Performance charts and cockpit instruments are notably exotic components of the world of pilot training. Beginning students invest much time learning how to interpret both the instruments and the charts. Cross-country flight planning, knowledge tests, and practical tests focus heavily on these skill areas. But there's a chart and instrument that doesn't command much attention. Ironically, the same chart and instrument pair is the place to look for early warning signs if one is to have the best chance of avoiding certain problems. And if the habit of looking is not there, the risk factor increases.
The actual hazard to guard against--carburetor icing--is no stranger. It hides in plain sight every time the pilot of a float-carburetor-equipped aircraft makes a power change or wonders what may be behind an unexpected drop in manifold pressure or rpm. The newest of student pilots, flying most common trainers, learns that below certain recommended power settings, carburetor heat must be applied. It's a familiar part of the ritual when reducing power on the downwind leg of the traffic pattern to prevent carb ice by applying carburetor heat. We also learn that part of a proper takeoff or go-around requires removing carb heat. That's because the warmer air it produces is less dense, causing the engine to produce (noticeably) less climb power, as one accident described here underscores.
Despite the preventive measures learned and constantly practiced, carburetor ice surprises some pilots because the conditions that cause it frequently occur when the ambient air temperature is well above freezing. The evidence vanishes, so to speak, leaving only conjecture as to whether any accident had an iced-up carburetor at its core.
Have you guessed the obscure instrument and chart previously described? The instrument is the outside air temperature gauge (OAT). It too hides in plain sight, typically located out of your main panel scan but easily kept in mind once you become conscious of its presence. Read it in consideration of the information you glean from studying a chart found in textbooks and pilot's operating handbooks. That table or graph depicts the general likelihood of carburetor ice occurring at various combinations of temperature and humidity.
Do you recall that touch of surprise you felt when you read that carb ice best likes to strike when the temperature is above freezing--sometimes well above freezing? Chapter 5 of the Pilot's Handbook of Aeronautical Knowledge says, "Carburetor ice is most likely to occur when temperatures are below 70 degrees Fahrenheit (21 degrees Celsius) and the relative humidity is above 80 percent. However, because of the sudden cooling that takes place in the carburetor, icing can occur even with temperatures as high as 100 degrees F (38 degrees C) and humidity as low as 50 percent. This temperature drop can be as much as 60 to 70 degrees F."
Ideal conditions for carb ice are very likely in late summer to early fall. So when a Cessna 150 flying in the traffic pattern lost power and landed short of the runway in Urbana, Ohio, on September 10, 2008, the National Transportation Safety Board's accident summary noted that "the temperature and dew point in the vicinity of the accident site were 17 degrees and 12 degrees Celsius, respectively. Calculations using a carburetor icing chart indicate the possibility of moderate carburetor icing at cruise power and serious icing at descent power under the indicated conditions."
At low power in the pattern, carb heat evidently came too late. "The pilot reported that as she approached the destination airport after a one-hour cross-country flight, she reduced engine power to approximately 2,300 rpm in order to descend to traffic pattern altitude. During the downwind leg, she further reduced engine power to 2,000 rpm to slow the airplane for the approach. As she initiated a turn to base leg, she made a final reduction of engine power and activated the carburetor heat. During the turn to base leg, the engine began to lose power. The loss of engine power was reported to be gradual, not instantaneous, occurring over a period of approximately five seconds. The engine did not respond to throttle and mixture control movements. The pilot stated that the loss of engine power occurred 700 to 800 feet above ground level while on base leg. She said that she was worried about stalling the airplane and established a descent to remain above stall speed. The airplane landed in a grassy area about 50 feet short of the runway. Shortly after touchdown the nose gear collapsed and the airplane came to an abrupt stop."
The probable accident cause--"a loss of engine power due to carburetor icing"--made note of the place on the carb-icing probability chart at which this accident took place: "Contributing to the accident were environmental conditions conducive to carburetor icing."
It seems unlikely that a fatal accident with carburetor icing at its root might be identified as such, absent the details that a pilot could provide. But a GPS unit on board the aircraft gave investigators information that proved useful when reconstructing an accident in Wichita Falls, Texas. A private pilot flying an American Champion 7GCBC, intending to remain in the traffic pattern, ran into immediate trouble. This pilot, also faced with a forced landing, did not prevent the low-altitude stall.
"Winds were from 010 degrees at 16 knots gusting to 20 knots. A carburetor icing chart showed the potential for moderate icing at cruise power or serious icing at descent power. Through a GPS and engine monitoring system, the accident flight was able to be partially reconstructed. Upon takeoff the airplane turned left and entered the downwind leg. As the airplane was rolling out on the downwind, a throttle reduction was recorded. The airplane continued to the south-southwest on the downwind leg for at least 14 seconds. As the airplane approached the base turn, the first recorded attempt to actuate the throttle is recorded as an increase in manifold pressure and spike in fuel flow. Manifold pressure fluctuates as the airplane makes the base turn to final. Approximately three-quarters through the base turn, the airplane rolled out and flew a straight approach to the open field. Several witnesses observed the airplane approaching the open field without engine power. After 'barely clearing' a transmission wire, the airplane was observed to quickly roll right and impact the ground 'nose first,'" said the NTSB account, finding the final seconds of the flight "consistent with the airplane entering a stall/spin."
Probable cause: "The pilot's failure to maintain safe flying airspeed resulting in an inadvertent stall during the forced landing. Contributing to the accident was the loss of engine power due to carburetor icing."
The likelihood of carb icing based on charted conditions and manufacturer's information should be reviewed as part of thorough preflight planning, and made part of in-flight situational awareness. Develop the habit of keeping an eye on the OAT along with rpm or manifold pressure gauges, while tuning the ear to engine smoothness.
Another lesson of Urbana is that carb heat may not save the day if added too late or too low. Follow recommended technique--are you using full carb heat if recommended to so do, and applying it at the proper time? Carb ice is another reason to be alert to that possible forced landing that your instructor loves to nag you about during your flight lessons.
Dan Namowitz is an aviation writer and flight instructor. A pilot since 1985 and an instructor since 1990, he resides in Maine.
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