When is it Reverse Sensing?

[Ryan B]

Are there any rule of thumbs for HSI and traditional NAV instruments for when the needle is reverse sensing? For HSI I read you should always put the front course in.

[Peter Grey]

Here is the simplest rule of all for reverse sensing:

Traditional CDI: If the heading you are flying does not generally match the course you have put in you will have reverse sensing (small deviations for bracketing/intercepting won't do it, we are talking >90 error). This assumes you always put the front course in on an ILS/LOC.

HSI: Never, except if you don't put the front course in on an ILS/LOC.

[Ryan B]

Ok so HSI flying the pub missed ILS 20R at SNA - what do I turn my OBS to?

[Keith Smith]

you wouldn't need to change it, just keep tracking the same course.

[Keith Smith]

reverse sensing can be described as the case where you need to fly away from the needle to bring it closer, as viewed on the hsi.

normally if you have a left deflection, you fly left of the course to bring it back. with reverse sensing you'd have to turn right.

try the loc dme bc into smx, you'll see.

[Ryan B]

Yeah I think (or thought lol) I understand what it is but perhaps I turned my OBS to the wrong course while flying the missed on the S CRS off the 20R localizer.

So basically I'll leave 196 in but as I'm flying southbound won't the needle be reversed? Because the localizer antenna points north instead of south? Or am I overcomplicating things. I might just be because at Duluth we cannot have both localizers up at the same time.... But we've had a few pilots question it lately like it's the norm for them to receive both ends of Ry 9/27. Since I've only ever worked in DLH I don't know what other facilities do with their antennas.

[Peter Grey]

My rule of thumb covers this case. Assuming you have put in the front ILS course in (which you should always do, putting in the back course never helps and CAN hurt with an HSI).

Using the 20R ILS as the example (and the 2L BC for the opposing approach):

Flying inbound on the front course (the normal approach): CDI Course Set: 196. Heading 196: Result: Normal Sensing
Flying outbound on the back course (the missed approach): CDI Course Set: 196. Heading 196: Result: Normal Sensing
Flying outbound on the front course (outbound leg at SAGER): CDI Course Set: 196. Heading 016: Result: Reverse Sensing
Flying inbound on the back course (LOC BC 2L approach): CDI Course Set: 196. Heading 016. Result: Reverse Sensing

Putting the back course of 016 in a CDI will not change these facts, but will make it a lot more confusing.

Putting the back course of 016 in a HSI will REVERSE everything listed above (which is BAD).

Regardless of which antenna is on it's normal to receive a localizer signal on both ends of the runway. The back course may or may not be good enough to be useful but it is always there.

Let me know if this helps, if not I can go into the technical reasons why but that's really more likely to confuse then help on this one.

[Ryan B]

I think I should never go for my instrument ticket because that hurt my brain haha

Is it true that you do only set the inbound course on an HSI?

What course should one set on a traditional nav1 instrument?

[Peter Grey]

When navigating on a VOR regardless of instrument used (CDI or HSI) you should always set a course that will match the track you intend to fly once established on the radial.

So if you are flying inbound on the 300 radial you set 120. If you are flying outbound on the 300 radial you set 300.

When navigating on a localizer you always set the localizer front course regardless of instrument used (CDI or HSI).