A trim-steady digital autopilot</4h>
Digital autopilots have been around since the Apollo Space Program in the early 1960s, when Grumman developed an autopilot using a digital microprocessor to manage analog inputs from the control and navigation systems on board the lunar module. But while computers have helped steer the course for decades, we're now on the cusp of having completely digital autopilot systems flying our light singles and twins as we enjoy a cup of coffee.
The FAA has granted Chelton Aviation supplemental type certificate (STC) approval for its digital autopilot in Piper Cherokee Six and Cessna 210 aircraft, following STCs in other Cessna, Beechcraft, and Piper singles and light twins. The AP-3C is a three-component autopilot built for the general aviation market using solid-state electronics to achieve a lightweight, yet high-quality design.
A fully analog autopilot takes all heading, navigation, and attitude information from analog instruments. Course corrections are based on input from the heading indicator bug or the needle(s) of a VOR's course deviation indicator (CDI). Roll rate comes from either the turn coordinator or attitude indicator, depending on whether your autopilot is rate- or attitude-based. Altitude corrections come from the needle positions on altimeter, and climb and descent rates are set via the vertical speed indicator. While altitude hold is fair using this method — except in varying levels of turbulence — course correction via analog inputs alone can trace a weaving path.
A digital autopilot takes its course information through the ARINC 429 data line from a GPS/nav/com in the panel for more precise tracking, and uses a microprocessor to sort the digital and analog inputs in order to command the airplane. And soon navigators such as the Garmin GNS 480 and Chelton FlightLogic EFIS (electronic flight information system) will be ready to send vertical information from the GPS (known as GPS-V), and the AP-3C will be ready to put these inputs to use.
The end result should be a much better ride. Think about it this way — can you hold your altitude within a couple of feet using an analog altimeter? You don't even know your altitude within a few feet. Well, the autopilot can't do much better with this information. But if the autopilot can register the altitude within a couple of feet, it can anticipate altitude deviations more quickly and make small corrections to stay on target. And those small corrections will be less noticeable.
I flight-tested the AP-3C in October. We flew from California's Long Beach/Daugherty Field in Chelton Aviation President Jim Smisek's Grumman Cheetah. Smisek uses the Cheetah to commute to work from his home at Lake Texoma's Diamond J Airpark to Denton, Texas, where Chelton Aviation Corporation (CAC) is based. CAC is a subsidiary of Chelton Avionics with Chelton Flight Systems, which produces the FlightLogic EFIS (see " On Display: Chelton's FlightLogic Synthetic Vision," May 2003 Pilot).
The AP-3C consists of a panel-mount controller, a roll servo, and a pitch servo. Attitude information can consist of digital or analog input from an attitude indicator (vacuum or electric) or AHRS (attitude heading reference system), in addition to an altitude pressure sensor through a separate static port on the controller. If you have a backup system installed, whether a standby vacuum pump or backup attitude indicator, an external switch can toggle between the sources for the autopilot.
The standout feature is the pitch servo, which uses the pitch trim actuator to manage aircraft pitch rather than an elevator servo. Operation was smooth, and I was impressed with the slick transition when I disengaged the autopilot — the airplane remains in trim throughout, so there is no pitch excursion when the autopilot abdicates control. The roll servo uses heading information from the attitude indicator, as noted above, rather than a rate-based source (such as a turn coordinator), and GPS roll steering is available (during my test, in conjunction with the Garmin AT CNX 80, with other interfaces to come). Altitude holding information from the attitude indicator also should dampen the effects of turbulence — although it was a smooth day when we flew.
The AP-3C has standard modes, although activating approach mode involves coordinated use of the Nav and GS (glideslope) buttons rather than a dedicated approach button. For now, the AP-3C uses left/right input from the navigation source and sums it with course error from the heading bug (or course bug on a horizontal situation indicator) to avoid the dreaded S-turning you sometimes experience while an autopilot tries to track a jiggling course needle. Rather than control wheel steering for temporary changes in flight path, the autopilot uses a manual mode (accessed by pressing one of the arrow keys), and you can set the bank angle (up to roughly a standard rate — on the Cheetah it was about 20 degrees) and leave it, allowing you to orbit while maintaining altitude hold. If you get disoriented and enter an unusual attitude, you can enable an adverse attitude recovery mode by depressing the left and right arrow buttons for one second — the mode uprights the airplane. This mode shouldn't be engaged if the attitude indicator is suspect — but then, neither should the autopilot unless you have an alternate attitude source.
At the time of our test, Chelton was adding a trio of additional functions, including altitude preselect, voice annunciation of autopilot modes, and a go-around mode accessed by a button on the throttle or throttle quadrant. Certification was in December 2004. Smisek noted that the company's Beech Baron has an AP-3C coupled to a GNS 480 and is using the pair to fly LPV approaches.
Altitude preselect works by entering the desired altitude; upon reaching that altitude, the voice annunciator calls out "altitude hold," and the altitude is captured. The go-around mode is engaged by pressing the dedicated button while applying go-around power. The autopilot enters a 3-degree pitch-up attitude and engages the heading mode. This is to keep your hands on the most important place — the throttle — during the initiation of a go-around.
Chelton expects approval shortly in the Mooney M series, Beechcraft Bonanza, Cessna 340, and our 2005 Sweepstakes Countdown Commander 112A — with other STCs to come (see " The Commander Countdown Has Begun," page 87).
Price: $11,000
Contact: 940/320-3330; www.cheltonaviation.com
E-mail the author at [email protected].
