by Larry Anglisano
Compared to navigators and displays, autopilot technology doesnt advance rapidly. Not every airplane has flight control and the market is intolerant of the cost of full-system autopilot replacement. Realizing that many owners wont drop 40 grand into an autopilot upgrade, autopilot veteran S-TEC has dominated the small- to mid-size airplane autopilot market with their turn-coordinator-driven, rate-based systems. Theyve certainly snatched a lot of business away from Bendix/King with these well-regarded autopilots.
But buyers have been waiting for an S-TEC competitor for years, hopefully one that might offer an affordable, attitude-based autopilot at rate-based pricing: the Chelton AP-3C system. It has created a bit of a buzz in the autopilot community and is winning STC approvals in impressive numbers after just a year on the market. The digital AP-3C seems to deliver it all at a price that seems too good to be true. (List is $11,000.)
Dating back to 1947, Texas-based Chelton is no stranger to the aviation market, although many GA owners think its a new company, given its involvement in cutting-edge primary flight displays. Original-equipment Chelton components ride aboard aircraft from Cessnas to Embraers and everything in between. Theres no shortage of funding at the Chelton Aviation division either, a subsidiary of parent company Cobham plc, a $2 billion aerospace conglomerate which also includes companies such as Wulfsberg Electronics, which manufactures many of the AP-3C raw components, antenna manufacturer Comant Industries, Artex and Northern Airborne Technologies, to name a few. With its seemingly stable business platform, Chelton seems the perfect company to succeed with a modern alternative to the S-TEC and Century models.
Speaking of Century, they introduced the digital Triden system at Sun n Fun in 2000 and while there are several STCs in place, the attitude-based Century 2000 is still the only Century option for most aircraft, because of its longer STC list.
While the smaller and more modern Triden is STCd for the Mooney M20J, Cessna 172R and some Cessna Centurions, it lacks approval for many popular Beechcraft and Piper airframes, just the models most likely to need autopilot upgrades and flown by owners who can afford them. These aircraft are the exact target market for Cheltons AP-3C. Chelton openly claims that the flagship S-TEC 55X and Honeywell KFC225 are their main competitors.
The certification process is often a limiting factor in bringing new systems to market and Chelton is covering the bases as quickly as it can, securing TSO approval for common models of piston singles and some light twins, too. At press time, the AP-3C is STCd for about 20 aircraft models, covering many popular single-engine Piper, Mooney and some Beechcraft airplanes with the goal of securing some 50 STCs in 2006. While these numbers might seem low, FAA certification moves at a snails pace, but Chelton seems dedicated and serious. And so is the modern design of the AP-3C, which is surprisingly simple for a modern flight control system.
According to Chelton, the AP-3C evolved from an autopilot called the AP-1, which dates to 1973. It was designed by Giuseppe Basile, who was then teaching electronics and control automation at the University of Bologna, in Italy. Basile developed several iterations and improvements of the AP-1, which were certified by the FAA but which never made market inroads in the U.S. Chelton bought the design in 2001 and redesigned and improved it to create the AP-3C.
Thanks to the many components that make up an autopilot system, troubleshooting older flight control systems is challenging and costly. In response to this, the AP-3C has just three major components-a computer/programmer, a roll servo and a pitch-trim servo, which together add about 10 pounds to the airframe.
Unlike other systems, the AP-3C doesnt use a pitch servo, complex altitude-hold chambers (singing cans) or a static system interface to fly the pitch axis. Instead, a pitch-trim servo replaces the common elevator servo that has been used in small airplanes for years. Flying the airplane with pitch trim control is the way some large-airplane autopilots fly and aside from the benefits of weight reduction and reduced install time, the aircraft always flies in trim. This is most noticeable when the autopilot is disengaged, either intentionally or inadvertently.
Because the trim servo, via command from the flight computer, constantly trims the airplane correctly, autopilot disconnection is a smooth event without the abrupt pitch changes or the need to feed trim to maintain altitude. Plus, with a closed-loop feedback design between the servo and computer, the system can provide an increased amount of control stability compared to other systems, according to Chelton.
Chelton also suggests that their system does a better job of flying the airplane in turbulence than do other systems that have a separate pitch servo. But, in our view, since you really dont know what the airframe is doing when its getting tossed around in turbulent air, you really dont know how good a job the autopilot is doing, either.
Still, we like the simplicity of a single servo controlling the pitch axis, especially for troubleshooting and for installation simplicity. Like any other autopilot, it takes an attentive eye to watch the systems every move while its flying. Some argue that its easy to overpower the control forces of a light airplane, either with or without the autopilot engaged, but try a balked landing with full flaps in a pitch-heavy airplane like a Cessna 210 and youll think otherwise. Youll also respect how hard autopilot servos really work.
The Chelton servos are of modern design, too, intended to work long and hard using a magnetic clutch mechanism and a separable capstan, aiding in quicker servo removal from the aircraft control cables during maintenance.For some airplane models, the removal of a servo from the control cables for maintenance often means the aircraft is grounded. With a separable capstan plate assembly, this could alleviate the hassles and downtime of autopilot maintenance and make life easier for techs and easier on an owners wallet when the shop clock is ticking.
The combination of aluminum and steel materials used in the Chelton servos are intended to promote durability while keeping weight in check. Chelton tells us that a newer servo will use a brushless motor and, interestingly, this one-size-fits-all servo will work in any airplane, regardless of the model. We welcome that kind of smart design thinking.
Often, a major expense in installing a new attitude-based autopilot is buying a new attitude gyro. But Chelton says that most attitude gyros with roll and pitch pickoff (AC or DC in many cases) will drive the AP-3C, with the exception of Cessna/Sperry autopilot gyros that utilize phase-shifting capacitors. Most AHRS systems that output appropriate roll and pitch in ARINC 429 format will also work, ensuring that the AP-3C will have a long future as small airplanes slowly mature from steam gauges. As the industry still waits for a reasonably priced and easily certified electronic attitude and heading system suitable for retrofit, the AP-3C was born ready to dance.
Still, Chelton says its working to acquire a number of gyro options to brand under the Chelton name, in case the airplanes existing gyros cant be used. We caution about using existing used gyros with any new autopilot system, unless they are healthy and low time. Chelton offers similar caveats.We have seen too many poor performing autopilots that are driven by marginal and ill-performing gyros.
Similarly, heading command for the AP-3C is derived from most DGs and HSI systems with either AC or DC outputs, with the exception of the Bendix 800 series gyro. Despite the fact that the Chelton system is relatively low cost, using vacuum gyros to drive it might be a short-term savings that metastasizes into a long-term disaster, as it is with any attitude-based autopilot. Its unlikely that an autopilot output gyro will cost a mere couple of hundred dollars, but could run into the thousands.
The AP-3Cs panel-mounted control console was designed for straightforward operation with a digital display to show which mode is engaged. It has the pushbuttons for engaging familiar autopilot modes of heading command, nav intercept/tracking as well as glideslope intercept and capture, all standard functions of the AP-3C, with the added ability for pilot command of the system, too.
For example, when operating in manual mode, the pilot can command the system to fly the airplane in the current configuration, a spinoff of the familiar big-airplane control wheel steering mode, but with up/down, left /right arrow keys to command turns and climbs.
This might be useful for busy terminal operations when youre manually maneuvering the airplane for a visual approach with your index finger, freeing up your other hand to attend to other cockpit duties. Existing systems allow similar operation in the pitch axis with a pitch command control, but the AP-3C allows manual control in both pitch and roll. When operating the airplane using the AP-3C, the pilot truly becomes a button pusher.
The AP-3C has a certain amount of automation that many owners have been lusting for, such as the ability to couple and fly vertical GPS approaches from WAAS navigators. Using GPS roll and pitch steering, the system can capture and fly a precision GPS approach using ARINC 429 outputs from a GPS navigator for complete flight plan tracking. The system will fly a glideslope, too, using old fashioned-analog raw nav data and for those who know the drill, a localizer back course.
The system also has a basic integral altitude pre-select function, but unlike other systems, it doesnt reduce the rate of climb or descent when approaching the target altitude. In some airplanes, this could result in overshoots of the selected altitude. This might be one detriment of flying by pitch trim alone, in our estimation, but we suspect that software can fix this wart.
Chelton tells us that in a few months, an improved version of the AP-3C will be introduced, which they call a Series II, and it will incorporate audible voice annunciation, a go-around function, dual ARINC channels and the aforementioned brushless servo motors.
The rate-based S-TEC 55X autopilot with automatic pitch trim option has a list price of $20,280, not including a gyro for heading command. At $11,200 not including gyros, the AP-3C is bargain priced and from that view alone and considering its full featureset, it looks promising.
We are finding that once owners upgrade to an ultra-modern nav management system like a GNS480, for example, they expect more out of their old autopilot than it was designed to do and for the untrained, this can jeopardize safety. We recall one operator who couldnt understand why, after several hair-raising attempts, his 30-year-old autopilot wouldnt fully couple to his programmed flight plan, including flying the procedure turn and a GPS approach with vertical guidance right to the pavement. He assumed that since his fancy GPS navigator had the ability to display it all, his existing autopilot would fly it all.
He learned that a modern autopilot is every bit of half the interface in a modern digital avionics suite. In our estimation, the Chelton AP-3C, with its relatively simple operating logic, state-of-the-art circuit design and straightforward installation, should fit well into the gap between modern navigators and aging autopilots overdue for replacement.
What we dont know, of course, is how well the AP-3C will fare in the rough-and-tumble of the real world. Durability is always an unknown with new avionics and autopilots are famous for sucking up shop time chasing gremlins.The AP-3Cs simple design bodes well for maintenance considerations, in our view. Second, Bendix/King, Century and S-TEC have figured out technical service for shops. Chelton is new to the game and has to demonstrate it can run with the big dogs. Its not enough to simply sell a good autopilot. A company has to answer the phone when a shop calls for help.
Also With This Article
“AP-3C Competition and Prices”
“AP-3C Control Set”
-Larry Anglisano is Aviation Consumers avionics editor. He works at Exxel Avionics in Hartford, Connecicut.