My response was indeed to dispel your attempt to link lack of active control margin feedback to the accident. It is important for current operators of that aircraft to know there is more than adequate control margin throughout the operational flight envelope.
For those not familiar with the term, “unique trim” refers to a controller which resembles a joy stick. When relaxed, it is spring/detent centered. Inputs move away from the center detent by a small displacement, typically one to three inches.
Now to your other comments. I was a presenter at the early certification meetings to educate the FAA as to the Sikorsky FBW approach. There is nothing in the FARs that would prevent implementation of a unique trim controller. The main issues with the non-traditional controller approach were in demonstration of FARs such as longitudinal static stability. With automatic trim follow up (a fundamental element of unique trim), the stick is always in the same position regardless of trimmed speed. So we came up with an alternate method to demonstrate stability.
If Bell decided against a unique trim approach, that was their decision based on an assessment of the effort required to meet the certification challenges, not an FAA mandate. BTW, all the publicly accessible photos of the 525 cockpit show a sidestick with very little space for proportional control movement, at least with acceptable sensitivity. Makes one wonder, if it isn’t unique trim, what is it?
There are indeed benefits and challenges to the unique trim approach, and I have presented a couple of papers on the subject. The challenges, however, do not generally have to be mitigated by incorporation of active feedback. The control strategy with a unique trim stick is quite different than a proportional controller. If you look at time histories of a flight with a unique trim stick, you’ll notice the stick is in detent about 80% of the time. The stick moves in and out of detent at a relatively high frequency. Thus requiring the non-flying stick mirror the movement of the flying pilot is of questionable value.
Autopilot feedback? Think about what happens when you are coupled to a flight director airspeed and drag off speed with stick input. When you release the stick, do you respond to the subsequent pitch movement because the stick is moving forward or the nose is pitching down? I suggest it’s the latter.
I have considerable time in experimental helicopters with both active and passive unique trim sticks and, comparing the two, I didn’t find the active stick to provide a demonstrable improvement. And there are some nasty failure modes embedded in the active stick architecture that need to be addressed in emergency procedures.
Lastly, your contention the USMC “demanded installation of active BAE cyclics sticks” is false. Sikorsky convinced the Marines that there was potential in the active stick to incorporate tactile cueing, and if they didn’t invest in the technology now they might not be able to retrofit it later. The aircraft could handle the approximately 30 lbs weight penalty and the cost was just lost in the scope of the entire program. They are unique trim sticks and do not “mimic” the functions of mechanical controls.