Autopilot approaches - GPSS vs Approach modes

Waltla

I have an STEC 50 autopilot and working on my instrument rating have been doing approaches in Approach or Heading mode. I've heard GPSS aproach mode is much better especially in hard IFR. I've never been on a plane with this, what would GPSS add or do differently? Would like some information before I pursue having this added to my autopilot. Thanks.

Waltla

empannin

Hi Waltla,

GPSS mode has a GPS signal driving it and can provide more information to the AP including for turns to intercept as well as tracking taking into account GPS derived info on the course over ground.

If you have a GPS nav head it is possible you already have GPSS if you have the GPS steering module installed.

Here is what looks like a decent description of how this system would work on an STEC 50 - including GPSS. What GPS navigator do you have in the plane?

http://n5799p.morgan-systems.com/pubs/Unauth_Autopilot_Operation.pdf

jacobsja

To expand a bit on Eric's excellent input, in GPSS mode, the GPS will have the ability to adjust for winds and speed so that it is able to predict when to turn in order to arrive at the new leg's heading when appropriate.

GPSS mode also requires a constant trust-but-verify watch over the system as there are sadly too many factors which degrade GPS integrity ranging from 5G service, to inadvertent interference from ground based equipment (ie: system tests gone awry, or inadvertent bleed-over of spectrum), to outright nation-state manipulation / degradation (this risk is low in the US, but the military should (in theory) issue a NOTAM when they want to expose their troops to localized GPS reliability issues, so it does tend to occur around military installations without the required warning). Only mitigation to loss of GPS integrity is maintaining skills on navigating via radio and working with ATC if needing a vector heading assignment.

Just thought of another possible GPS degradation factor for somebody working on their IFR ticket: CFI's might kinda have a reputation for deliberately mucking with things to make sure the student is able to adjust. The military has a term for this: Semper Gumby.

Another factor is that the autopilot's POH will likely have limitations such as turbulence and minimum altitude which call for hand flying. So if the CFI is "pretending" to be bored, and starts reading the autopilot's POH, get ready 😉 Point here is, just make sure that the hand flying skills remain solid even if the autopilot is capable.

jacobsja

On a cold-read of my previous note, it might seem that I am a bit sour on GPSS use which is not the case. GPSS is a great tool.

Like all aspects flying, best to continually spread the focus and watch for anything that goes out of the normal range.

Waltla

Thank you both for your replies to my question.

Eric, I tried to open your link to the pdf file but it would not open.

I have a Garmin 430W navigator. I think I could benefit from adding the GPSS to my STEC 50. I wonder if any autopilot shops could install that and if the parts are still available?

WaltLA

morrisjd

I think that Garmin GPSS requires something like the Garmin GAD 29B unit that interfaces between the HSI and the Autopilot. The 430W does not talk directly to an STEC autopilot as far as I can determine. The autopilot gets its control information from the HSI. In our aircraft we have a Garmin 430W nav com, STEC 30 autopilot , and Garmin G5 configured as an HSI. Behind the scenes is the GAD 29B.

Jack

empannin

There is also an Stec box called a GPSS steering converter that can bridge to the GPS. In GPSS mode the HSI is bypassed.

If you search for "Flight Line Service Manual For Rate Based Autopilots" aeroelectric has the Stec install manual it but I will not link direct. I have attached the other file where you had issues with the link

Unauth_Autopilot_Operation.pdf

Waltla

Thank you very much for the information. Now I really want to have GPSS added, if I can find a shop that will do it and the parts are still available.

empannin

I think you could also use a Garmin Gad29D to interface with the AP. Your shop will know for sure. I would just tell them what you have and that you want to add GPSS.

https://www.aircraftspruce.com/catalog/avpages/gad29-11-15425.php