Modern autopilots are those that respond to digital signals to track pitch and roll commands from modern GPS receivers. They typically do this in conjunction with an ADAHRS, sometimes in the autopilot like the GFC600 and S-TEC 3100, or in a PFD or EFIS like the Aspen 1000, the Garmin G500, or the G1000 for the GFC 700 autopilot. The Avidyne DFC90 and DFC100 autopilots use an Entegra EFIS or an Aspen 1000 PFD to assist their operation.
Generally, these autopilots are attitude-based rather than rate-based, and they must be certified for each specific aircraft. You will need a compatible combination of autopilot, GPS, and EFIS for system operation. I explained in an earlier article how to use these autopilots in flight but this companion article expands on their capability and modes. Also, it explains how they treat VOR, LOC, and ILS courses, which create analog course deviation signals.
To understand how these units operate let’s first look at an overview of how GPS and VOR/ILS receivers create navigation signals for autopilot tracking, both analog and digital, from the active leg of a GPS flight plan or a VOR/ILS course. Figure 1 shows how analog signals from a VOR or GPS receiver drive an HSI or CDI to create off-course error signals used by an analog autopilot in NAV mode. That autopilot drives the pitch and roll servos to correct this error. Alternatively, you can use the HSI of a PFD to track the course. As you’ll see shortly, the PFD does much more than serve as an alternate HSI display.
Figure 1. Course correction signals from a GPS can be analog (through the CDI or HSI) to the autopilot, or digital with pitch and roll commands to the autopilot (assisted by a PFD and ADAHRS). Analog signals from a GPS or VOR receiver (VLOC and ILS) can also be digitized and used by a digital autopilot with the assistance of a PFD and ADAHRS.
But GPS receivers also create digital pitch and roll commands (GPSV/GPSS) that digital autopilots can track through a PFD and with the assistance of an ADAHRS unit, ignoring the analog commands it also generates. The upper portion of Figure 1 shows that path. It also shows how GPSS converters can translate the digital roll commands for GPS courses into analog heading commands so that an analog autopilot can track these in HDG mode, after selecting (with a GPSS switch) these signals over commands from a heading bug. Note that the GPSS selection can take place in a separate converter box or be built into a PFD or analog autopilot. Vertical pitch commands are also created to track sloped courses, such as the glidepaths from GPS approaches with vertical guidance or VNAV courses created from baro-altitudes at waypoints. The VNAV slopes on those legs can be adjusted through choosing either the vertical speed or angle.
Finally, note the option to send analog signals through a PFD to a digital autopilot. This option allows you to track analog signals. You can track VOR courses and approaches, selected GPS courses, and ILS approaches. Analog course error signals displayed on the PFD (receiver selected with the source switch) are digitized for use by the digital autopilot and to drive the HSI, your primary instrument for course tracking. The PFD (or EFIS), autopilot, and ADAHRS software works in concert to do the signal processing and display. If you’re tracking courses using pitch and roll commands, the PFD is also used to display the course on its HSI.
These modern autopilots have major advantages over analog units, including smoother and more accurate course intercepts, additional modes like IAS, LVL, VNAV, envelope protection for “out of bounds” attitudes or airspeeds, the ability to drive flight director (FD) bars for assistance in hand-flying the aircraft, and perhaps a switch-activated go-around mode (GA).
Digital Autopilot Modes
Different brands of digital autopilots share a lot of commonality of modes, including HDG, NAV, APR, VS, and ALT. Selected altitudes, vertical speeds, and indicated airspeeds can be set from the autopilot or PFD/EFIS. Also, they usually have indicated airspeed and level modes, and may have VNAV tracking, a track mode, yaw dampers, and back course selections. But each autopilot has its own buttons and display format to execute its available modes, so you should read your manual to learn which ones do what on your unit. They can be operated as an autopilot or a flight director. Selecting AP also turns on the FD bars.
Choosing NAV will usually connect to the roll commands, while selecting APR tracks those signals with greater sensitivity and arms a glideslope (GPS or ILS) and activates it on intercept to track the digital pitch commands. All units can track pitch and roll commands (GPSV/GPSS) but may not have specific buttons for these modes. The TruTrak Sorcerer does have buttons for each command and the DF 90 has GPSS. If they don’t have these buttons the modes will be accessed from the NAV and APR selections. The Sorcerer used with a Chelton EFIS is also unique in having the ability to do VNAV climbs. All other digital autopilot and EFIS combinations only have VNAV descents.
Before takeoff you might select an altitude, climb out at a comfortable pitch angle, then activate the AP choice so that your current pitch (PIT mode) and roll attitude (ROL mode) will be maintained. If you have control wheel steering, hold that button while changing to a new pitch and roll attitude, then release. If this pitch results in your normal climb speed, select the IAS mode to hold that speed until leveling at altitude. Speed and vertical speed adjustments can be made with a UP/DN rocker switch or roll wheel. You could also switch to VS mode to continue the climb, but the IAS mode protects against a stall.
At altitude you’ll engage NAV mode to track your flight plan, then later use the VS mode to descend at your desired rate. As you near your destination the task is to transition to an arrival, approach, or visual landing using appropriate modes. For approaches be prepared by briefing the missed, and perhaps to use a Go-Around button to initiate it.
Our first example is the Genesys S-TEC 3100 in Figure 2. Modes are announced in the display (and, usually, also on the PFD). In the display, lateral modes are on the left and vertical modes on the right. Active modes are on the top and armed modes are on the bottom. In this case we’ve selected APR as the lateral mode and since we’re on an approach, GPS Lateral (rather than GPSS) is announced. Vertically, we’re in the altitude hold mode, but GPSV is armed and will automatically become active when the glidepath is intercepted.
Figure 2. Pitch and Roll modes on the STEC-3100 digital autopilot. The lateral mode here is APR (Approach) mode which arms the glidepath and tracks the roll command on approach. Instead of GPSS, as is normal, their terminology is GPS Lateral for approaches (GPSL). In NAV mode it would display GPSS. The vertical mode is Altitude Hold, but the GPSV mode is armed because we’re in APR mode and it will become active upon intercepting the GPS glidepath.
The S-TEC 3100 can also operate in VNAV mode with the right GPS and EFIS combo. Then, a VNV button will be added (a bezel upgrade at Genesys) in place of the MNU button for setups that allow that.
A LVL mode is a safety feature usually included in a modern autopilot. When activated it returns the aircraft to straight and level flight from whatever attitude in which you found yourself. It can be a lifesaver if you become disoriented in IMC conditions. Envelope protection protects against under/overspeed and excessive bank by using an autopilot assist to return to acceptable limits.
As pointed out, you can track a VOR course, a VOR approach, a LOC approach, or an ILS with this autopilot. When you push the NAV button to track a GPS course the display is NAVGPSS. For a VOR course it is simply NAV, and for a LOC course it shows NAVLOC. To intercept a VOR course, after selecting it with the OBS knob, select the intercept heading then push HDG, followed by NAV to arm that mode on intercept.
To fly a VOR or ILS approach, use the APR button at the appropriate time. For a VOR approach the display is simply APR. For a localizer or ILS approach it is APRLOC, but the later arms the GS mode. When tracking the GPS legs leading to these approaches the NAV selection is announced as NAVGPSS but when you change to APR mode this changes to APRGPSSand when you turn onto final it becomes APRGPSL for GPS approaches and arms the GP mode. When you have no vertical guidance, you can set the selected altitude to the MDA then push VS at the faf and select a 500-fpm descent (or whatever is appropriate). This will arm the ALT HOLD mode and later activate it to level off at the MDA.
On LOC or ILS courses, on adding the approach the LOC frequency will be entered into the standby frequency and the OBS for the approach will be set in the PFD. Then, after you activate the frequency, the GPS will switch its CDI to VLOC if you set up “ILS CDI Switching” in setup and the leg to the final approach fix is active. When that switch takes place, the PFD source selection will also switch to VLOC. These very useful timesaving steps are automatic in most setups of a GPS and PFD.
Turning now to Garmin units, the GFC 500 (top) and GFC 600 (bottom) shown in Figure 3 have the familiar modes we described above. In addition, there is TRK on the GFC 500 and BC on the GFC 600. To use reverse sensing on a back course approach, select BC. On the S-TEC 3100 the unit recognizes when it is on a back course, and on pressing APR the reverse mode is automatically engaged, and REV appears on the display.
Figure 3. A Garmin GFC 500 autopilot controller (top) and GFC 600 controller (bottom) with the modes shown here. To track a VNAV course select VNAV (GFC 500) or VNV (GFC 600). The GFC 500 also has knobs you can push to sync or twist to set HDG/TRK or ALTS.
The GFC 600 has built-in ADAHRS while the GFC 500 does not. To follow a specific course use TRK mode. Both units can track a VNAV descent in VNV (GFC 600) or VNAV (GFC 500) mode. This button must be pushed within a certain time limit on the GFC 500 before reaching the Top of Descent (TOD), but the GFC 600 has no time limit.
The GFC 500 needs a separate mode display, while the GFC 600 displays them on its face. These are in addition to annunciations on a PFD or EFIS display. Here again on the GFC 600, active modes are on top and armed modes are below, and lateral modes are on the left while vertical modes are on the right. In Figure 3 the active lateral mode is HDG with GPS (GPSS) armed. The active vertical mode is IAS, and a selected altitude mode (ALTS) is armed. The selected IAS is 125 knots. Selecting NAV will couple to the roll commands and selecting APR will arm a glidepath if on a GPS approach. When flying a VNAV course to a final approach fix, switching from VNAV to GP or GS at the final approach fix may be automatic, but refer to your manual.
On this unit too, the NAV button has multiple uses; on VOR, LOC, or GPS courses the display will announce VOR, LOC, or GPS. Remember to set the bearing with the OBS if it is a VLOC course. If these courses are approaches, push the APR button when ready to intercept the final approach course. Then, for VOR, LOC, or GPS approaches respectively, the lateral display announcements are VAPP, LOC, or GPS. The vertical announcements are VS, GS, or GP. Be sure to also select the appropriate receiver for these using the source select switch on the PFD. If you select a VOR receiver this may also switch the CDI on your GPS. Also, since you’ll be tracking GPS flight legs prior to any approach, check whether your setup will auto-switch from GPS to VLOC, and under what conditions. Does it also switch the PFD source?
There are also digital autopilots from Century Flight Systems and Bendix King that aren’t described here, nor is the GFC 700 used with the G1000 EFIS. But the units we described here are representative of what modern autopilots offer. While they do similar things it’s important to read the manual carefully for your unit to go over the way to execute each of its modes and to learn about special features or conditions. How is a back course approach done? Can your unit auto-switch from GPS legs to VLOC/ILS courses on final? Does it switch from VNAV to GP or GS at the final approach fix? These are the kind of details that are specific to your configuration (AP, PFD, and GPS combo).
These modern autopilots offer significant advantages over the analog autopilots of the past. Their expanded mode set allows the autopilot to fly the aircraft from shortly after takeoff to near touchdown, allowing the monitoring pilot to better concentrate on navigation, weather, or emergency challenges. Significantly, the safety features of the LVL mode and envelope protection can be lifesaving and add comfort during the flight.
Dr. Thomassen has a PhD from Stanford and had a career in teaching (MIT, Stanford, UC Berkeley) and research in fusion energy (National Labs at Los Alamos and Livermore). He has been flying for 64 years, has the Wright Brothers Master Pilot Award, and is a current CFII. See his website (https://avionicswest.com/) on all his manuals plus numerous articles on GPS and other aviation topics.
