Gold Hill, CO, USA
N337KN
CESSNA T337G
The pilot and passengers departed on a local sightseeing flight in the twin-engine, centerline-thrust airplane. ADS-B data showed the airplane depart toward mountainous terrain in the general direction of the planned tour route. The airplane’s climb rate appeared appropriate and consistent with normal operation until the airplane leveled off about 11 minutes later, at an altitude about 9,500 ft mean sea level (msl) about 3,000 ft below a typical cruise altitude for that flight. A short time later, the airplane slowed and entered a left descending turn, reaching a descent rate of almost 6,000 ft per minute (fpm). Witnesses near the accident site reported observing the airplane flying low overhead. The right wing rose abruptly, dropped, and the left wing rose as the airplane entered a nose-down attitude and began to “corkscrew.” One witness stated that the engine noise stopped as the airplane entered the descent. The witness also stated that, up until that point, the engine(s) sounded like they were “struggling.” The airplane impacted wooded terrain about 7,800 ft msl and came to rest upright. The ground scars, impact signatures observed on the trees, and wreckage signatures indicated that the airplane impacted terrain in a nose-low attitude, consistent with a spin. A post-impact fire consumed most of the airframe. Both the front and rear engines sustained extensive thermal and impact damage, and the propeller for the rear engine was found in the feathered position. A postaccident examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation; however, the scope of the examination was limited due to the extensive thermal and impact damage, and a mechanical issue could not be ruled out. The feathered position of the rear engine’s propeller suggests that it is likely that the engine had lost power or been shut down; however, the reason could not be determined based on the available information. It is possible that the pilot was attempting to return to the airport when the accident occurred. Given the high density altitude conditions present at the time of the accident, the airplane’s single-engine climb performance would have been marginal. Additionally, although the pilot’s logbook was not available for review, he was recently hired by the accident operator and had accumulated about 3.5 total hours in the accident airplane since he was hired. According to the operator, the pilot’s training in the accident airplane did not include an engine shutdown; instead, a loss of engine power was simulated by reducing the throttle. The circumstances of the accident are consistent with the pilot’s exceedance of the airplane’s critical angle of attack while maneuvering with one engine inoperative, which resulted in an aerodynamic stall/spin and loss of control. Due to the extensive thermal and impact damage, the investigation was not able to determine why the rear engine was not operating.
HISTORY OF FLIGHTOn July 17, 2022, about 0938 mountain daylight time, a Cessna P337G, N337KN, was destroyed when it was involved in an accident near Gold Hill, Colorado. The pilot and three passengers were fatally injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 air tour flight. The owner of the company reported the accident flight was a local sightseeing flight, which was to remain within 25 nautical miles of the departure airport. ADS-B data, depicted in figure 1, showed that the airplane departed from Rocky Mountain Metro Airport (BJC), Broomfield, Colorado, (elevation 5,674 ft msl) and proceeded along a westerly course for about 4 minutes before entering a right turn to a northwesterly heading. Figure 1: ADS-B Flight Overview For the next 10 minutes and 40 seconds, the airplane maintained a ground speed between 90 and 105 knots (kts) while maintaining a climb rate of about 380 ft per minute (fpm). After leveling off at an ADS-B reported pressure altitude of 9,500 ft, the airplane was traveling about 95 kts. It slowed to about 92 kts while maintaining altitude, and 20 seconds later, began a turn to the left. The airplane’s rate of descent increased, and the airplane descended 1,100 ft over the next 10 seconds; the last recorded data point was located at an altitude of 8,400 ft about 579 ft northeast of the accident site. (See figure 2.) Figure 2: ADS-B Data - Last portion of the accident flight. A witness located near the accident site reported seeing the accident airplane descending as it neared his position. He reported that the airplane was very loud as if the engine was running at full power. The witness saw the airplane’s right wing rise abruptly before it entered a nose-down attitude and began to “corkscrew” until it descended out of view. Another witness stated that the airplane was flying “super low.” She reported that the engine “made a clickity-clackity-type of sound, like it was not running smooth.” The witness further stated that the right wing lifted, then quickly lowered, before the left wing lowered and the airplane entered a clockwise corkscrew descent before the tree line. She stated that, when the left wing lifted, the engine noise stopped and she was certain that the engine(s) sounded like they were “struggling.” PERSONNEL INFORMATIONThe pilot was hired by the operator as a private contractor about 6 weeks before the accident. The accident flight was his fourth tour flight with the company. His logbooks were not located, but a review of dispatch logs provided by the operator revealed that he had flown the accident airplane for a total of 3.5 hours, of which 1.1 hours were described as training. At the time of the pilot’s most recent Federal Aviation Administration (FAA) medical exam on February 17, 2022, he reported 550 total hours flight experience. The owner of the company that operated the flight reported that the pilot’s training included 3 hours of ground instruction via video conference, followed by training in the airplane, which was a combination of showing the pilot the route of the tours along with conducting some maneuvers. The owner stated that the training included several scenarios, including gear retraction failures, smoke in the cockpit, and pressurization issues. He added that during the flight, he would usually shut an engine down; however, he decided to simulate a loss of engine power by reducing what he recalled was the rear engine to zero thrust. The owner said it was a short checkout, but he was “extremely confident” to sign the pilot off. AIRCRAFT INFORMATIONThe operator, Bluebird Aviation, was operating under a 91.147 letter of authorization (LOA) issued by the FAA’s Denver Flight Standards District Office on January 6, 2022. The provisions of the LOA allowed the operator to provide nonstop commercial air tours provided they stay within a 25-statute-mile radius of the base airport. According to the operator, the ADS-B-depicted route of flight followed the general direction of a standard tour, and typically the maximum altitude attained during such a tour would be between 12,500 and 13,000 ft msl. The airplane owner’s manual contained two charts in reference to rate of climb, two-engine climb performance, and single-engine climb performance. The two-engine climb and single-engine climb performance charts contained sections for climb performance at 5,000 ft at 41°F and 10,000 ft at 23°F, with notes that stated, “for hot weather, decrease rate of climb 40 ft./min for each 10° above standard day temperature for particular altitude” and “for hot weather, decrease rate of climb 25 ft./min for each 10° above standard day temperature for particular altitude,” respectively. Using the reported temperature at the departure airport and a maximum gross weight of 4,700 lbs, two-engine climb performance charts indicated a climb rate of 1,060 fpm at 5,000 ft, or 895 fpm at 10,000 ft. The single-engine climb performance chart indicated a 215-fpm climb rate at 5,000 ft, and a 95-fpm climb rate at 10,000 ft. METEOROLOGICAL INFORMATIONUsing reported weather conditions, the calculated density altitude at the departure airport was 7,677 ft, with a pressure altitude of 5,012 ft. AIRPORT INFORMATIONThe operator, Bluebird Aviation, was operating under a 91.147 letter of authorization (LOA) issued by the FAA’s Denver Flight Standards District Office on January 6, 2022. The provisions of the LOA allowed the operator to provide nonstop commercial air tours provided they stay within a 25-statute-mile radius of the base airport. According to the operator, the ADS-B-depicted route of flight followed the general direction of a standard tour, and typically the maximum altitude attained during such a tour would be between 12,500 and 13,000 ft msl. The airplane owner’s manual contained two charts in reference to rate of climb, two-engine climb performance, and single-engine climb performance. The two-engine climb and single-engine climb performance charts contained sections for climb performance at 5,000 ft at 41°F and 10,000 ft at 23°F, with notes that stated, “for hot weather, decrease rate of climb 40 ft./min for each 10° above standard day temperature for particular altitude” and “for hot weather, decrease rate of climb 25 ft./min for each 10° above standard day temperature for particular altitude,” respectively. Using the reported temperature at the departure airport and a maximum gross weight of 4,700 lbs, two-engine climb performance charts indicated a climb rate of 1,060 fpm at 5,000 ft, or 895 fpm at 10,000 ft. The single-engine climb performance chart indicated a 215-fpm climb rate at 5,000 ft, and a 95-fpm climb rate at 10,000 ft. WRECKAGE AND IMPACT INFORMATIONThe airplane impacted mountainous terrain on a heading of about 283° magnetic at an elevation of 7,789 ft msl. The first identified point of contact was a cluster of trees that were severed or broken about 20 ft above the ground. These trees were grouped within an area of trees that were about 75 to 100 ft tall. The airplane came to rest mostly upright on a heading of about 019° magnetic. All major structural components of the airplane were located within about a 30 ft by 40 ft area. The area surrounding the wreckage was fire damaged. The fuselage was mostly consumed by fire, with no relative structure discernable. A portion of the instrument panel was observed with various avionics separated and thermally damaged. The throttle quadrant was located and exhibited impact and thermal damage. Flight control continuity was established from the middle of the fuselage to all primary flight control surfaces. Multiple areas of separation in the control cables were observed that exhibited splayed signatures consistent with tension overload. The front engine exhibited extensive thermal and impact damage throughout. Visual mechanical continuity was established throughout the engine (crankshaft, camshaft, connecting rods). The accessory housing was impact and thermally damaged, exposing the camshaft gears, crankshaft gears, and idler gears. The fuel pump was separated from its mounts. The starter adapter was impact and thermally damaged and separated from the engine. The throttle interconnect was intact and fire damaged and the throttle plate appeared to be in the full open position. The left and right magneto were not located, however, their drive couplings remained attached to the engine accessory drive gears. The fuel pump would not rotate by hand and exhibited impact damage. The drive coupling was present and impact damaged. The fuel pump was disassembled and found unremarkable. The fuel manifold valve was intact, however, exhibited thermal damage. The fuel manifold valve was disassembled. The diaphragm was thermally damaged, and the spring was intact. The screen contained burnt debris. The rear engine was separated from the engine mount. The engine exhibited varying degrees of thermal and impact damage throughout. The magnetos were not located. Visual mechanical continuity was established throughout the crankshaft, camshaft, and connecting rods when looking through the burned-away portion of the oil sump. Cylinder Nos. 1, 3, and 5 exhibited extensive impact and thermal damage. The induction system was impact and thermally damaged. The throttle plate was observed in an almost-closed position. The magnetos were not located. The starter was not located. Cylinders Nos. 2, 4, and 6 examined internally using a borescope, and were unremarkable with the exception of corrosion throughout the cylinders. The fuel pump was impact and thermally damaged. The aneroid valve was thermally damaged. The fuel pump drive coupling was intact. The fuel pump drive shaft would not rotate by hand. Disassembly of the fuel pump revealed corrosion and thermal damage throughout. The mixture control lever was separated, however, a portion of the lever remained attached to the mixture control cable. The fuel manifold valve remained attached to the engine and exhibited thermal damage. The fuel manifold valve was disassembled. The spring was intact. The diaphragm was thermally damaged and was unable to be removed from the fuel manifold valve cavity. The front propeller had chordwise scratches and S-bending. All six mounting holes were ovalized with internal threads ripped out. Witness marks from contact between the feathering spring and both blade root faces were observed. The orientation of the witness marks were consistent with both blades were in the normal operating range. The rear propeller was mostly intact and approximately 8 inches of the tip of one of the blades was melted and missing. Oil was present in the actuating cylinder, and plastic ball bearing separators were melted and solidified inside the hub. The rear propeller’s blades were in the feathered position, with little scratching or bending on the propeller blades. MEDICAL AND PATHOLOGICAL INFORMATIONAn autopsy of the pilot was performed by Boulder County Coroner, Boulder, Colorado, which listed the cause of death as “blunt force injuries.” Toxicology testing performed at the FAA Forensic Sciences Laboratory found no drugs of abuse.
The pilot’s exceedance of the airplane’s critical angle of attack while maneuvering with one engine inoperative, which resulted in an aerodynamic stall/spin and impact with terrain.
Source: NTSB Aviation Accident Database
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