This post was written by Mark Krebs, our Vice President of Engineering for Guidance and Controls, and can also be found on our Avidyne Live site.
Here at Avidyne, we have three aircraft in continuous flight test of our new DFC90/100 Autopilot and are proceeding towards certification at this time.
Since many of you may have the same ideas or questions as our test pilots, we thought it would be good to describe some of the key technical features of the autopilot in this forum. For many other "how does that work" questions, we're putting up a FAQ as well that you may want to peruse.
Several topics will be interesting to discuss here: I'll suggest a few to start:
- Envelope Protection
- Attitude recovery with the Straight & Level button.
- How do we perform better, with the same servos?
In this first post, I will discuss Unusual Attitude Recovery.
Attitude "recovery" is what you do after something undesirable has happened. Whether because of a stall, wake upset, loss of spatial orientation, or an extreme maneuver to avoid traffic, the point is you are now in a situation you need to correct. Coincidentally, Flying Magazine published an article on upset recovery in their December issue. Titled "The Leading Killer," it's definitely worth reading. Recovery from these situations can be a difficult thing to execute correctly under pressure, and it's a significant cause of accidents. The autopilot hasn't historically been a useful option in getting out of situations like this, but now it is. The Avidyne DFC90/100 autopilot can assist you in handling an upset, because it has recovery procedures programmed in.
The Straight & Level button may be pressed at any time. When you do, you will get a proportionate response to the situation, meaning that if you are flying in an ordinary flight condition, you will get a restrained response from the autopilot, gently leveling the wings and bringing the nose to just above the horizon.
However if you're in an unusual attitude, the servos will be commanded to their limits, recovering pitch and bank with "full control authority." The autopilot will do the right thing at extreme pitch and/or bank angles, even inverted. For instance, if pitched up too far, the aircraft doesn't risk stall if it flies ballistically over the top, and the ailerons work better too, if the wing is lightly loaded. In this situation, the autopilot will "push" to just 1/4g load factor, enough to keep the gas on the bottom of the tanks and the pencils on the floor, but substantially unweighted. Meanwhile it will be rolling as fast as the servos can go. Once righted, the pitch loops will then pull with maximum g authority we allow.
If this sounds a little forceful, it's meant to be, and again the Autopilot only reacts this way if the initial attitude is extreme. As soon as benign conditions are reached, the response is toned back down to ordinary levels. Also, like all autopilot modes, Straight & Level is guarded by Envelope Protection, so there's no danger of a stall. That's why it's always safe to start the autopilot in Straight & Level if you find it convenient to do so.
The autopilot does have to work with a few limitations that YOU don't have, namely that you can move the control yoke very quickly and powerfully, moreso than the servos. Because of this, if you are not disoriented and are confident with mild aerobatics, you should nominally be able to recover the aircraft quicker than the Straight & Level button. On the other hand, if something is going wrong, or even if you just need a minute to get your thoughts together to deal strategically with a developing situation, Straight and Level is a good way to hand the airplane to the autopilot, and certainly a preferable first step if you're considering the chute.
An airplane can often be an unstable system. What that means is that you have to "stay on top of it" all the time. The purpose of any autopilot is to reduce that pilot workload, and Avidyne aims to take that to a new level. Unusual attitude recovery and envelope protection are meant to change the complexion of autopilot flight from exacting management of a finicky and complex system to one of simple, reliable certainty. You shouldn't be able to "get in trouble" using your autopilot, or lack confidence in it's ability to handle a situation. With the DFC-90/100, Avidyne aims to provide high performance and compelling features, but most of all, we are trying to make flying safe and simple.