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.

don’t 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 you’ll 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, you’ll 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