Piper PA-28R N4081S accident near Crystal, Minnesota

Probable cause

The failure of the engine mount under normal operating loads due to the fracture of a vertical tube that was weakened by corrosion and fatigue of the internal surfaces.

NTSB analysis

During the landing flare, the airplane encountered a gust of wind, climbed about 3 ft, and landed on the runway centerline. The pilot described the landing as firm, but not hard. The airplane was refueled and returned to the hangar with no issues. The next day, the pilot’s son flew the airplane. The pilot’s son performed a preflight inspection with no anomalies noted. After the takeoff, he retracted the landing gear, and he noticed that the “gear in transit” light stayed illuminated. He extended the landing gear, and all three green landing gear lights were illuminated. The pilot’s son landed back at the airport and taxied back to the hangar with no issues.

The following day, the pilot’s son flew the airplane to another airport to have a mechanic inspect it. A postflight inspection by the mechanic revealed that the engine mount had fractured at the nose gear attachment point. The airplane’s engine mount sustained substantial damage.

Postaccident examination revealed that the engine mount likely failed under normal operating loads due to the fracture of a vertical tube that was weakened by corrosion and fatigue cracks. The corrosion occurred on the interior surface of the vertical tube, likely due to the presence of water and greasy deposits at the lower end of the tube. Although the corrosion damage had likely been developing for an extended time, it would not have been detectable by visual inspection. The corrosion damage included a loss of wall thickness that increased the average stress in the remaining cross-section, and local stresses were further increased by rough surfaces and pits that formed during the corrosion process. As a result, fatigue cracks initiated and grew from multiple origins at the corroded surface.

Although the total extent of fatigue cracking could not be determined due to post-fracture damage, it is likely that the tube was weakened sufficiently from the combination of wall thickness loss and fatigue crack growth to cause failure under normal loading conditions. The corrosion and fatigue cracks at the lower end of the tube were secondary to another failure that could not be determined from the examined pieces.

Although an incomplete weld or service damage such as a crack or hole in the tube could have allowed water to seep into the tube bore, causing the tube to corrode from the inside out, no such anomalies or cracks were observed below the cut location.

Text from the NTSB. Open the NTSB docket for CEN24LA342 for the full record.