N3962VCessna 170 · 1947

On August 13, 2020, about 1745 central daylight time, a Cessna 170, N3962V, was substantially damaged when it was involved in an accident near Ardmore, Tennessee. The pilot and passenger received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot, along with his uncle and a dog, were departing from a grass field on a friend's property when the accident occurred. The pilot reported that he purchased the airplane about five days before the accident, and this was the first time he had operated from the field. He had about 10 to 15 gallons of fuel per side (20 to 30 gallons total) onboard prior to takeoff. The grass was dry, and the wind was calm. The engine run-up was normal. During the takeoff roll, when the airplane reached 50 to 60 mph, he rotated and established a climb rate. The airspeed then began to decay, and there was inadequate thrust to achieve an acceptable rate of climb at an adequate airspeed, and the pilot performed a forced landing to a small pasture. The pilot did not recall any changes in engine noise prior to the accident.

Examination of photographs of the airplane provided by the Federal Aviation Administration (FAA) revealed that the left main landing gear collapsed, and the airplane impacted the ground with the left wingtip, nose, and right wingtip, and came to rest upright. The wing flaps were fully retracted, and the airplane incurred substantial damage to the engine mounts, firewall, wings, and fuselage.

Examination of the engine revealed that the crankshaft displayed a partial fracture just aft of the propeller flange. The fracture was visibly twisted, and the fracture face displayed a 45° angle to the longitudinal axis of the crankshaft. Further examination of the engine also did not reveal any noticeable issues and drivetrain continuity, from the front to the back of the engine was established. All the pistons would move when the crankshaft was rotated.

The top spark plugs appeared normal. The left magneto produced spark at all leads. The right magneto could not be turned by hand. Disassembly of the right magneto revealed that the impulse coupling shaft bearing was broken due to impact; however, it would move with difficulty by hand once the cap was removed. No damage to the coupling drive gear or any visible part of the gears seen through the accessory case was discovered. Fuel was present in the carburetor and the accelerator pump was functional. The fuel strainer was impact damaged.

The pilot believed that the field that he tried to depart from was about 2,000 ft long; however, review of satellite images of the accident location indicated that the field was about 1,230 ft long. The elevation was about 896’ above mean sea level. Obstacles existed along the departure path the pilot flew (about 180°), in the form of a driveway, trees, and buildings. Past the driveway, the terrain was about 25 ft higher than the field, and the trees were in close proximity to the area where the airplane came to rest.

Review of the Cessna Model 170 Owner’s Manual indicated that, for “shortest takeoff” the wing flaps should be in the “full down” (30°) position. For unusually short field takeoff, the application of wing flaps to the full down position should occur just before the airplane is ready to leave the ground, and the flaps should not be retracted until an altitude of at least 100 ft above the highest obstacle.

The recorded weather at Fayetteville Municipal Airport (FYM), Fayetteville, Tennessee, located 13 nautical miles east of the accident site, at 1755, included wind from 150° at 3 knots, 10 miles visibility, clear sky, temperature 31°C, dew point 21°C, and an altimeter setting of 29.96 inches of mercury. The density altitude around the time of the accident was about 3,224 ft.

Interpolation of performance information contained in the Cessna Model 170 Owner’s Manual revealed that, at maximum gross weight, zero wind, and flaps retracted, at the approximate elevation and temperature at the accident site, the takeoff distance to clear a 50-ft obstacle would have been about 2,205 ft with a ground run of 882 ft from a hard-surfaced runway.

Review of an FAA Koch Chart revealed that at a temperature of 31°C, the pilot would have experienced a takeoff distance approximately 34% longer with an associated 26% decrease in rate of climb. Under these conditions, a runway of approximately 1,300 feet would be equivalent to an international standard atmosphere sea level runway length of 970 feet.