S-TEK 65 with GPSS and Garmin GNS 430 WAAS

[Radiocommander]

I recently installed this configuration in my aircraft. I am seeking anyone familiar with this autopilot and GPS combo. Any input would be appreciated.

The avionics shop that did the install told me on GPS and LPV approaches I need to select "GPSS" on the Annunciator selector and "HDG" on the Autopilot controller. naturally I may use my altitude pre-select as necessary and Altitude controller as needed. The system works well with the turns to way points and the GPSS is anticipating the turns as it should. Where I am having an issue is on the final approach inbound. I was told that once established on the approach and still under the Glide Slope bars on my HSI I am to switch from the "HDG" mode to the "Nav" mode on the autopilot controller. Trouble is it does not capture the glide slope and continues flying the approach at the final approach fix altitude.
Am I missing something? I shot an approach 5 times today, and it captured the GS only once. Frankly, I don't know what I did differently the time it worked right, but at least I know it can work!:)
Any insight to the approach procedures using a GPS WAAS with an S-TEC 65 with GPSS would be appreciated. There are NO instructions for this combo either in the S-Tek Pilot operating handbook, the Garmin GNS 430WASS books, or anywhere on S-TEK's website or the internet that I can find.
Thanks!

[denlopviper]

switch the Autopilot to APR mode (approach mode). NAV mode doesnt shoot approaches, it'll just follow gps way point or the radial dialed in on the VOR/HSI

[Radiocommander]

The STEK 65 has no APPROACH MODE. It only has "HDG" and "NAV"....

It would be much easier if it did like the STEK 55....

Any other ideas?

[madlandrover]

Interesting. All the KAP, Stec, and Garmin autopilots I've flown with will happily descend below selected altitude in the particular case the OP is trying to achieve - flying a precision approach. They do have to have APP mode selected to capture the GS but once captured will descend from the selected altitude. Means you can then input the desired post-go around altitude in the altitude selector while flying the approach down to DA, then simply reselect ALT mode at an appropriate point. I've yet to fly one that attempts to do it in NAV mode though, I'm assuming the autopilot is actually certified for GS capture?

[madlandrover]

Ah ok, we did both mean the same thing ;) I wonder if the OPs issue is due to trying to intercept too tightly? The manual states "The aircraft must be 60% or more below the GS centerline during the approach to the intercept point". Might be worth trying a manual intercept as described in the next section.

[IO540]

I have little knowledge in what is (for Europe; the OP is American) a highly specialised area of avionics, but there are two separate issues:

1) Getting an autopilot to track a GPS approach laterally. This is easy. You traditionally do it in the NAV mode, and as one gets near, the full scale sensitivity automatically winds down from 5nm to 1.0nm to 0.3nm. If you have GPSS (a.k.a. roll steering) installed, the autopilot is either driven from a GPSS converter (whose output fakes the heading bug output of an HSI) and then the autopilot is used in the HDG mode, or it is driven directly digitally, using ARINC commands, from the GPS, in which case you use the NAV mode.

2) Getting an autopilot to track a GPS/LPV approach vertically i.e. a virtual glideslope. This doesn't yet exist in Europe because it needs EGNOS and that is still (AFAIK) buried in some c0ckups despite it being "certified for life critical uses" very recently. Exactly how you get a particular autopilot installation to track this virtual glideslope coming out of the GPS, is going to be rather installation specific. Ideally it will be again via digital ARINC commands, but more likely there will be some "fake" whereby the GPS outputs a glideslope-like signal and the autopilot is fooled into believing it is an ILS glideslope, with the appropriate sensitivity.

I think the autopilot will have to be in the APR mode for tracking an LPV approach; I don't think any autopilot will even look at any external VNAV signal when in the NAV or HDG modes.

But you do not fly normal (non LPV) GPS approaches in the APR mode; they are flown in NAV or HDG. So I am baffled....

Wigglyamp here should know exactly how this stuff is hacked together - even if nobody will have tested it in Europe (well not overtly, anyway :) ). I know some people in Spain have been testing LPV approaches for years, with a GNS480 with its firmware hacked to use EGNOS despite the Test flag being active.

[bookworm]

"The aircraft must be 60% or more below the GS centerline during the approach to the intercept point"

I've been similarly baffled by apparently intermittent success in coupling an STEC 60-2 to an ILS GS, which has a similar requirement. I think the condition you quote is the key. Thank you madlandrover, you've saved me some head scratching.

cjboy, as you've probably gathered from the manual, the STEC series have no "approach mode" as such, but the NAV mode flies the glideslope (and illuminates the APR light).

[IO540]

"The aircraft must be 60% or more below the GS centerline during the approach to the intercept point"

That is clearly not a universal thing, as my KFC225 will capture a glideslope from above or below - so long as one is reasonably close. I have actually done it and to my amazement it worked.

But here we are talking about an LPV glideslope, not an ILS one.

[bookworm]

That is clearly not a universal thing, as my KFC225 will capture a glideslope from above or below - so long as one is reasonably close.

And many years behind a KFC 200 a/p is my lame excuse for not knowing the SOPs and limitations for my current aircraft.

But here we are talking about an LPV glideslope, not an ILS one.

Would you expect a difference? The initial parts of an LPV approach are flown in GPSS mode, but the final approach segment is flown in NAV (APR) mode. I get the impression the a/p treats the LPV CDI and GDI just like an ILS, which probably means that having flown the first parts as if on rails, it starts weaving all over the place. ;)

[IO540]

That is a different kind of thing to LPV.

What you see there, I am pretty sure, is a GPS databse version of the continuous descent profile which is nowadays shown on a lot of Jepp charts for non precision approaches.

I know some people have discovered then can fly these CDAs with their existing GPS/autopilot installation right now.

The initial parts of an LPV approach are flown in GPSS mode, but the final approach segment is flown in NAV (APR) mode

I find that baffling, but I have never dug into this (no need to know) so I can't argue :) How does the AP switch over from say HDG to say APR mode?

[bookworm]

How does the AP switch over from say HDG to say APR mode?

As far as I can tell from the manual, by the pilot pressing the NAV button, just as when you switch from HDG/ALT modes when on an intercept track to an ILS.

[Big Pistons Forever]

A few questions for the original poster.

1) Are you trying to fly an ILS, a LNAV/VNAV or a LPV approach ?

2) If you do not couple the autopilot but just hand fly the flight director V bar do you have the same issue ?

3) Have you tried leaving it in HDG mode and see what happens ? Sounds crazy I know, but I occasionally fly a KFC 200 with G430 W equiped Cessna 340 and that is what you have to do if you want to fly a coupled approach off an RNAV star :confused:

[BigBen1251]

Glideslope capture is done in NAV mode. See 4.2.2.3 and 4.2.2.4 in the POH, which is linked to in another post. There are a series of conditions that must be met for 10 seconds before GS arming, mainly including being 60% below the glideslope. These are listed in 4.2.2.3. It's usually because one of these is missing that GS doesn't arm.

However, as 4.2.2.4 states you can force GS arming by pressing ALT once if you're already in ALT mode, which will usually be the case, or twice if you're in VS mode, which might occur if you're descending in VS mode to get down to the glideslope or glidepath.

To respond to some of the other comments, there is an APR mode, but it can't be selected manually--it's selected by the system when in NAV mode, and a LOC is detected. This is a confusing use of terminology by S-TEC, because APR mode in S-TEC is completely different from how other manufacturers use the term--it's only a higher sensitivity version of NAV mode, and always goes with NAV; that is, there's no stand-alone APR mode, only NAV APR.

Hope this helps.

[Tinstaafl]

This AP certainly has an Approach Mode. The trick is figuring out how to get to it. According to the linked manual it will automatically arm APR mode if a LOC frequency is set and then NAV mode is selected.

oops....what BigBen said...

You can also disable & re-enable GS arming (eg during a hold prior to the approach) by pressing & releasing the NAV button twice. Once re-intercepted the LOC then GS can be armed again by a further press of the NAV button. At least, that's what the manual seems to say. But then, it also says that pressing ALT when in ALT mode will also disable GS arming.

Frankly, I think it's a poor interface. I much prefer a discrete APR mode selector.