I have the plane in the avionics shop, and I'm wondering if this is an opportune time to install a GPSS roll steering unit. We have a turbo-Arrow with a G430W and a Century IIB autopilot with S-Tec ALT30 altitude hold.
I've searched the forum and the web for info. Can anyone answer the following questions:
1- What does GPSS stand for??
2- Why is it called roll steering? (what other kind of steering do we do in an airplane)?
2- I have found 3 available units being marketed: S-Tec ST901, DAC GDC31, and the Century AK1081. I know they'll work with the G430W, but are all of them compatible with an old Century IIB?
GPSS roll stearing basically allows the 430 to drive the AP directly, vs. driving the OBS or HSI which then drives the AP (I think?). In GPSS mode the AP (in my case STEC 55x) works very well with smooth turns to each waypoint.
The big advantage is for flying complex flight plans, holds (yes it will fly perfect race tracks if they are in the data base), or GPS approaches which can all be flown "automatically" w/o having to touch HDG or NAV modes.
With GPSS you can start the AP after take off and disconnect at landing.
The Garmin 430 is capable of working out your flight path through your entire flight plan - including heading changes.
When tracking a NAV source (old way), the autopilot behaves very badly at transitions over waypoints - mine would just take off some random direction, because of the rapid change in error signal.
The 530/430 predicts a turn ahead of time, and initiates the roll prior to the the waypoint. GPSS is able see and interpret this information through a digital connection between the GPSS unit and the Garmin.
The GPSS unit pretends (to an old autopilot) to be a heading bug. It commands changes by changing heading on this fake heading bug.
1. GPSS - GPSSteering.
2. It is called roll steering because it is the bank angle label used from the GPS ARINC 429 data bus.
3. STEC is not legal to install with a Century (99% sure it would work, but not STC'd for other AP's). GDC31 is OK to use with the Century IIB, as long as it has a heading bug input. And the Century AK1081 will work with the IIB, again, as long as it has a heading bug input. The Icarus SAM is also usable, and has alot of "extras", such as different warnings.
Further to everyone else's comments, roll steering adapters work by faking-out the A/P in HDG mode. Instead of the error value produced by the HSI as a result of deviation from required heading, the GPSS unit feeds in an error based on TRK vs. DTK, causing the A/P to provide the rate of turn that is required to make the desired turn. This avoids wind correction issues, and the need to reposition a NAV pointer.
I got your configuration in my arrow: gns430 with the century. I installed a DAC GPSS converter. It cost me around 2500 US installed with STC. Works great. I am using the S-Tec PSS 60 for altitude hold, gs and vs. The pss 60 is not connected to the gpss converter but to the 430 to receive the glideslope signal.
The whole set up flies any approach to MAP.
But before investing in a gpss nowadays I'd conmsider Aspen Avionics PFD 1000 Pro - it comes with a GPSS plus glass AI and DG.
Thanks for your replies, everyone! Great answers...very thorough!
My sense is that I will wait to fly with the existing autopilot a little while longer, and then either add the extra boxes to the existing system, upgrade to the Aspen or scrap the entire lot and install an S-Tec 55X.
It seems that as soon as you order a new avionic installation, even before it comes out of the box there is something newer and better just around the corner.
I just had a Garmin GPSS installed along with a 530W in my Arrow and all the comments above are exactly what I found. The original installation of the STEC 30 would "chase" the CDI needle at VOR crossings and had an issue handling wind correction angles when that would change frequently. The autopilot would hunt the course and eventually settle down if the required course heading stabilized. With the GPSS and the 530W you can see a dotted line curve that "cuts the corner" at heading change points and the GPSS causes the autopilot to smoothly turn and intercept the new heading. I haven't been able to fully check out the wind correction capability yet as I have had only two flights with the new setup and it has been extremely turbulent on both flights so maintaining a constant heading has not been possible.
GPSS and roll steering are two different things! As Denny said, GPSS equals GPS Steering - the GPS is driving the autopilot directly, instead of indirectly through the NAV deviation indicator. In contrast roll steering is a fundamental of how your autopilot keeps you right side up: autopilots can either look at the attitude indicator or the turn coordinator. If they look at the attitude indicator it's called an "attitude-based autopilot", if they look at the turn coordinator it's called roll steering (just like partial panel: stop the roll and the bank to stay upright). Roll steering is theoretically "better" because you can use the autopilot to keep the plane upright if your attitude indicator or vacuum system fail. Most GA autopilots, including the Century series, are attitude-based. STEC and the King KAP140 are roll-steering based. You can't convert an attitude based autopilot to roll-steering - you'd have to tear out the entire existing autopilot system and install a new one.
GPSS, in contrast, is easy to install. The DAC GDC31 and the Icarus SAM work by taking the steering signals from the GPS and turning them into very precise inputs for the autopilot. They emulate a heading bug so your autopilot needs a heading input. With GPSS the autopilot can follow course lines as before and can also fly holds and procedure turns. I installed a GDC31 - course deviation is now measured in single-digit hundredths of a NM.
Sorry, I don't know anything about the AK1081. And the STEC converter is only STC'd to work with their own autopilots.
