ERA20LA288 · Cessna 170 Undesignat

The pilot, a passenger, and a dog were departing from a grass field. The pilot had purchased the airplane about 5 days before and this was the first time he had operated from the field. He reported that, after takeoff, there was “inadequate thrust” to climb, and he performed a forced landing to a pasture, during which the airplane sustained substantial damage. Postaccident examination of the airplane revealed that the wing flaps were in the retracted position. Examination of the engine revealed 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. This was indicative of an overload type of failure consistent with the impact, and that the engine was producing power during the accident sequence. The right magneto was also damaged during the impact, which precluded functional testing. Overall, the examination of the engine did not reveal evidence of any preimpact failures or malfunctions that would have precluded normal operation. The pilot believed that the field was about 2,000 ft long; however, review of satellite images indicated that the field was only about 1,230 ft long and sloped uphill. Obstacles existed along the departure path in the form of a driveway, trees, and buildings. Review of the airplane owner’s manual indicated that, for shortest takeoff the wing flaps should be in the “full down” (30°) position. It also noted that for unusually short field takeoffs, the application of wing flaps 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 density altitude around the time of the accident was about 3,224 ft. Under such conditions, the pilot would have experienced a takeoff distance about 34% longer than normal and an approximate 26% reduction in rate of climb. Review of performance information contained in the airplane owner’s manual revealed that, under similar atmospheric conditions, at maximum gross weight with the flaps retracted, the airplane would have required a takeoff ground roll distance of about 880 ft; however, this information was valid for a level, hard-surfaced runway. The distance to clear a 50-ft obstacle was about 2,200 ft. No performance information was published for operations off turf/grass surfaces, which provide more rolling resistance (drag) than paved surfaces. Based on the available information, it is likely that the airplane’s takeoff and climb performance was reduced due to the elevated density altitude conditions and the additional drag provided by the grass. Additionally, the pilot did not use the short-field takeoff technique prescribed by the owner’s manual. Had the pilot more thoroughly familiarized himself with the constraints of the area he was attempting to take off from, as well as the information provided in the owner’s manual, he likely would have realized that departing from the field under the accident circumstances provided little margin for a successful takeoff and climbout.

Verbatim NTSB analysis and probable cause from the NTSB dataset