A hidden pitfall in buying a used airplane is underestimating the replacement cost of its autopilot. Worse yet, owners may pay little attention to the health of the existing system during the demo ride.

Dont be fooled by the sales ad saying the autopilot was a flagship model with huge capabilities. It could be as old as the airplane and ready to tank. (A 30-year-old piece of equipment is ancient in the electronics world.) Autopilots are major

systems and replacement cost can double that of a new engine.

You can eliminate this high-stakes gotcha during the pre-buy inspection, or better yet, during the aircraft search. At the least, autopilots can be helpful bargaining tools when closing the deal. Some systems youll want to think twice about getting involved with. Others are keepers. No matter what you choose, there are a couple of sound strategies for upgrading older systems.

Find Out Whats There

As we browse the used aircraft listings, were hard-pressed to find consistency among autopilot models for any given airframe. Aircraft builders have used models from almost every autopilot manufacturer. Searching for a mid-80s Bonanza, youll find models equipped with Century or a variety of Bendix/King models – each with different levels of utility. Cessna seems to have offered as many models of autopilots as they did airplanes.

As a general rule, autopilots that are rate-based – using a turn coordinator gyro for a primary reference – are less expensive to maintain given their simplicity. Attitude-based system which utilize the attitude gyro for roll and pitch commands can be expensive to maintain given their complexity and the high costs associated with most gyros.

Attitude-based systems generally offer more utility and features and a smoother ride. But as the system ages, circuitry that once contributed to a smooth ride can actually induce flaws and a troubleshooting tail-chase. Be prepared to play the role of test pilot as effective autopilot troubleshooting begins in flight.

Any work accomplished with the autopilot system should be logged in the airframe logbooks. Look for descriptive entries. For example, teardown reports of replacement servo motors and flight computers offer definitive clues as to the quality of the repair. Note that poor autopilot upgrades accomplished at the aftermarket can impact performance. Also, some flight instruments are components of the autopilot and you should know how their failure will affect the system. A record of their replacement history is helpful.

Pages and pages of logged autopilot repair should raise a red flag. Ask the seller

point blank if hes been battling autopilot problems. If so, were they resolved? Weve seen some owners give up on the repair effort given the costs, leaving the problem up to the next owner.

Avoid the Dogs

We consulted with several autopilot repair facilities and heard the same thing about older systems: Replacement parts are becoming scarce and expensive. Autopilots Central in Tulsa, OK, a well-respected and long-established autopilot repair facility, told us that many older systems can be successfully repaired. But, dont expect any freebies or a gentle invoice.

Mid Continent Instruments, another well-regarded and high-quality facility, notices a definite decrease in old autopilot repairs. They suggest finding an airplane thats had a recent autopilot makeover. As Mid-Continent put it, there are lots of pilots flying higher-end aircraft with half-functioning autopilots for lack of available replacement parts.

One system that several repair shops warned against is the Bendix M4D. This system can be found in some Beech King Airs, some bigger Cessna twins, Mitsubishi MU2s, and others. Servos for these systems can run close to $9000 each. Another potential dog is the Bendix FCS810-series system. Mechanical parts and electronic components for repairing these systems are scarce. How about the old pneumatic Brittain autopilot models? Dont even go there.

Acceptable Bendix/Kings

The KFC200 is a hugely popular system for a wide variety of airframes. Its attitude-based through the KI256 Flight Director horizon. The KI256 is still in

production because its an integral component of the KFC225 digital autopilot, the flagship of the Bendix/King line. This offers some good news for owners stepping up to the KFC225 from the KFC200.

While the KFC200 is long out of production, factory support (and aftermarket support, too) remains good, but expensive. The trouble with the KFC200 could rest in the KC295 remote-mounted flight guidance computer. A seasoned factory tech told us that the aging analogue circuitry in this computer makes it nearly impossible for KC295s to perform to their original specifications. Some early computers arent serviceable at all. Still, we wouldnt avoid the KFC200 as its a good flying autopilot with a solid performance record. The KFC150 and KAP150 (the latter has no flight director) and utilize a panel-mounted controller/flight guidance computer. They offer automatic pitch trim and can drive preselect/alerter systems. But all attitude-based units from Bendix/King can be prone to gyro-induced flaws. Some possible symptoms include shallow wing rocking and gentle pitch porpoising. The KI256 gyro could be the problem and its roughly $3600 on exchange.

Newer Cessnas feature the KAP140, a rate-based autopilot with solid performance and decent capabilities. There were, however, many service bulletins against these autopilots that required inspection and in some cases replacement of servo assemblies.

Many of these actions were once covered under warranty but as newer models in the Cessna single-engine line begin to age, the warranty might be spent. The same can be said for the KFC225, which had some issues early on. But we wouldnt avoid this model as its a solid, full-featured digital system.

Early Bendix systems, such as the FCS810 and Altimatic IV are ancient. We wouldnt throw our money at one.

S-TEC Simplicity

In our estimation, the S-TEC brand offers the easiest path for upgrade in its building block design. A basic entry-level S-TEC roll system can be upgraded for altitude hold while adding pitch axis capabilities, the shop will need to install a pitch servo, pressure transducer and remote pitch computer.

That said, basic systems including the System 20/30/40/50 cant be upgraded for glideslope capture and automatic electric pitch trim. Still, the roll servo and in some cases the turn coordinator can be retained even when stepping up to bigger models such as the 55X and System 60-2.

Older models, as in the System 55 (less the “X” suffix) dont have integrated digital roll steering, aka GPSS. They also had a washed-out display, in our opinion. Adding GPSS capabilities to the old 55 requires a stand-alone ST901 GPSS steering system, which could set you back $3000 or more including labor. Given the technology packed into modern GPS such as the Garmin GNS 400- and 500-series navigators, a lack of digital steering shortchanges an otherwise modern interface. There are aftermarket roll steering systems available from several manufacturers as an add-on to existing systems.

The System 60-2 is full-featured with several remote boxes including roll and pitch computers. We recall one owner of a Piper Navajo who dumped serious money into his 60-2 shortly after purchasing the airplane. Sadly, the seller told him there was an extended factory warranty in place. Unfortunately, S-TEC doesnt offer extended warranties.

While flight testing any S-TEC system that has altitude hold be sure to watch for pitch oscillations and subtle altitude deviations. Some early altitude hold transducers were prone to high instances of failure. And as with any autopilot, proper aircraft control cable rigging is imperative for proper performance.

ARC/Cessna Woes

The ARC/Cessna autopilot experience can be like opening the proverbial can of worms. Some ARC autopilots are fine, but there are many variables to deal with when it comes time for repair. Theres lots of variation within the autopilot model line, too.

As a general rule, the bigger the Cessna, the bigger and more complex its original equipment autopilot is. When you earn your place in the cabin-class and pressurized Cessna twins youll likely find the 800-series IFCS system. There are many remote components that are involved in this system, including an electric altimeter, preselect/alerting system, mode controller, altitude hold sensor, remote flight computer…the list goes on and on.

Youll want to go to a shop that specializes in Cessna flight control and you should expect an impressive repair invoice. Replace a G550A Attitude Director Indicator, for example, and fork over five grand. We recall a Cessna 340 owner who ponied this up every year.

Consider a particular gotcha when interfacing a 400A model with Garmins 430W or 530W WAAS navigators: This autopilot isnt approved for coupling to the units vertical GPS glideslope under the AML-STC. A fair number of 400A systems are still installed in Cessna airframes (generally in select 1969 through 1975 models). While the technical interface will work, additional FAA approvals might be necessary for a

legal GPS/glideslope interface.

The 400B is a scaled-down 800 system (less the flight director). One T210 owner recently confessed that the 400B system in his airplane performed better than the replacement S-TEC 55X he had installed. The old 400B, however, simply ran out of useful life.

One other tidbit to consider: Some Cessna autopilot Nav tracking circuitry and controls live within the original equipment Cessna audio switching panels. Retaining the autopilot but replacing the audio system will leave you without some functionality, including localizer back course tracking. On the other hand, we cant recall the last time we commanded an autopilot to fly a back course.

Lower-end Cessna autopilots are rate-based, with appropriate turn coordinators. While relatively simple, theyre aging and systems that function 100 percent are few and far between.

The Century Gamble

These autopilots are in a similar category as Cessna/ARC models. That is, there are a lot out there. Century made systems for Piper through the AutoControl and Autoflite nomenclature. These are attitude-based autopilots. Some support automatic pitch trim, some have flight directors and glideslope capturing. In our view, older Century systems can be a crap shoot. Like the Cessna models, when they work, they work well. When they break, they can be expensive.

The basic Century wing leveler is the model Century I, which is housed within a turn coordinator. Some models have nav tracking boards, others dont. The later model is the Century 2000, which can be as basic as a wing leveler, with heading command, or well-equipped with autotrim and flight director options. It utilizes a large controller console that takes lots of panel space.

The factory, Century Flight Systems in Texas, offers decent support on nearly all Century and Piper models but they advise that servos and other accessories for some models are scarce. Some arent available at all. Century tech support, in our view, is very good.

Gyros for Century and Piper autopilots are spendy and, of course, play an integral role in system performance. The larger Century systems use remote pitch trim amplifiers and altitude hold sensors, which can be troublesome. And then there are

those old split-pin- style interconnect connectors that cause huge amounts of intermittencies as any experienced shop can attest. They can be replaced, but not without serious labor.

When it comes time for repair, choose a shop with Century experience and the required test equipment. If your old Century autopilot fails hard, look at replacement options.

There is Still Hope

Autopilots are a core component of a modern avionics suite. For IFR flying theyre essential, in our view. If we could offer a single piece of autopilot advice for those serious about buying a used airplane its this: Know what you are buying. Too many times we see owners take the plunge into the ownership experience without doing the research.

When you go along on the demo ride, ask the demo pilot to conduct some autopilot-coupled instrument approaches and watch closely. If possible, complete a short cross-country trip with the autopilot engaged and watch for roll and pitch oscillations, course deviations or any other bad habits. Many times, you wont pick up on autopilot flaws until your first long trip in your new airplane. Its best to seek advice from an autopilot expert during your selection process.

Buying an airplane with the intentions of upgrading its old autopilot is OK, as long as you understand the costs associated with replacing the system.

Youll likely discover that autopilot upgrades are pricey and you might very well have to settle for a new system that has fewer functions than the antique it replaces. But at least it will work.