Peter wrote:
one of the last TBs made, in 2021
So at last you managed to have Daher put together one for you from the old parts in stock ;?
Ha 
I meant 2001.
To my understanding roll steering is beneficial when on an RNP approach based on ARINC 424 path and terminators. The older GPS don’t support the more complex path and terminators. On the SN3500 roll steering is enabled in HDG mode by pushing the HDG knob. Latest is ARINC424 is version 22 I believe. Older FMS cant digest that
The KLN90B I have has turn anticipation which work relatively well but its not computed bank angle but averaged. Even if the KLN94 would enable roll steering I don’t think it would offer significant improvements since it doesn’t support interpretation of the more complex path and terminators used for SBAS in the final approach segment.
This is from the newer 172 poh:
At 172R serial number 17281174 and On, automated Roll Steering
functionality has been added to the Bendix/King KLN 94 GPS
Navigation System and the KAP 140 2 Axis Autopilot System. Roll
Steering coupling between the GPS and the Autopilot provides area
navigation with automatic course changes at flight plan waypoints
similar to Flight Management System (FMS) operations, but without
vertical navigation capability. The Roll Steering function is similar
to “turn anticipation” for the autopilot.
At the noted serial effectivity, the KLN 94 GPS (ORS 03 or later)
has an added Roll Steering signal output. In order for the GPS Roll
Steering output to be utilized, the KAP 140 Autopilot (-7904 or later)
has an added input for the Roll Steering signal and additional
system wiring has been added to the airplane to connect the Roll
Steering signal output from the KLN 94 GPS to the Roll Steering
input of the KAP 140 Autopilot.
OK so the KAP140 has the analog roll steering input too.
The KLN90B I have has turn anticipation which work relatively well but its not computed bank angle but averaged. Even if the KLN94 would enable roll steering I don’t think it would offer significant improvements since it doesn’t support interpretation of the more complex path and terminators used for SBAS in the final approach segment.
The KLN94 + simple autopilot will fly, in NAV mode, any track sequence which the GPS can handle and display.
Perhaps sub optimally but mostly just fine.
