Russian Flat, MT, USA
N66255
CESSNA 150M
The commercial pilot attempted to take off from an uncontrolled grass runway with an uphill grade; the airplane subsequently impacted an open area of swamp land about 570 ft beyond the end of the runway. The airplane impacted in a right-wing, nose-low attitude; the impact damage was consistent with a near-vertical descent. Postaccident examination of the airframe and engine revealed no mechanical malfunctions or anomalies that would have precluded normal operation. The density altitude at the time of the accident was calculated to be over 8,200 ft. Review of airplane manufacturer's takeoff performance charts revealed that the conditions present at the time of the accident exceeded the airplane's takeoff performance limitations. It is likely that, as the pilot attempted to take off uphill in high density altitude conditions, he exceeded the airplane's critical angle of attack, and the airplane experienced an aerodynamic stall/spin at low altitude.
On July 21, 2014, about 1325 mountain daylight time, the wreckage of a Cessna 150M airplane, N66255, was discovered at the west end of Russian Flat Airport, Russian Flat, Montana, by a driver passing by on the nearby highway. The airplane was registered to Cowdog Flyers LLC, and operated by the pilot under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The commercial pilot, the sole occupant of the airplane, sustained fatal injuries and the airplane sustained substantial damage. Visual meteorological conditions prevailed at the time of the flight, and no flight plan had been filed. The flight originated from Bozeman, Montana, about 1236. A Federal Aviation Administration (FAA) aviation safety inspector from the Helena Flight Standards District Office who went on-scene reported that the airplane was located about 570 feet from the end of runway 25 at Russian Flat Airport. The wreckage was in an open area of swamp land with the engine and nose of the airplane partially submerged in water. The airplane impacted the water in a near vertical attitude and sustained substantial damage to the fuselage and wings. The airplane's right wing remained attached to the fuselage, and the entire leading edge exhibited extensive spanwise leading edge aft crushing. The lift strut remained attached to its attach points, and was free of impact damage. The right wing was displaced about 60 degrees aft of the lateral axis of the airplane. The airplane's left wing remained attached to the fuselage, and was relatively free of impact damage. The left wing was displaced about 60 degrees forward of the lateral axis of the airplane. Both left and right wing flaps were found in the retracted or "0" position. Due to a lack of tension on the direct control cables, flight control continuity could not be established on scene. The tail structure of the airplane was displaced about 50 degrees right of the longitudinal axis. The propeller remained attached to the engine crankshaft. One propeller blade exhibited substantial torsional "S" twisting and chordwise scratching. The other propeller blade exhibited leading edge gouging and chordwise scratching. The examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation. Convective rain shower and thunderstorm activity was prevalent around the accident site and central Montana at the time of the accident. Weather radar imagery showed a rain shower near the accident site around the same time as the accident and weather satellite data confirmed the movement of these rain showers/thunderstorms from west to east. Non-official surface observation data from a location next to the accident site indicated the strongest wind and gusts around the estimated accident time. When the rain showers/thunderstorms passed official sites there were surface wind gusts to 31 knots. The forecast valid at the accident time did warn of rain showers and thunderstorms in the central weather service unit advisory (CWA) and Area Forecast and closest terminal area forecast. A detailed weather study is located in the public docket for this accident. Remarks listed in the Airport Facilities Directory for Russian Flat Airport stated that Runway 25 had a 2% uphill slope to the west and recommended that pilots take off on runway 07 and land runway 25, conditions permitting. There was also a note cautioning pilots to check density altitude as Russian Flat airport was considered "high elevation." The closest non-official weather station, South Fork Judith, located about 1 mile east of the accident location, at an elevation of 6,300 feet, reported a temperature of 68 degrees F and a dewpoint of 47 degrees F. By utilizing the closest altimeter setting of 30.00 from the Lewistown Municipal Airport, 44 miles northeast of the accident site, combined with the Russian Flat Airport field elevation of 6,336 feet, the pressure altitude was about 6,256 feet. Pressure altitude is calculated using the following formula: [1000 x (standard pressure – current pressure) + field elevation] By imputing the derived pressure altitude into the density altitude formula, the density altitude at Russian Flat Airport at the time of the accident was about 8,296 feet. The density altitude formula, utilizing degrees Celsius is: Pressure Altitude + [120 x (OAT – ISA)] A review of the manufacturer's supplied Flaps Retracted Takeoff Distance chart, located in the Pilot's Operating Handbook revealed that the weather conditions present at the time of the accident exceeded the chart's performance parameters. As a result, takeoff performance calculations were not determined. The maximum altitude for which takeoff data was supplied was 7,500 feet, 796 feet lower than the density altitude at the time of the accident. Furthermore, the data provided did not include penalties or enhancements for sloped or wet grass runways. According to the FAA's Airplane Flying Handbook, FAA-H-8083-25, an increase in density altitude can produce a greater takeoff speed, decreased thrust and reduced net accelerating force. Also, "an increase in altitude above standard sea level will bring an immediate decrease in power output for the unsupercharged reciprocating engine.
The pilot's inadequate preflight performance planning and his operation of the airplane outside of the manufacturer's specified performance limitations, which resulted in his exceedance of the airplane's critical angle of attack and a subsequent aerodynamic stall/spin.
Source: NTSB Aviation Accident Database
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