A number of new digital autopilots (see examples in Figure 1) are now coming into the market, including those by Garmin and Genesys (formerly S-Tec). TruTrak and Trio autopilots, originally legal only for experimental planes, are now receiving approval to put new digital units in some certified aircraft. This flood of new digital autopilots, if nothing else, makes you aware of the revolution underway in navigation, which began slowly but has now more rapidly “gone digital.” What are these changes, and what do they mean to you? They deliver smoother and more accurate course tracking, as will be explained shortly, and simplified operation involving only autopilot mode selection.
To understand this revolution, let’s go back to the airway system of navigation using VORs, first deployed in the middle 1940s. Tracking an airway meant tuning in the station, selecting a course with the OBS selector, and flying to center the CDI (or HSI) needles. If you had an analog autopilot with a NAV button, it tracked the course by reading the displacement of the needle and sending a signal to the analog servos to zero out the deviation. At each change in direction of the airway you had to reset the OBS selection.
With the introduction of GPS, circa 2000, this same basic concept was used to generate an “error signal” between your satellite-derived position and the desired position on an active GPS flight leg for display on a CDI/HSI. You still need to set the course for the new segment (unless “autoslew” is available) and an analog autopilot could again drive servos to null this deviation. But it wasn’t long before someone figured out a different way to skin that cat.
A VOR course can be either To or From the VOR; your choice. But a GPS flight leg has a unique direction and there is no To/From option. So, when off course, your GPS receiver can determine which way to turn for tracking the “To” course. A calculation is made by the GPS at each moment to determine how much bank angle the autopilot should command to smoothly intercept and track the course. This “roll steering command” is digital, so one might consider this invention to be the initial step in the digital navigation revolution.
The beauty of this solution is that the command can be sent directly to a digital autopilot—bypassing the CDI, which has always been the middleman in autopilot operation (between the navigation receiver and the autopilot). No OBS to set, or need for autoslew. (A signal is still sent to the CDI for your viewing, since it is your primary navigation instrument.)
These roll commands are a much better way to track the course than the old analog way. GPS generated roll commands can also be read by an EFIS and then sent to the autopilot (the Chelton does this). The autopilot reads these roll commands and sends signals to the servos (which now are also digital) to generate the required bank angle. The whole process makes the intercept and tracking process much smoother.
The difficulty was that, early on, digital autopilots in the GA market were rare. Thus was born the GPS Steering (GPSS) convertor, which converts this digital signal to an analog heading command to use with your analog autopilot. You simply select the HDG mode on the autopilot, and flip a switch to choose this converted signal instead of the normal command from a heading bug. Some autopilots, like the S-Tec 55X (and now many others) have this convertor built in and the switch you flip is also in the autopilot (on the 55X, for example, press NAV twice.
Since your CDI (or HSI) is your primary navigation instrument it also needs to know your course deviation. In the GTN 750, roll commands can be converted (by that GPS) to an analog command for the old steam-gauge CDIs and HSIs. But certain EFIS units like the Garmin G500/600 or G3X now accept digital commands for the HSI displays on the PFD – so the HSI has also gone digital.
But now comes the interesting part. Why not create digital pitch commands to track flight legs that have a slope? You can, and we do. The question, of course, is where and how are these slopes created? For now let’s leave aside the final approach course leg, which does create a glideslope, and look at enroute flight legs. For those, Vertical Navigation (VNAV) systems have been around for a long time, way before GPS arrived. Sloped courses can be created from a waypoint having an altitude attached to it, using a so-called baro-VNAV system. Figure 2 shows the profile of an approach into Tucson that has multiple step-downs prior to the final approach segment. They can be tracked by a digital autopilot with VNAV capability (to track pitch commands) if the GPS or EFIS unit tied to the autopilot sends the appropriate signals.
At this time, only the Chelton EFIS or the G1000 can incorporate these VNAV altitudes into a flight plan and track them with a digital autopilot; a TruTrak Sorcerer (or DigiFlight with Vertical tracking) works well with the Chelton, and the G700 Garmin autopilot tracks VNAV descents set up in the G1000. With a Chelton (developed about 15 years ago) you can also create VNAV climbs, but no other system at the moment does that (I’m ignoring FMS systems used in transports and many business jets).
VNAV altitudes are either added automatically with Procedures, or can be attached manually to a waypoint by the pilot. I understand that Garmin is pondering the addition of VNAV to the GTN 750, since vertical tracking with digital autopilots is “here and now.” What’s lacking is more GPS systems that create enroute pitch commands from VNAV altitudes, i.e. provide a place for those VNAV altitudes in the flight plan legs.
An ILS system aside, the other way to create a sloped course is to define a GPS altitude at all points on a flight leg. This is the basis of APV approaches (approaches with vertical guidance), whether LPV or LNAV/VNAV, or adding vertical guidance to LNAV and LP approaches. So, are these flown using analog or digital commands? Does the autopilot look at the glideslope deviation on the CDI and command a correction, or get a digital pitch command from the GPS (or an EFIS)? Depends on your equipment, but even the venerable GNS 430 or 530 will send a digital pitch command if you can accept it. So, if you have a fully digital autopilot you will necessarily be tracking pitch commands and must have the compatible navigation equipment (GPS or EFIS) that sends those commands.
Looking at the buttons on the digital autopilots in Figure 1, it is not transparent what is being selected by each of them. Specifically, what do the NAV, APR, and VNAV buttons do? Tracking a GPS course in NAV mode means it is accepting the roll steering commands for lateral, while APR is used prior to the FAF to tell the unit to choose the pitch commands from the approach. VNAV is generally used to track baro-VNAV courses.
Finally, how do these digital autopilots track VOR or ILS courses? They are defined by analog signals, from VORs or the localizer and glideslope antennas, to a VLOC receiver. The G1000 and GTN 750 use these analog signals to compare your position relative to the course (level or sloped) and then the GPS (or EFIS receiving these signals) creates digital pitch and roll signals for the autopilot. They also send digital error signals to the HSI on an EFIS (like the G500, G1000, and G3X) so you can monitor your tracking of the approach.
We’ve progressed far from the original VOR tracking and ILS guidance of yesteryear, and are emerging into an admittedly more complex digital tracking world. In this world, compatibility of systems and a thorough understanding of “what’s going on” and “how do I do this” is vital. But the rewards are a simplified system (the CDI is out of it!) and much more accurate and smoother tracking.
Keith Thomassen, author of GPS manuals and workshop trainer, earned his first pilot’s certificate in 1958 and pursued a professional career of scientific research and teaching while expanding his aviation interests. With a PhD in Electrical Engineering from Stanford University, he carried out research and teaching at Stanford, MIT, Los Alamos National Laboratory, the Lawrence Livermore National Laboratory, UC Berkeley, UCLA and Harvard. He is a Flight Instructor with Airplane, Instrument and Glider ratings. Contact him at www.avionicswest.com.