In September 2023, a Kawasaki BK117 B-2 air ambulance helicopter was severely damaged when it crashed into terrain covered by dense native forest in New Zealand's North Island. Fortunately, the pilot and crew escaped without injury.

What happened on this mission?

The air ambulance was dispatched late afternoon from Hamilton Airport to retrieve an injured hiker on Mount Pirongia. Given the terrain, there was no suitable place to land, so it was determined that a winch extraction would be the most appropriate recovery method.

The helicopter rapidly and unexpectedly dropped in height while descending on the windward side of a ridge line. The aircraft almost certainly (> 99% probability of occurrence as stated by Transport Accident Investigation Commission (TAIC)) entered a Vortex Ring State (VRS) during its descent. VRS is a hazardous aerodynamic phenomenon that a helicopter can encounter during the flight's descent, landing phases, and manoeuvres at low forward speed. The onset of VRS can lead to a rapid loss of main rotor lift, resulting in a sudden increase in the helicopter’s rate of descent.

The helicopter’s height above ground level (AGL) at the onset of VRS left the pilot with inadequate time to recover, resulting in the helicopter impacting the terrain.

Flightcell DZMx’s Flight Data Recorder (FDR) feature was critical in the investigation

The helicopter was fitted with a Flightcell DZMx – a voice, data, and flight tracking system that uses its internal GPS and Multi Axis Motion Processing Unit to record flight data that complies with FAR 135.607 Flight Data Monitoring Requirement. This FAA requirement states that no person may operate a helicopter in air ambulance operations unless equipped with a compliant flight data monitoring system (FDMS), also known as a Flight Data Recorder (FDR). The Flightcell DZMx records this data at 1Hz-4HZ intervals, depending on operator setting requirements. This flight data recording function comes as standard with all Flightcell DZMx and DZMx PLUS units.

The Flightcell DZMx recorded a range of flight and GPS data every second, including:

• Flight path/trajectory

• GPS position

• Flight orientation – pitch, yaw, and bank

• Rate of descent

• Aircraft speed and altitude

The recorded data was extracted by Flightcell under supervision by Commission investigators, allowing the helicopter’s flight trajectory to be analyzed. Flightcell reconstructed an animation of the helicopter's flight path, which assisted the investigation. The flight data downloaded from the Flightcell DZMx showed a sudden drop in height that occurred at 17:06:55, when the helicopter was 291 feet AGL. The helicopter's trajectory, descent rate, and forward speed, with its pitch attitude and the presence of wind rising over the ridge, were conducive to the onset of VRS. With no time to recover, the helicopter crashed into the terrain below.

TAIC recommendations

TAIC provided multiple recommendations, with the primary recommendations including:

• Kawasaki Heavy Industries revise the Kawasaki BK117 B-2 Flight Manual to include specific data for Vortex Ring State to assist pilots in avoiding this phenomenon.

• The Civil Aviation Authority of New Zealand promotes, through the appropriate International Civil Aviation Organization (ICAO) forum:

1. Consideration is needed for the requirement and implementation of Vortex Ring State alerting systems to be installed in new helicopters.

2. Additional information should be included in all rotorcraft flight manuals to assist pilots in avoiding Vortex Ring State.