Fatigue as a cause of failure of aircraft engine cylinder head

Abstract

Aircrafts powered by piston engine represent the bulk of the fixed wing fleet. Large majorities of these aircrafts are equipped with single, air-cooled, and horizontally opposed, piston engines. The mechanical failure in any of the piston engine components, especially concerning fatigue failure of the cylinder head (CH) made of an aluminum-casting alloy may have serious or fatal consequences to the safety of the crew and the aircraft. Therefore, the aim of this study is to identify the root cause(s) of repetitive premature failures in an aircraft engine CH.Two piston engines of the training aircraft Utva-75 have malfunctioned during the flight mission due to the fatigue cracking of their aluminum cast CHs. Visual inspection revealed that transverse cracking had occurred between the fifth and the sixth deep integral cooling fins. Based on the fractographic analysis, observation of the characteristic ratchet and beach marks indicated the occurrence of fatigue failure. The crack initiated from multiple origins located on the inner flange fillet on the exhaust side of the CHs. The metallographic examination revealed that the fatigue was promoted from pre-existing material defect due to a high concentration of shrinkage pores at the initiation crack site and can be most likely associated with the manufacturing process of casting. The finite element method, utilized to determine the stress state of the cylinder assembly, also confirmed that the crack origin was located at the most stressed area of the cylinder assembly, i.e., on the inner flange fillet of the exhaust side of the CH.