Accident Details
On June 29, 2013, at approximately 1430 Alaska daylight time, a Cessna 175 airplane, registration N7698M, experienced a partial loss of engine power during departure from the Yentna Lodge Strip. The pilot, who held an airline transport pilot certificate, was operating the airplane under visual flight rules on a personal cross-country flight to Merrill Field in Anchorage, Alaska. Visual meteorological conditions prevailed and no flight plan was filed. The airplane was destroyed when the pilot executed a forced landing into a swamp, during which the aircraft nosed over, substantially damaging the wings and vertical stabilizer. Neither the pilot nor the single passenger sustained injuries.
According to the pilot, the engine began losing power approximately 100 feet above the ground after takeoff. Unable to maintain altitude, he selected a swampy area for landing. The pilot reported that the engine had recently undergone a major overhaul and had accumulated only about three hours of operation since being returned to service.
Engine Examination
On October 31, 2013, the National Transportation Safety Board investigator-in-charge, accompanied by an investigator from Continental Motors, examined the wreckage at Alaska Claims Services in Wasilla, Alaska. The examination focused on the engine and related systems.
Manual rotation of the propeller produced spark at each spark plug in firing order, except for the No. 6 top spark plug. When the No. 6 top spark plug was tested with a different ignition lead, it functioned properly. Testing of the magneto distributor cap indicated that the ignition tower for that plug was functional. However, when the No. 6 top spark plug's ignition lead was removed and repositioned on the distributor cap, the lead was found to be inoperative. Inspection revealed a permanently distorted and pinched area near the distributor cap end of the lead. The firewall aft of the magneto distributor cap exhibited noticeable buckling and denting, but investigators could not determine whether the damaged lead resulted from momentary contact with the firewall.
Fuel System Examination
The carburetor was removed and disassembled; no internal anomalies or debris were found. All orifices were clear, and the fuel inlet screen was free of debris. The glass fuel strainer was approximately one-third full with an amber-colored fluid that appeared dirty. A sample tested with water-finding paste (SAR-GEL) detected water. The fuel strainer screen was dirty but not blocked. Investigators could not determine whether the water was present in the strainer before the accident.
Recovery personnel reported removing at least 30 gallons of aviation fuel from the airplane after the accident. No other preaccident mechanical malfunctions or failures were identified.
Maintenance History
According to engine logbooks, the engine was removed from the accident airplane on May 4, 2011, at a tachometer reading of 209.2 hours. It was overhauled on March 1, 2012, and reinstalled on June 1, 2013, at the same tachometer reading. At the time of the postaccident examination, the tachometer indicated 212.5 hours, reflecting approximately 3.3 hours of operation since installation. No preaccident anomalies with the airframe were noted.
Postaccident Findings
Examination of the airframe revealed no preexisting problems. The engine examination did not uncover any additional mechanical malfunctions that would have precluded normal operation. The precise cause of the engine power loss remained undetermined, although the damaged ignition lead for the No. 6 top spark plug was noted as a potential contributor.