Garmin 430W autopilot operation

[Glasair470]

We have upgraded the Garmin 430 to WAAS in our homebuilt. We have the
Garmin talking to our STEC autopilot using its ARINC 429 roll steering
commands through the STEC GPSS interface. In the old 430 non-WAAS
unit, even if you selected an ILS approach on the Garmin and loaded
it, you would get roll steering commands down to the ground out of the
Garmin. Of course, you are using the VLOC receiver and the ground
stations for the approach but the autopilot would still fly the
magenta line on the screen that was provided "for guidance". I now
notice on the WAAS'd unit we got back that the roll steering commands
only seem to be provided until we get to the ILS final approach fix,
and then the STEC starts flashing to indicate that there is no more
roll steering. It drops out at the FAF.
We have to revert to HEADING mode if we still want to use the
autopilot on the approach or maybe NAV mode and track the localizer
needle, although I cut those wires figuring that roll steering is so
good I would never again want to track needles.
If we are flying a GPS or an RNAV approach on the 430W it does give
the roll steering signals all the way down to the ground or out to the
holding fix on the missed approach and will even fly the holding
pattern.
Anyone else can confirm this? I called Garmin technical support and
the guy really couldn't get what I was talking about. He was under
the impression that we should not be getting ANY roll steering on an
ILS approach but that was not the case with the 430 and semi-not the
case with the 430W.
Is this a deliberate feature programmed into the software to insure
that you are not flying the "guidance only" course line on an ILS
approach?

[Harvey Spencer]

Thanks to you guys for confirming my understanding of the problem. I only
flew 2 ILS approaches since I had the unit WAAS'd but
was familiar enough with the operation of the 430 before it got WAAS'd and
realized that at the FAF roll steering goes away with the
430W. I don't have GS coupling on my STEC so I have never had to worry
about that problem. It is incredible that with roll steering you
have to resort to the analog "fly the needles" with the autopilot.
Concerning the STC implications of the hookup of my Garmin to the autopilot
I chose to NOT hook it up according to the STC for the
430W, so GPSS is NOT disabled when the CDI is switched to VLOC. It is
because my autopilot flies like a pro with roll steering but
in NAV mode and following the needle, it S-turns like a drunken sailor.
To have GPSS roll steering active all the time, it just involves not hooking
up the inhibit wire on the autopilot
to the VLOC active line from the Garmin. If you hook up that line, then
when you switch the Garmin to VLOC it grounds that inhibit line which tells
the autopilot to forget about using roll steering.
I know that in my plane, roll steering is always following the magenta line,
because that is how I hooked it up. Pretty easy to remember. There are
actually very few times when my flight plan does not match where
I am going using the VLOC, so the magenta line is for all intents and
purposes the same as the VLOC radial or the LOC.

"Glasair470" > wrote in message
...
> We have upgraded the Garmin 430 to WAAS in our homebuilt. We have the
> Garmin talking to our STEC autopilot using its ARINC 429 roll steering
> commands through the STEC GPSS interface. In the old 430 non-WAAS
> unit, even if you selected an ILS approach on the Garmin and loaded
> it, you would get roll steering commands down to the ground out of the
> Garmin. Of course, you are using the VLOC receiver and the ground
> stations for the approach but the autopilot would still fly the
> magenta line on the screen that was provided "for guidance". I now
> notice on the WAAS'd unit we got back that the roll steering commands
> only seem to be provided until we get to the ILS final approach fix,
> and then the STEC starts flashing to indicate that there is no more
> roll steering. It drops out at the FAF.
> We have to revert to HEADING mode if we still want to use the
> autopilot on the approach or maybe NAV mode and track the localizer
> needle, although I cut those wires figuring that roll steering is so
> good I would never again want to track needles.
> If we are flying a GPS or an RNAV approach on the 430W it does give
> the roll steering signals all the way down to the ground or out to the
> holding fix on the missed approach and will even fly the holding
> pattern.
> Anyone else can confirm this? I called Garmin technical support and
> the guy really couldn't get what I was talking about. He was under
> the impression that we should not be getting ANY roll steering on an
> ILS approach but that was not the case with the 430 and semi-not the
> case with the 430W.
> Is this a deliberate feature programmed into the software to insure
> that you are not flying the "guidance only" course line on an ILS
> approach?

[Thomas Borchert]

Harvey,

> I chose to NOT hook it up according to the STC for the
> 430W, so GPSS is NOT disabled when the CDI is switched to VLOC.

Not many of us have that option. You're flying an experimental.

> It is
> because my autopilot flies like a pro with roll steering but
> in NAV mode and following the needle, it S-turns like a drunken sailor.

Not many of us have that problem. You're flying an experimental.

<gd&r>

What I'm trying to say is: If the S-TEC installation is properly done in
an aircraft it is designed for, it will follow the localizer in approach
mode just fine - and it'll be the legal way to fly that approach.

--
Thomas Borchert (EDDH)

[Thomas Borchert]

John,

> So far, those who have had this done have not had
> difficulty in obtaining a field approval for the change.

Very interesting, thanks! Our EASA is not that flexible, I'm afraid.

> Regardless of how the autopilot is determining the course, the
> pilot is required to be displaying the ILS localizer and GS on the CDI or
> HSI, and must use the CDI or HSI to determine if corrections need to be made
> or not.

Agree. Would you agree that a properly set-up S-TEC should be able to follow a
localizer without too much weaving about?

--
Thomas Borchert (EDDH)

[Glasair470]

I agree that it would be nice for the STEC in my Glasair to do a
better job of flying the analog needle output. I have worked on the
sensitivity tailoring by hanging internal resistors under the
direction of the technical service personnel at STEC and can only get
it a little better than when I started. Also, tinkered with the roll
centering that is a user adjustment (hidden under a panel screw) to
try and null that out as best as possible. The Glasair just has too
much roll sensitivity to do a very good job with this particular
autopilot in NAV mode, but in roll steering mode where, yes, it is
"faked" into thinking it is in an everchanging HDG mode, it works
great.
I also agree with the poster that you can drive the autopilot any way
you want when doing an approach using terrestrial nav aids and as long
as you are watching the appropriate needles it is legal. One way is
to just put it in HDG mode and keep tweaking the heading bug. I guess
that is a "brain-driven" coupled approach. It doesn't matter where
the autopilot gets its intellengence from as long as the aircraft is
flying on the right path.

[Peter Clark]

On Wed, 16 Jan 2008 15:03:59 +0100, Thomas Borchert
> wrote:


>Agree. Would you agree that a properly set-up S-TEC should be able to follow a
>localizer without too much weaving about?

Mine do. Check main aircraft rigging and cable tensions, upper end of
the limit seems to ork best, autopilot servo brible cable tensions,
and the servo start voltages are within spec. There can be a feedback
loop when one or more of the above is wrong, the computer thinks it
needs a correction, starts it, but due to slow or damped response
because of one of the items above being wrong it doesn't stop on time,
starts a correction back the other way, but lag due to start voltage
and/or having to take up the control slack makes it overcorrect the
other way, ever increasing oscilations.....

[Peter Clark]

On Wed, 16 Jan 2008 17:56:26 -0500, "John Collins"
> wrote:

>Peter,
>
>Besides cable tensions another biggie is a startup voltage for the servo is
>too high.

Yea, I had start voltage listed in the things to check ;)

[Ron Natalie]

The STEC GPSS (at least my 55x) do not work with vnav.
When you're converging on the final course you need to
go into regular NAV+APR+ALT mode.