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Aviation Meteorology

Airframe & engine icing

Rev. 9 — page content was last changed September 23, 2009
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10.1 Airframe icing

High humidity and low winter freezing levels in south-east Australia provide likely conditions for icing at low levels. Hopefully it is unlikely that an ultralight or VFR GA pilot would venture into possible icing conditions, but the pilot of an enclosed cockpit ultralight may be tempted to fly through freezing rain or drizzle. Aircraft cruising in VMC above the freezing level, and then descending through a cloud layer, may pick up ice.

The prerequisites for airframe icing are:
  • the aircraft must be flying through visible, supercooled liquid; i.e. cloud, rain or drizzle
  • the airframe temperature, at the point where the liquid strikes the surface, must be zero or sub-zero.
The severity of icing is dependent on the supercooled water content, the temperature and the size of the cloud droplets or raindrops. The terms used in the Australian Bureau of Meteorology icing forecasts are:
  • light: less than 0.5 g/m of supercooled water in the cloud — no change of course or altitude is considered necessary for an aircraft equipped to handle icing. No ultralight and very few light aircraft are equipped to handle any form of airframe ice
  • moderate: between 0.5 and 1.0 g/m — a diversion is desirable but the ice accretion is insufficient to affect safety if anti-icing/de-icing is used; unless the flight is continued for an extended period
  • severe: more than 1.0 g/m — a diversion is essential. The ice accretion is continuous and such that de-icing/anti-icing equipment will not control it and the condition is hazardous.
The diagram below shows the ice accretion in millimetres on a small probe, for the air miles flown in clouds with a liquid water content varying from 0.2 g/m to 1.5 g/m.

Ice accretion chart

The small, supercooled droplets in stratiform cloud tend to instantaneous freezing when disturbed and form rime ice — rough, white ice that appears opaque because of the entrapped air. In the stable conditions usually associated with stratiform cloud, icing will form where the outside air temperature [OAT] is in the range 0 C to –10 C. The continuous icing layer is usually 3000 to 4000 feet thick.

The larger, supercooled droplets in convective cloud tend to freeze more slowly when disturbed by the aircraft; the droplets spread back over the surface and form glossy clear or glaze ice. Moderate to severe icing may form in unstable air where the OAT is in the range –4 C to –20 C. Where temperature is between –20 C and –40 C the chances of moderate or severe icing are small except in CB CAL; i.e. newly developed cells. Icing is normally most severe between –4 C and –7 C where the concentration of free supercooled droplets is usually at maximum; i.e. the minimum number have turned to ice crystals. Refer to section 3.1 Cloud formation. Mixed rime and clear ice can build into a heavy, rough conglomerate.

Flying through snow crystals or snowflakes will not form ice, but may form a line of heavy frosting on the wing leading edge at the point of stagnation, which could increase stalling speed on landing. Flying through wet mushy snow, which is a mixture of snow crystals and supercooled raindrops, will form pack snow on the aircraft.

The degree and type of ice formation in cloud genera are:
  • CI, CS and CC; icing is rare but will be light should it occur
  • AC, AS and ST; usually light to moderate rime
  • SC; moderate rime
  • NS; moderate to severe rime, clear ice or mixed ice. As the vertical extent of NS plus AS may be 15 000 or 20 000 feet the tops of the cloud may still contain supercooled droplets at temperatures as low as –25 C
  • TCU and CB; rime, clear or mixed ice, possibly severe.
 Freezing rain creates the worst icing conditions, and occurs when the aircraft flies through supercooled rain or drizzle above the freezing level in CU or CB. The rain, striking an airframe at sub-zero temperature, freezes and glaze ice accumulates rapidly — as much as one centimetre per four air miles.

Freezing rain or drizzle, occurring in clear air below the cloud base, is the most likely airframe icing condition to be encountered by the VFR or ultralight pilot. As it is unlikely to occur much above 5000 feet amsl, choices for descent are possibly limited.

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10.2 Effect of airframe ice

Ice accretion on the wing leading edge is a major concern for aircraft not equipped with anti-icing or de-icing. Airflow disruption will reduce the maximum lift coefficient attainable by as much as 30–50%, thus raising the stalling speed considerably. Because the aircraft has to fly at a greater angle of attack to maintain lift, the induced drag also increases and the aircraft continues to lose airspeed, making it impossible to sustain altitude if the stall is to be avoided. Fuel consumption will also increase considerably.
  • The weight of 25 mm of ice on a small GA aircraft might be about 30 to 40 kg but the increased weight is usually a lesser problem than the change in weight distribution. Also, accretion is often not symmetrical, which adds to increasing uncontrollability.
  • Forward visibility may be lost as ice forms on the windshield.
  • Icing of the propeller blades reduces thrust and may cause dangerous imbalance.
  • Ice may jam or restrict control and trim surface movement; or may unbalance the control surface and possibly lead to the development of flutter.
  • Communication antennae may be rendered ineffective or even snapped off.
  • Extension of flaps may result in rudder ineffectiveness or even increase the stalling speed.
  • Aircraft operating from high-altitude airfields in freezing conditions may be affected by picking up runway snow or slush, which subsequently forms ice and possibly causes problems such as engine induction icing or frozen brakes.
Engine air intake icing
Impact icing may occur at the engine air intake filter. If 'alternate air' (which draws air from within the engine cowling) is not selected or is ineffective, power loss will ensue. When air is near freezing, movement of water molecules over an object such as the air filter may sometimes cause instantaneous freezing. Ice may also form on the cowling intakes and cause engine overheating.
Pitot or static vent icing
Pitot or static vent blockage will seriously affect the ASI, VSI and altimeter, as shown in the table below, but be aware that blockage of the static vent tubing from causes other than icing — water for example — will render the ASI, VSI and altimeter useless, unless the aircraft is fitted with an alternative static source.

If the static vent is totally blocked by ice —
Flight stageAltimeter readingVSI readingASI reading
During climbconstantzerounder
During descentconstantzeroover
During cruise+constantzeroOK
On take-offconstantzerounder

If the pitot tube is totally blocked —
Flight stageAltimeter readingVSI readingASI reading
During climbno effectno effectover*
During descentno effectno effectunder*
During cruiseno effectno effectconstant*
On take-offno effectno effectzero*

If the pitot tube is partially blocked —
Flight stageAltimeter readingVSI readingASI reading
During climbconstantzerounder*
During descentconstantzerounder*
During cruise+constantzerounder*
On take-offconstantzerounder*

* and/or fluctuating

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10.3 Ice jamming control surfaces and cables

Many aircraft are prone to accumulation of water from dew or rain in areas which, if that water freezes during flight, will inhibit control movement and affect hinge, cable or torque tube movement. This particularly applies to ailerons and elevators if the gap between the control surface and main structure contains some form of flexible seal (to improve aerodynamic efficiency) that allows accumulation of water. Engine controls may also be affected if exposed cables or cable runs are wet and subsequently ice up.

If water has accumulated within a control surface and frozen before it has the opportunity to drain, then the mass balance of the surface will be degraded and there is a possibility of flutter development.

Before flight, water should be removed from areas that may affect controls. Care must be taken to avoid flight into freezing conditions after flying through rain.

10.4 Hoar frost obscuring vision on take-off

In frosty, still, early morning, winter conditions the air layer adjacent to the ground will be much colder and drier than the air just 10 or 20 feet higher. Pilots planning a post-first light departure in these conditions should be aware that, while on the ground, the airframe will have cooled to freezing point or below. On take-off, the aircraft will quickly rise into the warmer, moister air and it is quite possible, in an unheated cockpit, that atmospheric moisture condensing onto the cold canopy will immediately form an external light, crystalline hoar frost; refer to 'Atmospheric moisture'. The hoar frost will suddenly and completely wipe out vision through the canopy for a short period, and at a most critical time.

Under slightly warmer conditions it is possible that a dense internal fogging of the canopy and instrument faces will occur during take-off, which will also wipe out forward vision for a short, but critical, period.

If dewpoint is below freezing, hoar frost may be deposited on parked aircraft in clear humid conditions at night when the skin temperature falls below 0 C. Rime ice will form on parked aircraft in freezing fog.

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10.5 Carburettor icing

Ice is formed in venturi-type and slide-type carburettors in ambient air temperatures ranging from about –10 C to +30 C if refrigeration and adiabatic cooling within the airways are sufficient to lower the air/fuel mixture temperature — and consequently the metal of the carburettor — below the freezing point. There must also be sufficient moisture in the air, but this need not be visible moisture. Ice may form at the fuel inlet, around the valve or slide, in the venturi and in curved passages, choking off the engine's air supply. If icing continues, this will cause the engine to stop. Carburettor ice may form in flight or when taxying; the latter event will severely degrade take-off performance.
Temperature reduction within the carburettor
Adiabatic cooling — in the induction system the constrictions at the throttle valve and choke venturi cause a local increase in air velocity, with consequent increase in dynamic pressure and decrease in static pressure. Density remains constant, so the temperature instantly decreases in line with the decrease in static pressure, refer to section 1.2 Equation of state. This adiabatic cooling is more noticeable when the throttle is closed or partly closed for extended periods, but it is unlikely to be more than a 5 C drop at the coldest part, and probably much less — say 2 to 3 C.

Refrigeration cooling — when fuel is injected into the airstream a certain amount evaporates. The latent heat for fuel evaporation is taken from the surrounding air and metal, which is already being cooled adiabatically. The temperature drop caused by refrigeration may be as much as 15 C, giving a total drop within the carburettor as high as 20 C. If the metal of the carburettor is thus reduced to a temperature at or below freezing then cooled or supercooled water droplets will freeze on contact — as in airframe icing.
Sublimation of water vapour
Even if there is no visible water in the air, the temperature reduction may cause ice to be deposited on the freezing metal by sublimation of the water vapour in contact with it; refer to sections 1.5 Atmospheric moisture and 1.6 Evaporation and latent heat. The amount forming depends on the absolute humidity of the atmosphere. Normally the higher the temperature, the greater the absolute humidity can be. Thus it is possible that when flying in OAT as high as 20 C, even 25 C, carburettor ice can form. Air with a relative humidity of 25% at 20 C, or 50% at 10 C, will reach saturation at 0 C.

However, an OAT range of 0 C to 25 C, peaking at around 10 C to 15 C and with relative humidity exceeding 60%, are the most significant conditions for moderate to severe clear air icing — particularly at low throttle openings — as shown in the probability diagram below. Note that the region to the left of the 100% relative humidity line would be visible moisture — mist, fog and cloud.

Carburettor icing probability chart

Locally high absolute humidity may also occur in the following conditions:
  • poor atmospheric visibility at low levels, especially early morning and late evening
  • after heavy rainfall in light wind conditions
  • in clear air just after morning fog has dispersed
  • just below a stratiform cloud base.
When flying through visible moisture, cloud patches or light rain, some of this moisture will evaporate in the carburettor, further reducing the temperature in the airstream. The drop is slight but may be enough to tip the scales. The probability of icing is increased if fuel flow is not leaned — the excess fuel injected into the intake airstream increases the refrigeration.
Combatting carburettor icing
The formation of carburettor ice is indicated by a slow decrease in manifold pressure in aircraft equipped with a constant speed propeller, or a decrease in rpm in fixed-pitch aircraft, probably with ensuing rough running as the ice build-up further restricts the airflow and enriches the mixture. Corrective action is usually by FULL application of carburettor heat, which pre-heats the air entering the carburettor. Full carburettor heat should also be applied in conditions conducive to icing, particularly at low throttle settings such as on descent or taxying, but never on take-off. Carburettor heat will increase the fuel vaporisation in a cold engine. Application of partial heat may cause otherwise harmless ice crystals in the airstream to melt then refreeze on contact with freezing metal.

Rough running may increase temporarily after application of full heat, as the less dense air will further enrich an over-rich mixture; however, full heat must be maintained until the engine eventually settles into smooth running.

Pre take-off checks: note the rpm and apply full heat — the rpm should drop. Return the heat to the cold position — the rpm should return to the initial reading. If a higher reading is obtained, then icing was — and is — present.

Non-venturi carburettors, such as the various slide types attached to two-stroke engines — the throttle slide performs as a throttle valve and venturi — are considered, for various reasons, not to be very susceptible to icing. Consequently, they are usually not fitted for carburettor heat, or intake air heating, on the principle that any ice formed will be immediately downstream of the slide, or multi-hole spray bar, or around the main jet, and movement of the throttle slide will dislodge it. This is provided of course, that the rpm drop is noticed before things get out of hand.

Next — atmospheric electricity The next section of the Aviation Meteorology ground school covers atmospheric electricity

Aviation meteorology guide modules

| Meteorology guide contents | The atmosphere and thermodynamics (part 1) | Thermodynamics (2) and dynamics |

| Effects of altitude — contained in the Flight Theory Guide module 2 & module 3 |

| Cloud, fog and precipitation | Planetary-scale tropospheric systems | Synoptic scale systems |

| Southern hemisphere winds | Mesoscale systems | Micrometeorology — atmospheric hazards |

| Airframe and engine icing | Atmospheric electricity | Atmospheric light phenomena |

| Aviation weather reports and forecasts |

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