TECHNICAL


Carb ice? S


Given that it’s summer, you might think that carb icing isn’t going to be a problem – but you’d be wrong


trangely, the most likely days for carb icing aren’t in the winter as many people might expect, but tend to come in the warmer


months of the year. That’s not to say it can’t happen at any


time, but with the amount of potential water vapour held in clear air being related directly to temperature, and carburettors being capable of reducing temperatures internally by well over 20°C, the most likely days for rapid carburettor icing in clear air are the warmer ones because in the UK’s climate there will often be more water mass available to freeze out from any given volume of clear air. Judging by the large number of


successful flights in the UK using engines with carburettors we have shown that we can cope, but still the accident reports come in. Perhaps more dangerous than a total engine failure is the insidious problem of reduced power, especially while still at low level on the climb-out. Engine failures happen too in the


cruise, or a reduced power descent, often through a more rapid build up of carb ice than normal. This might come about due to local or widespread atmospheric conditions combined with the engine handling by the pilot. The key to reducing carburettor icing


incidents is to improve weaker areas of engine handling, perhaps through discussions with instructors either during initial or biennial training sessions, on improved engine handling.


EARLY DETECTION Ice can be detected (and dealt with) by efficient checks long before late symptoms such as a rough-running engine. Applying the carburettor heat and watching the RPM drop (assuming a fixed-pitch propeller) is a check often carried out, but far too many pilots believe the RPM drop itself


combined with a rough running engine is what they are looking for to detect ice. Such a situation indicates ice has been allowed to develop for far too long already. The key to detecting ice early is to look for


any subsequent rise in RPM after an initial drop, either dynamically during warm air application, or by comparing values before and after the process. Even a small rise in RPM indicates the ice build up has started, and that’s the time to apply more heat to remove it all. Many pilots never look for the RPM rise, and miss the opportunity to recognise high risk icing conditions early and the need to reduce the time interval between checks significantly.


ICE BUILD-UP As pointed out in the intro, with the amount of potential water vapour held in clear air being related directly to temperature, the most likely days for rapid carburettor icing in clear air are the warmer ones, because of the UK’s climate. The closer the outside air temperature is


to its dewpoint, (and therefore the closer the relative humidity is to 100%), the easier it is for a carburettor to form ice rapidly, so care needs to be taken for example near the cloudbase itself, where by definition, temperature must equal dewpoint. Although cold air cannot hold as much


water vapour as warm air per given volume, a rapid ice build up can happen still happen, for example, after taxying across grass on cool mornings, the dew being stirred up as minute droplets into the atmosphere near ground level, perhaps more easily thought of as a form of super- saturation. So after crossing wet or damp grass


to line up, further ice removal should be attempted before take-off, and the attitude, speed, and power monitored carefully during the climb-out for any differences from normal.


Yet it only takes less than a


second to apply carburettor heat


ENGINE FAILURE Asked what they would do when an engine failure occurs, some pilots would rightly say ‘control the aircraft, adjust the attitude for best glide, then trim’, and sometime later include ‘restart checks’. Observing many pilots in practice engine


failures, one hand adjusts the attitude, the other the trim control. Trimming can take a reasonable time because it requires the aircraft to be stable at the required speed, yet it takes less than a second to apply carburettor heat, so it would make sense for the carburettor heat to be applied before the hand moves to the trimmer. It is probably the only ‘restart check’ that has a time dependency for it to work.


For more on the subject there is a CAA Safety Sense Leaflet on Piston Engine Icing (No. 14), see www.caa.co.uk/safetysense revised as recently as this year.


Summer 2018 CLUED UP 37


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