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EAA AirVenture Oshkosh RSS FeedAvidyne Autopilot Prevents Stall-Spin
 
Avidyne has just earned FAA certification for its new DFC90 autopilot that can prevent a stall if the distracted pilot allows airspeed to get too slow, or can also slow the airplane if it is approaching the airframe’s high-speed limitations.

The autopilot is approved for Cirrus SR22 and SR20 piston singles equipped with Avidyne’s Entegra avionics systems and the S-Tec55X autopilot.

Avidyne equipped its Entegra avionics system with a data recorder that notes performance values such as speed, altitude, track and so on. Avidyne engineers studied the recorded information from several Cirrus accident airplanes and found that in a significant number of events the autopilot flew the airplane into a stall which resulted in the loss of control.

In a typical scenario the pilot would have the autopilot engaged in descent, the autopilot captures an assigned altitude, and the pilot fails to add power. The airplane then slows to a stall as the autopilot holds the assigned altitude.

Or, the autopilot could be engaged in a climb with a selected vertical speed. At some point there may not be sufficient power to maintain the selected climb rate but the autopilot will continue to raise the nose to try to hold the selected vertical speed until the airplane stalls.

“George” gets stall-savvy
After studying the accident data Avidyne engineers realized they could build a smarter autopilot.

Instead of a system single-mindedly flying the airplane into a dangerous situation because that’s what the human pilot asked it to do, a smart autopilot could protect the human pilot from disaster and give him a chance to realize and correct his error.

I had a chance to fly the DFC90 autopilot with Avidyne founder Dan Schwinn in an SR22. Dan and I played the distracted pilot by having the new autopilot capture an initial approach altitude after descent, but we didn’t add power.

The autopilot kept pulling the nose up to maintain altitude but airspeed was bleeding off quickly.

As the airspeed dropped to about 1.2 times stalling speed, the autopilot announced “under speed” in plain and very clear language and flashed the same warning words on the primary flight display (PFD).

Under any normal circumstance, the warning should certainly be enough to alert the pilot to add power. But Dan and I just sat there and watched.

As the DFC90 continued its repeated warning that we were flying too slowly, the autopilot very gradually lowered the nose to maintain a safe airspeed.

It was trading the minimum amount of altitude for a safe margin above the stall.

In heading mode we commanded the autopilot to turn to a new heading, which it did, but used very shallow bank angles to preserve altitude and airspeed.

The DFC90 cannot add power, but it can prevent a stall. In a worst-case scenario such as pilot incapacitation, the autopilot would maintain control above the stall even after fuel was exhausted.

A forced landing under control at minimum safe airspeed would at least give the chance for survival.

Slow down, you move too fast…
Exceeding the design airspeed limits, particularly in turbulence, can be as disastrous as a stall and spin. So Avidyne engineers gave the DFC90 the smarts to slow down when things get too fast.

Protecting the airplane from exceeding its design speed limits is less dramatic because all the system needs to do is raise the nose and slow the descent or even climb while announcing the over speed condition.

The DFC90 also has a “straight and level” mode button so that if a pilot becomes confused about what mode is engaged, or what the autopilot is doing, a single button press commands level flight.

Of course, the envelope protection would still be continuously available as it is in any selected mode.

Smarter, smoother, more capable – and plug-and-play
Even if you never need the envelope protection of the DFC90 you will get vastly improved autopilot performance because the system uses attitude, not just air data, to fly the airplane.

The S-Tec55X uses airspeed and vertical speed inputs along with rate of turn to create an approximation of aircraft attitude.
The Avidyne DFC90, conversely, gets exact attitude information from the digital electronic attitude-heading reference system (AHRS) that is fundamental to the Entegra system.

All phases of flight are more precise, and the flight director commands are perfectly stable.

The DFC90 slides directly into the mounting tray of the S-Tec autopilot computer and uses the existing servos. That holds installation costs down to a minimum.

However, the Entegra PFD will need to be modified so it can export the attitude data to the DFC90.

The DFC90 autopilot is priced at $9,995 and modifying the Entegra PFD costs from $3,895 to $5,395 depending on software status of the PFD. Avidyne has already received many orders and the system will begin shipping soon. Avidyne’s exhibit is inside and outside of Exhibit Hangar B.

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