Southern Microlight Club of Victoria
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Carb Icing by Steve Bell
With a high pressure centred over Victoria and a nice sunny day forecast Ken and I thought to ourselves lets go flying. And so it was at 7am on a fresh August morning that we took off from Latrobe Valley heading for Dixons Creek as our first stop-over on our way through to Yarrawonga. As usual the Latrobe Valley has lots of foggy areas and heaps of moisture in the morning air. We didn’t really know for sure if we would be able to negotiate our way through all this cloud. Ken called Reg at Dixons creek before we left and he assured that it was nice and sunny there, so off we went.
The cooling ponds around the Power Stations are responsible for much of the moisture that goes into the Latrobe Valley air and the resultant morning fog. We were hoping that as we leave the power stations behind us, so would we be also leaving the moisture and fog.
So taking off into light misty fog and heading north west we needed to fly at various altitudes to about 3,500 feet in order to cross the ranges. At this time of day the cloud base was quite low and we were trying to balance our need to fly a safe margin above the terrain but still under the cloud base. The air temperature was brass monkey at zero degree’s and we kept telling ourselves that as the sun rises and the morning and our flight progresses everything will just get better.
We were crossing over the tiger country around Noojee when my engine had a spit, just a momentary spit, I was concentrating on staying warm and watching the cloud and where Ken was, I didn’t really think anything of it as it seemed to recover imediately and was running very nicely, I had more pressing concerns as my fingers were starting to go numb despite the fact that I was wearing gloves in addition to my bar mits. But when the engine had a few more spits I realised that perhaps my carbs were starting to be effected by ice.
By this stage we had been flying for about 45 - 60 minutes and quite a way from home, Ken was a mile or so ahead of me and had concerns about the low lying cloud that was blocking our path. Reluctantly we were forced to turn back as the cloud was looking inpenetrable.
Just after we turned around the revs on my engine just fell away, shit whats going on, I couldn’t afford to lose any altitude as it was all tiger country, but I was losing altitude. I decided to push it harder to gain some altitude and then if it konked out at least I’d have a longer glide and more landing options. As soon as I put the foot down the spluttering stopped and it ran stronger and smoother. I was trying to remember all the stuff I’d read about carby icing but I think my brain as well as my hands were frozen. All I knew was that the engine ran smoother and I felt more assured with more revs.
My streak 2B normally trims at 55 knots with the 582 at 4,800 revs, I flew all the way back to LV with the bar pulled in and around 65 – 70 knots airspeed and 5,300 revs, frozen hands, fog and the terror of not knowing if my carbs were going to give it up on me at any moment. It was such a relief to drop into warmer 7 and 8 degree air as we approached Latrobe and I honestly don’t know if it was my imagination but the engine seemed to immediately sound and feel stronger in the warmer air. On landing Ken told me how surprised he was that I was right behind him as he also had extra revs and airspeed up on his Streak 3 912 as he had some icing of his own to contend with.
Well that’s my story and the reason why I have collected a little further information for anyone who may be interested to learn a little more about Carby Icing, what causes it what to look out for and what to do about it when it comes along.
Carburetor (or carb) icing is an icing condition which can affect any carburetor under certain atmospheric conditions. Carb icing occurs when there is humid air, and the temperature drop in the venturi causes the water vapor to freeze. The ice will form on the surfaces of the carburetor throat, further restricting it. This may increase the Venturi effect initially, but eventually restricts airflow, perhaps even causing a complete blockage of the carburetor. Icing may also cause jamming of the mechanical parts of the carburetor, such as the throttle butterfly valve.
While it applies to all carburetors, it is of particular concern in association with piston-powered aircraft, especially small, single-engined light aircraft. Aircraft powered by carbureted engines are equipped with carburetor heat systems to overcome the icing problem. In cars, carb icing can occasionally be a nuisance. The inlet manifold and parts of the carburetor often have warm water from the cooling system or exhaust circulating through them to combat this problem. Motorcycles can also suffer from carb icing, although some engine designs are more susceptible to it than others. Air cooled engines may be more prone to icing, due to the absence of warm coolant circulating through the engine.
By Mike Watson
ATSB's air safety investigator, Mike Watson, in his unique style, discusses the insidious dangers of carburettor icing.
The aircraft was on short final for runway 29L when the pilot made a brief Mayday call. The aircraft was then observed to land in a car-yard, short of the runway. Both occupants managed to evacuate without injury.
The pilot later reported that the engine did not respond when an increase in RPM was required, as the aircraft was undershooting the approach. The aircraft subsequently collided with a fence, short of the runway.
Weather conditions at the time were conducive to severe carburettor icing at descent power. It is likely that carburettor icing occurred during the low power descent and precluded the engine accelerating above idle power on the final approach.
If I were to stuff a gag forcibly down your throat, you would not be able to get air into your lungs, and after quite a short time, your body would stop working. The same is true of aircraft engines: if I were to block their air intakes, they would also stop working.
The easiest way to block an engines air intake is to freeze water and simply choke the engine, so that it can no longer breathe.
Can this happen to my aircraft? Yes. Let us look at how water can find its way into the air intake when we least expect it. To do so, we need to examine how water is carried in the atmosphere and how it can choke a carburettor.
Water is dissolved in the air that both we and our engines breathe, in much the same way as sugar can be dissolved into a cup of tea. It is much easier to dissolve sugar into a hot cup of tea than a cold cuppa, and likewise it is easier to dissolve more water in warm air than into cold air. Water that has been dissolved into the atmosphere is actually a gas that you cannot see, and it is always present in the atmosphere.
Let us take a hot cup of tea, stir in as much sugar as we can, and then put the cup in the fridge. Once the tea has chilled, you will see that some of the sugar is no longer dissolved in the tea, but has formed crystals of sugar in the cup.
In the same way, if you take a cup of warm, humid air, (lots of water dissolved in it), and cool it, you will see that some of the water that was dissolved in the air as a gas will change back into a liquid. Normally, this can be seen is as tiny droplets like those found in a cloud. Many clouds are formed in exactly this way: as humid air rises and cools, it cannot hold all its dissolved water, and some of the water condenses into a cumulus-type cloud.
How will this affect the engine in your aircraft? When air passes through the carburettor on the way to the engine, fuel is evaporated into the carburettor. This chills the air, in just the same way as evaporating water chills a swimmer leaving the ocean for the beach. If this chilled air was previously humid, then some of the water dissolved in the air will immediately change into cloud-type water droplets. If the chilling effect of the fuel was sufficient to cool the carburettor below freezing level, then when these water droplets hit the sides of the venturi (the part where the air passes through), or the throttle valve, the water droplets will freeze in place. This will start the process of choking the engine. Eventually, if the process is allowed to continue, it will no longer be able to breathe, and the engine will stop.
The problem will be more pronounced if the engine is operating at a low power setting. In this case, the airflow through the carburettor will be partially impeded by the throttle valve. This valve not only provides more area for the ice to form: it also increases the partial vacuum downstream of the valve, and that will cause a further chilling of the air and the water droplets.
Even at temperatures exceeding 25 degrees Celsius, air passing through a carburettor may form ice that can choke your engine. The more humid the air in which your aircraft is flying, the more likely it is that ice will form in the air-intake system.
Remember, note the air temperature: the most severe icing will occur at temperatures up to around 20 degrees Celsius, and the severity will decrease slowly as the temperature increases. The other major factor is the humidity in the air. If the air feels muggy, it is humid; if perspiration does not dry rapidly off your body, it is humid; if a breeze does not cool you on a warm day, it is humid.
When you are flying, remember that the air gets cooler with an increase in altitude, and this can increase the humidity. If you are flying near clouds, then the air is likely to be humid, (the relative humidity in a cloud is normally 100 per cent).
Prevent carburettor icing at the first indication, rather than leave it until the engine is choked by ice!
With a high pressure centred over Victoria and a nice sunny day forecast Ken and I thought to ourselves lets go flying. And so it was at 7am on a fresh August morning that we took off from Latrobe Valley heading for Dixons Creek as our first stop-over on our way through to Yarrawonga. As usual the Latrobe Valley has lots of foggy areas and heaps of moisture in the morning air. We didn’t really know for sure if we would be able to negotiate our way through all this cloud. Ken called Reg at Dixons creek before we left and he assured that it was nice and sunny there, so off we went.
The cooling ponds around the Power Stations are responsible for much of the moisture that goes into the Latrobe Valley air and the resultant morning fog. We were hoping that as we leave the power stations behind us, so would we be also leaving the moisture and fog.
So taking off into light misty fog and heading north west we needed to fly at various altitudes to about 3,500 feet in order to cross the ranges. At this time of day the cloud base was quite low and we were trying to balance our need to fly a safe margin above the terrain but still under the cloud base. The air temperature was brass monkey at zero degree’s and we kept telling ourselves that as the sun rises and the morning and our flight progresses everything will just get better.
We were crossing over the tiger country around Noojee when my engine had a spit, just a momentary spit, I was concentrating on staying warm and watching the cloud and where Ken was, I didn’t really think anything of it as it seemed to recover imediately and was running very nicely, I had more pressing concerns as my fingers were starting to go numb despite the fact that I was wearing gloves in addition to my bar mits. But when the engine had a few more spits I realised that perhaps my carbs were starting to be effected by ice.
By this stage we had been flying for about 45 - 60 minutes and quite a way from home, Ken was a mile or so ahead of me and had concerns about the low lying cloud that was blocking our path. Reluctantly we were forced to turn back as the cloud was looking inpenetrable.
Just after we turned around the revs on my engine just fell away, shit whats going on, I couldn’t afford to lose any altitude as it was all tiger country, but I was losing altitude. I decided to push it harder to gain some altitude and then if it konked out at least I’d have a longer glide and more landing options. As soon as I put the foot down the spluttering stopped and it ran stronger and smoother. I was trying to remember all the stuff I’d read about carby icing but I think my brain as well as my hands were frozen. All I knew was that the engine ran smoother and I felt more assured with more revs.
My streak 2B normally trims at 55 knots with the 582 at 4,800 revs, I flew all the way back to LV with the bar pulled in and around 65 – 70 knots airspeed and 5,300 revs, frozen hands, fog and the terror of not knowing if my carbs were going to give it up on me at any moment. It was such a relief to drop into warmer 7 and 8 degree air as we approached Latrobe and I honestly don’t know if it was my imagination but the engine seemed to immediately sound and feel stronger in the warmer air. On landing Ken told me how surprised he was that I was right behind him as he also had extra revs and airspeed up on his Streak 3 912 as he had some icing of his own to contend with.
Well that’s my story and the reason why I have collected a little further information for anyone who may be interested to learn a little more about Carby Icing, what causes it what to look out for and what to do about it when it comes along.
Carburetor (or carb) icing is an icing condition which can affect any carburetor under certain atmospheric conditions. Carb icing occurs when there is humid air, and the temperature drop in the venturi causes the water vapor to freeze. The ice will form on the surfaces of the carburetor throat, further restricting it. This may increase the Venturi effect initially, but eventually restricts airflow, perhaps even causing a complete blockage of the carburetor. Icing may also cause jamming of the mechanical parts of the carburetor, such as the throttle butterfly valve.
While it applies to all carburetors, it is of particular concern in association with piston-powered aircraft, especially small, single-engined light aircraft. Aircraft powered by carbureted engines are equipped with carburetor heat systems to overcome the icing problem. In cars, carb icing can occasionally be a nuisance. The inlet manifold and parts of the carburetor often have warm water from the cooling system or exhaust circulating through them to combat this problem. Motorcycles can also suffer from carb icing, although some engine designs are more susceptible to it than others. Air cooled engines may be more prone to icing, due to the absence of warm coolant circulating through the engine.
By Mike Watson
ATSB's air safety investigator, Mike Watson, in his unique style, discusses the insidious dangers of carburettor icing.
The aircraft was on short final for runway 29L when the pilot made a brief Mayday call. The aircraft was then observed to land in a car-yard, short of the runway. Both occupants managed to evacuate without injury.
The pilot later reported that the engine did not respond when an increase in RPM was required, as the aircraft was undershooting the approach. The aircraft subsequently collided with a fence, short of the runway.
Weather conditions at the time were conducive to severe carburettor icing at descent power. It is likely that carburettor icing occurred during the low power descent and precluded the engine accelerating above idle power on the final approach.
If I were to stuff a gag forcibly down your throat, you would not be able to get air into your lungs, and after quite a short time, your body would stop working. The same is true of aircraft engines: if I were to block their air intakes, they would also stop working.
The easiest way to block an engines air intake is to freeze water and simply choke the engine, so that it can no longer breathe.
Can this happen to my aircraft? Yes. Let us look at how water can find its way into the air intake when we least expect it. To do so, we need to examine how water is carried in the atmosphere and how it can choke a carburettor.
Water is dissolved in the air that both we and our engines breathe, in much the same way as sugar can be dissolved into a cup of tea. It is much easier to dissolve sugar into a hot cup of tea than a cold cuppa, and likewise it is easier to dissolve more water in warm air than into cold air. Water that has been dissolved into the atmosphere is actually a gas that you cannot see, and it is always present in the atmosphere.
Let us take a hot cup of tea, stir in as much sugar as we can, and then put the cup in the fridge. Once the tea has chilled, you will see that some of the sugar is no longer dissolved in the tea, but has formed crystals of sugar in the cup.
In the same way, if you take a cup of warm, humid air, (lots of water dissolved in it), and cool it, you will see that some of the water that was dissolved in the air as a gas will change back into a liquid. Normally, this can be seen is as tiny droplets like those found in a cloud. Many clouds are formed in exactly this way: as humid air rises and cools, it cannot hold all its dissolved water, and some of the water condenses into a cumulus-type cloud.
How will this affect the engine in your aircraft? When air passes through the carburettor on the way to the engine, fuel is evaporated into the carburettor. This chills the air, in just the same way as evaporating water chills a swimmer leaving the ocean for the beach. If this chilled air was previously humid, then some of the water dissolved in the air will immediately change into cloud-type water droplets. If the chilling effect of the fuel was sufficient to cool the carburettor below freezing level, then when these water droplets hit the sides of the venturi (the part where the air passes through), or the throttle valve, the water droplets will freeze in place. This will start the process of choking the engine. Eventually, if the process is allowed to continue, it will no longer be able to breathe, and the engine will stop.
The problem will be more pronounced if the engine is operating at a low power setting. In this case, the airflow through the carburettor will be partially impeded by the throttle valve. This valve not only provides more area for the ice to form: it also increases the partial vacuum downstream of the valve, and that will cause a further chilling of the air and the water droplets.
Even at temperatures exceeding 25 degrees Celsius, air passing through a carburettor may form ice that can choke your engine. The more humid the air in which your aircraft is flying, the more likely it is that ice will form in the air-intake system.
Remember, note the air temperature: the most severe icing will occur at temperatures up to around 20 degrees Celsius, and the severity will decrease slowly as the temperature increases. The other major factor is the humidity in the air. If the air feels muggy, it is humid; if perspiration does not dry rapidly off your body, it is humid; if a breeze does not cool you on a warm day, it is humid.
When you are flying, remember that the air gets cooler with an increase in altitude, and this can increase the humidity. If you are flying near clouds, then the air is likely to be humid, (the relative humidity in a cloud is normally 100 per cent).
Prevent carburettor icing at the first indication, rather than leave it until the engine is choked by ice!