Incident
On 23 January 2015, a Robinson R44 helicopter, registration ZK-HPC, was conducting agricultural spraying operations near Waikaia, New Zealand. The pilot had performed a pre-flight inspection of the helicopter, including the rotor blades, with no irregularities noted. After approximately three hours of flying that morning, during a downhill spraying run with a light load, the pilot made a turn at about 30 knots airspeed. When pulling out of the turn at 10-15 knots, the pilot felt an unusual and severe vibration but maintained control. The helicopter was about 200 metres from the landing site, so the pilot chose to land there. Ground staff observed abnormal movement of one main rotor blade, prompting the pilot to shut down the engine. Inspection revealed a large crack on the lower surface of one main rotor blade, extending from the trailing edge to just behind the D spar leading edge.
Investigation
The Transport Accident Investigation Commission (Commission) opened an inquiry on 25 February 2015. The Civil Aviation Authority had already sent both main rotor blades to the United States for inspection by the manufacturer, Robinson Helicopter Company. The National Transportation Safety Board (NTSB) appointed an Accredited Representative. Visual inspection at the Robinson factory showed the crack originated in a radius at the trailing edge where the chord length increased, known as the chord transition. A section of the cracked blade (part number C016-7, serial number 2031) and a similar section from the opposite blade were examined at the NTSB laboratory. No material defects or manufacturing anomalies were found. The chord transition radius on the opposite blade conformed to design specifications.
For comparison, two C016-7 main rotor blades that had been used exclusively for agricultural operations and reached their 2,200-hour service life were inspected using eddy current and X-ray methods. No cracks were found, though minor corrosion was detected at the trailing edge of one blade. Robinson conducted flight tests with strain gauges to measure stresses during normal flight and simulated agricultural spraying. The Defence Technology Agency of New Zealand reviewed the NTSB report.
Findings
The Commission determined that the crack point of origin had features consistent with metal fatigue, and the crack was not the result of any manufacturing defects in materials or construction. The helicopter had been operated primarily for agricultural flying, with engine power likely to have exceeded the allowable limit at times. The types of turns used during spraying operations, especially when near the maximum permitted all-up weight, likely subjected the main rotor blades to additional stresses not envisaged by the manufacturer. The helicopter was not designed specifically for agricultural flying, so the manufacturer had not been required to consider the increased loads and cycles of agricultural flying when calculating the service life of the rotor blades.
Safety Issues
A safety issue identified was that aircraft design organisations did not have to consider whether proposed major modifications to an aircraft would significantly alter the use of the aircraft or adversely affect the service life of any component. The Commission recommended that the Director of Civil Aviation consult the original equipment manufacturer when considering a modification or supplemental type certificate that could result in an aircraft being used in a way significantly different from that originally modelled for determining component fatigue life and maintenance programmes.
Key Lessons Identified
Key lessons from the inquiry included: metal fatigue occurs continuously in dynamic components; a fatigue crack can lengthen rapidly and the component lose structural strength. If an unusual or severe vibration develops in flight, the pilot should land immediately and have the helicopter inspected before further flight. The key to minimising stress in dynamic components is to fly conservatively, especially near published weight, speed, and power limits. Operating an aircraft outside published limitations erodes safety margins in component service lives. Operators and maintainers of aircraft subjected to cycles or flight profiles significantly different from those envisaged during certification should consider implementing shorter inspection intervals and earlier component replacement times.