STEC ST-901 GPSS Steering Module ?

[scottb613]

Hi Folks,

Just curious - did you ever consider including an STEC ST-901 GPSS module in the Comanche - a little variety from the autopilot in the Cherokee would have been welcome or maybe some future release...

Regards,
Scott

[AKar]

These GPSS modules are rather interesting both functionally and in their benefits.

Essentially, when selected on, they replace the heading mode with a steering command calculated by the navigation system. Autopilot 'sees' a regular heading bug, the signal of which is emulated by the roll steering box. In principle, only thing that is required from the existing autopilot installation is a heading mode, using supported kind of signal from the DG's heading bug to transmit the heading error. And of course, a GPS navigation system that outputs a GPSS signal is needed.

A major benefit is that a proper GPS route following, with interceptions, tracking and everything, can be used in simplest autopilots which may lack any kind of tracking function on their own. The steering converter, such as this one, feature a button that allows one to switch in between the heading mode to follow the bug as usual, and the GPSS mode, in which the bug signal is driven by the box. In typical installation, the NAV mode would not be used to track the GPS route if the airplane is GPSS-equipped, the tracking being entirely independent of the HSI needle (and of NAV/GPS selection if such is provided).

These kinds of installations that found their way into classic instrument panels the A2A GA fleet represents would be great to have included as they are increasingly common in RL airplanes.

-Esa

[scottb613]

Hi Esa,

Yeah - I've chatted with a real world guy on his performance with an STEC-30 - he stated it works fine for lateral navigation on an approach but the NAV Mode when going between enroute waypoints it is very poor and oscillates all over the place (could have been localized issue but he mentioned others with similar results) - he said once he added GPSS it was like a whole new world and turned his very simple autopilot into a fairly robust one - he said he could NEVER go back... GPSS will fly complete holds and procedure turns - pretty impressive for a simple STEC-30 and a lot of bang for your buck at around 3K...

It would add some different button pushes and such - spice - to the Comanche - 3D model-wise it's just a simple lighted push button... Optional equipment upgrade via the hangar - LOL - just thinking out loud ?


Thanks for commenting...

Regards,
Scott

[AKar]

Yeah - I've chatted with a real world guy on his performance with an STEC-30 - he stated it works fine for lateral navigation on an approach but the NAV Mode when going between enroute waypoints it is very poor and oscillates all over the place (could have been localized issue but he mentioned others with similar results)

Would that have been on GPS nav source or on VOR? Not knowing about real-life performance of the S-TEC 30, I'd experiment some with the HI and LO TRK. Which gains produce the best results on a given nav source could very well be dependent of the particular installation. If we had a scalloping VOR signal for instance, it would take some experiment to see whether it couples through either tracking mode, or in worst case, through both.

Not speaking about A2A simulation of S-TEC 30, but in general many FSX/P3D autopilots are somewhat lazy to correct in the first order of the control loop, so to say, or in other words to counter the immediate disturbance, but then they often attempt to be excessively "accurate" in tracking of signals. This is 'allowed' for by our very FSX/P3D platform in that these signals, be them the altitude, airspeed, VOR, GS or whatever are perfectly robust whereas in reality they exhibit more or less natural fluctuation due to various phenomena affecting the readings. The control design in an autoflight system is necessarily required to meet these criteria: be sensitive enough to immediately counter for any starting deviation, but again, be smooth enough to not 'hunt' for any signal-induced oscillations. I am aware of cases where GA autopilots, which are rather simple in design and often utilize sensory inputs of less than the best possible quality, do things like scalloping in altitude hold and so on. Reasons could be as simple as bad cable tensions or up to and including a faulty servo.

Anyways, about GNSS...I am not personally too... "approving" towards most airplane modifications when I first hear them, but GPSS has been one I've always liked as an idea: it is simple, utilizing existing systems while allowing the interfacing of the new ones, a nice on/off pilot interface, and very cost-effective in comparison to its benefits if the navigation system was updated to a reasonably modern GPS level anyway.

-Esa

[scottb613]

Hi Esa,

It's funny that the newer tech seems to have issues tracking a NAV source - I've flown with and Century I driven by a Northstar Loran unit - windy or not - the darn thing would track absolutely perfect even on a 300 mile leg...


Regards,
Scott

[AKar]

When systems are created to achieve increased short-term accuracy, they are almost inherently more prone to tuning issues, resulting in errors most often oscillatory in nature. In small GAs, the system integration tends to be of rather low level: they are mostly made of multiple components of different eras and manufacturers, interfacing via analog voltage signals. Airframes themselves are all different. And of course, the signals themselves (such as VOR) can be less than perfect as well. It is a system level problem mostly, not easily isolated. That said, if a system can't produce the expected results, it should be considered faulty, and the issue be tracked down, no matter what kind of troubleshooting hell that leads into. Obviously, one needs to remain realistic about the expected performance as well.

-Esa

[Scott - A2A]

I've only used the STEC-30 in VOR or HEADING tracking. Heading sticks like a rock while VOR does oscillate.

Keep in mind the STEC-30 is very simple in that it can only track a straight course and can't make turns to a route if the turn is anything significant. It will just see a "turn slightly to stay on course" and will eventually end up blowing through the coarse at a harsh angle, then see a "turn slightly back the other way" command. And will just end up flying all over the place always being way behind the curve. However, this simplicity IMO is what makes it such a great autopilot - the thing is so reliable.

Scott.

[Oracle427]

Used to have an STEC 30 on a plane I flew, great little autopilot but it definitely couldn't handle intercepts or big course changes. Not a big deal, all one had to do was hand fly the change and then turn it back on.

We now have an STEC 55X in a different plane. It behaves nearly the same as the 30. The only difference is that it has a vertical speed mode without altitude capture, but we can get an upgrade to add altitude capture at a low cost.

We did integrate it to our 430W such that we have GPSS and oh my! The thing can follow holds, make intercepts, fly over, fly by without batting an eyelid. It is wonderful. However I find it still has a tendency to oscillate a little more than a KAP140 or GFC700. The STEC with GPSS is a great tool.

The STEC has been a very reliable unit. The KAP, not so much... I hope never to find out what repairs to a GFC cost, so far it's been trouble free.

[scottb613]

Hi Folks,

Yeah - my only experience is with the Century I and now I have the STEC-65 with the ST-360 Altitude Preselect Module... Overkill for my plane but a nice toy - especially in very busy places like inside the DC Speed Ring... Both always seem to track very well - I've recorded plenty of breadcrumb trails to check after a flight... I don't have the GPSS module yet - but it's on the list as it will be a good tool when flying single pilot IFR - anything to reduce the workload...

Regards,
Scott

[AKar]

However I find it still has a tendency to oscillate a little more than a KAP140 or GFC700. The STEC with GPSS is a great tool.

Speaking of KAP, I never ran into this particular configuration in real life, but studied it some in simulator context some time back; the KAP 140 after certain mod status readily accept roll steering commands apparently in Bendix-King's own signalling from B/K KLN GPS units that some Cessnas apparently shipped with as factory equipment. This however appears not compatible with, say, Garmins that are more common navigation systems these days; such installations would require an added GPSS roll steering converter which would translate the Garmin's roll steering output into emulated heading bug. As discussed previously on the forums, it appears that in this specific sense, the current simulation of GPS NAV tracking as simulated in A2A Cessnas (172 & 182) take some artistic license, functioning somewhat as if we used the Bendix-King KLN+KAP roll steering scheme, even if we necessarily used some sort of Garmin. It is somewhat functionally different in reality, unless my documentation is significantly dated (I don't suppose it would be).

-Esa

[zazaboeing]

It would really make a difference having a better autopilot (more modern one) in the Comanche as an option. I would personally love it!