Definition

Often referred to as “Carburettor Icing”, Induction Icing is the build-up of ice in the fuel induction system and can affect all types of piston engined aeroplanes, helicopters and gyroplanes.

Description

There are 3 types of fuel induction system icing that may affect piston engines:

1. Impact Ice. This is formed by the impact of moist air at temperatures between -10°C and 0°C on air scoops, throttle plates, heat valves, etc. It usually forms when visible moisture such as rain, snow, sleet, or clouds are present. Most rapid accumulation can be anticipated at -4°C. This type of icing can affect fuel injection systems as well as carburettor systems and is also the main type of icing hazard for turbocharged engines.
2. Fuel Ice. This forms at, and downstream from, the point that fuel is introduced into the carburettor when the moisture content of the air freezes as a result of the cooling caused by fuel vaporisation. It generally occurs between +4°C to +27°C, but may occur at even higher temperatures. It can occur whenever the relative humidity is more than 50%.
3. Throttle Ice. This is the most common, earliest to show and most serious carburettor icing. It is formed at or near a partly closed throttle valve (sometimes called the “butterfly valve”). The water vapour in the induction air condenses and freezes due to the venturi effect cooling the air as it passes the throttle valve. Since the temperature drop is usually around 3°C, the optimum temperature for forming throttle ice is between 0°C to +3°C although a combination of fuel and throttle ice could occur at higher ambient temperatures.

Although “Carburettor Icing” is most likely to occur when the temperature and humidity are in the ranges indicated above, it can also occur under conditions not depicted. A more detailed chart is available in CAA AIC 077/2009 (see Further Reading).

“Carburettor Icing” is much more likely at reduced power, so select carburettor heat before power is reduced for the descent, especially if you are intending to lift off again e.g. a practice forced landing or helicopter autorotation.

Effects

The first signs of carburettor icing are normally:

• Fixed Pitch Propeller - a reduction in available power measured by RPM
• Constant Speed Propeller - a reduction in manifold pressure

If left untreated:

• Rough running engine
• Vibration
• Further loss of performance
• ENGINE STOPS due to fuel starvation

Defences

Prevention is better than the Cure.

For extended periods of flight with reduced power settings, the power should be routinely increased and FULL carburettor heat applied for around 15 seconds to ensure that the engine stays sufficiently warm to melt any ice.

If flying in conditions that appear conducive to the development of carburettor ice, carburettor heat should be applied at regular intervals as both a preventative measure against the formation of ice and a method to test whether or not carburettor ice is developing. If a normal drop in RPM or manifold pressure accompanies the application of carburettor heat, it is reasonably certain that ice is not forming in the carburettor.

But…

The use of carburettor heat will decrease engine performance by up to 15% so pilots should beware of flying around with it continuously selected; the aircraft will use more fuel than planned for and this practice could potentially decrease the life of the engine due to an inappropriate mixture setting.

Best Practice

• Review the difference between the OAT and the Dew Point when obtaining your meteorological briefing.
• Monitor the OAT gauge fitted to the aircraft
• Monitor the carburettor mixture temperature gauge (if fitted)
• Monitor the carburettor air inlet temperature gauge(if fitted)
• Conduct a carburettor heat ground check immediately prior to take-off
• Use carburettor heat when operating within the icing range
• Use carburettor heat on approach and descent

Remember that, as a rule-of-thumb: Power OFF = Carb Heat ON

Accidents and Incidents

Contributory Factors

• Carburettor icing depends on atmospheric conditions, the fuel in use and the detailed design of the carburettor. There is no scientific justification for a diagram which seeks to represent the dependency of carburettor icing on any one of those three factors without reference to the other two. The question is whether the deposition of a small amount of ice causes sufficient distortion of the flow of air into the carburettor that further deposition of ice becomes more likely. Clearly this depends on the detailed geometry of the air intake, as well as atmospheric conditions.
• Outside air temperatures in the ranges above combined with moist conditions are ideal for the formation of carburettor ice.
• All aircraft types/fuel combinations can have slightly different characteristics associated with engine icing; an aircraft operating on AVGAS will have different characteristics to the same type operating on Motor Gasoline. Pilots should know and understand what happens to their particular aircraft/fuel combination in various flight conditions.

Solutions

In the event that carburettor icing is encountered, full carburettor heat should be applied immediately. Engine performance may initially get worse but the ice will begin to clear after about 30 seconds and power will return to normal.

Related Articles

• In-Flight Icing
• Ice Formation on Aircraft
• Bernoulli's Principle

Further Reading

• Aircraft Flight Manual (AFM) and Pilot’s Operating Handbook (POH) specific to aircraft type.
• UK CAA AIC 077/2009. However, the diagram in section 3.4 is considered to be useful for a rather small range of aircraft type/fuel combinations and its application to other aircraft type/fuel combinations may be misleading.
• Aircraft Icing Handbook, Version 1 by Civil Aviation Authority of New Zealand.
• EGAST Safety Promotion Leaflet for GA operations: Piston Engine Icing, 10 July 2013. However, the diagram in section 4 is considered to be useful for a rather small range of aircraft type/fuel combinations and its application to other aircraft type/fuel combinations may be misleading.
• SafetySense Leaflet: Piston Engine Icing, UK CAA, January 2013.