Thales/StandardAero StableLight Autopilot Targets AStar

Introduced in 2021, the Thales/StandardAero StableLight autopilot is the first four-axis autopilot for light helicopters, and the two companies have targeted the popular Airbus Helicopters AS350/H125 AStar as a large market opportunity for the upgrade. The autopilot received FAA supplemental type certificate (STC) approval in late 2023.

Heli Austria is the European launch customer and will install the autopilot once EASA validation of the U.S. STC is completed. StableLight is approved by the FAA, Transport Canada, Brazil’s ANAC, and Mexico’s AFAC.

Unlike helicopter autopilots that are adapted from fixed-wing products, the StableLight autopilot derives from four-axis autopilots designed for larger IFR helicopters such as Sikorsky’s S-76 series.

The StableLight autopilot incorporates stability augmentation without feedback in the flight controls, according to StandardAero. Features include stabilized climb, flight attitude recovery, auto pull-up terrain avoidance, approach to hover, groundspeed hold, and other typical helicopter autopilot modes such as GPS nav and lateral and vertical approach. The autopilot also has protections such as coupled collective power limiting.

“There is a lot of interest,” said James Sleigh, Thales engineering test pilot, avionics systems. “The feedback has been excellent with everyone we’ve flown and engaged on it.”

Sleigh explained the development of the StableLight autopilot as a “descendant of our Part 29 IFR single-pilot autopilot,” which is installed in the S-76D. “It does follow a traditional autopilot from the IFR helicopter world.”

Light helicopter pilots who have flown with autopilots, but not a heavier machine such as the S-76, likely were exposed to what Sleigh refers to as “outer loop” autopilots, typically two-axis configurations. “By outer loop, I mean an autopilot that physically moves the flight controls in your hand, and it’s very simplistic,” he said. “You can engage heading hold, and you can engage an altitude or airspeed on pitch. And those are great; they obviously reduce workload, but their performance is limited to the actuation and the design that they have.”

Moving the cyclic with an actuator can be counterproductive in some cases, he explained. “How does an autopilot that has to move the stick truly help a pilot in a high-demand operation such as hovering, or in long lining, or in precision flying where the pilot is very attentive or is very precisely controlling the aircraft?”

A two-axis autopilot controls pitch and bank, and the third axis adds yaw control. Four-axis autopilots in helicopters take on collective control, which manages power in a helicopter, enabling full control in a fairly wide operating envelope.

Cockpit Agnostic

A goal of the Thales/StandardAero team was to develop a four-axis autopilot that could be installed in a variety of helicopters, no matter what kind of avionics are onboard. “We’re cockpit agnostic,” he said. “One of the design rules for the philosophy of our autopilot was we don’t want to demand a customer have a specific existing avionics configuration or that they install a glass display. For instance, you can buy a Garmin autopilot, but you’d better have a GTN [navigator] or a G500 [display]. Our mandate was that a [customer] may have a KCS 55 mechanical horizontal situation indicator (HSI) and an Aspen display and want to use our autopilot, or even have steam gauges. That was the design philosophy of making it standalone and cockpit agnostic, and divorced from relying on integration into an electronic display for things like targets, for mode annunciation, and that kind of thing.”

The StableLight autopilot does include a control panel and a Mid-Continent Instruments and Avionics Flex MD23 2-inch instrument as the autopilot’s mode annunciator.

The one control that StableLight retains for all installations is a heading select knob, which is a universal avionics convention, whether the HSI is mechanical or a glass display. But for other parameters such as airspeed and altitude, Sleigh said, “We have a philosophy that is called a virtual copilot.”

While the StableLight autopilot has a stability augmentation system (SAS) mode with rate damping, which is used for flying sling loads or other utility work, the normal mode pilots would use is ATT or attitude mode. In this mode, the pilot is still hand-flying but using movements of the beep trim button on the cyclic to make small changes in pitch and roll, or the collective button for yaw and collective control. In ATT mode, the autopilot will hold the helicopter in whatever attitude it is in when the pilot lets go of the controls.

“The idea is that if I set the aircraft in a 5-degree left bank, 2 degrees nose up, and I let go, it’s going to hold that attitude,” Sleigh said. “If we get a gust or a disturbance, perturbation, it’s going to bring it back to that attitude.” To make a change using the flight controls, the pilot simply presses the force-trim release button, moves the control, then releases the button.

The flight director modes act similarly. Instead of setting a selectable altitude, target airspeed, or vertical speed as on a traditional autopilot, StableLight allows the pilot to fly to the desired altitude, airspeed, or vertical speed, then select a mode on the control panel and let go of the controls, and it will hold that target. This can be done with small changes using beep trim or large changes using the flight controls while pressing force-trim release.

“To pull into a climb, engage vertical speed, and it’s going to hold that vertical speed,” he said. “Then use the beeper to make fine adjustments. If the altitude isn’t exactly where you want it, the airspeed isn’t within a knot of where you want it, you can use [beep trim] to make fine adjustments. That’s the way we’re teaching that, to use gross adjustments by hand, fine adjustments by beep.”

While this applies to the VFR version of StableLight, there is an IFR version that does have an altitude select and level-off feature, but that is easier to incorporate in a particular avionics suite for IFR operations. In any case, StableLight embodies the concept of pilot-in-the-loop during autopilot operations.

Inner and Outer Loops

As a flight test engineer, Sleigh looks at autopilots as “outer loop” or “inner loop,” depending on how they work. An outer loop autopilot has actuators that directly move the controls, generally the cyclic and pedals (in a three-axis autopilot). “That’s a totally viable means of inputting autopilot commands to the system,” he said.

“However, what you’ll find on more complex, [such as] IFR helicopters and our autopilot is we have a series actuator, an inner loop actuator, and this provides inputs between the pilot’s hand and the swashplate. In essence, it’s adding and subtracting control inputs from my hand position to the final input to the swashplate, thereby putting in—transparent to the pilot—high-rate, limited-authority autopilot inputs into the flight controls.

“So what does that do? It allows me to turn on SAS and fly a precision task—say, a precision hover task. And that autopilot is working in the background to bring a much higher degree of stability to the aircraft. From a pilot perspective, it feels like you’re just flying a more stable helicopter. Again, it’s transparent to the pilot. That’s why…if I’m doing a precision long line and I want to put in the control [movement] myself, but still benefit from that rate damping, SAS mode is good. For everyday, normal up and away flying, I want to be able to set an attitude, pick up my iPad, and know that if I get a perturbation of some kind, I’m going to come back to my wings-level attitude or my 2-degrees nose-down attitude. That’s the basic flying mode.”

In the flight director mode, StableLight will hold a heading, altitude, airspeed, vertical speed, velocity (current groundspeed, heading, and track), or radar altimeter height. “Those go on top of that basic stability mode,” he said. “It’s an autopilot where you put the aircraft where you want it, and let the autopilot hold that. It’s far more pilot-in-the-loop than, say, just set an altitude preselect and let it climb and level off.” StableLight also allows the pilot to climb in an airspeed-hold mode.

When selecting an airspeed and altitude upper mode, the collective (fourth axis) will also be engaged, and in this case, the beep trim on the collective is used to make small altitude changes. If altitude but not airspeed is engaged, then the cyclic beep trim can be used for altitude changes. The same is true with vertical speed: when coupled with pitch, the cyclic beep trim adjusts vertical speed, but when coupled on the collective (with airspeed), its beep trim is used. 

“I do want to talk about the importance of the four-axis versus two- and three-axis autopilots,” Sleigh said. “In a helicopter, more so than in an airplane, we tend to fly…back side of the power curve. The collective is altitude, [that is] power is altitude, pitch is airspeed. In other [three-axis] autopilots, where they only have a pitch-axis controller and not the collective, you have to put vertical modes on the cyclic.

“Now what happens is, I put altitude on pitch, for instance, like on a Garmin or a [Genesys] HeliSAS, and all of a sudden things become backwards in the helicopter cockpit, because now my collective, in essence, becomes an airspeed controller. Because if I don’t have enough power, I’m going to start decelerating. That aircraft is going to pitch up to hold altitude and keep pitching up to hold altitude.

“Eventually, I fall out the back side of the airspeed, and [the autopilot] decouples. And the pilot is confused. Why the heck did it decouple? Well, because it was incumbent on the pilot to put power in to hold altitude, or vertical speed. By coupling up the fourth axis, we put the flight modes where a pilot would fly and then where his brain is used to having them flow.”

Training on StableLight

When it comes to learning how to use the StableLight autopilot, Sleigh believes that pilots with any autopilot experience will have an advantage, even with an outer-loop system, “because they understand automation management and the basics of an autopilot. I think they’re easier to teach.”

There is a learning curve getting used to flying a four-axis autopilot using beep trim, he admitted, but “it’s pretty quick. They understand the value. The first half hour, they’re sitting there, flying with the force trim releases matched the whole time, and they’re trying to fly it like a non-trim aircraft. Talking them through it, getting them to fly the beep trim…if you’re in attitude mode and you just need to make a small turn, just use the beeper. Roll yourself into a 10-degree turn, then beep it out. Once they do that a few times and the light goes on, they understand the value of it relatively quickly.

“The other one is getting people to take their feet off the pedals. You’ve hard-coded in your brain doing that, that correlation between power and yaw, you’re the human mixer. One of the favorite things we do in demos is [have you] put your feet on the floor, and we do a takeoff, and whoa! How the heck do you do a takeoff with your feet on the floor? And then we do a landing with our feet on the floor.

“Getting people to realize…take your feet off the floor, let the helicopter do heading hold when you’re in the low-speed regimes. For sideways flight, take your feet off the pedals. It’s going to hold the heading as I translate 10 knots right, 10 knots left. If I want to make a heading change, go ahead and make it [with the pedals]. Then take your feet off the pedals. Once they learn it, do it, and fly it [they say], ‘Wow! Now I realize my workload is a lot lower. I don’t have to constantly be assigning some [brain] CPUs to doing that yaw control.’”

For pilots used to flying with four-axis autopilots in IFR helicopters, transitioning to StableLight is much easier. “They feel right at home because they understand four-axis autopilots,” Sleigh explained. “They understand…that you can put your feet on the floor. In fact, it’s best to fly it with your feet on the floor. They understand force trim release and beeping references. There is definitely a learning curve for the first cohort that comes out of simple autopilots but…just like when [they] end up in an IFR twin and they realize the capability and the power of such an autopilot, they really come to appreciate it.”

Another StableLight feature that pilots will appreciate is the go-around button on the collective, which engages the auto pull-up feature. The pilot can engage auto pull-up by pushing the go-around button, or the helicopter’s terrain awareness and warning system will automatically trigger auto pull-up when necessary. This uses pitch and the collective axes to start climbing to a target of 65 knots and at 1,000 fpm, and roll and yaw axes to set zero bank or a selected heading in heading mode.

“We teach when in doubt, push that go-around button, and the same is true in loss of spatial orientation in hover,” Sleigh said. “White-out, brown-out, night-vision-goggle failure: push the button, get away from Mother Earth, and then sort it out.”

Sleigh admits that selling AStar owners and operators on StableLight is challenging. “[They say] ‘Your system is expensive and it’s heavy,’” he said, “but it’s not even comparable [with outer-loop autopilots], it’s simply not the same thing. Come fly ours. Come see the functions, features, and capabilities that it has, and you’ll see it’s really transformative for this level of helicopter.”