Accident Overview
Atlantic Southeast Airline Flight 529 was climbing through 18,000 feet when a blade from the left propeller separated. The separation resulted in distortion of the left engine nacelle, excessive drag, loss of wing lift, and reduced directional control. The degraded performance forced the crew to conduct a forced landing. During the landing, the airplane passed through trees, impacted the ground, and was further damaged by a post-impact fire. The captain and seven passengers were killed.
Propeller Blade Examination
An examination of the left propeller revealed that the blade had failed due to a fatigue crack. The crack originated from multiple corrosion pits in the taper bore surface of the blade spar. The crack propagated toward the outside of the blade and around both sides of the taper bore.
Previous Blade Failures and Inspection Procedures
Due to two previous blade failures (separations), a borescope inspection procedure had been developed by Hamilton Standard to inspect returned blades that had rejectable ultrasonic indications for evidence of cracks, pits, and corrosion. The accident blade was one of 490 rejected blades that had been sent to Hamilton Standard for further evaluation and possible repair.
Maintenance and Inspection Lapses
Maintenance technicians who inspected the blade lacked proper NDI familiarization training and specific equipment to identify the corrosion that resulted in fatigue.
Probable Cause
The in-flight fatigue fracture and separation of a propeller blade resulted in distortion of the left engine nacelle, causing excessive drag, loss of wing lift, and reduced directional control of the airplane. The fracture was caused by a fatigue crack from multiple corrosion pits that were not discovered by Hamilton Standard because of inadequate and ineffective corporate inspection and repair techniques, training, documentation, and communications. Contributing to the accident was Hamilton Standard's and FAA's failure to require recurrent on-wing ultrasonic inspections of the affected propellers. Contributing to the severity of the accident was the overcast cloud ceiling at the accident site.