Since the invention of the wing leveler, most of the roll control done by general aviation autopilots was either wings level for heading hold or heading command. We didnt use the VOR or NAV modes very much.
The reason is simple. They didnt work very well. VOR signals arent straight but scallop and snake slightly through the ether. Even the best VOR receivers werent all that accurate, although the newer digital models may be. The result was that when needle tracking, the airplane would occasionally dip a wing if not roll into an impressive automatic Dutch roll. After we discovered with loran, if the avionics shop connected the CDI to the box, most pilots still didnt use nav tracking function much, more out of habit and ignorance, since loran solved the scalloping problem in most autopilots.
Direct-to navigation was possible, but not practical with the limitations of ATC routing. Experienced instructors who understood the limits of long-distance direct-to navigation were rare.
Its All Changed
Then along came GPS. The rock-solid signals, reliable accuracy and direct-to navigation made it a natural for autopilot interface. But the problem is this: The autopilot NAV modes were designed for linear meter movements, the kind of needles we have flown for the past 50 years.
They worked fairly well in a Band-Aid-and-patch sort of way, but even later generation autopilots were quite ready for the marvels of GPS enroute and non-precision approaches, at least in little airplanes.
For some time, corporate jets have had true autopilot interface with their FMS and long-range nav systems. On a North Atlantic track, a few degrees here and there can put you in Boston instead of Gander, which is kinda hard on the fuel planning.
So, the FMS and autopilot share heading and course deviation on something called composite roll steering. Roll steering provides commands not only to maintain the course, but also accommodates turn anticipation and processes both lateral deviation and course information. In short, it reacts to multiple stimuli such as winds, as well as the flightplan, and flies like any good pilot, ahead of the airplane.
Beginning with the first round of IFR-approved GPS navigators certified under TSO C129, GPS units have been required to have turn anticipation, which essentially smoothes out a course change so that the needle sees it as a smooth curve, not an abrupt angular change, as with conventional VOR.
Todays GPS navigators, along with being the most accurate systems, have capabilities most of us dont even see. With databases full of waypoints waiting to be visited, calculations of winds aloft and great-circle circumnavigation, todays navigators are clever. But until recently, autopilots simply werent able to keep up, manhandling the airplane through the electronic equivalent of a buggy whip.
S for Steering
S-Tecs new GPSS does the trick, however. The S in GPSS is for steering and the system is an add-on feature that smartens up S-Tecs already well-regarded autopilots. The GPSS is smart enough to translate the roll-steering commands from the GPS into a heading error command for the autopilot.
We all know that the bank angle for a standard rate turn increases proportionally to airspeed. General aviation autopilots dont have an air data computer that knows the current IAS, or even true airspeed. So autopilots are designed to use a turn rate (derived from bank angle) compromise targeted at the critical airspeed, approach.
S-Tecs GPSS is able to compute the desired bank angle based on more than just heading error. In the rate-based S-Tec autopilots, it can actually plot the necessary course over the ground to transition from one leg to the next.
Usually, autopilots are designed with a fixed turn rate and bank angle that favors approach airspeeds. By having the actual bank angle and speed-made-good available, the autopilot can gauge precisely the amount of turn needed in any configuration or situation without slamming the bank to the stops, which was common with needle trackers. Your autopilot and GPS work together to fly smoothly and efficiently.
What is this magic called GPSS? Its an electronic circuit thats either embedded in the newest S-Tec System 40x and System 50x, or built into a stand-alone unit to be added into other S-Tec systems.
The ST-109 GPS Steering Converter ($695 list price) is a deceptively simple black box, or more precisely, a gold box about the size of a pack of cigarettes.
The GPSS interfaces to the GPS system via an ARINC (Aeronautical Radio Incorporated) 429 serial data line. This interface contains digital words that contain lots of information generated by the navigation system, similar to the RS232 data bus that the GPS uses to talk to moving maps. The difference is that the RS232 GPS information lacks certain critical information that the ARINC 429 has,such as groundspeed and bank-angle commands that are part of the composite roll steering signal.
GPSS computes the commands it needs to send to the autopilot, based on the desired GPS course. The autopilot is actually placed in the heading command mode, but instead of John Q. Pilot yanking the bug, the GPSS steers the autopilot.
The beauty of the GPSS System is that it will work with any S-Tec autopilot. The disadvantage is that it only works with a few navigators, the ones that send the necessary words in their position outputs.
If you think youll be able to hook a GPSS up to your STS loran and experience the joy of composite roll-steering flight, youre going to be disappointed. Until GPSS came along, the GPS manufacturers didnt give much thought to having the roll steering-specific data available.
If theres nobody around to listen, why bother to send it? Heretofore, the only autopilots capable of roll steering were cabin class corporate jets, although the Bendix/King KLN 90B had and has roll steering capability as does the high-dollar Dzus-rail KLN 900.
At press time, Garmin was the best represented of the GPS manufacturers. All of its panel-mounted GPS-from the 150-series on, have ARINC 429 output capability. UPSAT/Apollo has a roll-steering unit, too, the NMS2101. But its newer stuff lacks this feature. (For now, stay tuned, however.)
S-Tec doesnt specifically recommend or even follow which GPS systems have 429 outputs. In GA, its rare. But its getting less rare. Fortunately, the single most popular system today, the Garmin 430, does have the required serial data.
As we said, the GPSS circuitry is available in both an entire system and as a stand-alone system to be integrated with existing S-Tec autopilots. This is in keeping with S-Tecs philosophy of building-block design.
There are three versions of the stand-alone GPSS. Two are self-contained, a vertically mounted and a horizontally mounted configuration. If space is a factor, the remote version puts a 1-inch square button on the panel, with the rest of the system buried in the aircraft.
The ST-109 GPSS system is extraordinarily simple to use, assuming you have the right autopilot and GPS on either side of it. Push the button to switch from conventional heading mode to GPS Steering and the autopilot, in heading mode, will fly the smooth, corrected path from here to there. Theres a LED to indicate in which mode the system is operating. And thats about it.
Since the autopilot is responding directly to the GPS, the pilot doesnt have to set up an intercept, or get established on course. The GPS is computing the maneuvers needed to get on, and stay on course, by sending a bank angle command to the GPSS.
According to S-Tec, the typical bank angle created by the GNS430, in enroute mode, is about 15 degrees, a bit less than standard rate at Bonanza-type speeds and a very smooth ride indeed. Since the S-Tec is a rate-based system, it automatically translates the commands into a turn rate for the servos and turn coordinator gyro. A roll-steering autopilot deviates from the standard version because it uses the necessary roll authority at the appropriate time. Since the S-Tec is a rate-sensing AP anyway, its well qualified to fly using the rate of turn created by the GPS, without need for gyro input.
System 50X and 40X
For those in the market for an autopilot upgrade to go with your GNS 430 installation, S-Tec has GPS versions of their popular radio rack-mounted autopilots. The System 40X ($5495, list) is a roll-axis only autopilot with HDG (with optional DG), conventional NAV (VOR), localizer approach and back-course localizer.
The System 50X ($7995 list) adds the pitch axis, with altitude and vertical speed hold functions, which are modifiable from the CWS (Control Wheel Steering) switch.
Activation of the integrated GPSS function is simple, just push the NAV button twice. The autopilot will indicate the GPSS mode, and use the 429 input to drive the controls.
GPSS represents a subtle but significant advancement in the state of little airplane autopilot technology. The concept is elegant and simple.
The single deficiency is the lack of nav systems that have ARINC 429 output. That limits the applications, not because of S-Tecs shortsight, but because the GPS manufacturers didnt have that sort of turbine (not like a swami, but a jet) vision.
It isnt the GPS manufacturers fault, either. ARINC 429 is like Esperanto. Fifteen years ago, the avionics industry thought that all avionics would speak 429. Yet the transition from maritime loran to marine GPS to aviation GPS brought the consumer electronic serial data bus RS232 or NMEA 189 to market, and left the complicated and expensive ARINC standard for the airline and corporate avionics. These are the airplanes that have the roll steering commands.
What remains is for companies to put this information onto the existing bus structure, and for S-Tec to allow the ST-109 GPSS converter to listen to either the ARINC or the RS 232 data bus.
Contact- S-Tec Corporation; One S-Tec Way; Mineral Wells, TX 76067; 1-800-827-7832; www.s-tec.com.
by Gary Picou
Gary Picou is an Aviation Consumer avionics editor. He works for PSEngineering, an intercom and audio panel maker.