Autopilot anomaly

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.

Small update. In a close reading of the 55 manual versus the 55x, I found an interesting difference–the 55 says that if you make an en route turn of more than 10 degrees, you should re-arm NAV mode to restart the capture sequence (there are three levels of aggressiveness to the autopilot; it starts very aggressive to capture and intercept, then it loosens up so that it doesn’t do S-turns along the en route course; APR mode locks it in the intermediate state, among other things.)

The 55x manual basically says that if you turn more than 10 degrees, the unit will automatically re-arm capture mode (presumably it notices when you twist the knob.)

Having said that, the behavior of the system is still anomalous; it was clearly following the course needle and turning right up until the point where the 430 sequenced to the next leg. I’ll keep pursuing this.

Garmin tells me that they have a software fix that may address this problem, so I’ll break down and get my 430s upgraded and give it another try.

For the record, both Garmin and S-Tec immediately replied to my emails, and the chief test pilot at S-Tec sent me his office number and we discussed the situation directly. Excellent support from both sides!