I was showing off the GNS430/S-Tec 55 combo to one of the buyers of my SR20. He had just gotten done sweating through the KWVI NDB-B approach in our venerable 172, so I thought I’d show him how it would be done in the 20th century. I selected the approach on the 430 (direct to the NDB, which is the IAF) and engaged the 55. As we approached the waypoint, the 430 said “Turn to 197” so I cranked the course needle around and the aircraft started to turn. Halfway through the turn, the GPS sequenced to the outbound leg, and then I uttered the words that show up in too many CVR tapes: “Where is it taking us now?”
It seems that when the GPS sequenced the approach, it somehow managed to confuse the autopilot, which bailed out to wings-level rather than completing the turn. Re-arming APR mode (by switching to NAV and then back to APR) brought the autopilot back to its senses.
I’ve used the 430/S-Tec 55 combo extensively and have never seen this problem before. On the other hand, I’ve never flown this approach before. What is interesting about it is that, while tracking outbound, the waypoint is behind you (the NDB) but the desired track is forward (outbound) so the TO/FROM flag is in the FROM position (pretty unusual for GPS tracking.)
I chatted with a gentleman at S-Tec and he didn’t have an explanation for this, other than that some transient needle movement was convincing the a/p that it was on course. (The 55 has only the course pointer and the needle offset as input, so when the GPS sequences, the only possible thing that the a/p can see is a needle transition.)
I’m going to press this with Garmin and see if they have any explanation for it, and I’ll probably go play with it in the air a few more times and see if I can gather any more data, but this is yet another heads-up about a/p use, particularly in IMC–monitor the systems, and know what to do (and quickly) when it starts to do something odd.