Carburetor Ice Conundrum

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When you’re flying be prepared for an emergency, it can happen to you. Practice your emergency procedures often to keep your skills sharp. I learned how important keeping the emergency skills honed actually is with only 90 hours under my belt. I had a partial power loss that exercised my fledgling emergency skills pretty hard.

Carburetor Ice Diagram It was a nice toasty 82 degree day in McKinney, Texas on April 19, 2008 with about 35% humidity when I was flying N7929U, a Cessna 150, up to McAlester Oklahoma. Just as I was crossing the Red River at cruise power (practically full power in that bird) at 5000 feet, the RPMs suddenly dropped. The drop was not as gradual as I would imagine for carb ice…and the drop was definitely enough to put me in a decent. Where was that panic button again? It was really happening to me, not some other person out there.

As I was working through the emergency checks, I also called McKinney tower back at the home airport and reported my position and situation. I was about 12 miles north of Bonham F00 airport at the time. As I was working through my emergency check flow, I came to the carb heat and was hoping that this strange situation was carb ice and that power would be recovered slowly as the ice melts off.

In my racing emergency-mode mind, I’m thinking it’s not too likely to be carb ice because the RPM drop definitely wasn’t gradual, the power was at pretty high RPM for cruise, and it was 90 degrees with only 30% humidity. The second I pulled on the carb heat, the RPMs jumped back to life, almost full power…no rough running engine, and no gradual RPM increase. Being that the symptoms didn’t match up with carb ice, I headed straight to the nearest airport to make a precautionary landing. Bonham (F00) was the closest airport.

McKinney tower made sure there was someone there to greet me at Bonham airport. As I got closer to the airport, I saw an abundance of fire trucks and police cars waiting patiently for me. I definitely wanted to bust out a nice landing for them since they took the time to come out there. I was definitely relieved to be on the ground safely.

Looking back at the situation I learned that carb ice may not always behave like they say in the text books. You’ve got to be ready for anything. I mean seriously… 82 degree temperature, only 35% humidity, and cruise power…and still carb ice! At 5000 feet with a 3.5 degree F per 1000 ft temperature lapse rate, the temperature aloft was about 72 degrees F. Apparently, a temperature of around 72 degrees and a little extra moisture while crossing the Red River were just the right ingredients to develop some nasty carb ice.

Something they may not tell you in the aviation books is that for any unexplained loss of RPMs, carb heat should be one of the first things to try. After you do your immediate emergency checks and find your emergency landing destination, it can make your life much easier to let someone know where you are, whether it be the local tower or on the emergency frequency, 121.5. McKinney Tower was extremely helpful…it was so nice to hear their familiar voices on the radio.

Carburetor Ice Graph

Check out this graph that shows when carburetor ice is likely to happen. Some of these conditions may be surprising. For my situation, the temperature was 82 degrees F and the dewpoint was 43 degrees F. According to the graph, that definitely puts me in the icing during glide and cruise category. Had I paid more attention to this graph before that flight, I might have known to expect it and been more prepared. For some reason, I was under the misconception that carb ice only happened at low power settings…wrong.

Carb ice can happen at any temperature any time of the year and at any power setting, so be prepared and be safe.

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