SEA03LA097

The pilot of the carburetor equipped Cessna 172H was approximately 9 nautical miles west of his first fuel stop and commenced a let down through a hole in a broken cloud layer. The surface weather observation at his intended destination recorded several minutes after the accident reported two broken cloud layers at 2,600 feet and 3,400 feet, an overcast cloud layer at 7,000 feet, and temperature 53 degrees F. and dew point 45 degrees F. He reported applying carburetor heat and reducing engine RPM, and then having to descend faster and reduce engine RPM further. He continued a "gliding" descent to 4,000 feet and then re-applied power with no effect. He then executed a forced landing to a muddy, wet agricultural field during which the nose gear collapsed and the aircraft nosed over. FAA Advisory Circular 61-23B, "Pilot's Handbook of Aeronautical Knowledge," provides guidance for operations in carburetor icing conditions. Specifically, the reference states in part "... carburetor heat should be turned to "full-on" before closing the throttle, and left on during the closed-throttle operation...." and "...Periodically, however, the throttle should be opened smoothly for a few seconds to keep the engine warm, otherwise the carburetor heater may not provide enough heat to prevent icing...."

On June 7, 2003, approximately 0945 mountain daylight time, a Cessna 172H, N2786L, registered to Grenfell, Inc., and being operated/flown by a private pilot, sustained substantial damage during a forced landing and subsequent nose over following a total loss of power on descent into the Glasgow International airport, Glasgow, Montana. The aircraft crashed approximately 8 miles west of Glasgow and the pilot was uninjured. Visual meteorological conditions prevailed and no flight plan had been filed. The flight, which was personal, was operated under 14CFR91, and had originated at Ione, Washington, approximately 0530.

The pilot reported in a telephone interview that during his descent approaching Glasgow, he retarded the throttle to 1900 RPM. He indicated that he was not monitoring the outside air temperature but had applied carburetor heat. When he re-applied power, the engine did not respond and the pilot then selected an agricultural field in which to perform a forced landing. Approximately 100 feet above ground he realized the field was wet and muddy and he "pan caked" the aircraft onto the terrain during which the nose gear collapsed and the aircraft nosed over. The pilot reported no mechanical malfunction with the aircraft's powerplant and reported suspecting carburetor ice.

The pilot provided additional detail in his submitted written statement (refer to attached NTSB Form 6120.1). He related that as he was flying eastbound and approaching Glasgow there was a broken cloud layer at 6,000 feet. He further reported "...There was a hole in front of me approximately 1 mile in diameter, that I let down through. I put the carb heat on, reduced the RPM's of the engine and started [a] descent down through the hole. As I got closer to the hole, I descended faster, at a faster rate, which required me to reduce the engine RPM's down slower. I glided down through the hole and continued gliding towards the ground till I was at about 4,000 feet. When I added throttle and nothing happened, I realized I had carb ice...."

An inspector assigned to the Federal Aviation Administration's Helena Flight Standards District Office examined the aircraft following the accident and reported finding fuel in both fuel tanks.

The aviation surface weather observation taken at the Glasgow International airport (2,294 feet above mean sea level) at 0953 on the morning of the accident reported the following conditions:

Visibility 10 statute miles, two broken cloud layers at 2,600 feet and 3,400 feet, an overcast cloud layer at 7,000 feet. Temperature 12 degrees Centigrade (53 degrees F.), dew point 7 degrees Centigrade (45 degrees F.), winds from 310 degrees magnetic at 14 knots and altimeter 30.08 inches of Mercury (refer to ATTACHMENT IP-I, ICING PROBABILITY chart).

The "Pilot's Handbook of Aeronautical Knowledge," (Advisory Circular 61-23B) published by the Federal Aviation Administration discusses carburetor icing. Specifically, the handbook states:

"Whenever the throttle is closed during flight, the engine cools rapidly and vaporization of the fuel is less complete than if the engine is warm. Also, in this condition the engine is more susceptible to carburetor icing. Therefore, if the pilot suspects carburetor-icing conditions and anticipates closed-throttle operation, the carburetor heat should be turned to "full-on" before closing the throttle, and left on during the closed-throttle operation. The heat will aid in vaporizing the fuel and preventing carburetor ice. Periodically, however, the throttle should be opened smoothly for a few seconds to keep the engine warm, otherwise the carburetor heater may not provide enough heat to prevent icing." Refer to ATTACHMENT PH-I.