Failure analysis of an aircraft engine cylinder head

• The cylinder head of the aircraft piston engine cracked during the flight mission.
• Malfunction of the cylinder head occurred due to the fatigue crack.
• The crack initiated from multiple origins on the inner fillet of the cylinder head.
• The fatigue promoted from material defect due to presence of shrinkage pores.
• The finite element analysis showed the highest stress in the crack origin area.

The piston engine of the training aircraft malfunctioned during the flight due to the cracking of its cylinder head (CH), which is manufactured from an aluminum casting alloy. Based on the fractographic examination of the mating fracture surfaces, the characteristic ratchet and beach marks were observed indicating the occurrence of fatigue failure. The crack was initiated from multiple origins located on the inner flange fillet on the exhaust side of the CH. The metallography examination has shown that the fatigue was promoted from pre-existing material defect due to an elevated presence of shrinkage pores at the crack initiation zone and was most likely associated with the manufacturing process of casting. The finite element (FE) method, utilized to determine the stress state of the CH subjected to gas pressure, also confirmed that the crack origin was located at the most stress area.