ERA14LA253 · Piper J3C - 65

The pilot reported that before taking off for a local flight, when he conducted the engine run up, the magneto drop for the right magneto was 150 rpm, which was 100 rpm greater than the drop for the left magneto (the pilot later stated that the "engine ran up just fine"). The pilot applied takeoff power and, based on his years of experience, he estimated that the engine was developing full power. The airplane became airborne in about the usual time, just before midfield. He began climbing out between 40 to 50 mph, which he said was normal; about 10 to 15 seconds later, when the airplane was about 100 feet above ground level and with about 1/4 of the runway remaining, the pilot noticed that the engine was not developing full power, which was usually 2400 rpm; the tachometer indicated 1,900 rpm. He verified that the throttle was full forward, the mixture control was full rich, and the carburetor heat was off. At 1,900 rpm, there was just enough power to maintain altitude but not enough to climb. The pilot initiated a shallow turn to try landing on a taxiway, but during the turn, he lost altitude and abandoned the plan. He maneuvered the airplane for a forced landing on a road. During the forced landing attempt, the right wing hit a tree, causing the airplane to be pulled to the right over a guardrail. Inspection of the engine following recovery revealed the right magneto was timed 14 degrees after the specified amount, or closer to top dead center, and discrepancies were noted with both spark plugs in the No. 4 cylinder.

Although the pilot indicated the engine run-up was satisfactory, the excessive right magneto drop was likely due to the mistimed right magneto and the spark plug discrepancies and should have been grounds to cancel the flight and investigate the reason for the excessive drop.

Verbatim NTSB analysis and probable cause from the NTSB dataset