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It Can Happen to You
One of the many things you learn about in basic pilot training is Carburetor (Carb) Icing and how it affects the ability of a carbureted engine to provide peak performance, or can cause it to quit entirely. But depending on where you fly and what aircraft and engine combination you operate, you may never experience carb icing in clear air in Visual Flight Rules (VFR) conditions. I primarily fly in Virginia and the surrounding states and, although Virginia gets chilly a few months out of the year, we’re not often subject to the heavy snow and ice conditions experienced in more frigid climates. In three and a half years of flying, I don’t think I have ever experienced carb icing…well…until the other night. Much of this is due to the fact that I am not yet instrument rated, so I don’t fly through visible moisture (as a side note I am nearly checkride ready and should have my instrument rating in the next month or so). So let me tell you a little story about how I was reminded that carb icing is real, even in clear air, and how it can catch you off guard especially if it is a novel occurrence for you.
As I mentioned above, I am currently working on my instrument rating, which means spending a lot of time in simulated instrument conditions. For the non-pilot community, or those unaware, that means intentionally restricting my view to the instruments alone using a view limiting device that forces me to focus only on the instruments and not outside the airplane. Because simulated instrument training takes place in VFR conditions, an additional pilot is required on board to serve as a safety pilot. The safety pilot is tasked with looking outside for other aircraft, clouds, obstacles, or anything else that could affect safety of flight while the pilot manipulating the controls flies only by reference to the instruments.
It was after dark in mid-December when I launched, safety pilot aboard, in my flight club’s Piper Archer to shoot some VFR practice approaches and accumulate some simulated instrument time. For a few days preceding this flight, the weather had been quite rainy and cold, low ceilings and visibility. This had been the first nice weather day, the ceilings were gone, the temperature on the ground was around 11 degrees Celsius (52 degrees Fahrenheit), and the dew point was 0 degrees. But, due to the precipitation in the preceding days, it was still quite humid. Because it is relevant to this article, generally the temperature lapses (declines) at a rate of 2 degrees Celsius (3.5 degrees Fahrenheit) per 1,000 feet of altitude.
The takeoff was normal and we planned on heading southwest to an airport a little over 60 nautical miles away. Due to the southwest heading, and the altitude required for the feeder route on the approach I planned to use, I planned a climb to an altitude of 6,500 feet. As we climbed, I began to lean the mixture as normal and appropriate for the altitudes we were reaching. We leveled off at 6,500 feet and I configured the airplane for cruise flight by trimming, adjusting the mixture and checking the exhaust gas temperature, and setting the throttle. All was right with the world…for a moment.
I first noticed a slow loss of altitude, at the rate of roughly 50 to 100 feet a minute, and so I checked the power and retrimmed. The altitude continued to leak, so I repeated the procedure. Despite these efforts, I could not seem to stop the slow sink rate we were experiencing. The engine sounded fine and wasn’t running rough, though I noted that the revolutions per minute (RPM) was a little over 2,300, lower than I had configured for and expected. I checked the mixture, and although it appeared leaned appropriately, I made slight adjustments to see if it made any appreciable change in the engine power output, but it did not.
I inquired of my safety pilot if we had gone through any visible moisture (remember, I am focusing on the instruments with a view limiting device), and was assured we had not. And although I doubted the existence of structural icing, I asked if it appeared we had somehow accumulated any. Collectively, we looked out at the wings and could see nothing. It was at this point I audibly said, “something is wrong here.”
The altitude continued to slip. We had not flown through any visible moisture, there was no structural icing, and despite trimming, adjusting the mixture, and adjusting the power settings, we could not seem to hold our altitude. Since we had lost a few hundred feet at this point, I decided it was time to go full power and climb back to 6,500. I moved the throttle to the fully open position, and noticed no change to the RPM. The altitude continued to slip and within seconds the RPM began dropping to 2,250; then to 2,200; and 2,150; and finally, to almost 2,100. I audibly said again “we got a problem here.” I began running through checklists and flows, wondering what we could be missing, and I began thinking of closer airports in case we lost the engine when suddenly, I had the lightbulb moment. Carb icing!
That little switch for carb heat, the one we test during every run up, maybe it was the solution. I shouted “CARB ICING” and reached for the switch. I flipped the switch to the on position, and watched as our RPM dropped some more (normal when turning on carb heat) and then reversed and began to climb and then climb more. Before I knew it, we were full throttle putting out close to 2,600 RPM again, had returned to our cruise altitude of 6,500 feet, and had begun reducing the power again and re-configuring for cruise.
The rest of the flight was uneventful. Now aware that the conditions were ripe for carb icing, I was cautious to ensure I used the carb heat during the approach, especially at the lower power settings, and to turn it off just before landing in case we needed to go missed.
This was a wake-up call for both me, and my safety pilot. Although I am a little embarrassed that it took as long as it did for me to realize this may have been the issue, I’m quite glad it finally dawned on me and was the solution to the problem. I share this story as a reminder that, even when you don’t realize it is possible, carb ice can sneak on up you.
If you have had an experience with carb icing, especially when you didn’t expect it, I’d love to hear your story. Please reach out to Jeff@angryoctopusaviation.com and let me know how it happened to you, and how you caught it.
Keeping the carb warm,
Jeff
