Aerospatiale AS 355F1 N58020 accident near West Windsor, New Jersey

Probable cause

Disconnection of the upper rod end from the fore/aft servo due to severely worn threads, which resulted in a loss of control and separation of a main rotor blade during cruise flight. Contributing to the accident were incorrect maintenance procedures and inadequate maintenance inspections performed by the operator, and insufficient inspection criteria provided by the helicopter manufacturer.

NTSB analysis

During cruise flight, witnesses on the ground reported hearing a grinding or popping noise, which was followed by the separation of the yellow main rotor blade. Examination of the fore/aft servo revealed that the internal threads of the upper rod end fitting on the servo were severely worn. The lower threaded portion of the upper rod end was not found secured into the servo's upper end fitting and was not recovered. Additionally, the upper end fitting was packed with soil as the lower threaded portion of the upper rod end had separated from it prior to ground impact. A 100-hour maintenance inspection of the accident helicopter had been completed on the morning of the accident and a 600-hour inspection of the accident helicopter was completed about 3 months prior to the accident. Neither inspection detected the worn threads on the fore/aft servo upper end fitting. The fore/aft servo had been overhauled about 4 years prior to the accident. Additionally, the fore/aft servo was repaired 10 months prior to the accident and no anomalies were observed with the threads at that time.

The operator used Mastinox, a corrosion inhibiting compound, during installation of the upper rod ends to the right-roll, left-roll, and fore/aft servos. The helicopter manufacturer's maintenance manual listed only G.355 grease and did not prescribe for the use of Mastinox. However, the standard practices manual stated that a torque correction factor of 0.4 is used for G.355 grease, but not for Mastinox, since the latter is not a lubricant. The torque value listed in the manual for the rod ends already took a torque correction factor into account. A higher torque value would theoretically have been necessary when Mastinox was used in place of G.355 grease. The operator stated an adjusted torque value was not used during installation of the upper rod ends using Mastinox. Evidence of sealant at the junction of the upper rod end and nut, which was required to be applied during servo installation per the maintenance procedures, was not found on either the right-roll or left-roll servos. While the lack of sealant may not result in a catastrophic event, its breakage or absence (and/or radial play of a servo end bearing) noted during a maintenance inspection could be indicative of a loss of torque.

Review of the helicopter manufacturer's checklists and maintenance manuals revealed some guidance for servo inspections. The daily operating check (after the last flight of the day) included an instruction to check the main rotor servos for security and absence of leaks. The 600-hour inspection called for checking the radial play of the end bearings; however, there were no instructions to specifically check the threads of the servo end fitting or the torque of the rod end nut.

Text from the NTSB. Open the NTSB docket for ERA12FA563 for the full record.