WEEK 27
H2F brings you the ‘helicopter mayday of the week’ with a short accident report summary. I tell you what happened. You think more about why it happened. We all learn from it. Because that’s what accident reporting is for.
TITLE
Vortex ring state (VRS) during mountain winch approach results in collision with terrain.
WHAT?
A BK117 was dispatched from to recover an injured hiker from Mount Pirongia, New Zealand. The crew located the patient on a walking track below a ridgeline and determined that the terrain prevented landing, requiring a winch extraction. After landing nearby to reconfigure the cabin and brief the operation, the helicopter returned to the site. During the final approach it suddenly developed a high rate of descent and struck densely forested terrain.
WHERE?
The accident occurred on the western side of Mount Pirongia in New Zealand’s Waikato region, on a steep west-facing escarpment covered by dense forest.
WHEN?
19 September 2023. Final report published on 17 April 2025.
HOW?
The pilot climbed to 3,000 ft before turning and descending toward the patient on the windward side of the ridge. The operator had a recommended standard winch circuit, but the pilot modified this profile and the pre-winch tasks remained incomplete as the helicopter entered final approach. Airspeed needed to reduce below 60 kt to allow the rear door to be opened. Shortly after responding to the crew member’s request to open the door, the pilot experienced an unexpected rapid loss of height. The pilot initially attempted to arrest the descent, but, seeing the treetops approaching, changed focus to controlling the impact and minimising injury. The helicopter descended through the canopy and struck the hillside.
CONDITIONS?
Weather was generally fine, but the mountainous environment produced significant local effects. Westerly to north-westerly winds were around 20 with gusts to 30 kt, with cloud cascading over the summit. On the windward side of the ridge, the airflow produced orographic uplift and light turbulence.
A combination of the helicopter’s descent rate, reducing forward speed, a pitch up attitude and rising airflow over the ridge unexpectedly and suddenly created the conditions for to VRS. The accident occurred close to steep, forested terrain, leaving little height available for recovery.
OUTCOME?
The helicopter penetrated the tree canopy, impacted the steep escarpment and was severely damaged. The pilot, winch operator and paramedic escaped without injury. The paramedic immediately continued attending to the original patient, and two other rescue helicopters subsequently extracted the crew, patient and patient’s companion.
WHY?
Investigators concluded that it the helicopter almost certainly entered vortex ring state (VRS). Several operational and human-performance factors combined to create the conditions for its onset:
The helicopter’s reducing airspeed and increasing descent rate, combined with its decelarative pitch up attitude, as well as orographic uplift across the ridge, created conditions conducive to VRS and contributed to its early onset.
The pilot modified the operator’s recommended standard winch circuit. Pre-winch tasks remained incomplete and the helicopter was not established on a stable approach profile.
The pilot’s visual attention was focused primarily outside the cockpit on the patient’s location. The resulting reduced instrument scan meant that the developing combination of airspeed and descent rate conducive to VRS was not immediately recognised.
Once VRS developed, the helicopter’s low height above the terrain provided insufficient opportunity for an effective recovery.
The investigation also noted that although pilots received generic VRS training, the BK117 Flight Manual did not provide type-specific information necessary to identify the flight parameters associated with VRS onset. Investigators considered that reliance on generic rules of thumb inadequate and recommended improved flight-manual information and consideration of VRS alerting systems.
REFERENCE?
Transport Accident Investigation Commission. (2025). Kawasaki BK117 B-2, ZK-HHJ: Collision with terrain, Mount Pirongia, 19 September 2023 (Final Report AO-2023-010). Transport Accident Investigation Commission.
Note:
Accident reports selected from the following open source databases: US NTSB; UK AAIB; Flight Safety Foundation’s Aviation Safety Network; Australia’s ATSB. Ireland’s AAIU; Taiwan’s TTSB; France’s BEA; Spain’s CIAIAC. Germany’s BFU.
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