Narrative
On 19 September 2023, a Kawasaki BK117 B-2 air ambulance helicopter, registration ZK-HHJ, operated by Search and Rescue Services Limited, was dispatched from Hamilton Airport to recover a trauma patient on Mount Pirongia in the Waikato region. The crew consisted of a pilot, a crew member serving as hoist operator, and a paramedic.
The helicopter departed Hamilton at 1620 after an earlier uneventful inter-hospital transfer. The pilot entered the patient's GPS coordinates, which directed them to Mount Pirongia. The crew located the patient and their partner on the Mahaukura Track at about 1645. Unable to land near the patient, they decided on a winch extraction. The helicopter landed at Pirongia Forest Park Lodge to reconfigure the cabin for winching.
At 1702, the helicopter departed the lodge and flew up the western valley of Wharauroa Lookout. After passing abeam the lookout, the pilot executed a 180-degree climbing left turn to 3000 feet, then began a descent on final approach to the patient on the windward side of a ridge. The crew member requested the rear sliding door be opened, requiring airspeed under 60 knots. The pilot responded, and shortly after, the helicopter suddenly and unexpectedly dropped. The pilot attempted to arrest the descent, but upon seeing treetops in peripheral vision, shifted focus to controlling the impact. At 1707, the helicopter descended through the tree canopy and impacted terrain, sliding forward about 1 meter before coming to rest 7 meters upslope of a pre-existing landslide. The crew exited uninjured, and the paramedic attended to the patient. Between 1802 and 1840, the crew, patient, and partner were evacuated by two rescue helicopters.
Why It Happened
The investigation determined that it is virtually certain the helicopter entered Vortex Ring State (VRS) during the descent on the windward side of the ridge. VRS is a hazardous aerodynamic phenomenon causing rapid loss of main rotor lift and increased rate of descent. The helicopter's trajectory—specifically descent rate, forward speed, pitch attitude, and the presence of orographic uplift—was conducive to VRS onset.
During the final approach, the pilot's visual attention was primarily focused outside the cockpit on the patient's location, leading to a reduced instrument scan. As a result, indications that forward speed and descent rate were conducive to VRS were not immediately recognized. The helicopter's height above ground during VRS onset provided insufficient time for an effective recovery.
The investigation also noted that while helicopter flight training provides generic VRS tuition, the helicopter's Flight Manual did not contain the necessary information to identify flight conditions and parameters conducive to VRS for that type.
Personnel Information
The pilot held a Commercial Pilot Licence for helicopters and had accumulated 7257 total flying hours, including 6895.3 hours in helicopters and about 1928 hours on the BK117 B-2 type. The pilot was current in all required competency assessments and had completed a winching competency check on 9 February 2023. The pilot reported being well-rested and in good health, with 9 hours of sleep before the shift. Drug and alcohol tests conducted after the accident returned negative results.
Aircraft Information
The Kawasaki BK117 B-2, serial number 1109, registration ZK-HHJ, was constructed in 1996 by Kawasaki Heavy Industries Ltd. It was fitted with two Honeywell LTS101-850B-2 turboshaft engines and a Breeze-Eastern 600-pound hoist. The helicopter had a valid Certificate of Airworthiness and all scheduled maintenance was current with no recorded defects. The rotor system is hingeless with four composite main rotor blades.
Findings from the Investigation
The investigation concluded that the omission of guidance from the helicopter manufacturer on avoidance and recovery from VRS posed a significant risk. It emphasized that helicopter pilots must understand VRS and how to avoid it. Flying in mountainous terrain presents challenges including misinterpreting the air environment; pilots should use all available information to avoid visual illusions that may cause misjudgment of speed and descent rate. Standard operating procedures help ensure safe operation, and modifying them increases risk of adverse outcomes.