Failure analysis of X-Msn oil cooler fan blade on helicopter

An analysis on cracks found on AZ91C-T6 X-MSN oil cooler fan blades of a HH-47 helicopter was performed. Cracks on oil cooler fan blades, which are conditional parts, are variously distributed between 199 and 1023 operating hours. All cracks initiated from the blade root mid span as a form of multi-origin, and then developed by HAF (high amplitude fatigue). Cracks were initiated from gas porosity formed in the casting process, a shrinkage cavity and a pit instigated between base metal and coating layer. The blade with 1238 operating hours had a crack 42 mm out of the total length of 45 mm. This crack began from the convex side and penetrated through the concave side causing blade fracture. Therefore, improvement of blade manufacturing process is required and blades in operation are recommended to be eddying current inspected after paint coating removal every 250 h and to be replaced every 1000 operation hours to prevent fracture.