ERA16LA172

A partial loss of engine power during cruise flight for reasons that could not be determined because postaccident examination of the engine revealed no anomalies that would have precluded normal operation.

The airline transport pilot was conducting a revenue sightseeing flight with one passenger onboard. During cruise flight, while the airplane was level at 3,000 ft mean sea level, the engine began to lose power. The pilot applied carburetor heat with no improvement; the engine speed remained at 2,000 rpm. He then turned the carburetor heat off and checked the magnetos and noted no significant change in performance. He then reapplied carburetor heat with no improvement; the pilot left the heat on for the remainder of the flight. The airplane would not maintain altitude, so he conducted a forced landing in a grass field. After touchdown, the airplane nosed down in high vegetation, and the main landing gear collapsed. The airplane sustained structural damage to the forward fuselage and engine firewall. Postaccident examination of the airframe and engine revealed no evidence of preimpact mechanical failures or malfunctions that would have precluded normal operation, and there was sufficient fuel onboard at the time of the accident. Although the weather conditions at the time of the accident were conducive to serious carburetor icing at glide power, the pilot applied carburetor heat twice, which should have melted any existing ice, but power was not restored. Therefore, the investigation could not determine the reason for the partial loss of engine power.

On April 30, 2016, about 1057 eastern daylight time, a Champion 7BCM, N7620B, was substantially damaged following a partial loss of engine power and forced landing to a grass field in Tyrone, Georgia. The airline transport pilot and one passenger were not injured. The airplane was registered to a corporation and was operated by the Dixie Wing of the Commemorative Air Force under the provisions of 14 Code of Federal Regulations Part 91 as a revenue sightseeing flight. Day, visual meteorological conditions prevailed, and no flight plan was filed. The local flight from Atlanta Regional Airport – Falcon Field (FFC), Peachtree City, Georgia originated about 1040.According to the pilot, the airplane was level at 3,000 feet above mean sea level when the engine began to lose power. He selected carburetor heat and no improvement was observed; the engine speed remained at 2,000 rpm. Carburetor heat was then turned off. The magnetos were checked and there was no significant change in performance noted. Carburetor heat was re-applied with no improvement; the pilot left it on for the remainder of the flight. The airplane would not maintain altitude, so the pilot configured the airplane for a forced landing in a field. After touchdown, the airplane nosed down in high vegetation and the main landing gear collapsed. The pilot and passenger exited the airplane and were assisted by first responders. An inspector with the Federal Aviation Administration (FAA) responded to the accident site and examined the wreckage. The airplane came to rest upright. Structural damage to the engine firewall and forward fuselage was evident. The main landing gear was collapsed under the airframe. The fuselage-mounted fuel tank contained an adequate amount of fuel. An initial cursory examination of the engine revealed no evidence of a mechanical failure. The wreckage was recovered to the operator's facility for additional examination. The examination was conducted by two FAA inspectors and the NTSB investigator-in-charge. The engine was found intact and still attached to the airframe by the engine mount. External examination of the engine case and cylinders revealed no evidence of rupture or breach. All four top spark plugs were removed. The electrodes were normal in wear when compared to a Champion Check-A-Plug chart. The electrodes were coated with a thin layer of black soot. The engine was turned through manually using the propeller. Suction and compression were observed on all cylinders and valve action was correct. The carburetor was removed and disassembled. The fuel bowl was clean and free of contaminants. No water was noted. The bowl contained a small amount of clean fuel that was light blue in color. All carburetor components were normal in appearance. The air intake to the carburetor was unobstructed. The magnetos were turned manually and produced a spark at all leads. The ignition harness was normal in appearance with no fraying or cuts noted. According to the 1053 weather observation at FFC, located about 7 miles south of the accident site, the temperature and dew point were 79 degrees F and 63 degrees F, respectively. According to the carburetor icing probability chart in FAA Special Airworthiness Information Bulletin CE-09-35 (Carburetor Icing Prevention), dated June 30, 2009, the temperature/dew point at the time of the accident was in the area of serious icing at glide power.