Localizer Back Course Approach?

I have an IFD540 installed with an Aspen PFD and Century 2000 autopilot. I just did a practice localizer back course approach with this setup. I found the system behavior to be interesting.

Normally, with a back course approach, you set the HSI course to the bearing of the front course. That means that the HSI course selector points backwards and, since the signal is backwards too, you don't get reverse sensing, like you would with a non-HSI nav indicator.

When you are shooting front course localizer/ILS approaches with the IFD540 and the Aspen, the 540 automatically sets the bearing selector on the Aspen HSI, so you don't have to touch it. I was expecting it to do the same on the back course, and thought it would be smart enough to flip the course selector upside down.

Not so.

While we were being vectored for the approach, the 540 was in GPS mode and it set the course selector to the inbound course of the back course. (Not upside down.) When we got close to the final approach course it went to GPS->VLOC mode, as expected.

When we intercepted the final approach, the 540 went to green VLOC mode, as it should. However, it did NOT flip the OBS upside down. So we got reverse sensing and the autopilot promptly went the wrong way and we went way off course. I was able to use the course selector knob on the Aspen to flip the OBS manually and then I selected REV mode on the autopilot. Everything was fine after that.

The thing that was unexpected was the requirement to flip the course selector manually just as the 540 goes to VLOC mode. That's kind of a busy time. I don't know if that's documented anywhere.

I recognize that backcourse approaches are probably being buried with the dinosaurs. The runway that I was on had a GPS approach which I would have used if I hadn't been practicing. But, as an old pilot I knew used to say when we practiced with ridiculous crosswinds, "some day you might really have to do it."

Maybe this is something Avidyne should look into on the next release. What do you think the OBS should do? Stay the same or flip automatically?

Mike

I was watching the OBS and I knew that the 540 had it set to the back course inbound course. I knew it hadn't flipped it. But at first I thought that the 540 was doing some sort of internal magic to reverse the signal, since it was programmed for a back course. It knew we were on a back course, so maybe it would reverse the sensing automatically.

It doesn't.

Mike

Unlike a VOR, an ILS has only one course, the front course.  That course is set by the ILS transmitter not the OBS.  The setting of the OBS is independent of the ILS and has no effect on the CDI.  Regardless of the OBS setting, the CDI will always show deviation in relation to the front course.  Regardless of the aircraft position or heading, it will always show deviation in relation to the front course.  There is no such thing as reverse sensing.  The autopilot will sense the deviation in regard to the front course and correct accordingly; the setting of the OBS be damned. 

The statement:  “…[The 540] did NOT flip the OBS upside down. So we got reverse sensing and the autopilot promptly went the wrong way and we went way off course…” makes a connection between the OBS and autopilot that does not exist.  With the OBS pointer “flipped upside down” on the reciprocal of the back course, the autopilot will still turn in the wrong direction even though, to the pilot, the “picture” of the CDI looks correct.

mfb wrote: Normally, with a back course approach, you set the HSI course to the bearing of the front course. That means that the HSI course selector points backwards and, since the signal is backwards too, you don't get reverse sensing, like you would with a non-HSI nav indicator.