Ice Protection System - Airframe Icing

Airframe Icing

Ice accumulates on the leading edges of wings, tailplanes, and vertical stabilizers as an aircraft flies through a cloud containing super-cooled water droplets. Super-cooled water is water that is below freezing, but still a liquid. Normally, this water would turn to ice at 32°F (0°C), but there are no "contaminants" (ice nucleus) on which the drops can freeze. When the airplane flies through the super-cooled water droplets, the plane becomes the droplet nucleus, allowing the water to freeze on the surface. This process is known as accretion.

Droplets of supercooled water often exist in stratiform and cumulus clouds.

A popular misconception is that aircraft icing events result from the weight of accreted ice on the airframe. This is not the case. Rather, airframe icing causes problems by modifying the airflow over flight surfaces upon which the ice accretes. When ice accretes on aerodynamic lift surfaces, such as the wing and tailplane, the modification of airflow changes the aerodynamics of the surfaces by modifying both their shape and their surface roughness, typically increasing their drag and decreasing their lift. The particular effect of icing on the aerodynamics of a lift surface is a complicated function of the ice shape and location as well as of the amount of ice. These characteristics in turn depend in a complicated fashion on atmospheric conditions such as the amount, temperature, and droplet size of water in the air. The composite effect of this aerodynamic deterioration over all lift surfaces is a degradation of aircraft flight dynamics. In severe atmospheric conditions, dangerous levels of icing can be obtained in as little as five minutes. Small to moderate amounts of icing generally cause a reduction in aircraft performance in terms of climb rates, range, endurance, and maximum speed and acceleration. Icing effects of this type are known as performance events. As icing increases, separation of air flow from the flight surfaces can cause loss of pilot control and even wildly unstable behaviour. These more severe icing events, known as handling events, are often precipitated by a change in the aircraft configuration or an aircraft maneuver effected by a pilot unaware of the flight-dynamics degradation. This was the case with American Eagle Flight 4184, where the aircraft experienced an uncontrolled roll of 120 degrees in five seconds after the pilot initiated a flap retraction. Another icing event that led to a major crash was the Aero Caribbean Flight 883 that experienced icing conditions at 20,000 feet height after a crew request of course change. They lost control of the aircraft after they initiated a roll to change the aircraft's direction. This loss of control can be defined as a handling event. Handling events generally can be classified into either tailplane stall, where the aircraft pitches forward, or asymmetric wing effects causing a roll upset (or roll snatch) as in the American Eagle Flight 4184 accident.