What happened
On April 30, 2016, a Champion 7BCM, registration N7620B, was performing a revenue sightseeing flight from Atlanta Regional Airport – Falcon Field (FFC) when the engine began to lose power. The aircraft was level at 3,000 feet mean sea level when the power loss occurred.
The pilot attempted to resolve the issue by applying carburetor heat, but engine speed remained at 2,000 rpm. After turning the heat off and checking the magnetos with no significant change in performance, the pilot reapplied the carburetor heat and left it on for the duration of the flight. Because the airplane could not maintain altitude, the pilot executed a forced landing in a grass field in Tyrone, Georgia. Upon touchdown, the aircraft nosed into high vegetation, causing the main landing gear to collapse. The pilot and one passenger were not injured.
The investigation
An FAA inspector examined the wreckage at the accident site, noting the aircraft rested upright with structural damage to the engine firewall and forward fuselage. The main landing gear had collapsed under the airframe, and the fuselage-mounted fuel tank contained an adequate amount of fuel.
Subsequent examination of the engine by FAA inspectors and the NTSB investigator-in-charge found the engine intact and attached to the airframe. There was no evidence of rupture or breach in the engine case or cylinders. Spark plug electrodes showed normal wear, and manual rotation of the engine showed correct valve action, suction, and compression on all cylinders. The carburetor was found to be clean and free of contaminants or water, and the air intake was unobstructed. The magnetos produced a spark at all leads, and the ignition harness showed no fraying or cuts.
Weather observations at FFC indicated a temperature of 79 degrees F and a dew point of 63 degrees F. According to FAA carburetor icing probability charts, these conditions were conducive to serious icing at glide power.