GPSS / roll steering

chflyer wrote:

Avidyne recommends changing the STEC AP mode from HD (ie using GPSS) to HI-TRK, which uses CDI deviation, when flying both LPV and ILS approaches. Your comments would indicate that leaving the AP in HD+GPSS mode for lateral tracking would give better performance, essentially using GPS for lateral guidance rather than LOC in the case of an ILS. I’m certainly going to go out and compare the two!

Handling of GPSS guidance on an ILS is different, dependent on how the system is wired and which GPS is used.

The GNS430, GNS530, and GNS480 continue to output roll steering on the final approach segment of an ILS or Localizer approach.
The GNS430W, GNS530W, GTN650, GTN750 suspend sending the roll steering output on the final approach segment of an ILS or Localizer Approach
IFD440 and IFD540 – I don’t know what they do

Each GPSs system can work differently. The Stec units install manual says to wire VLOC annunciator to the unit so that anytime VLOC is selected, the converter is disabled. I don’t recommend connecting this wire.

EHSI devices also work differently. The Garmin units require that GPS be the selected source for GPSS to be provided to the autopilot. Aspen units (configuration dependent) allow the CDI selected display source to be different than the GPSS source. Each device is different and you really have to understand the specific system and in some cases how it is installed or configured. Software versions may also make a difference.

AlexTB20 wrote:

I wonder if my Sandel 3308 has the same ‘software’ enabled roll steering…

Something I’d check with your installer. If so, it might save you the GPSS converter. Actually if it does, you should already have it now as the 430 and 530 can do GPSS roll steering as well, not only the WAAS units.

If it is so, that would be great. Otherwise, the question might be if you want to upgrade to a 3500 which does have it instead of buying the converter.

Question for avionics and autopilot gurus…

I’ve been testing an ST-901 GPSS module through which my GTN 650 can drive the S-TEC 60-2 autopilot. GPSS transforms the accuracy with which the AP follows a procedure or route and once it has captured a straight leg it follows it like a laser, but there’s still a couple of hundred yards overshoot on a 90 degree corner:

It looks as if the GTN 650 is anticipating the turn, as it is supposed to do, but just a few seconds too late.

Is this to be expected? I wonder whether it has to do with the Maule-specific installation in which the “roll” servo operates on the rudder, or perhaps the S-TEC AP limiting roll to 90% (?) of a standard rate turn?

Of course, a couple of hundred yards overshoot doesn’t matter in practice, but it looks a bit sloppy to anyone watching a radar screen. I would hate ATC to think that any Maule pilot would be so busy drinking beer as to fly a procedure like that.

This came up before although I think that thread is a bit inconclusive in that nobody interpreted my Q exactly (or so it appears).

AFAIK roll steering does a Rate 1 turn (or less for small angles) and then when it gets near the final heading it reduces the roll angle proportionally to the remaining error. Obviously, the loop gain is chosen to avoid instability in the control loop.

Throughout, the GPS transmits ARINC429 packets containing the desired instantaneous roll angle to the autopilot, in a rapid succession.

But the GPS doesn’t calculate the precise trajectory to generate an exact intercept.

I have the exact same behavior on my STEC 60-2, where the GPSS data is provided by a GNS530 non WAAS and it’s converted internally by a Sandel SN3308 EHSI before it is sent to the autopilot.

It will be nice to hear someone had this fixed in any way.

Rob wrote:

For example.
You are using GPSS to fly a RNAV approach finalising in a ILS, at what time and how do transit from GPSS roll steering to ILS. Do you at some stage change to heading mode and arm the autopilot to capture the ILS as you would normally do.

You are mostly radar-vectored to intercept the ILS anyway…

During autopilot flight, one usually flies towards the LOC in the HDG mode, with the AP armed to intercept the LOC.

GPS flight is normally in NAV mode (although some roll steering converters fake a heading bug and then the AP is in the HDG mode again) and how one transitions from NAV (GPS) to APR (ILS) is going to be system dependent. @rob you need to post details of your avionics.

Multiple roll steering threads merged.

Sorry Rob I don’t want to sound unhelpful but if you are looking for good responses it won’t work to post 1 line and expect people to jump in and write an essay. This has been going on for years, here and elsewhere.