Christmas Giveaway 2023: Question 7 - Carburettor Icing
Question
Using the carburettor risk graph below, if the air temperature is +22°C and the dewpoint is +5°C, what level of icing is a pilot likely to experience and under what power settings?
- Serious icing, all power settings
- Serious icing, descent power
- Moderate icing, descent power
- Light icing, cruise power
Explanation
One of the most significant risks for a general aviation pilot flying an aircraft fitted with a carburettor is 'carb-icing'. You may have encountered the carb icing risk graph below, which helps a pilot determine the likelihood of carburettor ice forming at certain stages of flight based on the temperature and dewpoint.
To determine the likelihood of carburettor icing using the risk graph, you firstly need to read the air temperature from the horizontal axis followed by the dewpoint from the vertical axis. Identify the colour at the point at which the two lines cross and then read off the corresponding carb icing risk from the chart key. For our scenario, with an air temperature of +22°C and a dewpoint of +5°C, the two lines cross in the orange section of the chart. Using the key, we can see that we may experience serious icing in the descent power configuration in todays conditions.
Carburettor icing is caused by the lowering of the pressure (and therefore temperature) in the aircraft’s carburettor as the fuel air mixture is drawn through the venturi and towards the throttle valve. As the temperature lowers, the water vapour in the fuel air mixture will condense and freeze, preventing delivery of the mixture to the engine. The temperature in the carburettor can be as much as 35ºC less than the ambient temperature! Power is often lost gradually as the ice builds, going unnoticed until a significant amount is present.
Carburettor icing is not restricted to cold weather and can occur on warm days if the humidity is high enough, especially at low power settings. Cold days during the winter with low humidity may be less hazardous than warm humid days in the summer. High humidity is common in the UK, with signs including a close temperature and dew point, visible moisture on the ground, rain, fog or a low cloud base.
Pilots flying in the UK should always be alert for the signs of carburettor icing. A loss of engine power and a decrease in aircraft performance are usually the first signs of a build up of icing. Signs in the cockpit may include a drop in RPM with an associated loss of airspeed and/or height. As the ice continues to build, the engine will start running rough, causing vibration and a further RPM loss, and if not addressed the ice build-up could cause a complete engine failure.
To clear a build-up of carburettor icing, you should always follow the guidance issued in your AFM or POH. For most aircraft fitted with a carburettor this will involve the use of carburettor heat, heating the intake air in an exhaust heat exchanger before it reaches the carburettor. This heated air will help melt any ice build up present. When using carburettor heat, be sure to select it for long enough (usually at least 15 seconds) to allow any ice to melt.
Source: CAA Safety Sense Leaflet 14 - Page 6 (June 2023)
As well as taking action when we suspect carburettor icing, we can also take preventative measures to reduce the likelihood of an icing build-up during the flight and at critical stages where icing may be more likely. Engine power checks on the ground, as well as in-flight checks such as FREDA and approach checks such as BUMFITCH, can aid pilots in preventing and identifying carburettor icing early.
You can download our FREDA and BUMFITCH postcards below, to help you memorise these vital checks!
