Atlantic Southeast Airline Flight 529 was climbing through 18,000', when a blade from the left propeller separated. This resulted in distortion of the left engine nacelle, excessive drag, loss of wing lift, & reduced directional control. The degraded performance resulted in a forced landing. While landing, the airplane passed through trees, impacted the ground, & was further damaged by postimpact fire. An exam of the left propeller revealed the blade had failed due to a fatigue crack that originated from multiple corrosion pits in the taper bore surface of the blade spar. The crack had propagated toward the outside of the blade & around both sides of the taper bore. Due to 2 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 & corrosion. The accident blade was one of 490 rejected blades that had been sent to Hamilton Standard for further evaluation & possible repair. Maintenance technicians, who inspected the blade, lacked proper NDI familiarization training & specific equipment to identify the corrosion that resulted in fatigue. (See: NTSB/AAR-96/06 for additional information.)