Innovative Solutions & Support (IS&S) has received FAA certification for an autothrottle system for the Pratt & Whitney Canada PT6-powered Pilatus PC-12 turboprop.

While IS&S’s first autothrottle is standard equipment on the Eclipse 550 and optional on the Eclipse SE upgrade, that system, like many other autothrottle designs, is connected to engines with full-authority digital electronic controls (Fadec). What makes the PT6 autothrottle stand out is that it is able to fully control an engine with a hydromechanical fuel control, and the IS&S PT6 autothrottle also provides protections that prevent over-torquing during takeoff or over-temping in climb or at high altitudes as well as overspeed and underspeed protection.

The new autothrottle system opens up a vast new market for PT6-powered aircraft, and IS&S is planning further approvals for other airplane types. The FAA supplemental type certificate (STC) covers all legacy (non-NG) PC-12s, and NG models will be added by the end of this year’s fourth quarter. IS&S will also seek EASA approval.

System Design

Because the autothrottle requires processing power to make it work, IS&S employs its own primary flight display (PFD) to run the autothrottle. Also required is an engine data concentrator unit (EDCU), a remote-mounted box that connects the PT6 with the IS&S displays. For PC-12 NGs, the autothrottle can be installed by adding an IS&S integrated standby unit (ISU) and the EDCU. The ISU is basically like a small PFD; it has plenty of processing power to run the autothrottle and it can also replace an existing standby display.

The autothrottle itself mounts inside the forward end of the console between the seats, and it attaches to the PC-12’s power control lever (PCL). IS&S has applied for a patent for its autothrottle, which features a clutchless design using mechanical components, and an electric motor and control electronics. Essentially, said IS&S president Shahram Askarpour, “it is a linear actuator that converts rotation into linear movement.”

He pointed out that the FAA Aircraft Evaluation Group, which sets standards for charter operations, evaluated the IS&S PT6 autothrottle system. “They determined there are no special pilot training requirements for Part 135,” he said, “and no limitations.”

Like all autothrottles, the IS&S system must be designed so that pilots can override it by pushing or pulling on the throttle lever(s) or PCL. In the PC-12, the IS&S autothrottle takes less than 10 pounds of force to break away, and it automatically shuts off when overridden.

Flying with the Autothrottle

During a flight in early June from Centennial Airport in Englewood, Colo., IS&S chief pilot Eric Smedberg demonstrated the autothrottle in action, as well as its interface with a full-up IS&S PC-12 integrated flight deck.

For takeoff, Smedberg switched the autothrottle on using the PFD controls, but there is also an on/off switch on the center console forward of the PCL. A disconnect switch is installed on the PCL. During takeoff, the autothrottle smoothly moved the PCL forward and accelerated the PT6 to maximum power.

When using the autothrottle PFD control, the pilot can select torque or airspeed hold mode. Airspeed hold uses power to control airspeed, unlike the IAS (indicated airspeed) hold button on the PC-12’s autopilot, which uses pitch to manage airspeed. With the IS&S autothrottle, PC-12 pilots should avoid using the autopilot’s IAS hold mode because that will shut off the autothrottle, Smedberg explained.

The autothrottle on the Pilatus can be used from takeoff to touchdown. The PC-12’s IS&S avionics calculate the airport’s density altitude and the takeoff power (torque) setting. Four minutes after takeoff, the autothrottle pulls power back to 36.9 psi, to keep the engine below the five-minute max power limitation. It is possible to continue climbing in torque hold mode; when the engine reaches its temperature limit, the autothrottle will maintain that setting during the remainder of the climb and also during cruise. To switch to airspeed mode, it’s just a matter of pushing airspeed hold on the PFD, and the autothrottle will maintain that airspeed, as long as there is enough power to do so. The autothrottle synchronizes to the current airspeed when the airspeed hold mode is engaged, and a bug matching the set speed is displayed on the airspeed tape.

As do the Eclipse 500/550 autothrottles, the PC-12 system has automatic protections designed to help pilots avoid overspeed, underspeed, overtorque and overtemp. These protections are independent of the autopilot and don’t use the PC-12’s autopilot to keep the airplane within its flight envelope.

For example, Smedberg demonstrated a high-speed event and allowed the PC-12 to accelerate to redline airspeed. The autothrottle automatically pulled back the PCL to reduce power and bring the airspeed back below redline. At about three knots below redline, the autothrottle reverts to airspeed hold mode and remains set at that airspeed. But if the airplane is still accelerating, the autothrottle kicks off because it senses it can’t slow the airplane enough, Smedberg explained. And it doesn’t use the autopilot to pull the nose up.

Faced with an underspeed, the autothrottle adds power to try to keep the PC-12 from stalling, and it does this before the airplane slows to stick-shaker speed. This protection will help pilots who forget to add power after leveling off during a descent or on an instrument approach. However, he added, if the PC-12 stick pusher activates, the autothrottle will kick off.

For a go-around, the pilot just needs to click the go-around button on the PCL and the autothrottle calculates the go-around torque then sets the power, leaving the pilot to raise the landing gear and flaps.

Where Smedberg has come to truly appreciate the autothrottle in the PC-12 is flying in busy airspace, especially in the New York City and Boston areas, where air traffic controllers issue rapid-fire and frequent speed and altitude changes. On a flight into Boston’s Logan International Airport, for example, he was told to fly at 180 knots at 10,000 feet, then descend to 8,000 feet at 170 knots, then back to 180 knots. Before the autothrottle installation, he would have to manipulate the power constantly to meet those instructions, a challenge during single-pilot operations while trying to fly the airplane, brief the approach, talk to ATC and adhere to the constantly changing instructions. Now he can just set the airspeed to the desired number and forget about having to adjust the PCL and not worry about aggravating the controller by inadvertently busting the speed restriction. “It’s a true workload-reduction item,” he said. “It all comes back to safety.”

IS&S is distributing the PC-12 autothrottle system through its dealer network, which will also provide price information. The price varies depending on whether the PC-12 is already equipped with IS&S displays or needs them installed, and display installations can range from adding an ISU (in the NG model) to a full cockpit.

Beyond the PC-12 market, which IS&S believes is significant, there are many PT6-powered airplanes that could benefit from an autothrottle system. “We feel the King Air will be a big [market],” said Smedberg. “It will provide a true safety envelope for a twin.” The advantage of an autothrottle on a King Air is that if an engine fails, the remaining engine can automatically be set to the correct maximum power setting, allowing the pilot to focus on flying the airplane. If the max power setting is matched to the minimum airspeed for the single-engine condition, then the autothrottle can also bump up the power in a turn as lift drops. “In the twin-engine world the autothrottle plays a different role,” he said. “It’s a much more direct line to safety.”

The other benefit for a King Air is that its complicated power charts and engine limitations are loaded into memory (like those of any airplane equipped with autothrottles), and this will greatly reduce the pilot’s workload. “We’ve mastered the ability to control non-Fadec engines,” Smedberg said. “There are no obstacles to [equipping] other PT6-powered airplanes.”

He added that IS&S is discussing the autothrottle system with aircraft manufacturers. “There’s no reason this shouldn’t apply to other aircraft,” he said, “and it’s a lot cheaper than developing a Fadec and then adding autothrottles.”