When planning major avionics upgrades, autopilots rank near the top. A panel full of integrated glass or even a single-screen Aspen will be lacking without autopilot integration. But adding a new autopilot could be a budget-blowing proposition. For most go-places aircraft the investment is worth it, especially with modern AHRS-driven interfaces upping the ante on automation and long-term reliability. Heres an overview of market offerings for retrofit autopilots. One size wont fit all, and thats without covering systems for LSAs or Experimentals. we’ll look at those in a future article.
How Much Control?
When shopping for a system, you’ll 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 there’s pitch trim, both manual electric and automatic electric. Autotrim automatically commands elevator trim when the autopilot makes pitch changes. Some lighter aircraft don’t need autotrim, and low-end systems like the S-TEC Thirty don’t even support it. But higher-performance aircraft need it. Autotrim requires a dedicated trim servo which adds to the bottom line. There’s also trim prompting, which prompts the pilot to trim if it senses an out-of-trim condition.
Cobham S-TEC
With more autopilots and approved applications than any manufacturer, the Cobham-owned line of S-TEC products is we’ll 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 there’s 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.