When shopping for a system, youll need to get your axis terminology correct. A single-axis system controls the roll axis of the aircraft only, essentially leveling the wings and driving the ailerons to a selected heading and, if interfaced, tracking or intercepting nav and GPS signals.

A dual-axis system includes pitch control or basic altitude hold. A yaw damper adds the third axis-not heading command as some are led to believe-and uses a dedicated servo to the rudder for coordinated flight.

Then theres pitch trim, both manual electric and automatic electric. Autotrim automatically commands elevator trim when the autopilot makes pitch changes. Some lighter aircraft dont need autotrim, and low-end systems like the S-TEC Thirty dont even support it. But higher-performance aircraft need it. Autotrim requires a dedicated trim servo which adds to the bottom line. Theres also trim prompting, which prompts the pilot to trim if it senses an out-of-trim condition.

With more autopilots and approved applications than any manufacturer, the Cobham-owned line of S-TEC products is well proven, and easier to install and maintain than other brands. These rate-based systems use a custom turn coordinator to drive the roll axis of the aircraft. They offer backup for vacuum failures since the turn coordinator is electric.

S-TEC claims their rate gyros have a mean time before failure of over 8000 hours because the rotor spins at one third the speed of the average attitude gyro. But theres a catch. Faster and heavier craft with rate-based autopilots often struggle with turbulence and coupled approaches in gusty winds. Its widely accepted that attitude-based autopilots offer a superior ride, but they cost more to install and maintain.

All S-TEC systems use common hardware, servos and sensors. In our view, this is a big advantage that allows you to upgrade or add to the system in stages. Another S-TEC advantage is the ability to save panel space with the S-TEC System Twenty and System Thirty systems, which contain autopilot controls on a replacement turn coordinator.

All systems can be interfaced with an optional heading system (HSI, DG or PFD) for heading command. GPSS is an option that requires a panel-mounted command switch and remote converter box.

The S-TEC line is so rich with options, its confusing (see chart on page 11). The entry-level models include the System Twenty and Thirty, as well as the ATI panel-mounted System Forty, which uses a separate turn coordinator and controller. Function-wise, this systems are nearly identical. The System Thirty and System 50 have altitude hold capability and a manual electric trim option. The way we see it, theres little advantage going with the two-piece Forty or 50 system unless you want localizer backcourse tracking, a function thats lacking in the Twenty and Thirty.

All S-TECs will interface for lateral navigation and GPS tracking. But the Twenty/Thirty/Forty/Fifty-Five systems wont intercept a course at sharp angles. Instead, you manually put the aircraft on course-including an approach path-and then engage the autopilots nav mode to stay there. They wont capture a glidepath for an ILS or LPV approach. But they can fly the lateral part of the approach while the pilot controls the vertical. None offer autotrim.

STECs higher-end systems include the Fifty-Five X, which is the familiar radio rack-mounted controller that has vertical speed command, navigation and glideslope capturing, GPSS and supports autotrim. It also works with the flight director for the Aspen and KI256 (through a separate annunciator). Theres an optional remote altitude preselect/alerter with voice callouts. It also includes a new encoding altimeter that commands level-offs and climbs and descents.

A vintage model that remains in the S-TEC line is the System Sixty-Two. It was initially aimed at bigger aircraft and has nearly all the functionality of the Fifty-Five X except integral GPSS. The ST-901 GPSS system can be interfaced with it (and any other S-TEC system). The Sixty-Two also utilizes remote roll and pitch guidance computers that take space in the airframe. Racking them in the avionics bay or in the nose of a twin is in order. The Sixty-Two is a solid performer despite being rate-based, with good nav and glideslope intercepting. The Sixty-One is a single-axis version of the system and a relatively uncommon choice.

The Fifty-Five X and Sixty-series also support control wheel steering (requiring another button on the yoke) and dual-mode intercept, meaning you can fly selected heading to automatically capture a course.

All S-TEC systems call for optional yoke or panel switches for autopilot disconnect and altitude engage/disengage. S-TEC has options for yokes that dont have space for these.

Introduced in 1999, Honeywell dropped the digital KFC225 flight control system in

2007 because of low sales volume. This higher-end system was a stretch for the average budget and with limited STCs. Honeywell says demand is back and is returning the KFC225 to market.

A big part of the KFC225 expense was that the STCs included the pricey KCS55A HSI/compass system. But now that Aspen and Garmin have STCs to interface their displays to the KFC225-removing the need for the KCS55A and KI256 Flight Director-the KFC225 is a viable option.

Picking up where the excellent-performing analog KFC150 autopilot left off, the digital 225 is packed with features and a proven performer. In our experience, it outperforms any rate-based system in all modes. The system is intuitive, has GPSS, autotrim, voice alert automation and an integrated altitude preselector; and is equally at home in a Baron or single-engine Rockwell. Thats a lot of punch in a single package. A yaw damper is the only major option.

Digital servo motors have a lot to do with the KFC225s tight and authoritative command. It aggressively grabs the localizer and glideslope even in high winds and best-forward speed-exactly the conditions where an S-TEC system might disappoint.

This isnt to say that the KFC225 wont fail, but we dont see nearly as much as with the lesser and discontinued KAP140. We like the maintenance-friendly servo installation that allows for motor and clutch removal while the capstan assembly stays connected to the controls. This time-saver makes avionics techs smile. Further, the KFC225 stores diagnostic codes that techs access through a digital interface, speeding troubleshooting efforts.

Honeywell hasnt firmed up pricing for the KFC225. Applications will include a healthy list of Beechcraft Barons, Bonanzas, Piper Navajos, some twin Cessnas. Were told the Cessna 210 and others may be added at a later date.

Seemingly fallen off the face of the earth, Century is still around and offers good repair service on most vintage models. They recently announced a new autopilot:

the C4000.

On paper (weve yet to see one) the 4000 system looks high-end, with a Dzus-mount control head and lots of mode lights and buttons. The C4000 is attitude-based and driven by the 52D267, a pricey old-school gyro.

Standard on the 4000 is heading hold, nav/approach intercept and tracking (including ILS, VOR and localizer back course) and GPSS steering. The altitude function offers trim prompting or autotrim, pitch attitude hold, vertical speed select and hold and glideslope from above or below the glide slope.

The base C4000 system is $19,995. Century offered us little information on STC approvals but they dont appear to be extensive. We get the feeling that if you wanted a new C4000 for your aircraft, Century would work with you on the STC. Plan on hauling the aircraft to Centurys headquarters in Texas for the project.

We covered the Avidyne DFC90 retrofit system in the September 2010 issue of Aviation Consumer, and its a reasonably easy yet pricey upgrade to the stock S-TEC Fifty-Five X found in many Avidyne Entegra (PFD/MFD) installations. Its an attitude-based autopilot driven by the Entegra PFD and therefore only available for those systems. The DFC90 addresses the griping many Cirrus owners have with the Fifty-Five Xs performance, particularly in the SR22. It also adds safety features, including speed protection and an emergency return-to-level button.

The DFC90 is $9990. The modification to the PFD to send new signals on the old S-TEC wiring harness is $3600 if youre already running Release 7 (WAAS) software or $5800 for earlier PFDs. Installation runs between three and six hours, including removal of the PFD for the upgrade and the flight test. Whether the performance gain is worth the $15,000 upgrade is questionable, in our view. Currently the STCs are for only Cirrus aircraft, but Avidyne tells us they are expanding the list. The DFC90 will not be offered for non-Enterga aircraft.

Its important to consider future upgrade plans when upgrading the autopilot now. For example, if your plans eventually include an Aspen or Garmin G500, installing a $4000 GPSS system and $2000 directional gyro will be wasted when the PFD is installed since that functionality will be built in. Also remember that a rate-based system installed in a Navajo wont yield the performance of an attitude-based one.

If we have to pick an autopilot for higher-end aircraft singles and twins, Bendix/Kings KFC225 wins hands down. We wish there were more STCs available but were told there will be in the future. For the less-speedy crowd of Skyhawks et al., our top pick is the S-TEC System Thirty. Its a no-frills navigation solution but offers reliability and enough performance for most missions for around $15K, and can offer GPSS with or without a glass panel.