Attitude-Based Digital Autopilot
The Avidyne DFC90 exceeds the functionality of the current and competitive systems in the following ways:
EP (Avidyne's Envelope Protection) is a unique and proprietary Avidyne autopilot feature which prevents the pilot from commanding the autopilot to stall or overspeed the aircraft.
The term “Flight Envelope” normally implies airspeed and altitude but the system is significantly more capable than that. While indicated airspeed limits are met by our EP, neither the aircraft’s safe operating range, nor Avidyne’s autopilot protections, are that simple.
EP monitors and limits the instantaneous lift demand on the aircraft. It is active in both Autopilot Engaged and Flight Director modes. Lift is constrained by saturating pitch rate command to the maximum value that will not exceed the wing g loading such that the sum of kinematic and (projected) gravitational g load remain within 90% of the maximum available. The available lift calculation is sensitive to flaps, speed brakes, weight, temperature, speed, and incorporate margins purposely designed to mitigate ice.
The term “kinematic g” takes into account the turning of the aircraft’s velocity vector. Any attitude whatsoever is automatically mathematically incorporated by this method. Next, gravity is vectorially added to the lift demand to the extent it projects on the wing normal.
Thus, the autopilot knows how much g is needed to maintain level turn or constant climb rate at any attitude, and that g request is “fed forward” into the control loop request before it even can generate a loop error. This means that the lift demand is pre-biased to account for the extra wing loading needed in turns. As an example, at 75 deg bank, the starting point for g request from the pitch inner loop is 3.9g. This is then augmented by any control loop tracking error (the closed loop part of the autopilot) and then limited (if necessary) by EP.
Last, pitch rate is the “lever” by which the EP is implemented. By imagining flight through a zero-g pushover, it is possible to see that EP can arbitrarily constrain lift demand by modulating pitch rate: a steady “push” holds lift to zero. This is exactly how the autopilot works. Besides being “all attitude” capable, this method additionally has the collateral consequence of controlling steady level flight to 1.1Vstall. In turning flight, a higher speed will result, but it’s all implemented naturally through pitch rate. The significant principal benefit is that EP acts with all the bandwidth (speed of response) of the pitch rate control loop, which is the most reactive and capable loop in any autopilot, and limited only by servo performance. Speed or attitude controls (such as used by others) are in comparison relatively ineffectual. This implementation allows EP to respond successfully to all but the most severe initial conditions avoiding stall in almost any circumstance. While override is always at the pilot’s discretion, all modes, including Straight and Level and Flight Director, are envelope protected.
Full Time Envelope Alerting is active even when the DFC90/100 autopilot or flight director modes are not engaged and alerts the pilot to conditions that can lead to stalling or overspeeding the aircraft.
With the AP disengaged, the system is still much more than a flight director. The DFC90/100 Flight Control system encourages safe and effective operations in all modes of flight. The flight director is fully envelope protected, just as is AP-engaged flight, and can be triggered to take into account the aircraft’s reduced capability under icing conditions.
When disconnected, the DFC90/100 autopilot system continues to calculate lift, monitoring speed, pitch, bank and commands for potential envelope exceedances. The Flight Director Command following performance is also tracked, and persistent errors, whether caused by automation or piloting, can trigger helpful alerts. Any hazardous conditions are announced by built-in audio alerting and by external telemetry data (available for display). These are significant benefits to the single pilot whose attention is demanded over long hours, and potentially distracted by many information sources.
EP® and EA® are registered trademarks of Avidyne Corporation.