Overview
On the morning of the accident, a Bell 47-G2 helicopter (serial number 1281) was engaged in aerial crop spraying operations. While pulling up to commence a 180-degree turn at approximately 100 feet above ground level, the pilot heard a loud bang. The helicopter rolled left, pitched uncontrollably, and descended nose-down, striking the ground on a 120-degree magnetic heading. A post-crash fire consumed most of the engine and cabin area. The pilot, seriously injured, was rescued by bystanders from a nearby road.
Sequence of Events
The pilot transported the helicopter by trailer from Chatham, Ontario, to the farm that morning. After a daily inspection, the pilot flew a circuit to inspect the field, landed, loaded fuel and chemical, and completed one spray swath. When initiating the turn, the main rotor blade separation occurred. The blue blade (part number 47-110-120-50, serial number P9551) was found 400 feet south of the wreckage, while the yellow blade remained attached to the rotor head and was recovered 100 feet north. The main rotor head, mast, and transmission separated intact before ground impact. The tail boom and tail rotor remained intact with the main wreckage.
Findings
Examination revealed that the blue blade separated at the inboard end within the internal threaded region of the blade grip (part number 47-120-135-5, serial number A100). A section of the failed grip remained attached to the blade; the other section was not recovered. Fatigue cracking initiated in the threads at the last complete outboard-most thread, leading to instantaneous failure under normal service loading. Similar but less advanced cracks were found in the yellow blade grip. The threads in both grips conformed to standard UN fine thread form, but the root radii were much smaller than those of an exemplar spare. Extensive pitting was also present on both grips. Sharp root radii and pitting are known stress concentrators that facilitate fatigue crack initiation. No abnormal installation or operational conditions were identified. The blade grip failed 992 hours before the first prescribed mandatory inspection cycle. Despite inconsistent part markings and surface alterations, the grips were considered authentic parts supplied by the manufacturer.
Probable Cause
One of the helicopter main rotor blades separated from the helicopter in flight when the blade grip failed due to fatigue cracking initiated within the thread area of the blade grip. The reason for the fatigue failure could not be determined.