This August 2010 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.
Aircraft: Piper J3C-65. Injuries: 1 Minor. Location: Williamstown, N.J. Aircraft damage: Substantial.
What reportedly happened: 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. However, while climbing out the pilot noticed that the engine had stopped producing full power. He decided that the airplane was too far down the runway to abort the takeoff and continued straight ahead. The plane cleared powerlines at the departure end of the runway and the pilot maneuvered to perform a forced landing in an adjacent field. The airplane hit trees and an unoccupied parked car.
The post-accident 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 four years prior to the accident.
Probable cause: The partial loss of engine power during takeoff due to weak spark plugs on the No. 1 cylinder.
For more information: NTSB.gov. NTSB Identification: ERA10LA437