Plane & Pilot
Tuesday, October 19, 2010

Advanced-Degree Autopilot

The DFC90 completes Avidyne’s integrated avionics suite with an increase in performance and safety

We familiarly call them "George" or "Otto." But Avidyne’s DFC90 autopilot makes a strong case for being called "Doctor" George or "Professor" Otto. The attitude-based digital autopilot represents the missing hardware link in the integrated avionics suite Avidyne has been building for 15 years. It delivers a dramatic increase in performance and safety for Cirrus pilots flying legacy Entegra panels using STEC 55X autopilots, which the DFC90 is designed to replace via retrofit. Even more impressive are the future capabilities this first Avidyne autopilot promises pilots who fly current (R9) and future Entegra flight decks.

"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.

he Avidyne DFC90 autopilot features Overspeed and Underspeed protection (top), as well as a Straight and Level button (above).

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.


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