Peter wrote:
Operationally, what is the difference between having roll steering and not?
That you fly PBN procedures with minimum flight technical error. This will become an increasing requirement.
Of course you can get by without SBAS and without roll steering, and may be able for some time to come. The same can be said for autopilot, f/d, HSI, RMI, glass, auto-slew, iPads etc but eventually the benefit exceeds the cost for each of us at a different point.
I don’t care whether you, @Peter, or anyone else gets these gizmos; for me, they make the experience of flying easier, safer, more expedient and more fun.
minimum flight technical error.
That is a circular argument because you are defining it in terms of a technical-jargon “requirement” 
Sorry. It is a term that we all are required to understand before August. It is essentially the difference between where the RNAV/PBN environment wants you to be and where you are. People define it, somewhat tongue in cheek, as the ability of the pilot to follow the flight director.
If a procedure has a fly-by waypoint, there is an expectation in PBN that you will respect that requirement and will start a turn before the waypoint and turn smoothly onto the outbound track, with the start of the turn and angle of bank based on airspeed, groundspeed and, even better, wind vector.
If you use a coupled autopilot without roll steering it will get nowhere near that requirement, at least on a substantial turn. Therefore, I assume that you come out of NAV mode, go into HDG mode and manually turn the bug to the new heading “when it feels about right”?
That’s fine, if that’s what you want to do, but roll steering just makes it easier. And roll steering attached to an SBAS box also makes it much smoother, particularly on procedures etc.
Timothy wrote:
If you use a coupled autopilot without roll steering it will get nowhere near that requirement, at least on a substantial turn. Therefore, I assume that you come out of NAV mode, go into HDG mode and manually turn the bug to the new heading “when it feels about right”?
Even if you do not have GPSS the GPS will provide information about turn anticipation with a countdown, the direction of the turn and the heading. When the countdown reaches 0 ( TURN NOW), the pilot switches to HDG mode and turns the heading bug in the right direction to the target heading. That is what GPSS steering would have done.
KAP140 installations on at least the older DA40 with G1000 do not have GPSS and will just follow the CDI. The CDI does not turn to the next heading until you reach the fix.
What I love about roll steering is that it does such a great job effectively determining wind correction, leading of turns, following curved paths, flying procedures such as holds. Anyone who has used an autopilot in Nav or APPR modes knows how the autopilot has to determine the wind correction by trial and error, scallops back and forth sometimes with major deviations to the point the pilot wants to help the autopilot figure things out. An ILS can get interesting when there are winds at altitude, the autopilot finally figures out the WCA, and then as you descend the winds change in velocity and direction, the autopilot by this time has reduced its sensitivity, and is behind the changing environment as it attempts to correct with S turns across the course as you near minimums. With roll steering on an LPV approach, all one notices is that the wind correction changes continuously as needed and you never see any deviation on the CDI. With the standard autopilot, the CDI has to drift off center before the autopilot will respond whereas with roll steering the instantaneous track just has to indicate a small drift in angle and correction is applied. No need to deviate from the course, just bank back to align the track with the desired track.
I had my GPSS installed in my Bonanza when it first came out in 2000, long before the WAAS upgrade. Since that point, I have never used NAV/APPR mode other than when flying an ILS. It was one of the two best avionics investments I made, the other adding a FS210 with Flightplan transfer.
Aviathor wrote:
When the countdown reaches 0 ( TURN NOW), the pilot switches to HDG mode and turns the heading bug in the right direction to the target heading.You don’t even have to switch to HDG mode — you just turn the heading bug and/or the OBS setting while still in NAV mode. At least that’s how it works with the two installations I’ve tried it. (GTN650+KAP140 and GNS430+Piper Autocontrol)
KAP140 installations on at least the older DA40 with G1000 do not have GPSS and will just follow the CDI.
That must depend on the particular installation. Our C172S with G1000 (non SBAS) + KAP140 does have roll steering.
Yes, you can fly IFR without GPSS. And you can fly IFR without SBAS. It would be ridiculous to suggest otherwise. It’s really a matter of how much automation you want and how much you appreciate it when you have it.
Turn this down a notch and we can discuss how much an autopilot is worth having, or an HSI. The whole thing is incremental and we each draw the line where we are happy.
But just like someone might extol an HSI over an OBS, those of us used to GPSS and SBAS are bound to say why it is better and how great it is to have.
When Peter writes:
The benefit is negligible
when he hasn’t got it and doesn’t know what it does, he is bound to get some pushback.
I was a little similar over GTN vs GNS. When my partner and I moved from the Aztec to the Navajo, I was all for keeping the two GNSs, on the grounds that they were “good enough”, but he much preferred the GTNs and I am now more than delighted that we made that change.
So, @Peter, keep your KLN 94. Enjoy. But one day you will discover the joys of more recent tech.
doesn’t know what it does
I happen to know exactly what it does, and how it works. I could even post the ARINC429 message format used for roll steering.
But one day you will discover the joys of more recent tech.
I have a great idea. How about paying for my WAAS upgrade Then I will be able to fully enjoy the new functionality, with no downside.
I’ve flown today an autopilot GPS (non WAAS) approach and switched to ILS a few miles before touch down.
To my surprise, my STEC 60-2 autopilot performed much better in GPS mode (served from a GNS530 without SBAS)! If on GPS the lateral deviation was negligible, when I switched to ILS the AP had a hard time trying to stay on the localizer. At 200 ft I found myself drifted to about 10m to the left of the runway. It’s true there was a gusty lateral wind, but that didn’t seem to affect the AP when driven by the GPS.
I can’t wait to see how it will perform after the upgrade!
Regarding @Peter ‘s comment about the cumbersome interfaces on the market today, I’ ve just receive this from Jeppesen yesterday:
So it looks they are slowly moving into the right direction, with no more card readers and PC needed… hopefully we could use our fast mobile devices at some time to feed the old units wireless, with no more hundreds of knob turnings and hit and miss screen touches in turbulence.
AlexTB20 wrote:
I can’t wait to see how it will perform after the upgrade!
One of the few issues I have with the upgrade of a GNS530 to a GNS530W is that on a localizer or ILS procedure, the W version suspends sending the roll steering label via the ARINC 429 port on passing the FAF, so you can’t use GPSS after that point. The GPSS will then simply blink at you and command wings level.
