The wrecked helicopter at the accident site being inspected by a TAIC investigator. Photo NZDF
Kawasaki BK117 B-2, ZK-HHJ, Collision with terrain, Mount Pirongia, 19 September 2023
Status
Closed
Occurrence Date
Report Publication Date
Jurisdiction
NZ
What happened
On 19 September 2023, an air ambulance helicopter was dispatched from Hamilton Airport to recover a trauma patient on Mount Pirongia. On board the Kawasaki BK117 B-2 helicopter were a pilot, a crew member and a paramedic.
While descending on the windward side of a ridge line to recover the patient by winch extraction, the helicopter rapidly and unexpectedly dropped in height. The helicopter impacted the terrain on a west-facing escarpment covered by dense native forest.
The helicopter was severely damaged. The pilot, crew member and paramedic escaped without injury.
Why it happened
It is virtually certain that the helicopter entered Vortex Ring State (VRS) while descending on the windward side of a ridge line to recover the patient. VRS is a hazardous aerodynamic phenomenon that a helicopter can encounter during the descent and landing phases of flight and during manoeuvres at low forward speed. The onset of VRS can lead to a rapid loss of main rotor lift and then to a sudden increase in the helicopter’s rate of descent.
The helicopter's trajectory, particularly its descent rate and forward speed, combined with its pitch attitude and the presence of orographic uplift across the ridge, was conducive to the onset of VRS.
While on final approach, the pilot assumed that they could continue the approach to the winch site in level flight. During this critical phase of flight, the pilot’s visual attention was primarily focused outside the cockpit on the patient’s location. As a result of a reduced instrument scan, indications that the helicopter’s forward speed and descent rate were conducive to VRS were not immediately recognised.
The helicopter’s height above ground level (AGL) during the onset of VRS provided the pilot with insufficient time to perform an effective recovery.
During helicopter flight training, pilots receive generic tuition on VRS. The helicopter’s Flight Manual did not contain the information necessary for the pilot to identify the flight conditions and parameters for the helicopter type that were conducive to VRS.
What we can learn
The omission of guidance by the helicopter manufacturer on the avoidance and recovery from VRS poses a significant risk to operators.
It is critical that helicopter pilots understand what VRS is and how they can avoid entering it.
Flying in mountainous terrain presents helicopter pilots with multiple challenges, including misinterpreting the air environment in which they are flying. Pilots need to use all available sources of information to ensure they are not subject to visual illusions that might lead them to misjudge their forward speed and rate of descent, particularly when operating close to the ground.
Standard operating procedures help ensure that an aircraft is operated and flown in as safe a manner as possible. Modifying these procedures can increase the risk of an adverse outcome.
Who may benefit
All helicopter pilots, operators, flight schools and manufacturers may benefit from the findings and recommendations in this report.
On 19 September 2023, an air ambulance helicopter was dispatched from Hamilton Airport to recover a trauma patient on Mount Pirongia. On board the Kawasaki BK117 B-2 helicopter were a pilot, a crew member and a paramedic.
While descending on the windward side of a ridge line to recover the patient by winch extraction, the helicopter rapidly and unexpectedly dropped in height. The helicopter impacted the terrain on a west-facing escarpment covered by dense native forest.
The helicopter was severely damaged. The pilot, crew member and paramedic escaped without injury.
Why it happened
It is virtually certain that the helicopter entered Vortex Ring State (VRS) while descending on the windward side of a ridge line to recover the patient. VRS is a hazardous aerodynamic phenomenon that a helicopter can encounter during the descent and landing phases of flight and during manoeuvres at low forward speed. The onset of VRS can lead to a rapid loss of main rotor lift and then to a sudden increase in the helicopter’s rate of descent.
The helicopter's trajectory, particularly its descent rate and forward speed, combined with its pitch attitude and the presence of orographic uplift across the ridge, was conducive to the onset of VRS.
While on final approach, the pilot assumed that they could continue the approach to the winch site in level flight. During this critical phase of flight, the pilot’s visual attention was primarily focused outside the cockpit on the patient’s location. As a result of a reduced instrument scan, indications that the helicopter’s forward speed and descent rate were conducive to VRS were not immediately recognised.
The helicopter’s height above ground level (AGL) during the onset of VRS provided the pilot with insufficient time to perform an effective recovery.
During helicopter flight training, pilots receive generic tuition on VRS. The helicopter’s Flight Manual did not contain the information necessary for the pilot to identify the flight conditions and parameters for the helicopter type that were conducive to VRS.
What we can learn
The omission of guidance by the helicopter manufacturer on the avoidance and recovery from VRS poses a significant risk to operators.
It is critical that helicopter pilots understand what VRS is and how they can avoid entering it.
Flying in mountainous terrain presents helicopter pilots with multiple challenges, including misinterpreting the air environment in which they are flying. Pilots need to use all available sources of information to ensure they are not subject to visual illusions that might lead them to misjudge their forward speed and rate of descent, particularly when operating close to the ground.
Standard operating procedures help ensure that an aircraft is operated and flown in as safe a manner as possible. Modifying these procedures can increase the risk of an adverse outcome.
Who may benefit
All helicopter pilots, operators, flight schools and manufacturers may benefit from the findings and recommendations in this report.