Pilot Debrief

MDHI 369D near Oso, WA — 2014-07-22

Final reportWPR14LA308
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Date
2014-07-22
Location
Oso, WA, USA
Aircraft
MDHI 369D
Registration
N5225C
Category
Helicopter
Highest injury
Serious
Fatalities
0

Probable cause

The pilot/mechanic’s failure to properly perform required inspections of the main rotor blades at the necessary intervals, which resulted in an in-flight separation of a main rotor blade due to disbonding and fatigue cracking. Contributing to the accident was the lack of clear guidance in the helicopter maintenance inspection instructions, which allowed for the possible misinterpretation by maintenance personnel of their intent.

Contributing factors

Contributing to the accident was the lack of clear guidance in the helicopter maintenance inspection instructions, which allowed for the possible misinterpretation by maintenance personnel of their intent.

NTSB narrative

The commercial pilot was performing external load operations in the helicopter when one main rotor blade separated, which resulted in a loss of control and collision with terrain. The helicopter rolled downhill, and the airframe sustained substantial damage. On-site examination of the wreckage revealed that four of the main rotor blades were fragmented into many pieces. The fifth main rotor blade was found about 900 ft from the main wreckage and exhibited less damage; postaccident examination of this rotor blade revealed that it had separated due to fatigue cracking that had initiated near the root end of the blade at the second-most outboard bolt hole through the spar, skin, and doubler. Further examination revealed that the fatigue cracks in the separated blade had initiated due to disbondment at the interface between the adhesive film on the blade subassembly and the upper and lower root fittings. Examination of the root fitting assembly revealed paint cracks along the root fitting/blade interfaces, indicative of complete or partial disbondment of the root fittings. When the remaining attachment bolts were removed, the root fittings cleanly separated from the blade subassembly. The helicopter manufacturer had previously issued a service bulletin indicating that disbondment of the root fitting was caused by a high number of torque events/external lifts per hour that exceeded the helicopter's "original design fatigue spectrum." Further, the Federal Aviation Administration had previously issued an airworthiness directive (AD), which instructed the operator to determine and record the number of torque events (TE) accumulated on each main rotor blade. It stated that, on or before accumulating an additional 200 TEs or at the end of each day's operations, whichever occurred first, the operator was required to record and update the total accumulated TEs. For each blade that had accumulated 13,720 or more TEs and 750 or more hours time in service (TIS), before further flight, unless accomplished previously, the operator was to perform a main rotor blade TE inspection. The AD also required a recurrent main rotor blade TE inspection at intervals not to exceed 200 TEs or 35 hours TIS, whichever occurred first. A review of maintenance records indicated that the blades on the helicopter had accumulated about 1,123 hours TIS and 232,674 TEs since installation. The examination of all of the blades suggested that initial indications of root fitting disbondment, specifically paint cracking around the root fitting/blade interface, occurred sufficiently early to have been detected if the inspections had been performed in accordance with the AD. The pilot stated that he obtained an airframe and powerplant certificate so that he could perform the TE inspections. He added that he averaged about 200 TEs per hour and that he tried to comply with the AD as best as he could on work sites. On a typical job, he usually did not perform the TE inspections until he got home at night. He read the AD and maintenance manual to determine how to inspect the blades and noted that the inspection procedures stated that the inspector should inspect the root end for cracks. He stated that he looked primarily at the root fitting and metal for cracks, not necessarily the bond line. He added that the intent of inspecting the bond line was not clear to him until the helicopter and blade manufacturers published service notices after the accident, which included color photographs; the photographs showed examples of possible evidence of initial failure of the adhesive bond between the main rotor blade and doubler. The fracture on the accident blade initially occurred at the second-most outboard bolt hole, which was outside the indicated inspection area for chordwise cracks. Because the crack was outside the indicated inspection area, it was possible that the crack could have been missed or misidentified during an inspection. However, the crack initiated at the disbondment of the root fittings, which was part of the inspection procedure. As a result of the accident, the helicopter and blade manufacturers changed the TE inspection procedure to inspect further inboard.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Pilot did not perform timely inspections of main rotor blades as required by AD.

NTSB coding

Evidence available

  • Photos
  • 8 docket documents
View NTSB final reportView NTSB docket

Docket documents8

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