ILS tracking with STEC 30

[AndreiC]

After flying quite a few approaches with GPS guidance from my GNS430W, which the autopilot (STEC/30) followed very well, I thought a few days ago to try an ILS. Luckily I was in good weather and only tried it for fun. The autopilot gets very easily confused, and either hunts badly (going from the left to the right of the localizer and back, never intercepting properly) or loses track of it completely (needle goes all the way to one side, and if I was not in good weather I'd have to go missed).

What I did was fly by hand until I had the localizer needle centered, put the heading bug on the runway heading, and engaged either TRK LO or TRK HI modes. The behavior above happened in both TRK LO and TRK HI modes. (I tried flying an ILS or a LOC approach 3-4 times.) My system has a GNS430W coupled to the autopilot via an STEC GPSS unit, but I thought that only affects the HDG mode. (In any case, the GPSS unit was on HDG mode.)

Am I doing something wrong, or is this autopilot known to be bad at tracking a localizer signal? In my previous plane, a Piper Cherokee, I had a Century autopilot which did very well on a localizer signal, so it's not just that it is an older analog autopilot.

[KSMooniac]

That has been my observation with a 530W/430W and STEC-30 setup.  I don't even try to get it to fly a VOR or Localizer any more and use GPSS 99% of the time unless I'm getting vectors and then use HDG mode.  I always request GPS approaches if I need to do one for real, and if I need to do an ILS I'll typically set one GNS to the ILS and leave the other on GPS.  The STEC will follow the GPS very well but I'll keep my needles centered on the Localizer/Glideslope CDI with the other GNS box.  

[M20S Driver]

Here is what I have been using and I have the same set up (gns530/430, Stec30 with GPSS).  It worked in my Eagle and my friend's J with the same set up.

Approach

• Activate/Load Approach---- (Activate leg)

• Load the ILS/VOR frequency (VLOC flip-flop)—or confirm

• Press “CDI” to select “VLOC” --- or confirm

• OBS: INBOUND COURSE

• Near FAF, set the GPSS to HDG and Stec30 to Hi-Trk

 

 

 

[gone fishing]

do you have the autopilot pre-flight test instructions? It's supposed to be done before every flight. 

STEC autopilots (except for the new 3100) are rate-based, so they will always play catch-up. So they need an occasional tune-up to work reasonably well. If the control head gyro speed is low, it will be lazy. If the servos aren't exercised regularly, they will also be lazy.

[DXB]

Seems like pretty typical behavior for the rate-based STEC-30. I've flown behind one for almost 10 years.  It is precise enough to follow a course enroute decently via your GPS.   Unless the wind is calm, it does a terrible job flying approaches (e.g. either RNAV or ILS) because of the greater precision needed.  I think the fact that it did a decent job with the RNAV was just luck.

[IvanP]

I had S-Tec 30 coupled to GNS 430 with GPSS in my prior plane and it was working fine both in GPS or VOR/ILS modes. I was using TRK HI setting for ILS approaches as it provided greater sensitivity. If I recall corectly, GPSS was in HDG mode for VOR/ILS guidance.  Couple times when it did not work well occurred due to operator error - I forgot to switch the autopilot to the correct input (NAV 1/NAV switch). VOR/LOC INtercepts were usually ok if less than 45 deg.

[Ragsf15e]

Mine worked ok but it wasn’t as good as in gps mode.  Gpss needs to be in heading (off) to track the ILS properly.
Another option is to just leave GPSS on and the guidance in GPS. This is not legal approach, guidance, but the auto pilot will be really close.
 

Use radio two tuned into the ILS (if you have a separate OBS). It will fly the GPS course close for you and you are really in control on the manual green needles on the other OBS.

[bigmo]

I have a different navigator (GNS480), but my STEC-30 tracks VORs and localizers exceptionally well with no scalloping.

First off, you definitely want TRK-HI for the localizer. I don’t do it super often (almost always fly RNAV approaches), but I turn off GPSS, make sure Ive got the approach selected and activated, and approach the inbound course at a non-ridiculous angle. As the needle centers (just before), I dial the inbound course on the bug and I pop the STEC to TRK-Hi. At that point it locks on and follows it really well.

Do you have a non Garmin option to test (ie a backup nav radio). 

[Ricky_231]

I've only had an STEC-30 for a few months now, but I find it very sloppy compared to the Century 41 I had in my old plane. It gets thrown off very easily by turbulence (slow to react to altitude changes) or strong quartering winds (feels like it has trouble countering the wind pushing on the rudder and it's always chasing the heading bug or magenta line). In fair weather it's awesome, but the Century would get me through rough weather and turbulence much more reliably. Until it died. And left me without an autopilot for 1.5 years because I couldn't find a single avionics shop that would touch it .

[skykrawler]

On 1/9/2025 at 8:01 PM, M20S Driver said:

Here is what I have been using and I have the same set up (gns530/430, Stec30 with GPSS).  It worked in my Eagle and my friend's J with the same set up.

Approach

• Activate/Load Approach---- (Activate leg)

• Load the ILS/VOR frequency (VLOC flip-flop)—or confirm

• Press “CDI” to select “VLOC” --- or confirm

• OBS: INBOUND COURSE

• Near FAF, set the GPSS to HDG and Stec30 to Hi-Trk

 

 

 

FWIW .....

Don't think it matters what the GPSS convertor is set to once the STEC30 is set to Hi-Trk.

In the case of a missed approach you might consider leaving it set to GPSS.  Then to follow the missed approach course disconnect the autopilot and re-engage autopilot in HDG to follow the GPSS steered MA course (somewhere in there the GPS has to be told to provide MA guidance).

[Immelman]

I have an STEC 50.

Similar observation. Great for en route to follow a heading.

IMO once you're set up, and all that is left to do is fly the approach, its just more fun to hand fly an ILS.