For student pilots who arent mechanically inclined-and even for many who are-some of the basic concepts in aviation are difficult to grasp. Recent advances in technology and aircraft design have resulted in new aircraft which more closely resemble the high-end luxury car the pilot may have driven to the airport. But thats pretty much where any similarities-accidental or purposeful-between automobiles and aircraft end.
As a primary student somewhat familiar with engines and other mechanical contrivances, one of the aviation-centric concepts I found challenging involved carburetor ice. Since most of my training took place during what I recall as a long, hot, humid summer in southern Georgia, the idea of any ice forming anywhere outside of a beer cooler was totally foreign. Being the dutiful student pilot, however, I readily accepted the instructors explanation of why and how to apply carburetor heat. Anything to get my hands on a mighty Cessna 150s controls and aim it skyward.
Later, when I started flying a Cherokee 140, was briefed that while it, too, had a carburetor and a carb heat control, the checklist didnt require its use as often as the two-seat Cessnas. The explanation had something to do with the carb heat system itself, along with the carburetors larger size and the induction systems design. Sure, whatever you say.
The central theme of all this, of course, was using carb heat to ensure the engine kept running at low power settings, like when gliding to a landing or doing stalls and slow flight. It wasnt until much later in my aviation career that I realized carburetor ice could form at higher power settings, those well within the green arc inscribed on the airplanes tachometer. My other realization involving carb ice involved the fact that, just because the tach reads in the green arc doesnt mean an engine driving a fixed-pitch propeller is turning at a high power setting. As this months accident demonstrates, its a concept others should understand.
Background
On August 3, 2006, at approximately 0925 Mountain time, a Piper PA-24-180 Comanche piloted by a private pilot, was destroyed when it collided with mountainous terrain during descent about 16 miles northwest of Mosca, Colo. Visual conditions prevailed at the time of the accident. The cross-country flight originated at Pueblo, Colo., at 0837, and was en route to Alamosa, Colo.
At 0710, the pilot contacted the Denver Automated Flight Service Station (AFSS) and filed a VFR flight plan calling 10,500 feet as an initial cruise altitude. During his weather briefing, he was told, “VFR flight [is] not recommended” due to a forecast of occasional IFR conditions until 1200 local time. Another forecast called for mountain obscuration through 1300. At the time of the weather briefing, current weather wasnt available, so the pilot was given the previous hours observations. The departure point and an en route station were reporting VFR. The aircraft took off at 0837. The last radio contact with the airplane was at 0856, when the pilot acknowledged that radar services had been terminated.
A witness, located about two miles from the accident site, said he heard an approaching airplane. He said the engine was “sputtering, like it was missing.” He did not see the airplane, but he did hear a loud “pop.” He went to a clearing and saw a fire on the mountainside.
The witness later said the sky was overcast and mountaintops were obscured. He estimated the point of impact to be 500 feet below the overcast.
Investigation
The accident site was at a GPS elevation of 11,589 feet msl. Ground scars indicate the airplane struck the mountain in a wings-level, nose-slightly-low attitude on a magnetic heading of 250 degrees. It then slid downhill approximately 75 feet. The landing gear and flaps were both retracted.
The wreckage was recovered from the mountain and the engine was disassembled and examined. No anomalies were noted. There was camshaft and crankshaft continuity, and all connecting rods were attached. The carburetor, which had separated from the engine, was examined. The bowl was empty, and the throttle valve was closed. Both the engine-driven and auxiliary fuel pumps were destroyed by fire.
At the time of the accident, weather recorded at the La Veta Mountain AWOS (VTP), located 37 miles south of the accident site, included a temperature of 15 degrees Celsius and a dew point of nine degrees C.
The Carburetor Icing Probability Chart was consulted. The temperature and dew point recorded at La Veta Mountain were conducive to “serious icing at cruise power.”
Probable Cause
The National Transportation Safety Board determined the probable cause of this accident to include “a non-mechanical partial loss of engine power due to carburetor ice, and the pilots failure to maintain clearance from terrain. Contributing factors in this accident were weather conditions conducive to carburetor icing and the pilot inadvertently flying into instrument meteorological conditions.”
Its rare for carburetor ice to down an airplane while in cruise but thats clearly what appears to have happened here. Its even more rare for an engine developing, say, more than 65 percent of its rated power, to be affected by carburetor ice. It can happen, but its rare. For a graphical presentation of the temperature and humidity (dewpoint) conditions under which carburetor ice is likely to form, consult the chart at the bottom of the opposite page.
Whats not rare is for a carbureted engine in a normally aspirated (NA) airplane cruising at a high altitude in humid conditions to develop ice. The reason? The higher altitude-remember, were talking about non-turbocharged engines-reduces available power, even if the engine produces its full rpm range. Basically, any NA engine operating above 8500 feet is incapable of producing more than 65 percent power. As the NA engines altitude increases, power decreases, enhancing carb ice susceptibility.
In this accident, the airplane likely was cruising at 12,500 feet or higher, since the accident site was at 11,589 feet. At that altitude and using common cruise power settings, the engine was probably developing around 50 percent power. Just about right for carb ice to form.
