Aspen and Avidyne Team for Digital Autopilot Upgrade
By J. Mac McClellan, Director of Publications, EAA 747337
April 2, 2012 - Aspen and Avidyne have teamed up to create an autopilot upgrade that offers owners of existing airplanes the latest autopilot performance and capability, and potentially lifesaving new features. Certification of the system is expected soon in Cessna 182s and Cirrus SR22s.
The Aspen Evolution Pro is a complete primary flight display (PFD) that includes non-moving electronic gyros (AHRS) and an air data computer packaged into a single instrument that fits in the panel space occupied by the conventional attitude and directional gyros. The Aspen PFD system is approved for installation in almost every type of GA airplane and thousands have been installed.
Avidyne has created the DFC90 digital electronic autopilot that is a direct replacement for the popular S-Tec 55 series of autopilots. When the DFC90 is installed with Aspen's PFD, the system is free of dependence on spinning rotor gyros and the vacuum pumps that power them. The DFC90 flies the airplane with the highest level of precision while protecting the pilot from an inadvertent stall while the autopilot is coupled. And, if the human pilot becomes disoriented, a single press of the "straight and level button" automatically levels the airplane, giving the pilot crucial time to become oriented to his situation.
The S-Tec autopilot the DFC90 replaces uses information from a turn coordinator gyro, and from air pressure sensors, to measure rate of change and estimate the attitude of the airplane. The DFC90 receives actual attitude information from the AHRS in the Aspen PFD. Instead of banking to find a standard rate of turn as the S-Tec system does, the DFC90 banks to a preset maximum angle in a turn, typically 17 or 18 degrees for more precise and comfortable flying.
The digital electronic brain of the DFC90 and the precision of the Aspen attitude and airspeed data allow the system to avoid a stall even when the pilot commands a flight path the airplane cannot fly. When the human pilot forgets to add power after a level off, for example, or commands a climb rate the airplane can't achieve, the DFC90 flies the airplane close to the stall, but then eases the nose down just enough to avoid the stall. All the while the system flashes a yellow "under speed" message on the Aspen PFD and aural voice alerts in your headset warn that you are flying too slowly.
To see the Aspen/Avidyne system in action, I flew a well-worn Cessna 182 during Sun 'n Fun last week. Avidyne decided to retain some of the odd S-Tec mode commands on the DFC90 - such as pushing heading and nav buttons simultaneously to intercept approach on a vector - so pilots familiar with the S-Tec system do not need to be retrained. I would rather have us all learn what are now conventional mode selections as the double-button pushes are leftovers from very long ago, but I understand the logic behind Avidyne's choice.
The system flew the 182 with excellent precision, holding heading and altitude solidly, and tracking both nav and approach courses exactly. The bank angle limits are just right and mimic the way an experienced human pilot flies. When I pulled the power back to idle with altitude hold engaged, the Skylane slowed to near a stall with the nose pitched way up, but it didn't stall. I commanded turns in the heading mode but the DFC90 kept the airplane flying just above the stall, gradually trading altitude for just enough airspeed while yelling at me to do something. So I did something - added full power. With the necessary thrust, the DFC90 maintained a climb airspeed of about 1.2 VS0 (stall speed) and clawed back up to the previously selected altitude. Impressive, and the airspeed control was very precise as we got bumped around in afternoon turbulence.
To give the "straight and level" recovery mode a test, I rolled the Skylane into about a 65-degree bank, 20 degrees nose down, and let the speed build up. When I hit what can accurately be called the panic button, the recovery was immediate and positive. I didn't have a g-meter available but the system did push me into the seat, pulling the airplane out of the beginning of a spiral dive. When control is lost there is no time to spare, and the DFC90 doesn't waste a second on the way back to straight and level.
The Aspen Evolution Pro PFD costs $10,180 plus installation. The DFC90 autopilot for a single engine airplane is $9,995. There is an additional software fee from Aspen of $1,995 to link the systems together.
Avidyne plans to certify the system in a broad range of airplanes. If an S-Tec autopilot is installed the servos can be retained. For those airplanes that do not have an S-Tec already installed Avidyne is creating its own line of servos to use with the DFC90. The new servo will match the physical size and mounting bolt hole pattern of servos used in the popular Bendix/King KFC series of autopilots so the mounting hardware and even the capstans that connect the servo to the control cables need not be replaced to upgrade.