VNAV ARINC 429 Autopilot resolved
Today I resolved the problem that I have been working on for quite some time with getting vertical navigation signals to the autopilot from the GNS430W. I have talked about this a lot but here is the recap again: The 430W should be able to output vertical navigation instructions to the TruTrak DF II VSGV autopilot via the ARINC 429 buss to drive the autopilot all the way down an IFR GPS approach. This wasn’t working for me. It worked great for lateral navigation down the approach, but the vertical just wasn’t there. Then I found that the GPS needs to output the “GAMA 429″ signal format and not “ARINC 429″. The GAMA format adds the vertical commands. Ok, so I set that. However, with only that change, I found that once a gps signal lock occurred, the GTX330 transponder was somehow, via it’s ARINC 429 connection to the 430W, commanding the 430W with a remote “joystick waypoint” entry command over-and-over continuously such that everything was unusable. That is where I have been stuck for the last few weeks.
Today I went back out to the airport committed to simply burn some more time on the problem. You know, sometimes if you just invest some more time with no expectation of real return, you can make at least some kind of progress, if only more visibility into the problem… So I spent about 1.5hr sitting in the cockpit messing with 430W and GTX330 configurations. After I had been through every config and verified that I understood everything and made sure my settings all made sense to me, I decided it was time to try the next step. You software engineers know where I’m headed with this (even though we usually won’t admit it). I began simply trying logical and illogical combinations of configurations of data formats and buss speeds on the ARINC configs. Yep, you guessed it, I changed the buss speed on a seemingly unrelated (input) ARINC interface on the GTX330 and the flood of joystick waypoint entry commands stopped on the 430W.
Cool. But of course I had to back track and try to figure out what that broke in the process. Hmmm… Nothing broke because of the change. Really? (head scratching). Yeap, seems like everything still works. All data is getting everywhere as normal. Sweet!
Time to hop in and take it for a spin to see the magic in action. I launched and setup a fully coupled GPS approach. I engaged the autopilot, switched directly to the GNS430W, let it fly the transition, and as it turned inbound for the 10mi. final, the autopilot went from “vnav arm” to flying the glide slope right down. Right about here, I’m just concentrating on containing my excitement as someone still needs to sit here and monitor the computer as it flies the final approach :). I just worked the throttle and trim. Down we went for a “perfect” landing. It works, It works!
Update: After a little research (root cause analysis) as to how I could have been so far off, I found that the ARINC 429 connection in question never made it into my final interconnect documentation. I designed it, I built it, I documented it. My fault, oops. I updated my schematic and all is well with one less mystery.