GPSS Doesn't Always Bypass HSI

I discovered something yesterday (at least I think I did) that might be common knowledge, but at least to me it came as a surprise. It does, in retrospect, make some sense.

I had always believed that the GPSS navigates the plane totally irrespective of the HSI. Not to dwel on a sore subject, but this is especially important for anyone who is “HSI impaired” or worried they might unexpectedly loose their HSI.

Well, yes and no. Cruising in the GPSS mode yesterday was a dream (off course by 1/100 of a mile!), even with my bad HSI’s CDI dancing to the left and back with no apparent reason. Then, we put the GPS in the OBS mode, to see what that did to the malfunctioning HSI. First, and not relevant, it didn’t fix the HSI problem. (HSI is being replace today anyway.) However, it sure did impact the autopilot. As I turned the OBS to try and center the needle, of course it moved the course that the autopilot was tracking and therefore caused the plane to bank and capture the new course.

Again, in retrospect the autopilot did what it was told to do. And literally, the OBS part of the HSI was giving info to the GPS which in turn was giving info to the autopilot.

My lesson, however, that if in the GPS OBS mode, a malfunction in the HSI could in fact impact the autopilot.


That makes sense because, in OBS mode, you are asking the GPS to fly a course dictated by the OBS setting on a VOR or HSI head. Without the HSI/VOR head working to tell the GPS which course to steer, it will not function as planned. This is totally independent of the GPSS steering.

Yep, all you did is add a bit more electronics between the HSI and the autopilot. I imagine that the a/p will try to set up a 45 degree intercept to the selected course until the needle centers (if ever.)