ATI Command Augmentation System #1
I am changing acronyms starting with this post. Previously I was calling this an SAS for “Stability Augmentation System” but someone pointed out that this is technically more of a CAS for “Command Augmentation System”.
SAS implies a direct connection between pilot input and control surface deflection with some additional stability augmentation mixed in. CAS implies that a flight computer is translating pilot inputs into a “request” and the flight computer then tries to satisfy that request, but there is no immediate direct mapping between stick deflection and control surface deflection.
Just to review, the ATI “CAS” system internally tracks a target bank angle and a target pitch angle. The pilot is “flying” these target bank/pitch angles and the flight computer is doing it’s best to match up with the request. The pilot changes the target bank angle by deflecting the stick; the greater the stick deflection, the faster the target bank angle changes. This is similar for the target pitch angle. When the pilot centers the stick, the target bank or pitch angle is held steady.
If the pilot banks into a 15 degree turn (target bank angle) and centers the stick, then the flight computer will hold that 15 degree bank indefinitely or until the pilot deflects the stick again. Likewise with the target pitch angle, the pilot pulls the nose up or down with the stick, but when the stick is centered, the flight computer holds that pitch angle indefinitely (as best as is possible for the available throttle and airspeed and maximum control surface deflections.)
The system can limit that maximum bank and pitch angles to stay within “nice” limits. In addition, the system can limit that maximum control surface deflections to avoid abrupt and rapid attitude changes. Currently the system is rigged so that even if the pilot commands maximum pitch up angle (+15 degrees) and pulls the throttle to idle, there is not enough allowed elevator authority to stall the aircraft. This makes the aircraft very safe to fly and very predictable.
Here is some real world flight video from the same flights shown in the first video. The CAS system is active for all but the initial take off in both of these videos. You can see in some situations the system is working quite well, in some situations the flight computer cannot completely compensate for the natural airframe dynamics and environment effects (turbulence, etc.) and in a few situations additional tuning will be required: