form when moist air is lifted by flowing up the side of a mountain. As the parcel of air is lifted, it cools and forms a cloud. Such clouds can contain a large volume of water and, in some cases, large drops.
(b) Wave clouds, recognized by their wavy tops, can have high LWCs. Continued flight along a wave may result in airframe icing.
(c) Cirrus clouds, found at very high, cold altitudes, are composed entirely of ice particles. Flight through these clouds should not result in structural icing, although the possibility exists for runback icing from the refreezing of particles that melted on thermally or aerodynamically heated surfaces.
d. Freezing Rain and Freezing Drizzle.
(1) Freezing rain forms when rain becomes supercooled by falling through a subfreezing layer of air. Ordinarily, air temperatures decrease with increasing altitude, but freezing rain requires a temperature inversion, which can occur when a warmer air mass overlies a colder air mass. This situation can occur along a warm front, where a warm air mass overruns a cold air mass. When flying in freezing rain, normally there is warm air (above 0 °C (32 °F)) above.
(2) Freezing raindrops are defined as drops of 500 micrometers (0.5 mm) diameter or larger. A typical diameter is 2 mm, and the few that grow much larger than about 6 mm tend to break up. Using 20 micrometers (0.02 mm) as a typical diameter for a cloud drop, the diameters of rain and cloud drops differ by a factor of approximately 100, and the volume and mass differ by a factor of about 1,000,000. Drop mass affects how far aft of the stagnation point (leading edge surfaces) drops will strike the aircraft. Subsequently, freezing rain will result in ice forming in areas far aft of where it would normally form in icing conditions without freezing rain.
(3) Drops of freezing drizzle consist of supercooled liquid water drops that have diameters smaller than 500 micrometers (0.5 mm) and greater than 50 micrometers (0.05 mm). While smaller than freezing rain, drops of freezing drizzle are still larger than regular cloud drops, and can form through the same process. It consists of supercooled liquid water drops that have diameters smaller than 500 micrometers (0.5 mm) and greater than 50 micrometers (0.05 mm). However, freezing drizzle is perhaps more commonly formed by a different process, known as the collision-coalescence process. When some drops in a cloud grow to approximately 30 micrometers in diameter through condensation, they begin to settle, falling fast enough so that they collide with some smaller drops. If the drops coalesce, the result is a larger drop, which now has an even better chance of capturing smaller drops. Under favorable conditions, this process can produce drizzle-size drops in a supercooled cloud, usually near the top, where the larger drops generally are found in any cloud. Statistics vary, but some studies have reported that freezing drizzle aloft forms more than 80 percent of the time by the collision-coalescence process in nonconvective clouds. Thus, in freezing drizzle, the pilot cannot assume that a warm layer (above 0 °C (32 °F)) exists above the aircraft. When drizzle drops are found within a supercooled cloud, they can result in accretions that cause very rapid and dangerous stall speed and drag increases for some aircraft and roll control anomalies for others. These situations may be caused by the roughness, shape, and extent of the accretion that forms. This is an instance of SLD icing as discussed earlier in this paragraph.
a. Formation. When air masses of differing temperatures, pressures, or relative humidity meet, a front is formed. If the front moves so that warmer air replaces colder air, it is called a warm front; if it moves so that colder air replaces warmer air, it is called a cold front. An occluded front forms when an air mass is trapped between two colder air masses and is forced to higher and higher levels. In all three cases, significant lifting occurs. If sufficient moisture and subfreezing temperatures are present, icing conditions are created.
b. Warm and Cold Fronts. Along a warm front, the warmer air tends to slide gradually over the cold front, forming stratus clouds conducive to icing (see Figure 2-1). In a cold front, the cold air plows under the warm air, lifting it more rapidly and resulting in the formation of cumulus clouds with high LWC if the lifted air is moist (see Figure 2-2). SLD in the form of freezing rain and freezing drizzle are sometimes found near fronts, as explained above.
c. Navigating a Front. Because of the icing and other hazards associated with some fronts, exposure to icing conditions of varying severity is possible. When flying through a front, the pilot should take the shortest route through the front, instead of flying along the front, to reduce the time spent in potential icing conditions.
2-4. CONVECTIVE WEATHER AND ICE CRYSTALS.
a. Convective Weather Systems. Convective weather systems, especially those associated with tropical weather fronts, can pump large quantities of moisture to high altitudes that freezes into ice crystals that can remain aloft. These ice crystals can remain as a cloud well after the convective system has decayed. Clouds and temperatures less than 10 °C are better indicators of the possible presence of ice crystals when near convective weather.
b. Hazards. Above flight level (FL) 250, clouds contain little liquid water and mostly contain ice particles. These clouds with no liquid water have about 20 times less radar reflectivity than rain drops, and therefore are difficult to detect. Airborne weather radar will receive little to no returns at these altitudes unless it is tilted down to lower altitudes near or below the freezing level. Strong returns from the lower altitudes indicate the possibility of hail, severe turbulence, or large quantities of ice crystals that could be encountered above and accrete inside turbine engines when overflying these areas. Large deposits may ultimately result in engine upset, engine damage from ice shedding, power loss, or engine shutdown.