Much of the U. S. is entering a period during which warmer weather can be expected. Warmer weather frequently brings along it's best buddy, higher humidity. When warm temperatures and higher humidity team up, their friend carburetor ice frequently tags along. This month's operational tip will be concerned with preventing accidents caused by carburetor ice so if you only fly a fuel injected or turbine powered airplane you can skip this section.

The combination of vaporizing fuel and decreased air pressure in the carburetor's venturi reduces the temperature by 60 to 70 degrees F. So even outside air with a temperature of 100 degrees F. could end up as 30 degree F. air in the carburetor. This rapid temperature decrease will wring out moisture from even moderately humid air. This moisture may form ice on the cold internal parts of the carburetor, primarily the throttle plate. As ice builds up the amount of air passing through the carburetor decreases and power is lost. Engine failure can result if the condition is left unchecked.

The consequences of carburetor icing are sufficiently severe that the FAA issued a Special Airworthiness Information Bulletin, SAIB CE-09-35 in June of last year. Click here to see the SAIB.

FAA SAIB CE-09-35

If you want to start a good argument around the FBO coffee pot, ask what temperatures are most conducive to carb ice formation. The fact is that carb ice is always a possibility when the ambient air temperature is above about 10 degrees F. and there is moisture present in the air. The probability of encountering this performance robbing phenomenon increases as the air temp increases above freezing and as the humidity increases. Some airplanes, I suppose by the design of the cowling, are more susceptible to carb ice than other models being powered by the same engine.

Taking the same airplane into different atmospheric conditions can also produce an unwelcome surprise. My early instructing took place in Cherokee 140s in upstate New York. The procedure I had been taught and passed on to my students was to only use the carb heat if there was an indication of carb ice. I moved to Florida to instruct at one of the large flight schools, also operating Cherokee 140s. Guess what? I quickly learned the hard way that in the Florida conditions it was prudent to always apply carb heat when the power was reduced.

Here are a few review points about preventing carb ice or dealing with it if it occurs. I will limit the discussion to airplanes without sophisticated engine monitoring systems or carburetor air temperature gages. It's probably a good idea to apply carb heat whenever the power is reduced much below cruise. It's also a good idea to check periodically for carb ice during cruise. We can do that by noting the engine power output relative to the tachometer for airplanes with fixed-pitch props or manifold pressure for those with constant speed props. Apply full carb heat and note the change in power output. The power output will decrease as the air entering the carburetor becomes warmer and therefore less dense. If the engine continues to run smoothly and the power remains at the reduced value, there is no carb ice present. If the engine loses power and begins to runs rough, possibly followed by a slight increase in power, ice is or was present. The roughness is caused by melting some of the ice and running it through the engine as water. The slight increase in power is caused by the increased airflow through the carburetor as the ice is removed. If ice was present, it will be back if the carb heat is turned off so leave it on.

Never apply partial carb heat unless you have a means of measuring the temperature in the carburetor. Consider that knob to be an on-off switch. Remember that the mixture will be richer while carb heat is applied because the warm air is less dense than the cooler ambient air. This will probably require the mixture to be leaned a bit. If it is necessary to continue with carb heat applied, remember that performance is going to be slightly reduced. Fuel consumption may be increased even though the mixture has been leaned so verify that the reserves are still valid. If a go-around is necessary, be sure to turn off the carb heat as it can be a real performance robber when calling for full power.

Always give carb ice the respect it deserves. Every year there are numerous accidents attributed to carb ice. Many more accidents are listed as "power loss for undetermined reasons." Some of those are probably carb ice related since the ice melts once the engine stops running.

This accident was a result of partial power loss due to carburetor ice. It involved a Beech Musketeer and occurred in Ohio during September of 2007. Fortunately, only minor injuries were suffered.

Photo courtesy of WTOV

The pilot reported that while on downwind he reduced engine power to initiate a descent. While on base leg the airplane descended below glide path. The engine did not respond to multiple throttle inputs and continued to operate at 1,500 rpm regardless of throttle position. The airplane was too low to make the airport and a forced landing was made to a cornfield. The airplane nosed over during landing.

Twelve gallons of fuel were drained from the airplane after the accident. The fuel selector was positioned on the left tank, the mixture control was mid-travel, and the carburetor heat was "off." The throttle, mixture, and carburetor heat controls functioned properly. A post accident inspection did not reveal any anomalies consistent with a loss of engine power. The temperature was 68 degrees F. and the dew point was 63 degrees F. Data indicated the possibility of moderate carburetor icing at cruise power and serious icing at descent power under those conditions.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows: The partial loss of engine power during approach due to carburetor ice and the uneven terrain encountered during the forced landing which caused the airplane to nose over. Weather conducive to carburetor icing was a contributing factor.

Last month's issue of this newsletter stressed situational awareness. Part of the preflight preparation for a flight in an airplane equipped with a carburetor should be an analysis of whether or not carburetor icing is likely and the formation of a plan to prevent it or to deal with it if it occurs. Awareness of this hazard is one more rung on the ladder to better situational awareness.

Click here to read the full accident report.

Here is an example of an accident that resulted from carburetor icing during an IFR flight. It involved a Mooney M20C and occurred in North Carolina in February of 2000. the pilot and sole occupant suffered only minor injuries.

The pilot was operating on an instrument flight plan while flying from Richmond Virginia to Knoxville Tennessee. According to the pilot, as he approached his destination, he saw that possible airframe icing conditions existed ahead, and he cancelled his instrument flight plan so that he could avoid those conditions and maneuver under VMC for a visual landing. He flew a circuitous route to remain in VFR conditions. While at 5,500 feet msl he experienced a sudden loss of engine power. He was over wooded, mountainous terrain. According to an FAA inspector, the pilot stated that he was not using carburetor heat.

The pilot estimated weather conditions at the time and location of the accident to be, about 2,000 feet overcast, 3 miles visibility in snow flurries, ambient temperature 30 degrees F, and moderate turbulence. According to Air Force Rescue Coordination Center, (AFRCC), the pilot communicated by cell phone that he was still with the downed aircraft, that his aircraft radio equipment was not working, that he did not know if the ELT was going off, and that he was in need of rescue. In response to AFRCC's query, the pilot reported he had a bump on the head and a sprained knee, and that he had only a jacket for warmth, and no survival gear.

Examination of the fuel tanks revealed about 12 gallons of fuel remaining in the right wing tank, and no fuel in the left wing tank. The cockpit fuel selector was found selected to the right tank. The throttle was found full open, the propeller full increase rpm, and the mixture full rich. The carburetor heat was in the full in or "off" position. This raises the possibility of running the left tank dry and not switching tanks in time to restart the engine, but the NTSB, with the benefit of more information than is typically included in the reports, dismissed that issue and attributed the accident to carburetor icing. The NTSB report includes the following: Reference to carburetor icing charts reveals that an ambient temperature of 30 degrees F, and a dew point temperature of 25 degrees F, as reported by the Knoxville FSS and the pilot, for the time of the accident, would put the flight's probability within the region labeled, "serious icing at cruise power".

The National Transportation Safety Board determines the probable cause(s) of this accident as follows: The pilot's failure to use carburetor heat in a timely manner while maneuvering in conditions conducive to carburetor icing, resulting in the loss of engine power and collision with trees during the subsequent emergency descent.

Here we see an example of carburetor icing occurring even when the ambient temperature is below freezing. Some pilots will say that is impossible because any moisture in the air is already frozen out. That is simply not the case. Carb ice can be found in ambient temps as low as about 10 degrees F.

What could the pilot have done differently? He probably was acting appropriately by deciding to avoid possible structural icing by remaining out of the clouds in freezing conditions. But, again, situational awareness should include environmental aspects including the possibility of encountering carb ice. The pilot further placed himself at risk by not having warm clothing aboard. A good practice is to always be prepared to spend the night in the terrain over which you are flying.

Click here to read the full accident report.

How many pilots are actively flying without a valid FAA medical certificate in their possession? I think the number might be higher than anyone would guess. How many pilots have given up flying their traditional airplanes and have moved into LSA because they know that they will be denied a medical certificate on their next visit to their AME? Again, I think the number is substantial.

How many of these pilots are medically unsafe to fly? There are certainly some who I wouldn't want to have flying my grandchildren around but I'm sure that others would be perfectly safe.

In the 48 years since I took my first aviation medical exam I have seen the FAA medical requirements change substantially. I can remember using several different techniques to squeak by the uncorrected vision requirement of 20/40 or better even though my eyes were corrected by glasses to better than 20/20. Now the corrected vision, barring other disorders, is not a limiting factor. I can remember when heart bypass surgery was new and anyone having the operation was saying good-by to their medical certificate forever. Now there are airline captains flying after successful bypass surgery. The FAA recently approved the use of certain medications, under certain conditions, to treat mild to moderate depression. Many other problem areas have become more lenient over time.

Is this a good thing? I think that it is. After all, very few airplanes crash because of pilot medical incapacitation on the part of a pilot holding a valid medical certificate. Yes, there have been cases of serious incapacitation or even death while at the controls, but I don't know of a single case in which the pilot was granted a medical certificate as a result of the more lenient rules of the past 40 years. There may very well be a few such cases. But, nearly all pilot incapacitation cases happen to pilots who are flying without a valid medical certificate, with a medical certificate obtained by making false statements or incomplete disclosure on the application, or who are stricken by a condition of which they were unaware.

I will praise the FAA for speeding up the process required to obtain a ruling from Oklahoma City in cases where the local AME is not authorized to act. I would encourage the FAA to look at other areas where the medical certificate requirements might be made more lenient. A pilot will be more likely to operate within the system if he or she believes the system is realistic.

It appears that the tide of health care reform in the U.S. is moving toward centralized medical records for everyone. Since we have to give permission to check our driving records when applying for a medical certificate do we think that we won't have to give permission to access all of our medical records? Let's get the system streamlined and realistic before that day comes or we will find many pilots grounded and even more operating without a valid medical certificate in their possession.