The Dangers of Aircraft Icing


Photo By: Shelby V.



Icing is one of the most dangerous things in aviation, it causes an increase in drag and weight, while decreasing lift and thrust. General aviation aircraft are small, light aircraft which are not certified to fly in any icing conditions, and must exit conditions immediately. Pilots need to always refer to the Aircraft Flight Manual/Pilots Operating Handbook (AFM/POH) for more details.


TYPES OF AIRCRAFT ICING

There are two types of icing that affect aircraft: Structural and Induction.


Structural Icing:

Structural icing is the type of icing that sticks to the outside of the aircraft, moister must be 0⁰C or lower. Aerodynamic cooling of the airfoil to 0⁰C or lower can cause moister to stick even though the ambient temperature is warmer. It is when supercooled water freezes on impact with an aircraft. Small or narrow objects such as; antennas, pitot tubes well ice up more rapidly. The tail plane section for an aircraft will collect ice faster than the wings.


The three main types of icing conditions that affect aircraft:


  • Clear Ice - Considered the Most Dangerous. It has a glossy, clear, even a translucent appearance. This ice can be denser, harder which can add additional amount of weight to the airplane. A portion freezes right away, while the rest streams or runs back slightly in the airflow and freezes. May form “horns” protruding from the leading edge, which severely disrupts airflow around the wing and can increase drag 300 to 500%.


  • Rime ice - Has a rough, milky, opaque look to it and is generally rougher in appearance than Clear ice. Small supercooled water drops freeze rapidly when they strike the aircraft. It accumulates on the wings leading edge, antennas, choking pitot tubes, etc. Has the ability to aerodynamically change the shape of the wings camber. Doesn’t weight as much as clear ice.


  • Mixed ice - Combination of clear ice and rime ice. Mixed ice can accumulate rapidly and becomes difficult to remove.


Clear ice can occur at any altitude above the freezing level. At higher altitudes, rime or mixed ice can occur from small water droplets.



Other types of Structural Icing:


  • Frost - is another type of structural icing, which can also be dangerous. Frost does not change the surface of the wing, but its roughness spoils the smooth air flow, causing airflow to slow down over the wing. Pilots should look for frost buildup during the taxi out for takeoff.

Remember, frost buildup with the thickness and roughness like medium coarse sandpaper, can reduce lift by 30%, and increase drag by 40%.


  • Freezing rain or drizzle - forms when rain droplets become supercooled when falling through a layer of subfreezing air. Freezing raindrops are drops of 500 micrometers (0.5 mm) diameter or larger. Freezing rain may form in areas far aft of the leading edge where normal icing conditions would form without the presents of freezing rain. Whereas, freezing drizzle is made of supercooled water drops that have a diameter smaller than 500 micrometers. When freezing drizzle is found within supercooled clouds, buildup can be very rapid which causes an increase in drag, make the stall speed dangerous. Roll control anomalies may also increase in some aircraft.


One of the most dangerous situations pilots are not well trained in, is when the tailplane of the airplane will ice up faster than the wings causing the tailplane to stall and the airplane to severely pitch down. If this situation where to occur, you as the pilot should:

  • Fly at a faster than normal speed.

  • IMMEDIATLY get out of any icing condition.

  • DO NOT lower the flaps. (By lowering the flaps air is deflected downward. Disrupting airflow over the tail.)

  • Always refer to the aircraft POH for proper procedures. In this case we used the Cessna 172 POH.




Induction Icing:

There are two types of Induction icing; carburetor icing which affects engines with carburetors and intake air filter blockage.


Carburetor icing is likely to form when the relative humidity is above 80%, and the outside air temperature (OAT) is between -7⁰C (20⁰F) and 21⁰C (70⁰F). That is not always the case. According to the FAA and the NTSB (National Transportation Safety Board), carburetor icing can occur when the relative humidity is low, at 50% and the temperature is as high as 38⁰C (100⁰F) when flying at low power settings. On carburetor engine aircraft, pilots should use carburetor heat when powering back from cruise power, even flying in clouds, rain or snow when temperatures are near freezing.


If flying an airplane with a fixed pitch propeller, the RPM will decrease followed by the engine running rough. Whereas, if flying an aircraft with a constant speed propeller, carburetor icing will show a decrease in manifold pressure with no change in RPM. Carburetor heat may also be used to melt ice buildup from the intake filter.




WHERE ICING CONDITIONS OCCUR

Icing occurs mostly on all aircraft when flying within supercooled clouds. Icing can form in Steam fog during the coldest times of the year. The most hazardous icing occurs in low clouds, which extend from the surface up to 6,500 feet AGL (above ground level). These clouds are mostly made of water droplets and possibly supercooled water drops. Middle clouds are from 6,500 feet up to 20,000 feet AGL, which may contain moderate icing.


The number one thing is to AVOID flying in any area where icing conditions are known to exist. DO NOT fly into rain, wet snow, any visible moisture of any kind or even cumulus clouds when the temperature is near or below 0⁰C (32⁰F).




ICING INTENSITY

Icing intensity is classified in three ways, light, moderate, and severe.

  • Sever Icing - If the aircraft encounters severe icing conditions, the rate of accumulation is such that the anti-icing and de-icing fails to reduce or control the icing buildup.


  • Moderate icing - Is encountered, the safest thing to do is divert, before the icing becomes a potential hazardous situation.


  • Light icing - is usually not an issue, unless the aircraft is exposed to the condition for a long period of time.

Of course, it depends on the type of aircraft and its de-icing or anti-icing equipment.

Symbols are used to identify the intensity of weather on weather charts. A symbol is placed on the chart where a PIREP (pilot report) was reported. Since we are talking about icing, below are the basic symbols shown for icing intensity.




HOW TO REMOVE ICE

Most small or light aircraft are only designed with electric heated Pitot tube and are not certified for flight into any icing conditions. Larger more complex aircraft are designed with ice removal/prevention systems. There are two ways to remove ice on aircraft.

  • De-ice

  • Anti-ice


De-Ice - boots are inflatable layers of rubber that are attached to the leading edge of the wings. The boots are inflated with high-pressure air which will crack the ice off.

NOTE: Don’t get the De-ice boots confused with the leading edge abrasion boots or leading edge tape installed on the horizontal stabilizer of some small aircraft or Cessna aircraft. Abrasion boots or tape is used to protect the tail and horizontal stabilizer leading edges against nicks, small dents caused by stones, gravel or other debris.



Anti-ice - can be accomplished a couple of different ways.

  • One is the weeping wing, which has tiny pores in the leading edge of the wing allowing TKS Fluid, a type of alcohol/ethylene glycol mixed solution, which comes out and flows of the wing surface. Propellers well have small nozzles near the prop hub that spray alcohol on the blades.


  • The Heated Wing or Heated Prop uses high pressure, high temperature bleed air from the turbine engines. The hot bleed air travels through tubes called Deicing air ducts to heat the wing to help prevent ice forming. Propellers blades, pitot tubes and static ports are heated electrically. Caution should be taken when touch this parts, they can burn your hands.


  • Carburetor heat is a type of anti-icing system used on older small aircraft. Carburetor or Carb. heat for short, is used to melt any ice buildup that has formed, and can be used as a preventive measure. Using the Carburetor heat can also melt an ice buildup on the intake filter.



RULES TO FOLLOW

  • We will repeat it again, AVOID! flying in any area where icing conditions are known to exist. DO NOT! fly into rain, wet snow or even cumulus clouds when the temperature in near or below 0⁰C (32⁰F).


  • If structural icing is visible and forming, get out of it immediately and advice ATC. It is also a good idea to establish a PIREP. Fly to an area with warmer temperatures, which is usually changing altitudes. If warmer temperatures are not found, the safest is to land immediately at the nearest airport and wait it out.


  • It is the pilot’s responsibility and required by FAR 91.103, to check all required information, along with current weather conditions. This should include all AIRMETs and SIGMETs, icing forecasts, freezing levels, any PIREPs (pilot reports), along with METAR to determine the local conditions, freezing levels and the altitudes for warmer air.


References:

Pilots Handbook of Aeronautical Knowledge (PHAK) Chapter 7

PHAK Chapter 12

AC91-74

Private Pilot ACS PA.I.G.K1j, PA.IX.C.K1b, PA.IX.C.K4

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