Accident
At approximately 1400 hours Mountain Standard Time, the pilot of a Hughes 369D helicopter (serial number 370102D) was conducting external load operations, slinging a rack of seismic equipment bags on a 50-foot lanyard. After positioning one bag, the pilot entered a hover at about 100 feet above ground level to release another bag when the cyclic control began to vibrate violently and the nose pitched down. The pilot applied full aft cyclic in an attempt to level the helicopter, but it descended and struck the ground heavily in a nose-down, left-side-low attitude. The helicopter came to rest on its left side, breaking the left cross tubes and left skid tube. The tail boom had separated in flight, and the main rotor blades were curled from ground contact. The fuel tank ruptured, leaking into the cockpit. Seismic workers called for medical assistance and helped remove the pilot, who was dazed, from the wreckage. The pilot was taken by ambulance to a local hospital. Later, a main rotor blade was located about 1,200 feet south of the accident site.
Aircraft Information
The Hughes 369D helicopter was equipped with five main rotor blades (Part No. 369D21100-517), installed as a set on 17 July 1995. Paint color on blade attachment nuts indicated installation location. At the time of the occurrence, the blade assembly had accumulated about 2,461 flight hours, except the blue blade which had 86 hours less due to out-of-service abrasion strip repairs. The blades have a normal service life of 3,530 hours. Records indicated the helicopter was certified, equipped, and maintained in accordance with regulations. Weight and balance were within limits. Airworthiness Directive 96-10-09, requiring examination of the root end fitting, had been carried out during the last 100-hour inspection on 26 November 1997, with no cracks found.
Pilot Information
The pilot was certified and qualified for the flight in accordance with regulations, having flown a total of 1,554 hours, of which 1,452 were on the Hughes 369D. The pilot was wearing a helmet and used the full seat-belt and shoulder harness. He occupied the left seat, as is normal in this helicopter type. The pilot was hospitalized with a collapsed left lung, minor abrasions to both ankles, chemical burns to his lower torso and left arm from fuel leakage, and a sore neck and tail bone.
Investigation
Examination revealed that the failed and separated green blade (serial no. 009999-H709) had chord-wise cracking just outboard of the lower blade attachment fitting. The spar, blade skin, and doubler showed indications typical of a fatigue-related failure. The crack had initiated in the lower inboard doubler and propagated chordwise through the blade skin and spar. Investigation found that a batch of doublers used for production blade assembly were initially rejected as non-conforming but were subsequently used. The non-conformance was a break or crease in the curvature of the doubler relative to the blade skin, which could allow variations in adhesive layer thickness and poor bonding. Stress analysis showed that the non-conforming doubler could introduce significant residual stresses in the doubler skin after blade assembly.
A similar crack was found in the same location on the blue blade (serial no. 009999-H706). Examination of all other blades revealed cracks: the white blade (serial no. 009999-H708) had a half-inch crack, and the red and yellow blades (serial nos. 009999-H705 and -H707) had micro-cracking.
The engine continued to run after impact and singed exposed grass below the exhaust tail pipes. The pilot's door was torn off its hinges, and the left upper plexiglass windscreen was broken during ground impact. There was no indication that main rotor blades struck the tail boom. The manufacturer's representative indicated that in previous occurrences involving severe main rotor imbalance, lateral vibration had resulted in tail boom structure failure.
Probable Cause
According to the Transportation Safety Board, the pilot lost control of the helicopter when a main rotor blade failed in fatigue and separated, resulting in severe vibration causing the tail boom to separate.
Safety Action
Examination of the failed blade parts by the manufacturer led to two Mandatory Service Bulletins: SB 369D-194 (24 December 1997) requiring visual inspection of the root fitting area at 25-hour intervals on blades with 1,500 or more hours; and SB 369D-195R1 (23 January 1998) requiring visual inspection at 25-hour intervals on specific model and serial number blades with 600 or more hours. The FAA issued priority letters AD 98-01-13 and AD 98-03-15 mandating compliance with these inspections.