Autopilot NAV Mode with RXP GTN 750

[skyhawkii]

I installed the RXP GTN 750 on the Comanche. When using the AP Nav mode to follow the GPS track the plane does not fly straight on course. It's making slight s-turns around the course. In the Bonanza and Skylane it's working fine. They flight straight on course. Anyone else experienced that and has an idea how to solve it?

[Al FR-153]

Yes. I recently switched over from F1 to Reality's GTN and saw the same exact thing. I know that the autopilot is constantly trying to search for the correct course, but this did seem a bit over the top and was very noticeable. I will have to check more closely to see if it is happening in the Cherokee as well.

EDIT/Update: Flew the Cherokee KBUU to KRAC and see the same thing but not nearly as severe. Of course, the speed wasn't quite as fast either. On the Cherokee, there wasn't enough deviation to change the course reading on the Reality XP GTN, but watching out the window, you could still see it.

For troubleshooting purposes, crank the simulation speed up to 4X and you will see it very clearly, especially on the Comanche.

Al

[skyhawkii]

Hi Al,

thanks for verifying this. It seems to occur in combination with the S-Tec AP and RXP GTN 750. I'll check this out on my next flight tomorrow.

Michael

[Al FR-153]

Agree Michael......As I said, I changed from the F1GTN 750 (also have the 650) to the RXP GTN 750 (also have the 650 there too) and I don't recall ever seeing the left/right drift with the F1. Therefore, not sure it is an A2A problem or the difference in tracking between the two GTN suppliers. I tend to think the later. But do agree that it is more noticeable in the S-Tec AP. I haven't seen it in any of the other A2A birds with the KAP style APs.

[skyhawkii]

I did a couple of tests during my flight today. I figured out that it's most pronounced by using the hi trk mode of the s-tec. Switching to lo trk mode makes it a lot better. Still noticable when you got your eyes on it, but it's down on a level that doesn't bother me. Using the lo trk mode only is my workaround for the moment.

[Paughco]

No RW experience to compare with the A2A Comanche in P3Dv4, but my N5665P is equipped with the RXP GTN 750, and it works alright, just not too well with the autopilot. Actually maybe the S-Tec 20 autopilot in the Comanche is more of a semi-autopilot, especially when you're using it with the flight plan on the GTN 750. My experience is that the S-Tec autopilot does not anticipate what's coming. You come to a turn in the flight plan, and the S-Tec seems to wonder what happened. By the time it figures out that the course has changed, the airplane is already a mile or so beyond the next segment of that magenta line. The S-Tec compensates, but overshoots, again, and again. I think this also happened with the A2A/Garmin GNS 400.

Here's a link to the S-Tec manual, as it seems like the one in the A2A Comanche Pilot's Manual is gone: http://www.mikeg.net/hobbies/aviation/a ... Manual.pdf.

Here's what the manual says:

GPS Tracking and GPS Approach
1. Enter desired waypoint in GPS receiver.
2. Set A/P NAV select switch to proper source, if so equipped.
3. Maneuver the aircraft to within +/- 1 needle width and within 10° of the course displayed on the GPS receiver.
4. Press and release Mode Select knob until HI-TRK lamp illuminates.
Note: This procedure is also applicable with Loran.
Note: When flying multiple waypoints repeat steps 3 &4 for each leg if it involves more than a 10° course change.

My usual procedure is to let the S-Tec follow the magenta line in HI TRK, but I keep the heading bug aligned with the route, and watch the GTN 750 for upcoming course changes in the flight plan. Then, a minute or so before a change in direction, I switch from HI TRK to HD and make the turn with the heading bug; once we're steady on the new course, I switch back to HI TRK.

ISYN. YMMV.

Seeya
ATB

[Al FR-153]

My usual procedure is to let the S-Tec follow the magenta line in HI TRK, but I keep the heading bug aligned with the route, and watch the GTN 750 for upcoming course changes in the flight plan. Then, a minute or so before a change in direction, I switch from HI TRK to HD and make the turn with the heading bug; once we're steady on the new course, I switch back to HI TRK.

I think we were both using HI TRK Paughco. Once on the magenta track the Comanche wanders to the left of track about 500-700 meters, then correct to the right, crossing the magenta line and continues to about 500-700 meters to the right of the magenta track line, then repeats, over and over. So, you are flying (very narrow) lazy S's at altitude, the full time you are on autopilot. Just flew a two hour jaunt from VHHH - ZJHK @ 5500' and it wandered the whole time. As said earlier, the Cherokee wanders as well, but not nearly as severe. This looks like a bug where the calculation of being on track never happens and never gets any closer to about 500-700 meters from happening. Don''t know for sure just who owns the bug Garmin' GTN set or S-TEC/A2A but it is annoying. Garmin just updated their portion of the GTN set (v2.7.0.0), and I have it installed. This bug was not fixed with that update.

Agree that the age of these S-TEC units were before they were capable of thinking ahead on a course change, but this wandering back and forth is a totally different issue. I would think that the S-TEC would measure the overshoot error, then correct back to course. The second time If there was still an overshoot from the course, it would 'half' the overshoot and measure again, eventually getting itself corrected to the course. That portion is not happening in the Comanche, particularly.