History of Flight
On July 22, 2014, about 1120 Pacific daylight time, an MDHI 369D helicopter, registration N5225C, collided with terrain and rolled downhill several times near Oso, Washington. Olympic Air operated the helicopter under Title 14 Code of Federal Regulations Part 133 for a local external load flight. The departure time was undetermined. Visual meteorological conditions prevailed, and no flight plan was filed. The commercial pilot sustained serious injuries, and the helicopter airframe sustained substantial damage.
Examination of Wreckage
A Federal Aviation Administration inspector examined the wreckage on site. The main rotor blades were fragmented into many pieces scattered along the hillside. Four of the five blades were identified initially; the fifth blade was later found about 900 feet away with much less damage.
On July 28, 2014, the National Transportation Safety Board investigator-in-charge, along with investigators from Boeing and Rolls-Royce, examined the wreckage in Auburn, Washington. No evidence of preimpact damage to the airframe was found, and no anomalies with the airframe or engine that would have precluded normal operation were identified.
Four of the five main rotor blades exhibited substantial damage, including bending, buckling, spar damage, leading edge gouges and dents, and trailing edge separation. Some blades were warped and fragmented. The fifth blade, serial number 091B, had substantially less damage. Its blade subassembly was liberated from the root assembly, fracturing with bonding separation at the root, while the blade airfoil remained relatively intact. All five blades were manufactured by Helicopter Technology Company, LLC, under FAA Parts Manufacturing Authority.
Main Rotor Blade Examination
Further examination on September 23, 2014, involved retrieving the blade roots of all five main rotor blades for analysis at the NTSB Materials Laboratory.
The main rotor blade assembly included skin doublers bonded to the root end, upper and lower root fittings joined with a blue-colored film adhesive on a scrim cloth carrier, and five attachment bolts. Three bolts were arranged spanwise, and two inboard bolts were arranged chordwise. The outboard bolt was labeled No. 1, and the adjacent bolt No. 2.
For blade SN 091B, visual examination before disassembly revealed a chordwise fracture through the second bolt hole from the outboard end of the root fitting. Blue adhesive remained on the blade subassembly where it had mated to the root fittings. Cracks in white paint were observed along the interface between the subassembly and the fittings. The No. 1 attachment bolt was missing; its hole in the upper fitting was enlarged, and in the lower fitting, it was elongated into a slot. The remaining four bolts were removed, after which the upper and lower fittings separated from the blade subassembly. No blue adhesive was found on the inner surfaces of the fittings, consistent with disbonding. Wear debris was observed around bolt holes. The fracture through the subassembly at the No. 2 bolt hole exhibited fatigue characteristics, with flat fracture faces and crack arrest marks. Fatigue initiated at multiple sites: the forward and aft edges of the No. 2 bolt hole on both sides, and the forward surface of the spar. No pre-existing damage or defects were found at these sites.
The disbondment between adhesive and the lower fitting was examined. Sealant was observed between the adhesive and fitting near the edge. Debris containing titanium, consistent with wear from the fitting, was found on top of the sealant.
Examination of the four intact blades revealed paint cracks around the perimeter of the fittings on blade SN 085B. After removing its attachment bolts, disbonding of the upper root fitting from the blade subassembly was found, similar to blade SN 091B. The lower fitting of SN 085B did not separate. The other three blades showed no separation after bolt removal.
Airworthiness Directives and Service Bulletins
FAA Airworthiness Directive 96-10-09 required initial and repetitive inspections every 100 flight hours for cracks, paint and sealant cracking, or separation between the lower surface root end fitting and the doubler on main rotor blades. The intent was to prevent blade failure and loss of control.
FAA AD 2005-21-02 required tracking torque events (TEs) on each blade. Inspections were required at defined TE thresholds. The accident helicopter's blades had accumulated about 1,123 hours and 232,674 TEs since installation.
MD Helicopters Service Bulletin SB369E-095R2 described causes of disbondment, inspection procedures, and intervals. It noted that a high number of torque events per hour could exceed the original design fatigue spectrum. The bulletin referenced inspection instructions including lifting the blade and inspecting for chordwise cracks using a bright light and 10x magnifying glass.
Following the accident, MD Helicopters issued Service Letter 14-SE-049 emphasizing the importance of performing main rotor blade inspections as prescribed. This included color photographs of adhesive/paint cracks that indicate blade rejection. Helicopter Technology Company issued Service Notice 2100-5 emphasizing proper tracking of torque events and the need for compliance with inspection requirements.
Pilot Statement
The pilot stated he obtained an airframe and powerplant certificate to perform torque event inspections. He averaged 200 torque events per hour (100 turns per hour) in clear cuts. He attempted to comply with the AD but often did not perform inspections until returning home at night, and did not always use the 10x magnifier in rain. He read the AD and maintenance manual but was primarily looking for cracks in the root fitting and metal, not necessarily the bond line. He noted that the intent of inspecting the bond line was not clear to him until the service notices published after the accident.
(Note: Probable cause is not stated in the source.)