Autothrottle Advances

Has the time finally come for automatic power control capability in light airplanes? We're getting closer all the time.

Autothrottle Feature

Autothrottle Feature

Significant advances in aviation technology usually arrive not with a brilliant flash of light and poof of smoke but over many years, in a slow, inexorable evolution of products that we sometimes don't even recognize as signifying a momentous change until, suddenly, the next incredible new capability is in our midst.

When we stop to think about all the marvelous innovations that have emerged in the last 20 years — precision satellite-based approach capability, synthetic-vision flight displays, integrated terrain and traffic alert systems, inflight datalink weather graphics — none of these appeared in our cockpits overnight. They migrated, drifted down, sometimes with imperceptible slowness from business jets and airliners, eventually into the airplanes we fly.

But isn't it kind of amazing that light general aviation airplanes by now don't come equipped with some sort of basic automated power control system? Cars have had cruise control since forever. Jet airliners have had rudimentary autothrust capability since the 1950s, starting with the Boeing 707. Can it really be that hard to add a simple linear actuator to a power lever to create an autothrottle system for piston airplanes and turboprops?

The answer is yes and no. Let's face it, autothrottle technology isn't quite as straightforward as the cruise control in your grandfather's 1958 Chrysler Imperial — but even still, the technology also really isn't that much more sophisticated than the computer-­driven active cruise control systems in cars in your local showroom today. So what will it take to bring autothrottle to the lighter side of GA?

Understanding Autothrottle

Autothrottles control power, and by extension airspeed, from takeoff to touchdown. In an autothrottle-equipped business jet or airliner, activating the autothrottle switches brings the thrust levers to life. On takeoff, they advance the thrust to the computed takeoff value automatically so the pilot can concentrate on flying the airplane instead of setting the power.

In other phases of flight, such as on initial climb, the pilot can also dial in the desired airspeed and watch as the throttles automatically move to maintain it. It's a great help in protecting against busting airspeed restrictions in controlled airspace. At cruise, power is continuously monitored and adjusted to maintain the selected airspeed as weight and temperature change. Pilot workload is greatly decreased, while fuel efficiency and performance improve too.

It used to be that the electronics and mechanical systems required to create an autothrottle system were so big and unwieldy that they would fit only on the largest airplanes. Well, we know what has happened with computer electronics over the years as they have become smaller, faster and less expensive. Overcoming the mechanical challenge of linking a throttle to the computers that control the engine really shouldn't be that hard, especially with fadec (full-authority digital engine control) coming to many turbine and now even piston engines.

Still, no light single-engine airplane has ever been certified with an autothrottle, in spite of the clear evidence of the safety benefits such a technology could bring to general aviation. Experts say there is a compelling case to be made for the benefits of automating power and therefore speed control in flight — especially in high-end piston airplanes and turboprops that are often flown by one nonprofessional pilot who must accomplish all the same inflight tasks as professional, two-person crews flying in the same busy airspace.

Safety Benefits

The crash of Asiana Airlines Flight 214 in San Francisco in July 2013 put auto­throttle technology squarely in the public spotlight — for all the wrong reasons. Automated speed controls are supposed to make flying safer, not completely befuddle an airline crew flying a visual approach on a glorious summer day to a long, wide runway at a major international airport.

Suddenly automation confusion was a term that appeared with regularity in the media as the National Transportation Safety Board admonished Boeing for disseminating documentation that "inadequately described" the 777's automated systems.

While confusion over automation is sometimes cited in high-profile accidents, like Asiana 214's, a closer look often tells a different story. The pilots of that 777 let the airspeed decay all the way to stick shaker before a pilot seated in the jumpseat behind them suggested they do something to correct the situation. In other words, there were bigger problems in that cockpit than simple confusion over the automated systems.

What rarely gets reported are all the instances in which autothrottle makes a pilot's job easier and ultimately safer by averting disaster. While the NTSB has yet to issue its final accident report, the crash of an Embraer Phenom 100 in Gaithersburg, Maryland, in December, which killed the three people on board the jet as well as a mother and her two small children in a house the airplane struck, calls into question whether autothrottle might have prevented the tragedy after the airplane got too slow on approach and stalled.

"We see autothrottle as being a huge safety benefit, and there are a couple of reasons for that," said Bill Stone, senior business development manager at Garmin. The use of autothrottle ensures more stabilized approaches by nailing speeds in the descent. "Another important benefit," he said, "is during high workload situations or pilot inattentiveness for whatever reason, when the autothrottle can prevent the airplane from getting dangerously slow."

Garmin isn't a name most people associate with autothrottle, but that's changing. The avionics maker certified its first autothrottle system aboard the new G5000-equipped Citation Sovereign+ business jet last year and is preparing for additional development programs in the near future.

Autothrottles aren't the only way to automatically control speed, but they're a critical part of any comprehensive speed control system.

Garmin actually has already developed a speed control system of sorts for piston airplanes, and it's called ESP (Electronic Stability and Protection). Built into the software of many of Garmin's latest integrated cockpits, the technology uses the autopilot to gently nudge the flight controls should the pilot stray outside the normal operating envelope — such as getting too slow on the base-to-final turn. ESP works in the background all the time, even with the autopilot turned off. All of these important but critical safety solutions ESP can accomplish without an autothrottle.

In a similar fashion, advanced autopilots make use of speed mode with the oddly named "flight level change," or FLCH, mode (sometimes called indicated airspeed mode) that lets the pilot select an airspeed the autopilot will hold. It is mainly used by pilots to control speed on climb-out so that, as available power declines with altitude, the system doesn't try to maintain an impossible rate of climb, which can lead to a stall.

Without an autothrottle such speed control systems are limited. For example, with the power set at cruise and a descent initiated, ESP will keep the airspeed from going into the red but it won't be able to do that while maintaining the descent. For that kind of control, only a power adjustment, either by the pilot or the autothrottle system, will do the trick.

Early Autothrottles

The first-ever autothrottle was a primitive system in the Word War II-era German Me-262 jet fighter. The first viable commercial system was installed in a DC-3 in 1956 (two years before the Speedostat cruise control was introduced in the Chrysler Imperial). That first system was called AutoPower. The inventor was Leonard Greene, who founded Safe Flight Instrument Corp. and was a pioneer in stall warning and angle-of-attack equipment.

Greene's speed control device connected servos to the throttles that automatically adjusted to maintain a given angle of attack — not an airspeed. When the pilots turned the system on to fly an approach, it commanded the power levers to maintain a speed corresponding with 1.3 times VSO (stall speed in the landing configuration). It worked, but the concept didn't really catch on until Safe Flight linked it to airspeed and not just angle of attack. The modern autothrottle was born.

An unusual element of Safe Flight's autothrottles is a patented safety feature called the "voter." It allows the auto­throttle system to compare the speed selected by the pilots with 1.3 VSO and automatically chooses the higher of the two. This prevents an airplane from stalling with the autothrottle engaged if the pilot dials in a speed below VREF, even at steep bank angles with high load factors or at heavy weights. All of Safe Flight's auto­throttles to this day, including the systems the company designed for the Gulfstream GII and GIII and in the Challenger 604 and 605 business jets (as well as military aircraft including the F-117 stealth fighter) employ the voter concept.

It wasn't long before competitors got into the autothrottle game as Sperry (now part of Honeywell) and Collins began developing autothrottle systems for a growing number of airliners and bizjets. Today autothrottle is standard on most large-cabin bizjets and airliners, and it's coming to more midsize jets as well. On these autothrottle equipped airplanes, the system can automatically fly the glideslope and maintain speed to within a hair's breadth of the target, leading to predictably safe and stable approaches (and landings).

Because of the clear safety and operational benefits autothrottle offers, it's only a matter of time before light jets, turboprops and even piston airplanes fly with the capability.

Autothrottle Dissected

There are two basic types of autothrottle systems, back-driven systems in which the mechanical movement of the throttle levers controls engine power and front-driven systems in which the computer regulates power regardless of whether the throttles move or not.

The introduction of fadec in modern engines ensures the system is smart enough not to exceed engine limits, which easily could happen if the autothrottle shoved the power levers to the stops. Fadec is another technology that we in the piston and turboprop realm don't sufficiently appreciate; only a few models come with fadec, a state of affairs that probably won't be changing soon.

Fadec consists of a digital computer, with sensors hooked up to the engine, that controls every aspect of engine performance, electronically monitoring and adjusting parameters for proper and smooth operation. So instead of the pilot manually adjusting the throttle, prop and mixture controls, he's left with single-lever control for all phases of flight.

"If you have one throttle lever, you only need a single actuator to move the lever, and it only needs very limited authority to allow the pilot to override the system should he need to," Stone said. "All of that is pretty simple in the grand scheme of things. We think we have a pretty good design to do it."

With fadec managing all aspects of engine operation, the autothrottle's mission is reduced to adjusting available power to maintain selected airspeed without worry of exceeding engine limits or calculating available power for takeoff or climb. Combined with other more recent inventions, such as low-priced digital air data computers and attitude and heading reference systems, the necessary raw data for the incorporation of autothrottle is now available in many modern airplanes — maybe even the one you fly.

"Bringing autothrottle to a turboprop or piston airplane, you really want single power lever control," said Paul Barnes, marketing manager for Rockwell Collins, which provides the autothrottles in a variety of jet transport aircraft and business jets including the Boeing 787 and Gulfstream G650. "Coupling fadec with air data and other pieces allows you to integrate autothrottle in a propeller airplane. It's a bit like cruise control in your car, but with more parameters to monitor."

Another important component of an autothrottle system is the mechanical actuator and clutch that allows it to be disconnected if the pilot reaches up and moves the throttle. Airbus has solved this issue by developing an autothrust system for its fly-by-wire airliners in which the thrust levers are advanced by the pilots into preset gates and left there. The thrust levers never physically move in autocruise setting as the computer manages engine power behind the scenes.

Autothrottle has been a standard feature of airliners for decades (TOP). So far the lightest airplane with Autothrottles is the remarkable Eclipse 550, which also features fadec (ABOVE).|

Autothrottle for the Masses

Such a design philosophy would reduce the complexity of an autothrottle system for light general aviation airplanes, but it also would introduce potential new problems. For instance, how does the pilot know how much power is being applied by the computers if the power lever doesn't move? And what happens when he or she reaches up and adjusts the lever or the autothrottle system suddenly disconnects?

And that's not the only obstacle designers face. Not only are the servos and clutches used in certified autothrottle systems complex, but they're also expensive. The challenge will be in creating an automatic mechanical throttle with a small and relatively simple mechanism that can be offered at prices GA pilots can afford.

"That's the area we're addressing now," Safe Flight president and CEO Randy Greene said. "When we look at the market for the TBM, the ­Meridian, the PC-12, those kinds of aircraft, and twin-engine turboprops as well, I think those markets are ready to accept autothrottles. I think today's pilots in today's cockpits are ready for the fourth axis of automatic flight control — which is power and speed."

Garmin's Stone hinted that the Olathe, Kansas, avionics maker has some interesting ideas that could lead to a breakthrough in the mechanical portion of the system and bring autothrottle down to, say, a new Cessna Skylane. Safe Flight's target market is significantly upstream from that, with turboprops the preferred goal.

"I think we need to bring the end-user cost of an autothrottle system from what it is in the bizjet world to single-engine turboprops down by probably 75 percent to be attractive in that market," Greene said. That would put the price of an autothrottle system "at around $50,000" in an airplane with a purchase price of, say, $3 million.

That's likely a bit higher a price than even the high end of the GA piston market could swallow. But eliminating the complex servos and clutches, or redesigning them to incorporate linear actuators and smaller, simpler safety mechanisms, could bring the price down further still.

Make no mistake, the mechanicals are an important element in autothrottles that use them. During an approach, especially in turbulence, the autothrottle will be making continuous small adjustments in power to remain on target speed. "No human pilot would fly that way because our tolerance for precise airspeed control isn't as tight as a modern auto­throttle system can maintain," said Rockwell Collins' Barnes. But it's a nice capability to have, he said.

What seems certain is that, once the first aircraft manufacturer announces the availability of autothrottle in a turboprop or high-performance piston airplane, competitors will feel pressure to jump on board. When might we see an autothrottle system certified in a Part 23 piston or turboprop airplane? That's hard to say. There really aren't any certification barriers since autothrottles have become so prevalent in the transport category Part 25 market, but as you've probably heard, nothing is happening very quickly on the certification front these days.

It's clear there are still significant challenges and obstacles to overcome before autothrottle makes its way into the cockpit of a piston airplane or turboprop single. It might take a few years, or it could take a decade or longer. But like most every groundbreaking aviation technology, it's one that is absolutely worth waiting for.

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