A European study into Vortex Ring State (VRS) – one of the helicopter industry’s most pernicious and persistent safety risks – has concluded there are only minor differences between the effectiveness of two different recovery techniques.
However, the research also highlights the higher workload involved in the so-called Vuichard recovery – named after its Swiss pilot inventor Captain Claude Vuichard – noting pilots found it “less intuitive” and “less comfortable” to perform, with a higher risk of exceeding aircraft power limitations.
VRS – where a helicopter with slow forward speed descends rapidly through its own downwash with no arresting effect from the application of more power – has been highlighted as the cause of multiple accidents over the years, most recently by New Zealand investigators in their report into the 2023 crash of a Kawasaki BK117 rescue helicopter.
Pilots are typically taught to exit VRS through the “forward” recovery technique which sees them reduce the collective, ease the cyclic forward to adopt a nose-down attitude, while maintaining the heading with the foot pedals. Once the airspeed is above that required for translational lift, pilots are told to increase the collective to maximum continuous power to climb.
While the project, supervised by the European Union Aviation Safety Agency (EASA), had set out to analyse the effectiveness of this method against the Vuichard technique, initial flights found strong correlation between the amount of torque applied and the height loss.
As such, the forward recovery was adapted so pilots simultaneously put the helicopter into a nose-down attitude and brought the power up to 70% by raising the collective.
In contrast, the Vuichard technique requires the pilot to raise the collective to obtain the maximum available power, while simultaneously applying lateral cyclic to roll in the direction of the tail rotor thrust and using the foot pedals to maintain heading. Once the descent is stopped, the cyclic is eased forward to regain airspeed.
Additionally, the study hoped to “experimentally determine as precisely as possible” the flight conditions where VRS starts to develop.
The research was conducted by the French aerospace research laboratory ONERA and saw flight tests performed by the DGA – Essais en Vol, the defence procurement body’s experimental test agency.
Flights were carried out at Istres air base in the south of France in late 2023 and early 2024 using an Airbus Helicopters AS550 Fennec light-single and a pair of AS365N3 Dauphin medium-twins at a safety altitude of 3,000ft.
A total of 229 runs were performed across eight flights – four for each type - of which 177 were finally validated: 146 recoveries were carried out where VRS was fully developed, with 31 at the condition’s onset; the flights also saw a roughly even split between forward and Vuichard recoveries.
Although noting the limitations of the study – the small number of flights across just two helicopter types with similar rotor systems – EASA still believes the results are a valuable contribution to the topic.
In particular, the test results for both types showed “good consistency” with ONERA’s predictions for VRS onset.
To measure the effectiveness of the two recovery techniques, the researchers used two criteria: the helicopter’s height loss during the manoeuvre and the time taken to complete it.
For the Fennec, the Vuichard technique showed “slightly better” performance once the VRS condition was established. On average, the helicopter lost 199ft and recovery was completed in 9.23s, against figures of 221ft and 9.55s for the forward technique.
At onset, the Vuichard technique was notably better, showing average figures of 82ft/6.5s against 168ft/9.3s for the forward technique.
“However, the Vuichard recoveries were generally performed with a higher average torque. When limiting the comparison to a similar range of average torque during the recovery, the performance of both methods becomes similar,” the report adds.
On the Dauphin, meanwhile, the Vuichard technique generated average figures for established and onset VRS recoveries of 256ft/11.6s and 179ft/10.8s, respectively, compared with 227ft/8.5s and 247ft/10.9s for forward recoveries.
Noting the 3s difference between the two techniques once VRS is fully developed, the report says this is “partly explained by the time needed to recover a positive [vertical speed] after exiting VRS and trying to come back to hover, due to the limited available power on the Dauphin.”
Pilots were also asked to evaluate the workload during each recovery manoeuvre. Although the study acknowledges their relative unfamiliarity with the Vuichard technique, even after they became more accustomed to the process this “did not change their feedback on the perceived workload”, it says.
“The pilots expressed a higher workload during the Vuichard recoveries, with high frequency actions on all three [control] axes and on the collective to avoid exceeding the [torque] limitations, and in a short time interval.”
On both helicopters, the pilots felt the Vuichard recoveries were “less intuitive than forward recoveries” – partly due to their training – “and less comfortable due to the sudden attitude changes often experienced especially on the Dauphin”.
Even when closely monitored by the co-pilot, take-off power limitations were “often approached”, it says, adding: “In operational conditions, for a pilot with normal skills immediately using all available power at the start of the recovery, there would be a high risk to exceed the aircraft limitations.”
The study suggests it is inadvisable to use the Vuichard technique on the Dauphin due to certain design limitations with the helicopter, unless the forward recovery cannot be performed, for instance due to obstacles.
For both types, “inverse” Vuichard recoveries – where the helicopter is rolled in to opposite direction to the tail rotor thrust – were also attempted, which showed “good performance”.
“While the sample size is small, this result is interesting and consistent with the pilot feedback which indicated a seemingly easier and more fluid inversed Vuichard recovery, compared to the ‘normal’ Vuichard recovery,” it says.
It calls for further flight-testing of this “inversed” technique to validate the perceived benefits.
