‘The shackles are off’: DFC90 autopilot to the test

September 21, 2010

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Avidyne’s new autopilots are equipped with a “straight & level” button designed to prevent pilots from becoming disoriented or losing control of their aircraft.

A pilot hand-flying an airplane with a DFC90 or DFC100 autopilot can simply push the S&L button, and that engages the autopilot and commands it to right the aircraft. But does the S&L feature work in all loss-of-control scenarios?

The FAA certifies autopilots to operate within 30 degrees of pitch and 60 degrees of roll. But a steep spiral or an encounter with severe turbulence or wing-tip vortices could exceed those limits. Would the S&L feature on Avidyne’s new attitude-based autopilots work in those scenarios? In an effort to better understand this new technology, AOPA Pilot Senior Editor Dave Hirschman took an Experimental-category Cirrus SR22-G3 into some extreme attitudes and used the S&L button on a DFC90 autopilot to recover. Here’s what he found:

“It goes against all of a pilot’s instincts and training to put the airplane in an unusual attitude and then let the autopilot take over,” said Hirschman, a veteran aerobatic pilot and Cirrus instructor. “But that’s exactly what we did here—and the results were both consistent and remarkable.”

Hirschman performed a series of aileron rolls to the left and right. Using a high-cruise power setting and an entry speed of 160 KIAS, he’d pitch the nose up 35 degrees, roll the airplane inverted, and engage the autopilot through a single push of the S&L button. Each time the autopilot guided the airplane back to level flight.

“The recoveries were smooth and authoritative and didn’t place extraordinary stress on the airframe,” Hirschman said.

According to the airplane’s flight data recorder, the autopilot took about two seconds to take over once the S&L button was pushed. Then it commanded rolls up to 25-degrees-per-second until the wings were level. The nose of the aircraft fell about 40 degrees during the recoveries, and indicated airspeed increased from 100 knots when the autopilot was engaged to a maximum 185 knots (VNE is 201 KIAS) during the pull-out. Acceleration during the recoveries ranged from negative 0.5 Gs to a positive 2.5 Gs, well within the aircraft’s normal operating envelope of negative 1.56 Gs to positive 3.89 Gs. Beginning at an altitude of 10,000 feet, the aircraft lost about 1,800 feet during each recovery. Hirschman manually reduced the throttle to avoid gaining excessive speed.

“The S&L button isn’t a ‘Get out of jail free card’ and it won’t solve every conceivable problem from in-flight loss of control,” Hirschman said. “An aerodynamically clean aircraft like the SR22 can develop excessive airspeed during the recovery, and altitude loss can be significant. Also, the autopilot isn’t linked to the rudder, so it won’t command a spin recovery because it can’t apply opposite rudder.  

“But the S&L feature works far beyond the pitch and roll limits for which autopilots are certified, and I wouldn’t hesitate to use it in an emergency—or to prevent an emergency situation from developing in the first place.”