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Handling Icing in Flight

Icing, a hazardous condition, leaves a coating or deposit of ice on airplanes. The cumulative weather hazard impairs aircraft efficiency by reducing lift, adding weight, and adding drag.

Combined, the coating and deposit reduce speed or force the aircraft downward while causing a deterioration in engine performance and potentially compromising brake, control surface, and landing gear operations. The situation requires two preconditions - the airplane must fly through cloud droplets, rain, or visible water. Then, the temperature with moisture must be zero degrees centigrade or less. In addition, aerodynamic cooling may result in airfoil temperatures below this critical threshold.

Risks of icing increase with rapid accretion when water becomes a supercooled or unstable liquid state. If an aircraft strikes it, a portion of the supercooled drop freezes instantaneously, generating a latent heat of fusion that increases the temperature of the rest of the drop above the melting point. Then, aerodynamics causes the remaining part to freeze, with the progression determining the type of icing.

There are three structural types of icing: clear, rime, or a mixture of both. Clear ice starts with a portion of large rain or cumuliform cloud droplets that freeze at initial impact, with the remaining liquid parts of the drop flowing out across the aircraft surface. It eventually freezes as a sheet of solid, smooth ice.

Rime ice occurs with drizzle, stratified clouds, and small drops. The remaining liquid portion of the drop freezes quickly after initial impact, with no time to spread out. The small droplets trap air as they freeze, resulting in ice with a white appearance, brittle, and easier to remove than clear ice. Rime ice has a rough surface and irregular shape, which magnifies its impact in degrading aerodynamic efficiency, increasing drag, and reducing lift.

Mixed ice involves raindrops of varying sizes, liquid drops mixed with ice or snow particles that can form extremely rapidly, and ice particles embed themselves in clear ice, forming a rough accumulation, sometimes in the form of a mushroom.

Induction system icing, one of the most dangerous forms of icing, affects the engine’s air intake and deprives it of the airflow needed to support combustion. Carburetor icing impacts engine performance and may reduce intake flow to a point where the engine can no longer operate.

Ice can also distort airflow over the wing, which impacts the aircraft’s handling qualities and maximum lift while adding drag. Tests in flight and wind tunnels have shown that sandpaper-thin accumulations of ice, snow, and frost on the wing's upper surface (leading edge) can increase drag by 40 percent and reduce lift by 30 percent. Larger accretions can create drag that is 80 percent higher than normal.

When flying through clouds, pilots must determine if they contain moisture. The Dakotas, with frigid winters, often have dry clouds. In contrast, wintertime clouds along the Appalachians and over the Great Lakes often have extremely wet and moisture-filled clouds, which present an elevated icing risk. Warm and cold fronts can cause freezing rain and freezing drizzle. Therefore, pilots should find the quickest route through the front rather than along its length.

Icing can also affect instruments. Therefore, pilots should turn on the heat, which keeps the pitot tube warm whenever icing occurs. Pilots should also maximize cabin heat and airflow and engage defrosters on windshields.

Although contemporary planes have anti-icing and de-icing equipment installed, the pilot should remain alert for loss of RPM in the engine, which may indicate a blocked air intake filter. Adjust the throttle to maintain a steady RPM, raise the carb heat, and utilize it until the engine regains normal power. When an extremely rapid ice build-up occurs and turning back is not an option, the pilot should plan a landing at the nearest airport.

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Risks Posed by Windshear When Flying

One of the primary causes of approach-and-landing accidents is adverse wind conditions, which include tailwind, strong crosswinds, and windshear. The latter involves a sudden change of wind direction or velocity over a short distance.

This typically reflects density gradients or temperature inversions, and may occur at any altitude. One indicator that a front has associated windshear is that it is moving at a speed of 30 knots or greater. In addition, the temperature difference across the surface of the front is often more than 10 degrees Fahrenheit.

While it can potentially occur in any direction, windshear is measured along a horizontal and vertical axis and is thus referred to as horizontal windshear or vertical windshear. When variations occur along the vertical axis, turbulence arises that impacts the aircraft as it descends or ascends through the wind shear layer.

Thunderstorms often have associated windshear, with large downdrafts from mature thunderstorms striking the ground and then spreading out horizontally and causing gusts and turbulence. Such winds may change direction as much as 180 degrees, at velocities of more than 100 knots. Particularly problematic is the initial 1,500 feet from the ground, with gust wind speed suddenly changing by up to 50 percent, and most of this change registered in the lowest 150 feet (critical to landings and takeoffs). With windshear occurring as much as 10 miles beyond the visible storm, the Federal Aviation Administration recommends keeping a distance of 20 miles or more from a mature or developing thunderstorm if possible.

While first gusts some distance from a storm pose a risk, these are less severe than the threat of the concentrated downburst. This is a localized, extremely intense downdraft that may entail 720-feet-per-minute vertical velocity and surpass even high performance jets’ climb capacities. Avoiding such downbursts starts with recognizing visual signs such as intense rainfall, roll clouds, and dust clouds.

Even more hazardous than downbursts, microbursts are severe downdrafts concentrated into a small space of between a few hundred and 3,000 feet across. Present in around 5 percent of thunderstorms, they may also occur in apparently clear conditions when the convective conditions are right. Signs that a microburst may be present include rapidly increasing headwinds and light to heavy precipitation.

Another type of localized windshear, posing a threat on landing approaches, is surface obstructions. These include nearby mountains and buildings and should be planned for in any situation where such obstructions are present and strong surface winds also prevail.

Temperature inversion windshear reflects overnight cooling at low altitude, coupled with a low-level jet stream with air of a different temperature. Particularly in the Southwest desert environment, this turbulent flow can escalate in just a few minutes, with surface winds increasing by 20 to 30 knots accompanied by a 90-degree shift.

Windshear risk is highest on landing, where there is not enough altitude, or adequate airspeed, to allow for recovery. It is similarly a concern on takeoffs. This makes preflight weather checks and pilot reports a must, as well as vigilance for visual indicators, gusty conditions, and increases in headwind speed. If there’s a good chance windshear is present, it’s advised to delay takeoff, or at least select a favorable runway, given the potential downburst and wind shear location. When taking off, the pilot should apply maximum takeoff thrust and be ready to engage the brakes if the takeoff must be aborted, provided the runway distance allows.