ERA17LA176 · Cessna 150M

The pilot was conducting a cross-country flight when, during cruise at 3,000 ft mean sea level, he heard an unusual sound from the engine and engine power decreased to 1,500 rpm. The pilot unsuccessfully attempted to restore engine power, then light smoke began to enter the cockpit. The pilot subsequently declared an emergency with air traffic control and selected a forced landing site. During touchdown on a soft field, the airplane nosed over and came to rest inverted, resulting in substantial damage.

Examination revealed that the engine's No. 4 cylinder had separated about halfway along the cylinder barrel from a fatigue crack that initiated at a cooling fin valley on the exterior surface. The fatigue crack grew around the circumference of the cylinder through approximately 30% of the cross section, and the remaining cross section succumbed to overstress. Once the cylinder fractured, the other components, such as the piston and connecting rod, sustained subsequent damage. Metallographic cross section of the cylinder at and near the fracture revealed the presence of corrosion pits under the remaining paint and primer.

While the alloy used in the engine’s cylinders was not necessarily susceptive to pitting, other issues, such as crevice corrosion near an unpainted area or use in salt environments, can lead to similar corrosion features. Corrosion pits can be a common cause of fatigue cracking. Although the accident engine had accrued about 2,249 hours since its most recent major overhaul about 21 years before the accident which exceeded the engine manufacturer's overhaul recommendations of 2,000 operating hours or 12 years in service, whichever occurred first, the No. 4 cylinder had been installed new about 9 years before the accident. However, the presence of pitting corrosion to the extent that it resulted in failure of a cylinder is consistent with inadequate inspection and maintenance of the engine.

Verbatim NTSB analysis and probable cause from the NTSB dataset