Bellanca 8GCBC N749SK accident near Frankford, Delaware

Probable cause

A partial loss of engine power resulting from cylinder detonation and a cracked spark plug, which resulted in a forced landing and impact with terrain.

NTSB analysis

The pilot was flying at 300 ft mean sea level (msl) while towing a banner when he noticed the engine oil temperature began to increase. The pilot informed the operator and climbed to 1,200 ft msl; however, the airplane would not maintain altitude and was slowly descending. The engine rpm began to decrease and the engine increasingly began to run rough. The pilot dropped the banner as the airplane descended through 700 ft msl, and he initiated a forced landing in a field that was bordered by trees. Unable to reach the field, the airplane impacted the top of trees. The airplane then descended to the ground, resulting in substantial airframe and wing damage.

Postaccident examination of the engine revealed damage to the No. 3 cylinder that was consistent with detonation in the form of erosion on the edge of the intake valve and valve seat insert, and a cracked lower spark plug that was likely the cause of the detonation. A review of the most recent annual inspection maintenance logs revealed that the No. 3 cylinder assembly was replaced with a new cylinder 45 days before the accident.

Text from the NTSB. Open the NTSB docket for ERA24LA299 for the full record.