Advanced-Degree Autopilot

"We've put in all kinds of building blocks within the DFC90," said Avidyne engineer and VP of Product Management Steve Jacobson---AviJake to his Twitter followers. "It's a platform that can go much farther than what it does now."

Not that its performance at the moment was anything to sniff at. We were under the control of a DFC90 in an SR22, N834LA, at 5,500 feet over New York's lower Hudson Valley, where the Professor had brought us after being handed command shortly after departure from Westchester County Airport (KHPN), climbing us to altitude at a preset airspeed (125 knots) on an NNE heading.

Entegra PFDs (primary flight displays) have had AHRS (attitude and heading reference systems) capable of driving an autopilot since early in the decade, but the rate-based STEC took its commands from an electric vacuum-driven turn coordinator. It's a given that a digital attitude-based system is much more precise and reliable than a mechanical system, and the DFC90 has a clear advantage in function-by-function comparison. (One telling data point: Max course intercept angle is 179.9 degrees---easily handled in a high-speed descent with flaps thrown in at the upper limits of the green arc without squirming.) But the real difference is in everything else the DFC90 does, which includes "Envelope Protection" to prevent stalls and overspeeds, and a potent "Straight and Level" panic button.

The STEC 55X was never accused of being an intuitive system, but the DFC90, as a retrofit product, has retained the STEC's look and feel. "Habit patterns are powerful, and we decided not to mess with the UI [user interface]," Jacobson explained.

Data fields on the PFD and value inputs are controlled primarily from the autopilot head. Lighting on the head and PFD indicate whether a function is armed (blue) or engaged (green). On-screen annunciations alert pilots to changes in the autopilot state.

New features unavailable in the 55X include the "Pitch and Roll" mode, which holds the aircraft in the attitude of the moment of engagement; vertical speed hold, for constant-speed climbs and descents and synched altimeter setting, which brings the aircraft to the proper altitude whenever the barometric pressure is reset. Jacobson demoed another use for the autopilot's ability to hold a specific airspeed when we reached altitude: He dialed in 88 knots and armed the IAS (indicated airspeed) mode. "That's the engine-out glide speed," he said. "If I lose the engine, I hit the IAS button and let the airplane handle the energy while I look for a place to go."

But an autopilot, like fire, can get out of hand if not properly monitored, and the DFC90's Envelope Protection---comprised of Overspeed and Underspeed protection---minimizes chances of the autopilot contributing to an accident. We simulated both sides of the envelope, first putting the airplane into a climbing turn, engaging the pitch and roll mode, and then pulling back the power, imagining our attention were elsewhere. As airspeed bled toward 80 knots, the system issued aural and on-screen alerts. At 82 knots the autopilot leveled the wings, and then lowered the nose to maintain airspeed. When we applied sufficient power, it resumed the climbing turn. We repeated the exercise with a descending high-speed turn. As the airspeed closed in on 200 knots, aural and visual alerts warned of the potential overspeed situation developing; when no outside corrective action was applied, at 201 knots the DFC90 reduced pitch down to keep the airspeed from increasing.

Avidyne's exhaustive study of Cirrus accident data found almost one-quarter of all mishaps involved overspeed or underspeed conditions, situations Envelope Protection can help prevent. (Envelope Protection in the DFC90 is active whenever the autopilot is engaged; in future iterations it may be active at all times.)

The Straight and Level (S&L) button in the center of the DFC90 is another potential lifesaver. This is hardly the first autopilot with a panic button that promises to recover an aircraft from an unusual attitude, but Dr. George is unusually robust. The user guide calls for maximum engagement limits of 60 degrees of bank and 30 degrees +/- of pitch, but the S&L button reportedly has been demonstrated to work when engaged in an inverted Cirrus. When S&L is engaged, the autopilot commands an unloaded roll and a 2.5 G pitch-up recovery.

The ideal demonstration of the DFC90's rock-solid approach mode would be a WAAS approach in a bucking crosswind. Where the 55X is known for fishtailing its way to the runway on final, the DFC90 flies the airplane like it's on a rail. But this was a fairly calm day and the panel wasn't WAAS-enabled. The ILS 6 approach at Poughkeepsie (KPOU) nonetheless showed the autopilot's deadeye aim. We looked straight down the runway from the moment the autopilot intercepted the final approach course more than six miles out, and other than getting larger, the target barely moved.

The Flight Director (FD) mode, which can be a struggle to use in the 55X owing to a jumpy command bar, is stable and functional in the 90. We hand-flew using the FD on our way back to HPN, and it made keeping the aircraft in the correct attitude for the desired flight path easy.

"Avidyne's finally in the flight control world," Jacobson summed up after the flight. "The DFC90 and 100 are core to our business. They really flesh out the whole Avidyne product offering and make us a real player."

The next step in Avidyne's autopilot evolution is certification of the DFC100 for Entegra R9 panels, both as a retrofit and in new installations. The R9 uses Avidyne's own FMS, rather than the Garmin 430s that have driven all earlier Entegra releases. The all-Avidyne architecture will give the DFC100 better and more powerful capabilities, including VNAV, enabling it to fly vertical, as well as lateral approach segments. Going forward, synthetic vision will be added to the Entegra mix, along with as yet unannounced capabilities that Jacobson promises will be a "game changer."

Meanwhile, more than 250 DFC90s ($9,995) have already been sold into a fleet with several thousand upgradable Entegras. The retrofit requires an Entegra R8 system; earlier releases can be updated to R8 with a PFD hardware and software upgrade (about $3,000), easily accomplished by an authorized dealer in a couple of hours. (Avidyne has offered discounts for combo Entegra/DFC90 retrofits.) The autopilot swap itself is a slide-out/slide-in replacement, which takes "four minutes with a slow turning wrench," Jacobson said. "The idea is to have a drive-by replacement---the guy flies in, borrows the crew car, goes get lunch, comes back, and the plane is ready."

For legacy Entegra flyers looking for a giant leap forward in performance and safety from their Cirruses, the DFC90 is just what the doctor ordered.