History of Flight
On January 26, 2015, at approximately 1202 mountain standard time, an Enstrom 280FX helicopter (registration N86235) was destroyed when it struck terrain during final approach to land at Erie Municipal Airport (EIK) in Erie, Colorado. The helicopter was operated by Mountain One Helicopters as a 14 Code of Federal Regulations Part 91 instructional flight. Both the instructor pilot and student pilot were fatally injured. Visual meteorological conditions prevailed, and the flight operated without a flight plan.
A witness reported that the helicopter was on final approach with a steep angle of descent. She heard a loud "pop," after which the helicopter began to rotate, and the main rotor (MR) blades separated from the top of the helicopter. The remaining airframe descended to the ground and then "exploded."
Aircraft Information
The helicopter, an Enstrom 280FX (serial number 2002), was a three-seat, single-engine aircraft manufactured in 1985. It was powered by a turbocharged Lycoming HIO-360-F1AD engine rated at 225 horsepower. The three-bladed main rotor system was attached to a hub via spindles and grips. According to maintenance records, the most recent 100-hour inspection was completed on March 17, 2014, at an airframe total time of 9,167 hours. During that inspection, all three Lamiflex bearings were replaced, and the main rotor grip assemblies were removed, cleaned, and reassembled per the maintenance manual. No other maintenance to the MR grips or blades was performed between that inspection and the accident. The #2 spindle (part number 28-14282-13, serial number 03-014-84N) had not been replaced since the helicopter's manufacture; at that time, spindles were not life-limited and had no recurrent inspections specified for their threads.
Wreckage and Impact
The helicopter came to rest upright on its right skid, east of the runway. The main wreckage, including the fuselage, skids, tail boom, and engine, exhibited severe damage from a high-energy vertical impact and post-impact fire. The main rotor head, with blades #1 and #3 still attached, and the MR gearbox were located about 299 feet from the main wreckage. The #2 MR blade assembly—including the blade, grip, threaded portion of the spindle, retention nut, and drag link—was found about 657 feet away. The #2 blade showed no damage or contact with other parts. An 18-inch section of the leading edge spar and balance weight from the #1 blade was found 493 feet away. Flight control continuity was established from the cockpit to the MR hub. The tailboom exhibited a blade strike, and the MR gearbox mounts were intact, with separation occurring at the pylon.
Examination of Components
All three spindles from the rotor arm assemblies were sent to the NTSB Materials Laboratory for examination. The threaded end of the #2 spindle had separated in-flight. Approximately 92% of its fracture surface was flat with beach marks consistent with fatigue cracking, emanating from the trailing edge side. Corrosion was present in the fatigue initiation area. The remaining 8% showed overload features. The average root radius of the threads was smaller than specified in the Enstrom spindle drawing. Threads appeared cut, not rolled, with tight radii in the corners. Magnetic particle inspection of the #1 and #3 spindles revealed multiple crack indications in the thread roots, including a bright linear crack on the #3 spindle at the same location as the #2 fatigue initiation. The #1 spindle also had a crack indication at the cotter pin hole. No metallurgical issues were found with other components.
Finite element modeling performed by the NTSB showed that the stress in the measured thread profile was nearly double that in the nominal profile under axial load. Additional modeling incorporating bending moments, later confirmed by Enstrom flight strain surveys, revealed that bending loads increased stresses by about 14% and had not been accounted for in the original spindle design. Computed tomography scans of the #1 and #3 spindles confirmed cracking but with some uncertainties.
Additional Information
In response to the accident, Enstrom issued Service Directive Bulletin (SDB) No. 0119 on February 11, 2015, requiring magnetic particle inspection (MPI) of main rotor spindles with more than 5,000 hours time-in-service (TIS). The FAA issued Emergency Airworthiness Directive (EAD) 2015-04-51 the following day. Based on MPI results showing cracks in about 20% of inspected spindles, the TIS threshold was progressively lowered to 1,500 hours, and the FAA superseded the EAD with EAD 2015-08-51. Enstrom revised the SDB multiple times, eventually setting a life limit of 1,500 hours TIS and a 500-hour repetitive inspection interval. Enstrom's flight strain surveys and crack growth analysis further characterized loads on the spindles.