Accident Overview
On July 1, 2021, at about 1400 Alaska daylight time, a Cessna 170B airplane, N2746D, was destroyed in an accident near North Pole, Alaska. The pilot and three passengers were seriously injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Departure and Initial Climb
The airplane departed from Dalrymple’s Airport (31AK) in Fairbanks, Alaska. The pilot reported that the aircraft was about 13 pounds heavier than its maximum gross takeoff weight. After completing a preflight check, the pilot initiated the takeoff roll with one notch of flaps deployed. When the airspeed reached about 40 to 45 mph, he added a second notch of flaps and climbed to tree-top level after using approximately 1,000 feet of runway.
Attempt to Clear Obstacles
The pilot was unable to increase airspeed or altitude without overly increasing the angle of attack. Recognizing that the airplane would not clear the trees off the end of the runway, he deployed full flaps and slowed to about 40 mph. The airplane then settled between trees about 200 feet beyond the runway end. It came to rest upright in a wooded area with the nose pointed back toward the runway. A postcrash fire occurred.
Witness Accounts
A witness reported observing the airplane take off and climb to about 100 feet above the trees at the departure end. She then saw the airplane make a sharp left turn, during which the left wing struck a tree, and the airplane descended into terrain. She noted that the nose did not drop during the turn and that the engine sounded normal throughout, with no unusual noises.
Pilot Statement
A law enforcement officer who responded to the accident reported that the pilot stated he was unsure what happened during takeoff. The pilot thought that the wind had shifted and that the airplane lost lift at the end of the runway.
Postaccident Examination
A postaccident examination of the airplane revealed no evidence of preaccident mechanical malfunctions or failures that would have precluded normal operation.
Performance Considerations
The estimated density altitude at 31AK at the time of the accident was 2,186 feet above mean sea level. According to the airplane owner’s manual, the airplane required an estimated takeoff distance of 2,250 feet to clear a 50-foot obstacle. Runway 31AK is 2,400 feet long and 100 feet wide, with a turf surface. The airplane manufacturer noted that the turf runway would likely increase the required takeoff distance.
FAA Guidance on Weight and Density Altitude
The Federal Aviation Administration (FAA) Pilot’s Handbook of Aeronautical Knowledge discusses the effects of weight on aircraft performance, noting that overloading can lead to longer takeoff runs, reduced climb rates, and higher stalling speeds. The FAA publication Density Altitude explains that high density altitude reduces air density, adversely affecting aerodynamic performance and engine power.