NTSB Notes Rotor Spindle Crack in Enstrom Crash

 - September 11, 2017, 3:05 PM

The fatal crash of a 1985 Enstrom 280FX in Erie, Colorado, during an instructional flight on Jan. 26, 2015, was the result of “an in-flight failure of the helicopter's number-two main rotor spindle due to undetected fatigue cracking, which resulted in an in-flight break up (of the main rotor system),” according to the NTSB. Contributing to the failure were “nonconforming thread root radius of the spindle and the manufacturer's failure to include a bending moment within the spindle threads when performing the fatigue analysis during initial design of the spindle.”

Witnesses to the accident reported that the main rotor blades separated from the helicopter before impact. Both occupants were killed. The NTSB found that “the helicopter was on approach to land when one of the three main rotor blades (Number two blade) separated from the main rotor head. The main transmission and the main rotor head (with Number One and Number three blades still attached) then separated from the helicopter, and the helicopter descended to ground impact.”

The accident triggered an emergency Airworthiness Directive in February 2015 mandating magnetic particle inspections of piston-model Enstrom main rotor blade spindles with more than 5,000 hours time in service (TIS) or unknown times in service. The AD also applied to a limited number of spindles installed on Enstrom 480 turbine singles. That emergency AD was modified in May 2015 to require inspections for all spindles with 1,500 hours TIS after results from the previous emergency AD found that 20 percent of the spindles inspected had evidence of cracks, including those with less than 5,000 hours TIS. Before the accident, the spindle was not a life-limited part and recurrent inspections were not specified for the spindle threads, making fatigue fracture detection unlikely, noted the NTSB.

Earlier this year, Enstrom noted that the spindle failure on the accident helicopter was the “first report of a failure of a main rotor spindle in Enstrom’s history of more than 50 years and, conservatively estimated, three million flight hours.” The company “found no evidence to support a design flaw or material defect that would result in the accident of 280FX” and pointed out that the full maintenance history of the accident aircraft is unknown. Specifically, Enstrom noted aircraft sale offer documents that contained spindle serial numbers that do not appear in the aircraft's records. 

The NTSB's metallurgical analysis revealed that the fractured spindle on the accident helicopter had “signatures consistent with a fatigue crack initiating from multiple origins that propagated across 92 percent of the cross-section; the remaining 8 percent of the fracture surface exhibited signatures consistent with overload. The high percentage of stable fatigue fracture growth compared with overload suggested that low-loading propagated the crack. Further, corrosion was visible on the fracture surface in the fatigue initiation area, which indicated that the crack had been present and growing for some time.” The NTSB noted, “The root radii of the thread on all three spindles did not meet the thread form specified on the manufacturer's drawing.”