Staying centered with GPSS

Old analog autopilots, their idiosyncrasies notwithstanding, typically can fly a heading. Twirl the HDG bug on your heading indicator or horizontal situation indicator (HSI), push HDG mode on the autopilot controller, and the device keeps you pointed in that direction. To track a course, confirm that the connected course deviation indicator (CDI) is properly set and select NAV mode. While in NAV mode, a legacy autopilot monitors the deflection of the VOR, LOC, or GPS needle and commands banks to keep you on the centerline.

That setup works well when you’re flying an airway, direct between fixes, and the straight segments of the initial or intermediate phases of an approach. But in NAV mode, older autopilots can’t follow curved paths, such as DME arcs, the turns at the ends of holding patterns, or radius-to-fix (RF) legs now associated with some RNAV (GPS) approaches. To fly those legs with an older autopilot, you usually revert to HDG mode and twist the HDG bug to maintain the charted paths.

When the CDI is set to GPS, newer autopilots, built with GPS in mind, track GPS courses in NAV mode (typically labeled LNAV in business jets and airliners). These autopilots have built-in GPS steering, or GPSS, sometimes called “roll steering.” When tracking the path set by your GPS, these autopilots don’t monitor deflections of the CDI. Because they instead follow the magenta line, these autopilots can intercept and track both straight and curved paths that are part of the active flight plan or procedure.

Some avionics manufacturers offer GPSS interfaces to make old autopilots smarter. Instead of interpreting CDI deflections, the GPSS interface detects deviations from the active GPS course and sends heading commands to the autopilot to intercept and track those courses. To take advantage of GPS steering with an older autopilot with a GPSS interface, you must set it to HDG (not NAV) mode. Typically, when you activate GPSS (see the pilot guides for the devices installed in your panel), the HDG bug on an electronic HSI becomes hollow. While in GPSS mode, you can spin a HDG bug at will, but the autopilot won’t follow the bug—it’s getting HDG signals via the GPSS interface, not the bug.

GPSS can still lead you astray, especially when flying approaches, and some of the new-generation autopilots come with limitations that can surprise you even when you’re flying a conventional procedure.

When you set the CDI to “green needles” (VOR or LOC), NAV mode, autopilots revert to watching for and correcting deflections of the CDI, sometimes using GPS information to complement the raw navaid data. In fact, the Garmin GFC 500 requires a valid GPS position to track a VOR or LOC course or an ILS glideslope. As the flight manual supplement for the GFC 500 warns: “If GPS position data is lost while the autopilot is tracking a GPS, VOR, LOC, or BC course, the autopilot will default to roll mode (ROL).” In ROL, the autopilot just maintains the current bank angle. It won’t keep you on course. If you lose the GPS signal, you can still hand-fly a conventional approach, but the autopilot won’t be available.

Selecting the correct mode on your autopilot becomes especially important when flying an approach. When you’re established on the final approach course of a procedure without vertical guidance, you could continue using either GPSS or NAV mode. (Note, however, per Aeronautical Information Manual paragraph 1-2-3, that you can’t use GPS or GPSS to track a localizer between the final approach fix and the runway.) To manage the descent, you can use vertical speed (VS) or indicated airspeed (IAS or flight level change) mode if your autopilot supports those functions.

If you want to follow an ILS glideslope or vertical guidance on an RNAV (GPS) approach, however, you must select APR mode (or its equivalent) on your autopilot and ensure that you have set the CDI to the correct source—LOC/VOR or GPS. An autopilot will intercept and track vertical guidance only when it’s in APR mode.

With many autopilots, switching the CDI nav source forces the autopilots to revert to ROL. That’s why it’s a good idea to center the HDG bug and temporarily switch to HDG mode before you change CDI sources. Select NAV or APR mode again after you confirm that you’re ready to have the autopilot follow courses.

Finally, note that some integrated systems can fly HDG legs of active procedures when the autopilot is in NAV mode. For example, if a departure begins with a HDG leg to an altitude or to intercept a course ahead of the aircraft, I can set my GFC 600 to NAV mode before takeoff and follow the flight director cues to maintain the charted until I reach the minimum autopilot engagement altitude—800 feet agl. Above 800 feet agl, the autopilot flies the correct heading until I join a published track, where it automatically begins tracking courses. A similar setup works during a missed approach that includes an initial HDG leg.

I can also use NAV mode continuously when flying an approach that involves a HDG leg, such as the ILS Runway 24 at Bowerman Airport (HQM) on the Washington coast. That procedure has HDG legs from ULESS and SOUPY. When I am cleared Direct to either of those fixes and then for the approach, I leave the autopilot/flight director in NAV mode, and the GFC 600 first flies the appropriate HDG leg and then provides precise lateral guidance through the remaining intermediate segments. I confirm APR mode near NEYDI to capture and track the LOC and GS.

Bruce Williams is a CFI. Find him at youtube.com/@BruceAirFlying and bruceair.wordpress.com