Carburettor Icing

From the 1960s onwards the new breed of imported American aircraft trainer and private aircraft were typically powered by Continental and Lycoming carburettor aero engines. All pilots became familiar with using carby heat. Today, most modern aircraft are fuel injected and do not have a carburettor. 

So, what is carburettor icing? This is a situation whereby ice develops in the throat of the float carburettor and effectively strangles the carby. A low temperature, moisture in the air and fuel vaporisation all assist in the formation of carby icing. If the dew point temperature and the outside air temperature are close be wary of possible carby icing. 

What are the typical symptoms? A rough running engine, rpm reducing, manifold pressure reducing, and air speed reducing. 

What are the cures? Promptly apply full carby heat and leave it on. As the heat taken from around a hot exhaust pipe is carried into the engine the ice will melt and the moisture run through the engine, giving a rough running engine. When the ice and moisture has cleared the engine will run smoothly but due to it breathing hot air will run at a lower rpm than originally selected. Then get rid of the carby heat and the power will go back to that previously selected. 

Of the two most popular brands of aero engines, small Continental engines seem to be more prone to carby icing than Lycoming engines. The small Continentals have the oil reservoir separate from the engine and thus there is less heat transfer to the carburettor unlike Lycoming engines, which have the carburettor mounted below the oil sump or reservoir. That assists with heat transfer to the carby. 

On the other hand, we have the Piper Seminole and Beech Duchess light twins with tightly cowled engines that have a carby heat control that is seldom used. 

Carburettor icing is something that might happen to catch out pilots trained on fuel injected aircraft 

In all my flight time over a good number of years I can say that I have only had carby icing three to four times and had to apply the carby heat in anger. The rest of the time is when the power has been reduced below 2,000 rpm. 

Carby icing can occur in the air or even on the ground during taxi out. During the pre-takeoff run-up one should ensure that application of carby heat does result in a 50-75 rpm drop. No drop could indicate that the carby heat control is not working. That in turn could result in a major issue in the air. No rpm drop, no fly!

‘If flying a carburetted engine aircraft beware! Carby icing is insidious’

If one had a forced landing and suspected carby icing, a look at the carby itself might find visible moisture and even frost on the carby body indicating ice inside the unit. Today a well-equipped carburetted aircraft may have a carby temp gauge on the panel. 

There are several things to note re carby icing. The carby heat control has two positions. Full off or full on. There is no half way position. If the engine starts to lose manifold pressure or rpm or runs rough, the first and best thing is to promptly apply the carby heat control. Carby ice can occur on the ground before take-off or in the air. The general rule is that below 2000 rpm apply carby heat, for example reducing power for the descent and landing. 

Applying carby heat on the ground means that the engine is breathing air that has by-passed the air filter and thus dust can get into the engine. Beware of this. Engines do not like to breathe unfiltered air. 

In this new age of fuel injected aero engines, pilots are losing touch with carburetted engines and the use of carby heat. It could well be that we will see an increase in carby icing engine issues in the future because pilots are unfamiliar with carby ice and carby heat application. 

If flying a carburetted engine aircraft beware! Carby icing is insidious. 

Instructor tip of the month 

Going to a new destination? Draw up a mud map. An A4 sheet with a straight line embracing the way points and the destination. Then draw in the abbreviations, frequencies, tracks, distances, nav aids etc. All in black or dark blue pen in slightly larger than normal size letters. That makes it all so easy to read in flight. No fumbling through charts or the iPad chasing frequencies or aerodrome elevations. Plan the flight, fly the plan.