Flight in icing conditions
Most light aircraft do not have any airframe or propeller de-icing or anti-icing protection and only limited protection for the engine. As such, if you read the Pilots Operating Handbook or Aircraft Flight Manual, you will probably find that your aircraft is not cleared for flight into icing conditions. However, especially in the UK climate, it is possible to experience icing at all times of the year and in various flight conditions so it is important for pilots to have an awareness of the kinds of icing you may experience.
Engine Icing
Typical forms of engine icing include:
Induction Icing. In carburetted piston engines, this is common referred to as ‘Carburettor Icing’. There is a sudden temperature drop due to fuel vaporisation and a further drop due to pressure reduction in the carburettor venturi. If the temperature drop cools the air mixture below its dew point, water in the air will form ice on the surface of the carburettor and can block or seriously disrupt the flow of the fuel/air mix through the carburettor and engine.
Impact Icing. This is ice that can build up on air intakes, filters and alternate air values and can starve the engine of the air needed for combustion. It forms when flying in snow, sleet, and sub-zero cloud and rain. Impact icing can affect fuel injection systems as well as carburettors and is a major hazard for turbocharged engines.
Fuel Icing. Water trapped in fuel may freeze in the induction piping, especially in the elbows formed by bends, and can starve the engine of the required fuel flow.
All these types of engine icing typically result in a progressive loss of power, rough running and possible engine stoppage. If you have a variable pitch propeller, the constant speed unit will try to maintain a constant RPM which can also make it difficult to detect a loss of power - you should watch for a drop in manifold pressure and a reduction in airspeed when level.
Airframe Icing
Ice will form on parts of the aircraft structure if liquid water hits a part of the airframe which has a temperature below freezing. Water droplets in and around clouds with a temperature below freezing do not always turn into ice, but they can remain supercooled in liquid form down to –40°C. The severity of icing conditions and ice accumulation on aircraft can vary enormously and much depends on the concentration of supercooled liquid water and the droplet size.
Airframe icing comes in three main types:
Rime Ice - Rime ice is formed when small supercooled water droplets freeze rapidly on contact with a sub-zero surface. The ice formed is rough and crystalline and opaque. Rime ice commonly forms on leading edges, and it can seriously affect the aerodynamic characteristics of both wings and tailplanes.
Clear Ice - Clear (sometimes also called Glaze ice) is formed by larger supercooled water droplets, but only a small portion of them freeze immediately on impact. This results in runback along the surface and progressive freezing of the remaining liquid. The accreted ice is transparent or translucent. Because the slower freezing allows the water to spread before evenly freezing, the resultant transparent ice may be difficult to visually detect.
Mixed Ice - As the name suggests, mixed ice is a combination of both rime ice and clear ice. It forms when both small and large supercooled droplets are present. The appearance of mixed ice is irregular, rough and whitish and it can form very rapidly.
Airframe Ice accumulation on an aerofoil adversely changes the shape and path of the airflow, reducing lift and increasing drag for a given angle of attack. Ice on a propeller blade reduces the efficiency of the propeller and reduces thrust. In addition, the weight of the ice increases the overall aircraft weight and hence also increases the lift required. Because the angle of attack must be increased to provide the desired lift, the critical (stalling) angle is reached at a higher airspeed than when ice is not present.
Instrument Icing
Icing up of the pitot tube can affect the ram air entry into the tube and give extremely erroneous airspeed indications. Icing and the blockage of an external static pressure port may also effect the indications of all instruments connected to the static system such as the ASI, VSI and Altimeter.
Where will I get icing?
Conditions most likely to cause icing include;
a. Flight through or descents through cloud or precipitation when the Outside Air Temperature (OAT) is close to, or less than 0ºC.
b. Flight between, or just below, cloud layers when the aircraft surfaces or OAT are below 0ºC.
c. Flight in snow, sleet and sub-zero cloud and rain.
However, carburettor icing is most definitely not restricted to these noticeably cold weather conditions. Carburettor icing can occur on warm days if humidity is high – even days when the OAT is 25 ºC can produce a risk of serious icing with a relative humidity as low as 30 ºC. Flying carburetted engines in the range of UK weather conditions and seasons requires constant vigilance for the risk of carburettor icing.
Avoiding Icing Conditions
Fronts and low-pressure areas are large ice producers, but isolated air mass instability with plenty of moisture can generate enough ice in clouds to make light aircraft flight dangerous. Icing is commonly picked up when flying in convective cloud which rises up through the freezing level on an unstable day. If your operating level is above the freezing level but below the tops, you face the potential for icing.
Freezing rain and drizzle can also make flight extremely dangerous or impossible in a matter of a few minutes. Freezing rain occurs when precipitation from warmer air aloft falls through a temperature inversion into below-freezing air underneath. Freezing drizzle is commonly formed when droplets collide and coalesce with other droplets. As the droplets grow in size, they begin to fall as drizzle. Freezing rain and freezing drizzle, including freezing drizzle aloft, are sometimes found in the vicinity of fronts and other hazardous weather conditions such as embedded thunderstorms.
Preparation, pre-flight planning and scrutiny of weather forecasts is a vital Threat and Error Management activity to try and avoid the risk of encountering icing conditions. In particular, a key pre-flight planning check is to compare the freezing level with your planned altitudes. When flying visually this level should be kept in mind when operating around or near water sources such as clouds and precipitation. If you are planning on operating in IMC, flight levels need to be selected with regard to temperature and terrain-safety. In flight, you should monitor the outside air temperature gauge, and if rain is experienced in sub-zero temperatures, try and reroute to leave the conditions immediately.
Inadvertent Flight in Icing Conditions
If icing conditions are encountered, the aircraft should ideally immediately be flown clear if safe to do with regard to terrain, airspace and weather. Monitor any accumulation carefully, remembering that ice could be building rapidly on parts of the airframe that are not easily visible. Support of ATC and an appropriate emergency or distress call should be made without delay – you may need to rapidly change levels or routing to exit the conditions. Also, if ice has accumulated, be prepared for a higher stall speed to help prevent a Loss of Control event.
If rough running, loss of power or a drop in manifold pressure is noticed, engine icing may be suspected even if you do not think you have inadvertently flown into icing conditions. Carburettor Heat (typically in carburetted engines) or Alternate Air (often in injected engines) should be selected to hot in accordance with your Flight Manual procedures. An alternate source of air, warmed by the engine, then bypasses the intake filter and allows an unrestricted supply of air into the induction system. The initial selection to hot may appear to make the situation worse if ice is present by increasing the rough running. This is due to the ice melting and passing through the engine; the temptation to return to cold should be resisted – be prepared for a forced landing, but with patience the engine performance should hopefully improve.
Summary
Icing is a complex subject for the light aircraft pilot and can be a significant threat in our UK island climate. This is a very brief overview and it is recommended that pilots undertake further reading appropriate to their aircraft and operating regime. It could also be a valuable subject to discuss on your next rating revalidation instructional flight or practice as an emergency, especially if you hold instrument qualifications and you regularly fly in IMC.
Further reading:
https://www.easa.europa.eu/sites/default/files/dfu/EGAST_GA5-Piston-Engine-Icing-final.pdf
https://www.easa.europa.eu/sites/default/files/dfu/198961_EASA_EGAST_GA10.pdf
https://www.skybrary.aero/index.php/In-Flight_Icing
https://www.aopa.org/-/media/files/AOPA/Home/Pilot-Resources/ASI/Safety-Advisors/sa11.pdf