As an example, consider the Automatic Flight Control System (AFCS) of the A7-E. The A7-E is a Vietnam era light-attack jet that was deployed until being replaced by the F-18. I suspect that the system used to maintain coordinated flight hasn't changed much since then.
The AFCS is a 3-axis autopilot with control augmentation. It includes automatic pitch trim, failure monitoring, with automatic disconnect and warning. There are 8 operational modes:
- Yaw stabilization
- Control augmentation
- Attitude hold
- Heading hold
- Heading select
- Altitude hold
- Automatic carrier landing system
- Ground controlled bombing
The operation of the AFCS required both electrical and hydraulic power.
The yaw stabilization mode provided coordinated turns, with an aileron-rudder interconnect, and provided up to 5 degrees rudder trim in either direction. The coordinated turn was automatic, without pilot input. The yaw computer got its signal from a lateral accelerometer, and movement of the rudder pedals placed this signal on hold. As previously noted, once airborne you kept your feet off the pedals. A pilot could engage and disengage the yaw stabilization mode.
I went to maintenance to read the maintenance log book on the aircraft. I had a launch, and was on my way out to the flight deck. This is from my memory of over 30 years ago, but nonetheless it is fairly accurate. There were pink slips and yellow slips. Pink slips were maintenance problems that had downed the aircraft at some point. I would read these to get an overall idea of the problems, recurring and otherwise. Yellow slips were, "Hey, keep an eye on this. We haven't downed the aircraft, but ..."
The mission included unusual attitudes, and low speed flight, common in air-combat maneuvering. I looked over the log book and noticed the AFCS had a yellow sheet, and stated that the system had initiated abrupt rudder control inputs at a point in a previous flight. It was yellow because maintenance was not able to reproduce the problem. I didn't connect the dots, although you might have.
I didn't think too much of the yellow sheet because I never had problems with the AFCS system. Additionally, it was automatic and so I rarely interacted with it except for hitting the switch that engaged it.
In the dog fight I ended up on my back with 0 airspeed at around 20,000 feet. The AFCS inadvertently commanded an abrupt rudder input. A departed aircraft, with a yaw input, are the conditions necessary for a spin.
And spin I did.
The AFCS yaw stabilization is great for allowing the pilot to concentrate on other tasks other than looking at the turn needle, like looking for the enemy at your 6. One must always remember, though, that automatic systems are often clever by one half.