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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
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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.
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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.
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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.
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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. 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. |
Photo by Mark Donovan
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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. |
Photo by Mark Donovan
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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!
Related Information
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