One example of such a full functioning autopilot is the S-TEC 55X shown above. It’s horizontal modes include HDG, NAV or APR, and GPSS.  Vertical modes that control pitch are ALT, GS, and VS.  This autopilot also has CWS (control wheel steering). 

Perhaps you learned about autopilots some year’s back from tracking a VOR or LOC course. Analog signals representing deviations from the localizer and glideslope, as seen for example on your HSI, told the autopilot which way to turn or to climb/descend. Analog autopilots still dominate the market, but there are now digital autopilots that don’t even look at the HSI information to tell the servos how to bank, and digital pitch control is coming. This overview of autopilots is prompted by the new vertical guidance GPS approaches, and to clarify autopilot use for tracking GPS courses. These general principles are illustrative of what autopilots do, but a word of caution is in order.  Your model or brand may not work exactly as described here, so for individual differences be sure to rely on the manual for your unit. What follows is specific for the S-TEC 55X. 

If you have a DG (no HSI), you must set the heading bug to the desired course.

Your position is the center of the HSI, and the split-out center portion of the yellow needle represents the selected radial from that VOR. In the NAV or APR mode, the angular deviation between the course and your present radial, and a right or left determination, commands a turn in a deviation-based autopilot to reduce the error (close the split). The turn coordinator gives input also, to limit the rate of turn.          

The APR mode has greater needle sensitivity and can be used for VOR or GPS tracking if you push APR rather than NAV.  When a localizer frequency is chosen and NAV is selected, APR will light up as well since localizer signals are more sensitive (less angular deviation for full scale deflection). This autopilot supports dual mode operation, pushing NAV and HDG together.  You then track the heading set by the HDG bug until intercepting the VOR (or GPS) course selected with the OBS, at which time the HDG light goes out and you remain in NAV mode. Again, if you have a DG, set the HDG bug to the new course.

Several vertical tracking modes are available with analog autopilots.  Altitude hold is one, as are VS commands to fly a given rate of climb or descent. With control wheel steering (CWS) you can suspend roll and pitch control by pushing a CWS switch on the yoke, and with both a roll and pitch mode activated, your autopilot will maintain current pitch and bank values on releasing the CWS switch.

On an ILS, which has vertical guidance, the glideslope can be tracked if it is captured.  It will be armed automatically in the 55X if you have a valid glideslope signal (no flag), have the NAV receiver tuned to the localizer, are in the NAV/APR/ALT modes, are sufficiently below the glideslope, and are within half a deviation of the localizer course --- all for 10 sec.  When armed, GS is illuminated and when captured the ALT illumination goes out.  If you have to hold at the OM, you can disable GS arming by pushing APR twice while inbound to the OM (GS will flash, ALT and DSBL come on). To re-establish, push APR again. If above the glideslope, you can manually arm it for capture by pushing ALT when in that mode

If you make a GPS flight plan, analog signals representing your cross track error (XTE), and whether left or right of course, are sent to the HSI (if you select GPS with the CDI key on the GNS 430/480/530 devices).  It’s also necessary to set the OBS course on your HSI (or HDG bug on the DG) to the DTK of the active leg, since both pieces of information are acted on --- XTE and the difference between your OBS setting and the DTK. Then, operation is no different than tracking a VOR course in the horizontal plane. Analog signals from the HSI to the autopilot command turns to reduce the deviation.

With the recently certified v2.0 software for the GNS480, it sends signals to the glideslope indicator so you can see if you are tracking the “GPS glideslope” associated with LPV and LNAV/VNAV GPS approaches. It also sends a “discrete” signal to arm autopilots, equivalent to arming the ILS glideslope. So, with the 55X for example you’ll fly these GPS approaches just like an ILS.Put the autopilot in the NAV/APR/ALT modes when a GPS approach with vertical guidance is selected, then sit back and watch it capture the GPS glideslope and fly it right to the runway! There are around 1800 vertical GPS approaches at this moment so GPS receivers with WAAS and certified software are now opening up very exciting new opportunities for precise approaches at airports without an ILS.

At the moment you can’t legally couple your S-Tec (or any other autopilot)  to the HSI for this operation because it is not described in their manual and approved by the FAA.  Whether or not autopilot manufactures amend their FMS (Flight Manual Supplements) to fly GPS approaches with vertical guidance remains to be seen.   

There are some new kids on the block, and they don’t play by the old rules.  Digital autopilots receive digital signals directly from your GPS navigator over ARINC-429 standard (two-way) digital links.  Analog information from the GPS to the HSI is still there for you to monitor, but the digital link drives the autopilot. When you make a GPS flight plan the active leg is defined and your position relative to it is known. So if off course, the GPS unit can process that information to compute a smooth closure track and command a specific bank angle at all times.    

The required bank angle computed by your GPS is called a roll steering command, and a digital autopilot can accept those commands directly from your GPS navigator. The autopilot then directs the servos in the wing to accomplish them.  This is known as GPS Steering, or GPSS.  On an S-TEC 55X autopilot, for example, you engage the GPSS mode by pushing the NAV button twice. The analog HSI signals are used for your information, but not for autopilot control in this mode.

To summarize, you have a choice of the analog mode if you push NAV once, and the digital mode if you push NAV twice.  In the NAV mode you operate through your HSI, and need to set the OBS pointer (or HDG bug on a DG) to the DTK of each leg.  In the GPSS mode the HSI is ignored for control, and the autopilot relies on the GPS computed roll steering commands (on a 429 digital link) to send bank commands to its roll servos.

On some installations, you may not need to switch your GPS unit to the GPS mode (depends on how it was wired). But if you stay in the VOR mode, the HSI information comes from your active VOR, while your autopilot is looking at GPS course and position data.  For that reason, some installations do not allow GPSS operation unless you switch to GPS with the CDI button on your GPS navigator.

Some autopilots have a switch to put you into the GPSS mode from the HDG mode, and back again.  In that circumstance, when in GPSS mode, pushing the NAV button has no affect on autopilot control.  It does send analog signals to the HSI, for you to monitor, but you have to go back to HDG with that GPSS switch before you can engage the NAV mode.

Dual mode operation with HDG and GPSS is available in the S-TEC 55X.  With this operation, you pick your intercept heading with the HDG bug, and the course is intercepted by the GPSS mode when within the criteria for that intercept. 

There are several digital autopilot manufacturers working on the vertical equivalent of GPSS, known as GPS-V. None are certified as of this writing, but with WAAS approved receivers already certified in the Chelton Flight Logic EFIS and the GNS480, it is only a matter of time before your autopilot could be fully digital.

The new Chelton AP-3C autopilot below now accepts GPS-V, but the GPS receivers are not yet certified to send these signals on the 429 data link. The Garmin GNS480 group is working on providing digital signals from their unit. For the experimental aircraft market, TruTrak has nearly completed their Sorcerer unit with GPS-V, and since it will not be certified it can be used for experimental aircraft as soon as they complete the hardware and software and bring it to market.

Digital autopilots are the brave new frontier. Capability available to us soon, together with WAAS receivers, will afford new and exciting opportunities for assistance in flying our aircraft on GPS approaches.