Tips and Tricks #32
Flying with a Digital Autopilot

Keith Thomassen, PhD, CFII

In an article here several years back, titled "Navigation gone Digital" (, the general impact of various digital navigation devices and the capabilities they encompass were described. This article deals specifically with using digital autopilots in typical flight situations, and illustrates the things they can do to ease pilot workload. It should be noted that digital autopilots have been available for over a decade from Garmin, TruTrak and Avidyne, but the combination of newer GPS navigators, EFIS systems, and digital autopilots from Garmin, BendixKing, and S-TEC have given their acquisition a "shot in the arm".

Digital autopilots use the digital pitch (GPSV) and roll (GPSS) commands from GPS navigators, unlike analog autopilots that cannot use pitch commands, and must have the roll commands first converted (sometimes internally) to heading commands to follow them in HDG mode, often by selecting them with a GPSS converter switch. Modes like IAS (indicated airspeed) and VNAV (vertical slopes from baro-altitude waypoints) are also features of these autopilots.

We'll use the S-TEC 3100 and GFC 500 autopilots in the examples here, as these are "representative" autopilots. There are several other brands, but while those have many similarities they may also have some differences or omit some modes. The S-TEC requires the Aspen EFIS as an interface to the GTN 750 used here, and the GFC500 requires a G5, G3X, or G500 EFIS to interface.

These autopilots also offer other modes of operation to assist in different flight situations. Altitude preselect is a common one, as is the ability to climb or descend at constant vertical rate or indicated airspeed. You may be able to follow a specific magnetic course over the ground (TRK mode), reverse the sensing of a back course approach (BC mode), or return to level flight (LVL mode). Some units offer envelope protection in the event that prescribed pitch or roll angles are exceeded. Enroute baro-VNAV courses from newer navigators can be tracked if your EFIS displays that vertical course on a VDI (vertical deviation indicator) as it does for GPS or ILS vertical courses. (This may not work with mixed manufacturers of the GPS/EFIS/Autopilot combos).

Generally, you can arm a mode while flying another mode. Typically, flying HDG mode towards a GPS flight leg, the course of that leg is "armed" by engaging the NAV button on the autopilot, and the automatic transition is made when you get close to the course. On the G5 display in Fig 1 (right), autopilot modes are displayed at the top of the unit. The division of lateral modes on the left of the

Figure 1

Figure 1. The S-TEC 3100 (top left) and GFC 500 (bottom left) control units are shown here, along with a G5 PFD (right) with autopilot mode annunciations on top.

AP (or Flight Director, FD) selection in the middle, and vertical modes on the right, is typical. That same left/right mode presentation is shown on the two autopilots in Fig 1 (left); the S-TEC 3100 on top and the control unit for the GFC 500 on the bottom. The G700 digital autopilot used with a G1000, and the GFC 600 autopilot use the same convention.

On the G5, the HDG mode is active (green) and the GPS mode is armed (white). On the right, the ALT (hold) mode is active, while a GPS glide path (GP) is armed. Similarly, if you set a preselected altitude in the autopilot, then engage IAS or VS, you will climb in those active modes while ALT is armed. Garmin uses ALTS for selected altitude and ALTV for a VNAV altitude, and a VNAV vertical course is called a VPTH (vertical path) and starts at the TOD (top of descent) point. Garmin navigators only provide VNAV descents, but Chelton EFIS systems create both VNAV climbs and descents. Currently, the S-TEC does not have the VNAV mode although it is a planned upgrade.

On the S-TEC 3100 in Fig 1, and on the GFC 600, mode annunciations are on the faceplates. Again, lateral modes are on the left, and vertical modes are on the right. Also, active modes are on top, and armed modes are below them. In Fig 1, the GPSV mode is armed in the 3100. Modes can also be shown on an Aspen, G3X or G500 EFIS, or on a separate unit like the GI 285.

Two other typical features are CWS (control wheel steering) and GA (go around) modes. If the autopilot is engaged and you want to temporarily disengage, use the CWS button on the yoke to do that. While depressed, you can change the roll/pitch attitude of the airplane, and when you let go the autopilot will engage in the new ROLL and PITCH attitudes. On the S-TEC 3100 and GFC 500, when the GA button is depressed to "go missed", the autopilot disconnects and commands the FD bars to a wings-level new pitch attitude for your aircraft.

So let's fly a typical flight; takeoff and climb to a selected altitude, select a lateral course, then cruise to a destination and shoot an approach to a landing and a missed approach. After start up, turn on the autopilot and trim master switches and allow the ADAHARS (in the S-TEC 3100 and GFC 600) to initialize. Before takeoff, be sure the controls are free and correct, then turn on the autopilot and do the lateral and vertical mode checks spelled out in your manual. Now, preselect an altitude.

After takeoff, you should climb to about 500' before engaging the autopilot (or FD if you want to hand fly). With the aircraft "cleaned up" and a climb attitude held, engage the AP to hold this attitude. It now shows Pitch and Roll as the vertical and lateral modes. Next, sync the HDG bug to current heading and then engage HDG mode on the autopilot. Then, select VS or IAS to climb to the selected altitude. Use the thumb wheel or Up/Dn buttons to adjust the climb rate or speed. Finally, select a lateral mode (or modes) to join your flight plan route.

To descend enroute and in the terminal area, use the altitude pre-select and the VS rate while tracking the GPS course in NAV mode. The 3100 will announce NAVGPSS, while in this mode. If you're tracking a VOR course it will simply show NAV. If you select APR before reaching the final approach course it will say APRGPSS, until reaching final, then it says APRGPSL, (L for lateral). If the approach were an ILS, it would say APRLOC. These different announcements for the NAV mode are very transparent; a nice feature of this autopilot.

Let's describe in more detail the steps for approaches into nearby Marysville (KMYV) from my home airport of KGOO (elev. 3158'). They have a RNAV 32 (LPV with 250' minimums) and an ILS 14 with 200' minimums. The flight plan is shown in Fig 2 on ForeFlight, along with the flight plan in the GTN 750 consisting solely of the approach. There's no need to make a flight plan with origin and destination on the 750, it can be a procedure all by itself. To do that, select PROC then Approach and fill in KMYV, RNAV 32 GPS LPV, and the transition at OTRUW. Now, Load and Activate the approach to proceed D-> OTRUW from KGOO, climbing to 4000'.

Figure 2

Figure 2. A flight plan from KGOO to KMYV on Foreflight (left), and the Active flight Plan (right) consisting solely of the RNAV 32 flight legs. Activating this approach creates the leg D-> OTRUW.

On the ground at KGOO we'll enter 4000' for a preselected altitude, set the HDG bug to runway heading, ensure that the 750 CDI selection is GPS, then climb 500' and engage the AP in HDG and VS modes. After a left turn, re-center the course by going D-> OTRUW, then engage NAV mode. On the 750, VNAV target altitudes are shown next to each waypoint on the approach in Fig 2. We continue to OTRUW at 4000', but shortly beyond that the 750 map shows a TOD point, which is where the first VPTH course begins, descending to VUJOR at 2000'. To couple to that slope, push the VNAV button on the GFC 500 to arm VPTH before reaching the TOD.

After descending to 2000' and turning the corner at VUJOR, engage the APR mode, which is necessary to arm the glide path or glideslope on all autopilots. Continuing on the VPTH to the faf, the GFC 500/600 will automatically switch to the GP mode between VUJOR and the faf if you have enabled auto switching on the 750 (On the Home page, go to System, then Setup). On the 3100, the descents should be made in VS mode to hit the target altitudes, and when APR is selected after VUJOR, the GPSV mode is armed as shown in Fig 1. If you are above the glide path, use the VS mode to descend to it, because when the GPSV mode is armed it can capture a GP or GS from above.

At the DA push the GA button to disconnect the autopilot, add power and pitch to the FD bars, raise the gear, then the flaps. Passing the map, choose to sequence to the missed approach on the 750 (Activate GPS Missed Approach), and after established in the climb engage the AP in pitch and roll modes in present attitude. Sync the heading bug to present runway heading and engage HDG mode. Set the altitude to the missed approach hold, then engage VS mode.

Since we're initially flying along the outbound ILS 14 course, we request vectors to the ILS. On the missed approach to RNAV 32 we can add a new approach without eliminating the remaining legs of the current approach. With the autopilot doing the flying, we select and "Load and Activate" the approach, which makes the VTF leg active. These busy moments are when an autopilot earns its keep. Follow ATC instructions using the HDG bug for vectors and use altitude preselect and IAS or VS to climb to assigned altitude.

The new approach is shown in Fig 3, after selecting ILS 14 and Vectors. On the left screen, after initiating the missed, we're on a 350° vector and our current active leg is vtf 141° -> CULUL as shown on both screens. Note that sequencing is suspended, as shown by the magenta SUSP next to the magenta GPS. On the right, the approach legs are shown including two of the on-line waypoints before the faf; ZUBUD and FONTS. Note on this screen that the MYV ILS frequency (110.50) has been automatically entered into Standby. Now is the time to switch it to Active, and select the ILS CRS of 141°.

Figure 3

Figure 3. On the missed approach at KMYV (left) after Vectors to the ILS 14 has been added to the flight plan (right).

Although sequencing is now suspended, after being vectored to within a 45° cone of the final approach course, sequencing will automatically be restored. During the vectoring process, preselect the assigned altitudes and use VS to climb or descend to those altitudes. As you're being vectored around in HDG mode, arm the APR mode as you're joining the final approach course. Now, with auto switching on the 750, the CDI will change from GPS to VLOC on entering final. When the APR mode becomes active, the GS is armed and you'll intercept it and track the LOC inbound in those modes.

These procedures demonstrate the utility of a modern digital autopilot. They significantly reduce pilot workload at a time when communicating, evaluating weather and planning options, are extremely important to the safety of flight.