Insecure Pitch Link Fatal R44 Accident
During a post-maintenance check flight the main rotor separated from an R44 after a maintenance assembly error.
The Severed Tail of R44 N3234U (Credit: KUTV2 News via Twitter)
A US National Transportation Safety Board (NTSB) report describes the fatal accident to Robinson R44 N3234U operated by Native Range Capture Services in Utah on 2 December 2014:
The helicopter’s owner stated that, during a flight in the accident helicopter the month before the accident, the helicopter “had a bit of a vertical [vertical vibrations]“. The blades also had noticeable slop and movement in the pitch change link bolt attachment to the swashplate. During a 100-hour inspection, on November 2, 2014, at 582 hours of helicopter total time, the owner and pilot/mechanic recommended sending the blades out for rework. …the [red] main rotor blade pitch horn was replaced… …the owner hired the pilot-rated mechanic to install the blades and track them in a heavier configuration.
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.
During the accident flight witnesses heard “popping” or “banging” sounds, before seeing the main rotor and empennage separate from the helicopter.
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 Seperated Main Rotor of R44 N3234U (Credit: Unknown)
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. …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.According to manufacturer’s maintenance instructions, the attachment bolts used at the upper and lower ends of the pitch change links are NAS6605 series bolts. The locknuts for the NAS6605 series bolts should be fastened to a dry torque value of 240 lb-in ± 24 lb-in, and palnuts used on NAS6605 series bolts should be applied with a dry torque of 20 lb-in to 40 lb-in. During installation, the palnuts are installed over the top of the locking nut.
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.
The National Transportation Safety Board determined the probable cause to be:
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.
There is no suggestion in the NTSB report that an independent inspection had been carried out, nor is one required under US regulation on this size of aircraft.
Robinson issued Safety Notice SN-43 in January 2015:
Original Article Courtesy of Aerosurrance