Cold Weather Flying in a Cessna Skyhawk 172

How Extreme Cold Weather can Affect Carburetor Performance in a Single Engine Aircraft

By Mark J. Donovan

I attempted to go flying a couple of weeks ago in a rental Cessna Skyhawk 172, the same exact plane I had trained on for the past year. It was a perfect morning for flying in all respects other than the fact that the air temperature was 2 degrees (F). The skies were clear, the wind was calm, and the visibility greater than 10 miles. It was a perfect cold weather flying day!

Having only gotten my private pilot’s license a couple of months ago I decided to do my homework on cold weather flying in a Cessna Skyhawk 172 prior to the planned flight.

All of my previous flight time had occurred in much balmier temperatures. Everything I read about cold weather flying in a Cessna 172 was pretty much the same process as flying it in warm weather.

About the only additional bit of advice given was to pretty much baby the engine a bit more by giving it more time to warm up. So this is pretty much what I did when I went out that cold morning and attempted to fly the plane. The only problem was I didn’t baby it enough due to one key assumption, the plane had been stored in a warm hangar all night.

After doing my preflight on the plane in the comfort of the warm hangar, I rolled the plane out of the hangar, jumped in the cockpit and began the Starting Engine checklist. The engine fired right up when I finally engaged the ignition switch and the engine ran smoothly. After completing the Starting Engine checklist I then taxied the plane to a compass rose and ran through the Before Take-off Checklist. In the process I did the standard magneto and carburetor heat checks. The engine ran smoothly with no signs of fouled plugs.

After completing the Before Take-Off checklist I taxied for about 3 minutes adjacent to the 6,000 foot runway. During the taxi the engine was set between 1,000 and 900 RPM. Again, the engine ran smoothly the entire time.

When I got to the hold line for the runway I performed the Normal Take-Off checklist. With the Take-Off checklist complete, I then radioed my intentions for departure and proceeded out to the runway for take-off.

Once lined up on the centerline I slowly applied full power and started to roll. That’s when the excitement, or should I say lack of excitement, began. As I brought the engine up to full power it immediately cut out.

Cold Weather Flying

The engine didn’t cough or sputter. It simply shut down abruptly. I had barely moved from my starting position. With no other signs of problems on the dash board, I decided to attempt to restart the engine. It started right up without any additional priming or any hesitation. However, in light of the fact that it was only 2 degrees (F) out and I just had an engine failure on take-off, even though I had barely made any forward progress on my take-off roll, I decided to pack it in for the day and go figure out what had caused my engine failure. While taxing back to the hangar I did notice that the engine oil temperature had not even gotten into the green, which was a lingering concern for me and something that I would come back to later in my exploration of this situation.

When I taxied back to the hangar the FBO’s mechanic and operator were there to greet me and to discuss what had happened. Our initial conclusion was that carburetor ice had formed in the carburetor during my long taxis roll at low RPM. I wasn’t totally convinced on this theory due to the fact that the air was so cold and dry that morning.

My training and understanding of carburetor ice concerns was that it was much less likely to be a problem at very cold temperatures than at moderate or high air temperature levels. So even though I fully trusted the experience of these two general aviation veterans I still felt compelled to dig a little deeper into what was the root cause of the engine failure.

Frozen lakes and pond on this flight.

Consequently I contacted my old flight instructor, as well as my father who had a 40 year career in aviation both as an A&P mechanic and private pilot, about the situation. My father commented that he remembers witnessing a similar situation with a flight instructor pilot in a Cessna 152. The day was very cold, however the engine started right up and ran smoothly, up until the run-up check where the engine proceeded to cut out. As the pilot went to start up the engine my father called for him to hold on for a minute as he saw fluid dripping from the air intake filter. It turned out that the fluid dripping was melting ice from the filter itself. Apparently a thin film of moisture on the air intake filter had frozen when the plane had been moved from the warm hangar to the cold outside. As a result, the air intake filter restricted the normal flow of air into the carburetor which resulted in the engine running rich. At low RPM the restricted air flow was not debilitating enough to cause the engine to cut out. However at higher RPMs associated with a Run-Up or at full power during a take-off, where more air and fuel are required, the air restriction caused by the frozen air intake filter caused the engine to effectively flood out. After identifying the fluid as melting ice the pilot attempted to restart the plane. Like my situation, the plane started up again flawlessly and the pilot continued on with his checklists and flight.

My instructor’s take on the situation, after speaking with the FBO’s mechanic, led to a similar conclusion, that the engine either was choked of fuel or flooded out due to a carburetor icing issue. Though my Skyhawk had been in a relatively warm hangar during the night, the hangar was apparently not heated all night long. As a result, the engine had effectively experienced a cold soak during the night, where the engine temperature probably got down to the mid-30’s based upon the 2 degree (F) outside air temperature.

Due to the fact that the engine was reasonably cold prior to bringing it out of the hangar and into the frigid outside air temperatures, and the fact that the carburetor on a Cessna 172 effectively hangs out of the bottom of the crankcase, the sudden and powerful throttle application, and the resulting massive suction of cold air into the engine, most likely froze the carburetor jet, but did not clog the venturi. As a result the engine was choked of fuel which caused the engine to cut out. If, however, the carburetor’s venturi did actually clog with ice, then the engine cut-out was caused by the engine effectively flooding due to insufficient air intake.

Whatever the exact problem was that caused the engine to cut out, it was most likely related to carburetor icing, since the plane flew several more times that day without incident. Preventing the situation I experienced is also fairly straight forward, if as mentioned earlier, the engine is truly babied during cold weather flight operations. It was the conclusion of all that it is imperative to allow the oil temperature to get into the green on very cold days before departure, as the warm oil circulating around the engine will eventually warm the carburetor via convective heating as the engine itself warms up. In addition, it was agreed that running the carburetor heat longer during the engine warm up period, while at the same time leaning the mixture appropriately to prevent the plugs from potentially fouling, was a wise idea. The only caveat about running the carburetor heat while on the ground for an extended period of time was that it should not be done in a dusty spot, as the engine could suck in dirty unfiltered air which could create another whole set of problems down the road.

So the take away message for me was – enjoy cold weather flying but don’t forget to baby your bird!

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