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 of
his installation, you can see his custom panels along with the nice stack of
radios. From the start, I could tell Mr. Sutton had his head on straight; he
wanted the Sandel, GNS 430 and the FS 450 fuel flow operator's manuals so to
study up on the operations before he picked up the flying machine. He also
down-loaded the GNS 430 simulator from the Garmin website and learned how to
operate it well before our test flight. Folks that take the time to study the
operation of the products they purchase sure make my life easier! On a Monday
morning Mike arrived to pick up the machine. I had asked him to be prepared to
spend the day with us for flying and testing out the equipment. The first thing we did was plug
the APU into the aircraft so Mike could get used to it on the ground and I also
asked him to program in a flight plan in the GNS 430 that would give the GPSS a
real test. After an hour or so Mr. Sutton felt he was ready to fly the Piper.
He fueled the aircraft and gave it a thorough preflight. The instant I got in
the machine with Mr. Sutton I could tell he was a good pilot (I'm not saying
this to make Mike feel good and he's already paid the bill):-) He ran through
is check list and started the big 235 up. Within seconds he was cranking in the
frequencies we would need and activating the GPS flight plan he had installed.
This guy actually took the time to study the manuals!
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.
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.
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 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. 
The first thing we did was plug
the APU into the aircraft so Mike could get used to it on the ground and I also
asked him to program in a flight plan in the GNS 430 that would give the GPSS a
real test. After an hour or so Mr. Sutton felt he was ready to fly the Piper.
He fueled the aircraft and gave it a thorough preflight. The instant I got in
the machine with Mr. Sutton I could tell he was a good pilot (I'm not saying
this to make Mike feel good and he's already paid the bill):-) He ran through
is check list and started the big 235 up. Within seconds he was cranking in the
frequencies we would need and activating the GPS flight plan he had installed.
This guy actually took the time to study the manuals! 
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.