What happened
On 19 April 1998, an Aerospatiale AS350D helicopter, registration ZK-HKU, was conducting a scenic flight near Mount Ngongotaha. The flight, operated by Marine Helicopters Limited, included the pilot and four passengers. While the aircraft was on approach to the Skyline Skyrides heliport in Rotorua, the pilot attempted to increase power to arrest a descent. Instead of the expected power increase, the pilot heard unusual noises and observed a rapid decay in rotor revolutions per minute (rrpm).
With only approximately ten seconds of flight time remaining, the pilot executed an emergency autorotational landing into a sloping, grassy paddock. The aircraft struck the ground heavily and skidded approximately 38 meters. While the helicopter sustained substantial damage to its tail boom, main rotor blades, and landing gear, there were no injuries to the occupants.
The investigation
Investigators examined the AlliedSignal LTS101 engine and the maintenance history of the aircraft. They discovered that an air pressure accumulator had become loose on the engine deck and that a B nut securing a pneumatic line to the overspeed governor was only finger tight.
Prior to the accident, the aircraft's manager had noted fluctuations in the measured gas temperature (MGT) the previous day. Although the engine had been inspected by a maintenance foreman, the investigation found that the symptoms had recurred during the flight to Rotorua, but the maintenance provider was not notified. Furthermore, the investigation looked into the operator's maintenance practices, specifically regarding the adherence to manufacturer-recommended engine performance trend monitoring.
Findings
- The primary cause of the power loss was excessively worn gas producer turbine rotor shroud sealing rings, which triggered an internal mechanical failure.
- The worn sealing rings were not detected because the required engine performance trend monitoring procedures were not being followed.
- The engine continued to operate despite displaying symptoms of impending failure, including temperature fluctuations noted the day before the accident.
- The maintenance company's chief engineer also served as the operator's maintenance controller, a dual role that lacked sufficient independent oversight.
Safety action
Following the inquiry, several safety recommendations were issued to the Director of Civil Aviation and the engine manufacturer, AlliedSignal Aerospace. These included requirements to inspect and tighten B nuts on pneumatic lines and to review the adequacy of securing accumulators. Additionally, recommendations were made to ensure all manufacturer-specified maintenance procedures, particularly performance trend monitoring, are strictly observed to detect component wear before failure occurs.