Icing on an aircraft is one of the most hazardous types of weather-related conditions encountered in the aviation industry. Icing can affect some regions of the country for over 6 months out of the year.
Certain critical areas on an aircraft are especially vulnerable to icing such as the wing flaps and ailerons, and horizontal and vertical stabilizers. Any accumulation of ice on these surfaces not only increases the weight of the aircraft, but most importantly, the ice changes the profile of the wing’s surface which disrupts the airflow around the wings which reduces lift, decreases thrust, and increases drag on the aircraft.
Even a trace amount of ice will disrupt the flow of air over the wings and could result in a loss of control of the aircraft. The handling and prevention of the build-up of ice is a very important part of aircraft maintenance and every precaution needs to be taken to remove icing before departure.
Effects of Ice on an Aircraft
Ice not only influences the function of the wings, but it has critical effects on other aircraft systems. Icing also seriously impairs aircraft engine performance either from icing on the propeller which can lead to a loss of power or when ice forms in the air intake of an engine reducing the air required to support proper combustion. Larger ice chunks could also break off and be sucked into the engine causing structural damage.
Ice build-up on the pilot tube and static pressure ports can change the air pressure around those instruments and create false indications on critical flight instruments such as the altimeter, airspeed indicator and rate of climb. Ice forming on the radio antenna can distort its shape which can impede VOR reception and result in communications issues between the aircraft and ATC. Ice forming on the windshield can reduce visibility and obscure the runway during critical times such as take-offs and landings.
Aircraft Icing Types
There are three main types of icing: Clear, Rime, and Mixed.
‘Clear ice’ is the most hazardous ice type. It forms when water droplets hit the aircraft and freezes slowly. It is the densest type of ice and because it freezes slowly, it can spread over a large part of the airframe’s surface and disrupt airflow.
‘Rime ice’ forms when water droplets freeze rapidly when contacting an aircraft. It has a milky color and the texture resembles a thick frost. This is the most common form of aircraft icing, but the least serious as it usually only forms on the leading edges of the wings or tail.
‘Mixed ice’ is a blending of clear and rime ice. Mixed ice can also spread over a large portion of the airframe.
When icing is predicted the best recommendations are to fly above the icing threat or fly around the hazardous weather activity. Unfortunately, an icing threat may still remain during takeoff and landing, as the pilot may need to fly through icing conditions. Pilots need to be aware of the effects of icing on aircraft systems and the proper procedures to be followed during encounters with icy weather conditions.
Aircraft Icing Prevention
The main way to prevent icing on the aircraft is to keep the aircraft away from the elements as much as possible before takeoff. Keeping the aircraft in a heated hangar is a common procedure for smaller general aviation aircraft. The use of wing covers or other temporary coverings will often reduce the amount of ice buildup and save the time that would be required for the deicing of the aircraft.
The procedures to be used for removal of ice from an aircraft vary, depending on the type of ice accumulation, the type of aircraft and the procedures recommended by the aircraft manufacturer, and by the fluid manufacturer, whose specific mixtures depend on the outside temperature.
There are two main steps employed in the removal of ice from the aircraft.
De-icing is the first step in the process. De-icing is used to quickly remove the ice that is already present on the aircraft. A propylene glycol/water mixture is heated and sprayed on the aircraft to melt the ice. The ratio of the mixture used depends on the temperature and weather conditions.
Anti-icing is used to prevent additional ice from adhering to the aircraft after it has been de-iced. The fluid is thicker than the deicing mixture to allow it to stick to the aircraft surface for a longer period of time and protect the aircraft during takeoff.
The ‘Holdover Time’ is the amount of time between when the de-icing or anti-icing is performed until the aircraft’s takeoff. For de-icing fluid, the holdover time is usually under 20 minutes, depending on the temperature and precipitation. Anti-icing of an aircraft allows for longer periods between the application of the mixture and takeoff.
A relatively short holdover time is a reason why de-icing is performed after all of the passengers are on board and the plane is ready for takeoff. At many airports, de-icing is performed away from the gate at an area called the de-ice pad. Many times, it is off of the taxiway so that the aircraft is already near the runway and can take off quickly after the de-icing. The pilot always has the final say as to whether to takeoff after de-icing and/or anti-icing the aircraft and must ensure that the de-icing was completed properly in accordance with FAA rules.
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