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- Apr 8, 2019
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Posting this in hopes that others might be able to learn from an experience I had last week. A reminder to take a look at your airbox to ensure your getting full carb-heat effectiveness.
I was out practicing IFR approaches last week in my Archer II (in the Great-Lakes area). Outside air temp was about 39F at 4000 ft. I was fully in IMC, and in/out of light rain showers for most of the flight, so no doubt the relative humidity was 100%. During one of my approach setups, I was starting to notice some engine roughness and a little abnormal vibe. I have experienced carb ice a few times before, so this was my first suspicion. I looked at my carb-temp on CGR-30P and saw it was measuring about 21F. When carb heat was applied fully, the temp would come up to about 27F. I do carb-heat check during runup prior to every flight and get a small drop in RPM so I was thinking it was operating fine. However, in this situation, I could tell the carb-heat was only marginally improving things. Eventually, the roughness and vibration kept getting worse, at some points almost seeming as though carb heat was making the situation worse. It was not effectively melting off the ice. To make a long story short, I eventually informed ATC of the suspected carb ice and they gave me vectors out of the precip, and priority handling to get it back on the ground quickly. After thawing out on the ground, an engine-runup was all clear and a few subsequent VMC flights were completely normal.
After the flight, I discussed with my A&P. He suggested looking at the carb-heat flapper in the airbox to see if it was closing completely. It seemed like carb-heat should warm the air up by more than 5 or 6 degrees when its turned on, but I didnt know how much is 'normal'. When we pulled the duct, we could see that the valve was fully-closed for the external/air-filter supply, but when switched to carb-heat on, it was only closing about 75% the other way. I attached some photos of what the flapper looked like both ways. A few simple cable adjustments, and it was back to closing fully.
I included a data-log which i hope could be helpful for others. I captured data before and after adjusting the flapper for comparison. In the chart, you can see before the adjustment, I was getting about 6-7 degrees F of warming with carb-heat on. After adjusting it to close fully, I was recording 32-33 degrees F of warming. Obviously much much better.
I was out practicing IFR approaches last week in my Archer II (in the Great-Lakes area). Outside air temp was about 39F at 4000 ft. I was fully in IMC, and in/out of light rain showers for most of the flight, so no doubt the relative humidity was 100%. During one of my approach setups, I was starting to notice some engine roughness and a little abnormal vibe. I have experienced carb ice a few times before, so this was my first suspicion. I looked at my carb-temp on CGR-30P and saw it was measuring about 21F. When carb heat was applied fully, the temp would come up to about 27F. I do carb-heat check during runup prior to every flight and get a small drop in RPM so I was thinking it was operating fine. However, in this situation, I could tell the carb-heat was only marginally improving things. Eventually, the roughness and vibration kept getting worse, at some points almost seeming as though carb heat was making the situation worse. It was not effectively melting off the ice. To make a long story short, I eventually informed ATC of the suspected carb ice and they gave me vectors out of the precip, and priority handling to get it back on the ground quickly. After thawing out on the ground, an engine-runup was all clear and a few subsequent VMC flights were completely normal.
After the flight, I discussed with my A&P. He suggested looking at the carb-heat flapper in the airbox to see if it was closing completely. It seemed like carb-heat should warm the air up by more than 5 or 6 degrees when its turned on, but I didnt know how much is 'normal'. When we pulled the duct, we could see that the valve was fully-closed for the external/air-filter supply, but when switched to carb-heat on, it was only closing about 75% the other way. I attached some photos of what the flapper looked like both ways. A few simple cable adjustments, and it was back to closing fully.
I included a data-log which i hope could be helpful for others. I captured data before and after adjusting the flapper for comparison. In the chart, you can see before the adjustment, I was getting about 6-7 degrees F of warming with carb-heat on. After adjusting it to close fully, I was recording 32-33 degrees F of warming. Obviously much much better.
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