Roll Steering With GPSS By S-Tec
When I first heard about the GPSS by S-Tec, my first thoughts were it's just another way to get navigation information to the autopilot and I doubted it would work any better than left/right information that was always available out of any panel mounted GPS. Another issue was the low cost; when the GPSS first came out, it sold in the neighborhood of $695.00 (price nowadays is $995.00). Let's face it, if the $6,000 roll autopilot couldn't keep you centered on a course, how could this cheap little box help? Yes, as most of you know, I'm a "Doubting" Thomas. Mike Sutton gave my boys the task of installing a GNS 430, Sandel EHSI, custom panels along with a host of other goodies. He asked us to install a GPSS that he had purchased elsewhere and for some reason the other shop didn't install it. Normally we install only the avionics we sell but I thought; Mike is willing to pay us to install the GPSS and if his GPSS doesn't work, I can point the finger at him! The deal was too good to pass so we agreed to install the little GPSS and see that happened. Besides, regardless of the outcome, it would give me some good article material.
Just what is the GPSS supposed to do? In simple terms, the panel mounted GPS knows where it is and based on the flight plan installed by the pilot, it knows where you hope to end up. "Real" panel mounted GPS systems have what we call a 429 buss output and on this buss is a digital composite roll steering information. When the GPSS is activated and a flight plan in the GPS is active, you do not need to set the course pointer to the station bearing, the GPSS does not look at the navigation indicator or HSI for any information. Personally I'd set the course pointer on the bearing the GPSS is flying just for a visual reference on what's going on but again, it's not required. Believe me, once you have a GPSS installed, you will be looking for something to do in the cockpit; after my test flight with Mike Sutton, I realize the GPSS can make you feel like the Maytag Repairman.
The GPSS indicator has two lamps on the faceplate. Normally this system will default to the heading mode. While in this mode, you can use the heading bug on the DG or HSI just as with any autopilot. Turn the heading bug and the aircraft will sharply follow it until the bug is almost centered then the aircraft will slow the gain so not to "S" turn when capturing the heading. While in the heading mode on the GPSS you can also use the standard "NAV" modes just like any other autopilot. In other words, when the GPSS is in the heading mode, your system acts just like it did prior to the GPSS being installed. What I've found with most autopilot systems, it's easier and more accurate to just set the heading bug to the bearing the GPS says and correct for the winds. Often VOR and GPS standard left/right signals will not keep the indicator needles centered and tend to make the autopilot wander all over the sky, which is not a good thing. Simply by pressing the GPSS button on the annunicator, your whole world changes. First, the "HDG" lamp goes out and the "GPSS" lamp illuminates. If you have an active flight plan in the GPS, the autopilot will instantly do whatever is required to intercept the proper bearing to the next waypoint. All you have to do is have the aircraft pointed less than 180 degrees of the next fix. More on this during our flight test
The GPSS comes in three flavors. Extremely compact (about the size of a deck of cards), the GPSS converter is 1" thick, 2.25" wide and 3.24" deep. the faceplate and control switch on the self-contained units, in either horizontal or vertical configuration is 1.5" by 3.25" If panel space is restricted and does not permit the use of a self-contained configuration, the control box can be remotely mounted with a 1.32" by .82" control switch which projects only 1" behind the panel. There's a GPSS made to fit just about any aircraft. Operating voltage is 14 or 28Vdc. If there is no 429 data buss output from the GPS, then the GPSS lamp will blink and can not be activated. At the present, most GPS systems 429 data buss output roll steering commands for enroute navigation and a few GPS overlays of DME ARC's approach transitions are currently available. In the near future, databases should be expanded to include full procedure approaches, transitions to approaches, procedure turns, published holding patterns and missed approach maneuvering. In a nut shell, soon you will be able to program your complete flight in the flight plan including the items mentioned above and have the GPSS fly the complete plan including the holding pattern if required. Can you imagine how much this will reduce pilot work load! If you have a GPS, a S-Tec autopilot, then the GPSS should be a "must" in my opinion, I wouldn't leave home without it. This system can be used connected to any S-Tec autopilot via a minor alteration, thus if you now have a S-Tec 20 or above, the GPSS may be of interest to you.
Let's go flying and see what the S-Tec GPSS really does.
We finished installing this system in Mike Sutton's Piper 235 and everything
checked out fine on the ground but as we all know, the proof is in the air. If
you would like to follow the installation of Mr. Sutton's Piper be sure to see
Our flight plan consisted of a straight out to Oceano (L52), with a 88 degree turn to the left to San Luis Obispo (SBP) and a 163 degree turn back to SMX. This should be a good workout for the new systems. As luck would have it as soon as we were off the ground ATC advised us to turn 50 degrees to the left to avoid incoming traffic. Mike turned on the S-Tec to the heading mode and cranked in the 50 degrees on the bug and the System 50 went after it. We had to climb up to around 6,500ft to clear the clouds and at that time altitude hold was turned on. Now we were about two miles off course due to the traffic deviation. I wanted to make a new course from our present position to the waypoint but Mike wanted to see if the GPSS would intercept the original track. I know in the regular nav mode, the autopilot would never capture the old course. With the S-Tec 50 system you have to get the aircraft on course and THEN engage the nav mode and the aircraft may keep the needle centered but don't count on it. I told Mike not to be too disappointed if the GPSS didn't do well at intercepting a course that was two miles off. He pressed the "GPSS" button and the aircraft instantly set up a 60 degree intercept. My thoughts were the blasted thing would shoot right through the course line and start "S" turning but as we approached the course line, the aircraft made a smooth transition to intercept the desired course. No wing rocking, no "S" turns, just a good intercept that you would expect from an autopilot costing in the neighborhood of $36,000.00. From there the GPSS kept the course line centered under the aircraft logo on the GNS 430 screen. Mike did not need to set the course pointer on the Sandel EHSI but he did just to have something to do and for a visual reference on what was going on. Our next turn was a 88 degree turn to SBP. I wondered how well the system would do, our ground speed was 130Kts. Once we neared L52, the aircraft made a standard rate turn to the next course which was a 88 degree turn. The aircraft briskly turned right and then slowly decreased the turn until we were straight and level again. The GPSS handled this major course change without a problem, I doubt if anyone could have hand flown the aircraft any better. The little GPSS kept the aircraft nailed on the bearing to the next waypoint and never once let us deviate more than a hundred feet horizontal. The next turn was the biggie, 163 degrees back to SMX. Once at SBP the aircraft started a sharp turn to the right, heading back to SMX. Now keep in mind, the autopilot will not allow any turns more than standard rate, thus the aircraft maintained standard rate until it set up what appeared to be a 60 degree intercept angle. Once the aircraft got near the course line, the intercept angle lessened as it should and we soon intercepted and the aircraft rolled out straight and level. I just couldn't believe this thing worked as good as it did. Friends I've flown just about every autopilot known to man but the S-Tec autopilot in conjunction with the GPSS is the most advanced roll steering system I've ever set behind, bar none (except some military types). Sure, go ahead and purchase a high dollar King autopilot, it won't work as good as this S-Tec set up. During the test flight Mr. Sutton and I checked out every mode of operation we could think of and all systems seemed to be up and running. The installation was a total success. Mr. Sutton stayed the night in SMX and headed home the next day. I received an e-mail from him stating just how happy he was with the products we sold him and the quality of the installation.
The GPSS will only work with most Garmin GPS's, King KLN-900 and a few KLN-90B GPS. Sure, S-Tec raised the price to $995.00 but for what this little box does, it's a steal. You can't purchase a good portable GPS for that price. After doubting the capability of the GPSS, now what do I think? In my opinion, the GPSS should win the Product of the Year award. As I mentioned, this makes any S-Tec autopilot (with the proper GPS) perform better than any other autopilot's roll commands, even the top dollar systems. As GPS receivers incorporate more data out on the 429 Buss (such as holding patterns and fixes) the GPSS will become the most valuable piece of equipment in the cockpit. Never in my aviation career have I ever seen a box that costs so little but does so much. The GPSS should be on your "Must Have" list.