A new report from New Zealand’s Transport Accident Investigation Commission (TAIC) provides deep insight into a helicopter accident associated with vortex ring state (VRS), including a detailed analysis of how upslope winds and the helicopter’s nose-high pitch attitude combined to make the entry into VRS more likely.
Emphasizing gaps in the information provided to pilots about VRS, the report encourages helicopter manufacturers to revise their flight manuals to include the VRS onset boundary specific to each model. In the absence of this information, the report says, “pilots commonly rely on rules of thumb to avoid VRS,” increasing their risk of inadvertently entering the state during routine flight operations.
The accident in question occurred in September 2023, when a Kawasaki BK117 B-2 air ambulance helicopter operated by Search and Rescue Services Ltd. impacted terrain on Mount Pirongia, on New Zealand’s North Island. Although the helicopter was severely damaged, the pilot, hoist operator, and paramedic on board all escaped without injury.
The helicopter had been descending on the windward side of a ridgeline to rescue an injured tramper, or hiker, using the hoist. When the aircraft was around 300 feet (91 meters) above the ground, it suddenly and unexpectedly dropped in height, leaving the pilot with insufficient time and space in which to recover.
TAIC investigators determined that this rapid descent was due to VRS, a hazardous aerodynamic phenomenon that helicopters can encounter when descending or maneuvering at low forward airspeeds. At higher forward airspeeds, the report explains, the cylindrical wake of the main rotor trails behind the helicopter, where it eventually breaks down into a turbulent, chaotic flow that does not appreciably affect the aerodynamics of the main rotor system.
However, within a specific range of descent rates and low forward airspeeds, the instability of the wake catches up with the main rotor, causing the wake to collapse into a toroidal, or donut-like, form. This results in a sudden reduction in the lift generated by the rotor, which usually causes the helicopter to descend rapidly unless action is taken to recover.
Pilots are commonly taught two main recovery techniques. The traditional recovery calls for forward cyclic control to increase forward airspeed, often accompanied by a decrease in collective control — and corresponding decrease in power — to remove energy from the toroidal wake structure.
An alternative recovery technique, called the Vuichard recovery, calls for lateral cyclic control, an increase in power through raising the collective, and a proportional increase in anti-torque pedal. When properly executed, the Vuichard recovery typically results in less altitude loss than the traditional method.
In the New Zealand accident, according to investigators, the pilot recognized that the cyclic and collective controls lacked responsiveness during the rapid descent, a hallmark of VRS. The pilot first attempted to apply the traditional recovery technique, then recognized there was insufficient height available and attempted the Vuichard recovery.
Once the pilot realized that a collision with terrain was imminent, they focused instead on controlling the descent through the tree canopy to minimize the impact forces. The report credits the crashworthy design of the BK117 for helping the crewmembers survive the impact.
Compounding factors
The experienced pilot had more than 7,000 hours of total flight time and had previously served as a senior instructor pilot and standardization instructor pilot for the U.S. Army. The pilot told investigators that to avoid VRS they used a rule of thumb they had learned in the Army based on speed, descent rates, and engine power.
During the final approach to the injured tramper’s location, according to the TAIC report, a combination of distractions led to the pilot reducing their scan of the flight instruments, meaning they did not immediately recognize that they were entering conditions conducive to VRS.
However, the report also notes that the pilot’s rule of thumb did not account for the primary factor determining the boundary of VRS onset: the type of helicopter they were flying and the actual disc loading of its rotor. Neither did it account for the influence of upslope winds or the helicopter’s nose-high pitch attitude, both of which led the helicopter to enter VRS earlier than it would have otherwise.
As the report explains, while all helicopters are susceptible to VRS, the specific combinations of airspeeds and descent rates where they will experience the phenomenon are determined principally by disc loading — a mathematical property calculated by dividing the helicopter’s weight by the total area covered by its lifting rotors. This varies by aircraft, meaning that the onset of VRS in a helicopter with higher disc loading, like the BK117, will occur at higher forward airspeeds and higher descent rates than in a helicopter with lower disc loading, like a Bell JetRanger.
Moreover, what determines the onset of VRS is the actual flow of air through the rotor system, which can differ significantly from the speed of the aircraft itself. Thus, while the VRS boundary is typically defined in terms of descent rates, anything that increases the upward flow of air through the rotor system — such as updrafts or a nose-high flare — will increase the effective descent rate and can hasten the onset of VRS.
The TAIC worked with a noted aerodynamicist, Richard Brown from Sophrodyne Aerospace, to locate the VRS onset boundary for the accident helicopter and determine the influence of mountain winds and pitch attitude in precipitating VRS. Based on the weight of the helicopter at the time of the accident, the density of the air at the accident site, and the helicopter’s trajectory as recorded by its onboard Flightcell DZMx tracking system, Brown determined that in the absence of other factors, the helicopter would have entered VRS at the point in its trajectory when it recorded a descent rate of 1,050 feet per minute (320 meters per minute) and a groundspeed of 18 knots (33 kilometers per hour).
However, an analysis of wind conditions at the time of the accident concluded that winds were from the west at 20 knots (37 km/h), gusting to 30 (56 km/h), and the topography would have induced a significant upward component of wind flow along the helicopter’s flight path. Factoring in the influence of wind, Brown established that the helicopter would have entered VRS two seconds earlier in its trajectory than in a no-wind condition, at which point it had a descent rate of 728 ft/min (222 m/min) and a groundspeed of 20 knots (37 km/h).
Shortly thereafter, the helicopter made a heading change that gave it a tailwind, effectively reducing its airspeed. According to the report, this caused the aircraft to penetrate more deeply into VRS than it would have if the heading had remained unchanged, exacerbating the symptoms of VRS and making recovery more difficult.
Finally, Brown considered the effects of the helicopter’s pitch attitude, which reached almost 20 degrees nose-up as it descended toward the patient, further increasing the upward flow of air through the rotor. When this factor was added to the analysis, he concluded that the helicopter would have entered VRS around four seconds earlier than if wind and pitch attitude had not been factors. This estimate corresponded to the actual point in the trajectory when the sudden drop in height occurred, at which time the helicopter had a descent rate of 649 ft/min (198 m/min) and a groundspeed of just under 26 knots (48 km.h).
Missing data
The TAIC report notes that current civil certification requirements do not require helicopter manufacturers to include any information in the rotorcraft flight manual regarding the flight conditions and parameters that are conducive to VRS. Neither the Kawasaki BK117 B-2 flight manual nor other rotorcraft flight manuals reviewed by investigators contained any information that would help pilots accurately determine the range of descent rates and forward speeds over which VRS might be encountered.
“Having to rely on ad-hoc procedures to avoid VRS is no substitute for having properly constructed graphs to hand that represent the location of the VRS onset boundary on the flight envelope as a function of the helicopter’s weight and the temperature-corrected altitude,” the report states. “The provision of this information in all rotorcraft flight manuals would give pilots the best possible chance to avoid VRS.”
According to the report, Kawasaki has accepted the TAIC’s recommendation that it revise its flight manual to include specific data regarding VRS. The company told investigators that it planned to start relevant discussions with the Japan Civil Aviation Bureau in April and complete the activity by June.
Meanwhile, the TAIC urged the Civil Aviation Authority (CAA) of New Zealand to work with the International Civil Aviation Organization (ICAO) to promote the need for such information to be included in all rotorcraft flight manuals. The CAA is taking this recommendation “under consideration,” noting that rotorcraft flight manuals are the responsibility of the original equipment manufacturers and approved by the country where each aircraft is initially certified in accordance with ICAO and bilateral agreements.
During its investigation, the TAIC also explored the use of VRS alerting systems, which are installed on some new helicopters to alert pilots to flight conditions that may develop into VRS. While the TAIC concluded that such systems would be technically difficult to retrofit on most older helicopter models, it recommended that the CAA promote a requirement for such systems to be installed in new helicopter models.
“CAA notes that [VRS] alerting systems are a developing technology,” the agency replied, adding that the technical feasibility, cost-effectiveness, and potential unintended consequences of these systems remain uncertain. “The CAA will collaborate with TAIC to determine the best method for this to be raised for discussion.”
