The yaw dampers that I have come across are purely to stop oscillation. It’s quite apparent in the back if the YD isn’t on in even a small Citation. There is no system to tell you about flying along with the ball off centre, other than the indication in the PFD which you can correct with the rudder trim.
Any airworthiness limitation will surely be about the traditional system to damp out oscillation in yaw, and the consequent dutch roll
This is an interesting product
The most interesting use is aligning high gain, narrow beam antennas for long range IP networks (offshore applications) that need to align two antennas very accurately and only a true North Compass can achieve that (without celestial navigation techniques e.g. theodolites etc).
You have some interesting ideas, and I think Embraer needs to hear from you about how to improve their flight control system.
what do you mean by “steady state yaw”?
Flying with the ball off to one side.
An AHRS has to provide an attitude solution for the aircraft.
Of course, but why is a full attitude solution required for yaw damping?
A yaw damper is effectively two separate systems.
One damps the Dutch roll (oscillatory) movement, and requires yaw rate information but not attitude.
The other is sideslip control (ball in the middle) and by definition you only need a single accelerometer as an input for this. No gyros and definitely no GPS. Some simple yaw dampers only do this without doing the Dutch roll bit.
So the Phenom GPS failure issue can only apply to the former, and I still don’t understand why this would ever need an attitude solution.
Peter, what do you mean by “steady state yaw”?
dnj wrote:
If the yaw rate is constant (no angular acceleration), there is nothing for the yaw damper to do, ignoring sideslip control which is a separate function to dutch roll damping and doesn’t use the gyro anyway.
I’m not rated for it, but IIRC, yaw damper on the Phenom 300 does both and I don’t recall any degraded mode where it would do just one of those. If any input the yaw damper needs becomes invalid, then it disengages.
Of course if you want to pony up for three ring laser gyros, you can dispense with GPS. MEMS just ain’t that good.
This isn’t complicated, and whether anyone understands or agrees with it or not, it’s still true. An AHRS has to provide an attitude solution for the aircraft. It can’t do that with MEMS alone, it needs external steady references to calibrate and remain calibrated during flight. If those external references were steady and smooth enough then there’d be no need for MEMS gyros. But they’re not. But MEMS gyros alone won’t do it.
Your AI uses gravity as an external reference; your Heading Indicator uses the compass as an external reference. For reasons already explained by me that’s not sufficient for a MEMS system so GPS is used to fill the gap.
Referring specifically to a yaw damper: it doesn’t merely" damp yaw" – it’s part of a whole system that adjusts the rudder to keep the ball in the middle. Quite obviously it can’t do that if if thinks the aircraft is continuously rotating about all three axes at the same time because the gyros aren’t mulled properly.
Indeed, I suspect that Embraer (who will surely be familiar with about 80 years of history of yaw damping) have done this for removing a steady-state yaw, rather than for removing yaw oscillation which is easily done with a cheap gyro, and while at it they used the same solution for removing yaw oscillation (software costs nothing) 
