Cessna 172N N74C accident near San Diego, California

Probable cause

A failure of the No. 2 cylinder due to fatigue cracking as the result of an undersized fillet radius around the edge of the cylinder head dome.

NTSB analysis

The pilot reported that he completed a preflight inspection and engine run-up with no anomalies noted. Shortly after takeoff, while making a right turn from the crosswind to downwind leg of the airport traffic pattern, he heard a loud “pop,” and noticed brown smoke coming from the engine compartment. The airplane began shaking and the engine subsequently lost total power. The pilot declared an emergency and landed on the runway; however, the airplane overran the departure end of the runway and nosed over, resulting in substantial damage to both wings and the engine mount.

Postaccident examination of the engine revealed that the No. 2 cylinder head cracked due to an undersized fillet radius around the edge of the dome. The undersized fillet increased local tensile stresses in the head under normal operating loads, which resulted in fatigue crack initiation and growth. The undersized fillet radius was less than that of the mating barrel corner radius, indicating that the undersized fillet was likely machined into the head at the time of manufacture. The remaining three cylinders from the accident engine were examined and revealed a discontinuous fillet in each of the heads that was inconsistent with the manufacturer’s engineering drawings.

At the time of the accident, the engine had over 1,400 hours in service in the 4 years since the cylinder was manufactured. The accident engine is likely a relatively high-time engine and could be among the fleet leaders for other engines operating with cylinders manufactured around the same time as the accident cylinders.

Text from the NTSB. Open the NTSB docket for WPR24LA229 for the full record.