Robinson Helicopter Company R44 N3234U accident near Bountiful, Utah

Probable cause

The pilot/mechanic's failure to properly secure the pitch link hardware of one main rotor blade to the rotating swash plate, which resulted in the pitch link separating in flight and a subsequent loss of control.

NTSB analysis

Before the accident flight, the commercial helicopter pilot, who was also a mechanic, had re-installed the main rotor blades, which had just been reworked, on the helicopter. The accident flight was a test flight to adjust the track and balance of the rotor blades. A mechanic, who spoke with the pilot a few weeks before the accident, stated that they had a discussion about an elongated pitch change link attachment hole on the accident helicopter and how to address it. On the night before the accident flight, they spoke again; the pilot was having trouble tracking the blades on the accident helicopter. The pilot stated that he could not get the blades to track any better, and that he was trying to change the track with the trim tabs.

Witnesses in the area of the accident site heard "popping" or "banging" sounds, then saw the main rotor and empennage separate from the helicopter. Several of the witnesses then saw the helicopter tumble in flight and impact the roof of a building. The main rotor and empennage came to rest on the ground a few hundred feet from the building. Witness statements and wreckage documentation were consistent with a main rotor blade striking the tail and subsequently, a mast bump, which resulted in the helicopter descending uncontrollably.

The damage observed on the components of the main rotor system was consistent with an in-flight separation of the pitch change link for the red blade, with separation occurring at the location where the pitch change link attached to the swashplate. The swashplate was free of contact marks corresponding to contact with the red pitch change link, which contrasted with the area around the blue blade pitch change link attachment, where multiple contact marks corresponding to contact with the blue pitch change link were observed. Also, the red pitch change link was intact and relatively straight, indicating that separation occurred under loads less than that required to buckle or fracture the pitch change link. The slight bending in the red pitch change links was likely secondary to the separation of the attachment at the lower end as evidenced by the location of the corresponding thread contact marks on the pitch horn. Finally, a series of impressions corresponding to contact with threads on the red blade pitch change link attachment bolt were observed on the attachment hole bore through the swashplate in an area that should have only contacted the grip portion of the bolt. Thus, it is likely that the intact bolt separated from the attachment due to loss of the lock nut and palnut.

Torque measurements were obtained on the locknuts installed on the three recovered pitch change link attachment bolts. All measured torque values were lower than that specified in the helicopter's maintenance manual, indicating that the fasteners were improperly torqued before the accident. While torque for the missing attachment bolt could not be measured, the torque measured on the remaining pitch change link attachment bolts and witness marks on the attachment hole bore in the swashplate suggest that the bolt likely separated due to insufficient torque applied at the time of installation, which led to the loss of the locknut and palnut due to vibrational loads under normal operation.

Text from the NTSB. Open the NTSB docket for WPR15FA051 for the full record.