A revised safety notice (SN) from Robinson Helicopter Company provides pilots with more information on flight in high winds and turbulence, including explicit instructions to slow down and disengage autopilot upper modes in moderate or greater turbulence.
SN-32 is one of three safety notices Robinson updated in July following an Australian Transport Safety Bureau (ATSB) investigation into the fatal in-flight break-up of a Robinson R66 helicopter near Hawks Nest, New South Wales in 2023. Others are SN-11 on low-G pushovers and SN-41 on pilot distractions.
As previously reported by Vertical, the Hawks Nest crash was the first Robinson mast bumping accident in which investigators had access to a cockpit video camera and flight data recorder, allowing them to conclusively establish the sequence of events that led to the in-flight break-up.
Mast bumping is a phenomenon in which the hub of a two-bladed main rotor system contacts the rotor mast, typically when a pilot inputs opposite cyclic to correct a roll after entering a low-G condition. While all two-bladed main rotor systems are susceptible to mast bumping, Robinson helicopters with the company’s original asymmetric horizontal stabilizer design can experience an aggravated roll to the right in low G, particularly at high airspeeds.
In the Hawks Nest crash, the helicopter was equipped with a Genesys HeliSAS two-axis autopilot capable of making pitch and roll inputs to the cyclic. It has a primary stability augmentation system (SAS) mode that maintains a steady helicopter attitude, as well as a number of “upper modes” including heading mode (which uses roll inputs to maintain a heading selected by the pilot) and altitude mode (which maintains a selected altitude using pitch control). It is designed to be easily overridden by the pilot with only light manual control inputs.
On the accident flight, the autopilot was maintaining heading and altitude at a relatively high airspeed of 115 knots when the helicopter flew into turbulent air, causing the aircraft to roll right, pitch nose down, and climb. The pilot was holding food in his right hand and overrode the autopilot by putting his left hand on the cyclic control cross bar.
For the short remainder of the flight, the pilot made reactionary cyclic inputs without moving the collective or pedals and without attempting to slow the helicopter. ATSB investigators determined that the aircraft ultimately entered a low-G condition as a result of both turbulence and pilot control inputs, after which it rolled through 285 degrees before breaking apart in flight.
The ATSB did not find any safety issues with the HeliSAS autopilot, but concluded that Robinson’s pilot operating handbook did not provide adequate guidance for appropriate control inputs in response to a turbulence-induced low-G situation. That was one of the findings that let Robinson to modify its relevant safety notices, including SN-32 on flight in high winds and turbulence.
SN-32 now clearly states that if moderate or greater turbulence is expected or encountered, pilots should reduce power and use a slower-than-normal cruise speed of 60 to 70 knots. They should also disengage autopilot upper modes such as altitude (ALT) and heading (HDG) hold.
The previous version of SN-32 did not contain any guidance about the use of autopilots in turbulence. It recommended reducing airspeed to 60 to 70 knots in “significant” turbulence, with the determination of significant left up to the pilot based on his or her experience and comfort level.
The revised safety notice aligns with the more widely accepted definition of “moderate” turbulence, which is turbulence that causes changes in altitude and/or attitude that do not result in loss of control of the aircraft but cause occupants to feel a definite strain against their seatbelts.
Enhanced protections
Robinson VP of engineering Sean Doyle told Vertical that the company made the recommendation to disengage autopilot upper modes because they will not provide useful control inputs in turbulence, when the priority is restoring aircraft attitude with smooth, gentle control inputs while accepting momentary excursions in airspeed, heading, altitude, and RPM.
He emphasized, however, that the autopilot is designed in such a way that it will not independently put the helicopter into low G. “Even the older HeliSAS . . . it will not fly you into a low-G condition,” Doyle said.
This built-in protection has been further enhanced on the Garmin GFC 600H advanced flight control system (AFCS) that was recently certified for the R66. The GFC 600H has an additional limit cueing feature that provides resistance to cyclic inputs when pitch, roll, airspeed, or G limits associated with safe flight are exceeded, with the amount of resistance provided proportional to the deviation beyond threshold values.
“It’s providing resistance for pilots trying to do something that is more dangerous than flying in a benign way, but it’s still intended that pilots have ultimate authority and control,” explained Robinson CEO David Smith. In a low-G scenario, he said, the AFCS will resist any rolling movement until the main rotor is safely reloaded, thus helping to counter any incorrect reaction from the pilot.
Other safety features on the GFC 600H include a level mode that establishes a safe flight attitude with a single button press, and low altitude protection that prevents inadvertent descent below 200 feet GPS height above terrain, with accompanying visual and aural “low altitude” annunciations to the pilot.
The GFC 600H also has low speed protection that prevents inadvertent deceleration below 45 knots while providing the pilot with “low speed” warnings. This prevents the helicopter from getting too far on the back side of the power curve, where a two- or three-axis AFCS cannot effectively maintain altitude or glidepath using pitch changes — a factor in some fatal accidents involving older-generation helicopter autopilots.
Smith said the GFC 600H is certified to the highest design assurance level, DAL A, for all flight critical functions, “so it has a very high design assurance and system reliability performance.” Robinson announced at Verticon earlier this year that it is making a two-axis (pitch and roll) version of the GFC 600H standard on all new-build R66 helicopters, with yaw axis control available as an option.
Looking to the future
Also at Verticon, Robinson unveiled plans for a new 10-seat utility helicopter, the R88, that will come standard with a four-axis autopilot. The R88 autopilot will be derived directly from the GFC 600H on the R66 and will have similar low-G protection, although exactly how the R88 autopilot and its safety features will function for utility operators is still up for discussion, Smith said.
“This is one of the discussions that we’re having with some of the bigger customers for firefighting, for example, is what do they want? Because they mostly don’t have this today. So it’s an open landscape in some respects.”
Adding a fourth axis of collective control on the R88 autopilot will enhance the types of safety features already found on the GFC 600H. It will also provide the groundwork for more advanced functionalities in the future, which could potentially include automated traffic or wire strike avoidance.
“There’s far too many wire strikes, there’s far too many mid-airs,” Smith said, explaining that the company is tracking the development of new technologies to detect wires and noncooperative traffic. A full-authority AFCS would support algorithms that direct the pilot on how to avoid them — or command the helicopter to do so automatically, once the technology is sufficiently advanced.
“Those are all the future,” he said. “And so for a new product, it’s important to lay the foundation for those things.”
