Piper J3C-65 N209RC accident near Williamstown, New Jersey

Probable cause

The partial loss of engine power during takeoff due to weak spark plugs on the No. 1 cylinder.

NTSB analysis

The airplane had not been flown recently so the pilot elected to conduct two preflight inspections, an engine run-up, and two full-power takeoff test runs where the airplane became momentarily airborne with no discrepancies noted. During the accident flight takeoff, the airplane accelerated and became airborne at the same point along the runway as it had during the two test runs. While climbing out on the runway heading, the pilot noticed that the engine was suddenly no longer developing full takeoff power. He decided that the airplane was too far down the runway to abort the takeoff and continued the forward flight path. The airplane cleared powerlines at the departure end of the runway and the pilot maneuvered to perform a forced landing in an adjacent field. The airplane then collided with the tops of trees and impacted an unoccupied parked car. A postaccident inspection of the engine revealed weak spark from both spark plugs of the No. 1 cylinder, and the bottom plugs in the remaining cylinders were oil soaked and exhibited weak spark. The cylinders were replaced about 4 years prior to the accident equating to 6 flight hours.

Text from the NTSB. Open the NTSB docket for ERA10LA437 for the full record.