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Localizer Back Course Approach?

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mfb View Drop Down
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    Posted: 11 Dec 2015 at 11:27am

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



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Post Options Post Options   Thanks (0) Thanks(0)   Quote oskrypuch Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2015 at 1:32pm
Now, of course you always need to check the CRS, whether it is auto-ed or not, but that is an interesting issue, it could very well create a gotcha.


* Orest

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Post Options Post Options   Thanks (0) Thanks(0)   Quote mfb Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2015 at 1:56pm

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


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Post Options Post Options   Thanks (0) Thanks(0)   Quote DavidBunin Quote  Post ReplyReply Direct Link To This Post Posted: 11 Dec 2015 at 6:39pm

When the pilot sets the OBS "down" instead of "up" on the CDI, that is the signal to the 540 that you are on a back course.  So regardless of what  procedure is selected in the flight plan, the 540 didn't "know" you were on a back course until you moved the OBS.  That is the cue for the 540.

David


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Post Options Post Options   Thanks (0) Thanks(0)   Quote SB Jim Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 2019 at 10:08pm
Old topic but interesting since my home field uses a back course localizer most of the time.

I’ve noticed my 540 won’t automatically go to VLOC at times. It does so when I hand fly the approach and flip the OBS to the front course on my HSI, but my Century III as luck would have it flies a beautiful back course in REV mode if I leave the OBS set to the back course.

I tried it today to verify. You guys are spot on.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SB Jim Quote  Post ReplyReply Direct Link To This Post Posted: 22 Jan 2019 at 10:09pm
Ps. Since the approach is called a back course I would’ve expected the box to know that.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rolfe_tessem Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2019 at 9:00am
I have re-read the above posts several times, and I'm still not clear.

Autopilot operation aside, do you set the HSI for the front course or the back course? My autopilot has a REV mode, so I would expect to engage that. 

Here in the northeast US, I don't know of an actual BC approach still in service, so the last one I did for real was several years ago in Tucson, AZ.

Rolfe

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Post Options Post Options   Thanks (0) Thanks(0)   Quote AZ Flyer Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2019 at 1:02pm
I have flown the BC at Tucson for training purposes a few times, and the operation of the IFD 540, Aspen and STEC 30 is indeed interesting.



Edited by AZ Flyer - 25 Jan 2019 at 2:43pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 25 Jan 2019 at 5:10pm

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.

Originally posted by mfb 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.
If the signal reversed, the deviation bar would move to the other side of the pointer. It doesn’t.  When the needle is reversed, the deviation bar stays on the same side of the pointer, but it is now oriented so that it matches the course that it is tracking and behaves as desired.  But the autopilot doesn’t know this and will continue to turn the aircraft as if the aircraft was actually on the front course heading in the opposite direction.  The REV feature of the autopilot is necessary to get the autopilot to turn in the direction opposite to what the CDI is telling it.  If you don’t have the REV feature, then fly with the heading bug or manually.



Edited by Bob H - 25 Jan 2019 at 6:06pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rolfe_tessem Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jan 2019 at 8:53am
Originally posted by Bob H Bob H wrote:

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. 



Yes, I get that but is it not true that the behavior of a standard CDI and an HSI is different? I have always been taught that the HSI does not have reverse sensing -- it is always set to the front course and the needle will behave normally (as it does on the front course). This is obviously not strictly an IFD thing, but it would be good to know what the interaction between the two actually is. ("set" in this case meaning for reference and possibly for the autopilot).

Rolfe

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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jan 2019 at 9:35am

Originally posted by rolfe_tessem rolfe_tessem wrote:

Yes, I get that but is it not true that the behavior of a standard CDI and an HSI is different? I have always been taught that the HSI does not have reverse sensing -- it is always set to the front course and the needle will behave normally (as it does on the front course). This is obviously not strictly an IFD thing, but it would be good to know what the interaction between the two actually is. ("set" in this case meaning for reference and possibly for the autopilot).
An HSI and CDI behave identically.  They both show deviation in reference to the front course.  However, the beauty of the HSI is that it gives a visual reference to the course being tracked.  Moving the HSI pointer to the front course being tracked while flying the back course, is like looking at the instrument in a mirror.  So, the behavior of the HSI deviation bar doesn’t change, but your view of it does.  The autopilot still receives the same deviation information regardless of your view of the deviation bar, so it turns in the wrong direction unless you use the REV feature.

Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 26 Jan 2019 at 2:12pm
Originally posted by SB Jim SB Jim wrote:

...my Century III as luck would have it flies a beautiful back course in REV mode if I leave the OBS set to the back course.
If I may, one more comment.  When in VLOC mode, your Century III will fly a beautiful back course in REV mode regardless of how the OBS is set, front course, back course, or somewhere in between.  Try it.  The OBS has no effect on the deviation signal sent to the autopilot.

Edited by Bob H - 26 Jan 2019 at 2:14pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote AZ Flyer Quote  Post ReplyReply Direct Link To This Post Posted: 27 Jan 2019 at 8:50am
This is very helpful, thanks.  It explains well why I can see the HSI moving as expected when set to the front course, but the autopilot (without REV function) doesn't turn the direction needed to stay on the localizer BC.  I had already pretty much concluded that the only way I could use the autopilot for the BC is with the heading bug, and this further confirms the correctness of that conclusion.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SB Jim Quote  Post ReplyReply Direct Link To This Post Posted: 31 Jan 2019 at 4:27pm
Originally posted by Bob H Bob H wrote:

Originally posted by SB Jim SB Jim wrote:

...my Century III as luck would have it flies a beautiful back course in REV mode if I leave the OBS set to the back course.
If I may, one more comment.  When in VLOC mode, your Century III will fly a beautiful back course in REV mode regardless of how the OBS is set, front course, back course, or somewhere in between.  Try it.  The OBS has no effect on the deviation signal sent to the autopilot.
Hi Bob,

I have tried it both ways practicing in VFR conditions. It does not work that way!

Forgetting the autopilot for a moment here’s how it works at my home field KSMX Santa Maria, Ca localizer DME back course alpha approach to runway 30:

1. Select approach and activate.
2. Dial front course 120 degrees on HSI OBS
3. Reversing the OBS triggers the auto localizer (gps to loc) on the 540
4. King HSI will then work normally. If needle goes left, fly left. Right for right.
5. Hand fly the approach. The Century III will not accurately track the course in this configuration regardless of which item is selected on the coupler.

If one wants to fly a coupled back course approach with the C-III and 540 it works as follows:

1. Select approach and activate.
2. Dial in back course 300 on HSI OBS.
3. When appropriate manually select VLOC on the 540.
4. Select REV on the autopilot and it will join and track the localizer back course.

This is in accordance with the C-III Pilots guide.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 01 Feb 2019 at 9:45pm
Originally posted by SB Jim SB Jim wrote:

I have tried it both ways practicing in VFR conditions. It does not work that way!

Forgetting the autopilot for a moment here’s how it works at my home field KSMX Santa Maria, Ca localizer DME back course alpha approach to runway 30:

1. Select approach and activate.
2. Dial front course 120 degrees on HSI OBS
3. Reversing the OBS triggers the auto localizer (gps to loc) on the 540
4. King HSI will then work normally. If needle goes left, fly left. Right for right.
5. Hand fly the approach. The Century III will not accurately track the course in this configuration regardless of which item is selected on the coupler.
Hi Jim,

#4: Are you saying that the HSI changes the way it operates when the OBS is reversed (#3)?  I agree that a reversed OBS properly represents the direction to turn, but only because the pilot's view of the deviation gets reversed along with the OBS pointer.
#5: Interesting.  Where was your heading bug set during this?  See below.
Originally posted by SB Jim SB Jim wrote:

If one wants to fly a coupled back course approach with the C-III and 540 it works as follows:

1. Select approach and activate.
2. Dial in back course 300 on HSI OBS.
3. When appropriate manually select VLOC on the 540.
4. Select REV on the autopilot and it will join and track the localizer back course.

This is in accordance with the C-III Pilots guide.
Thanks for trying this out in the airplane.  I've done some checking specifically on the C-III.  It appears that its computer does take some intelligence from the Heading Bug and OBS in order to properly finesse the intercept and tracking of a Localizer.  Tracking the Localizer requires that the OBS and Heading Bug be set to the inbound course.  So, clearly I was wrong in regard to the operation of the Century III.  I don't know how other autopilots work, but in hindsight it makes sense that the autopilot needs desired course information in order to do proper intercept angles and calculate rate of turns necessary.  But, I believe my primary points still hold:

1. Course deviation is independent of OBS setting.  An ILS or LOC back course can be flown with the OBS set to any magnetic bearing.
2. An HSI and standard CDI behave identically.  They always show deviation in regard to the front course.
3. On a backcourse, an HSI can be set to show proper deviation direction by setting the pointer to the front course.  This simply moves the CDI deviation bar from one side of the aircraft to the other.  The deviation itself remains on the same side of the OBS needle.  It is a visual perspective change only.

I'm open to a differing opinion if someone thinks I'm mistaken.  Jim, thanks for the C-III insight.


Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SB Jim Quote  Post ReplyReply Direct Link To This Post Posted: 02 Feb 2019 at 12:34pm
Bob,

I think we are saying the same thing with regards to the King HSI, but we’re using different terminology. I’m focused on what the pilot sees. Needle goes left, fly left to re-center the needle and be back on course.

Where we disagree is with regards to the Century III.

The C-III will NOT track a back course localizer in any coupler configuration (LOC NORM or LOC REV) if the OBS is set to the front course. The heading bug is not relevant.

The C-III will accurately track a back course localizer IF the OBS is set to the inbound back course (300 degrees in my specific example) and the pilot MANUALLY selects VLOC (because the 540 will not auto transition unless the FRONT course is dialed in AND the pilot selects LOC REV on the Century coupler.

The heading bug doesn’t matter much after intercept has been accomplished but I use it to remind myself of the appropriate inbound course.

Quite frankly these back course localizer approaches could be done much more easily using the GPS. In my specific example the waypoints already exist in the FAA database!

Then again I think the FAA could come up with an LPV that would replace the back course approach entirely (and it’s 3 step down fixes) that might result in lower minimums and would be easier to navigate.

The biggest problem I suspect most folks have with the back course approaches is they aren’t in widespread use so folks don’t get much practice with them.

I’ve owned my Bonanza for 29 years and only in the past year or so have I begun experimenting with using the autopilot for a B/C approach (in VFR conditions) trying to figure out how to make it work. 

Jim
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 02 Feb 2019 at 10:40pm
Jim,

I agree with you about the C-III.  Sorry if my note wasn’t clear.  I've never had to do a BC approach, but good to discuss the particulars.  There's always something to learn. 

I mentioned that the heading bug must be set to the inbound course because the C-III manual requires it.  “…DG must be set to correspond with the desired magnetic track…”  “…by setting the DG Course Indicator to match the Omni Course selection, all headings are then controlled by the radio signals.”   “…In principle, the radio coupler provides a heading signal which corresponds to the direction of the course to be flown.  This heading signal is coupled to a radio deviation signal in such a manner that any radio deviation will cause a proportional heading deviation.”

The two deviation signals work together to establish proper intercept angles and course capture without overshoot.  However, when the heading is within 15 deg of course, the heading deviation signal is removed from the circuit to allow tracking the course with a crosswind.  That allows the “radio deviation signal” to 100% control the heading within +/- 15 deg of the course.  So, within +/- 15 deg of course, the heading bug has no affect.  Outside of that 30 deg window, it certainly does.

Bob
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Post Options Post Options   Thanks (0) Thanks(0)   Quote nrproces Quote  Post ReplyReply Direct Link To This Post Posted: 03 Feb 2019 at 7:31am
FYI
Just a small reminder for those folks who have never "actually" done a BC approach.

Because the localizer antenna is now on your end of the runway the actual "physical" width of the course is smaller, relative to the position that you are on final, and thereby much more sensitive to changes in heading, a normal correction is going to appear to you, the pilot, as a much more aggressive change in CDI movement.

Please go out and fly some before you actually do it in the weather, so that you don't give your vestibular apparatus a serious once over near minimums.      JMO


(The width of the navigational ray can span from 3° to 6°, however mostly 5° are used. The output is set to secure a signal approximately 700 ft (213, 36 m) normal approach end of the runway, on the borderline . The width of the output magnifies, so at a distance of 10 nm (18,52 km) from the transmitter the output is about 1 nm (1,852 km) wide. So if you subtract that space from the distance on your runway [perhaps a mile or two less] you will understand why it is more sensitive when the antenna is on your end of the runway.)

Illustrates the localizer antenna radiation characteristics


Edited by nrproces - 03 Feb 2019 at 7:34am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote SB Jim Quote  Post ReplyReply Direct Link To This Post Posted: 03 Feb 2019 at 10:34am
Bob, here is a copy of the entire page of the C-III pilot’s guide regarding back course approaches. Note that the guide assumes that the aircraft begins the approach on the front course, flies past the airport, does a procedure turn, then returns on the back course.

ILS APPROACH--BACK COURSE (See Fig. 19)

1. To Intercept
A. Dial ILS Back Course outbound heading on Course Selector D.G. 
B. When stabilized, position mode selector to LOC NORM mode.

2. After interception and when beyond fix, position mode selector to HDG and dial outbound procedure turn heading.

3. After one minute, dial inbound procedure turn heading indirection of turn.

4. When between 90° and 45° to inbound course, dial inbound course on Course Selector D.G. and position mode selector on LOC REV mode.

5. Approximately 1⁄2 mile away from runway, position mode selector to HDG mode to prevent “S” turn over ILS station near runway threshold.

I can confirm that this procedure works as described in the aircraft.

Jim
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Bob H Quote  Post ReplyReply Direct Link To This Post Posted: 03 Feb 2019 at 7:45pm

Originally posted by SB Jim SB Jim wrote:

Bob, here is a copy of the entire page of the C-III pilot’s guide regarding back course approaches. Note that the guide assumes that the aircraft begins the approach on the front course, flies past the airport, does a procedure turn, then returns on the back course. 

ILS APPROACH--BACK COURSE (See Fig. 19) 

1. To Intercept

            A. Dial ILS Back Course outbound heading on Course Selector D.G. 

            B. When stabilized, position mode selector to LOC NORM mode. 

2. After interception and when beyond fix, position mode selector to HDG and dial outbound procedure turn heading. 

3. After one minute, dial inbound procedure turn heading indirection of turn. 

4. When between 90° and 45° to inbound course, dial inbound course on Course Selector D.G. and position mode selector on LOC REV mode. 

5. Approximately 1⁄2 mile away from runway, position mode selector to HDG mode to prevent “S” turn over ILS station near runway threshold. 

I can confirm that this procedure works as described in the aircraft.

Jim, this page from the C-III manual confirms that the heading bug is integral to the C-III tracking of a radio signal.  Unfortunately, the manual uses some ambiguous language.  The reference to “Course Selector D.G.” is a reference to the D.G. with heading bug.  See the definition on page 8.  #4 can be re-written: “When between 90° and 45° to inbound course, dial inbound course with heading bug and position mode selector on LOC REV mode.” 

Also see “OMNI MODE” on page 16, and “LOCALIZER (Normal) MODE” and “LOCALIZER (Reverse) MODE” on page 17.  All of these descriptions in the manual state that the “Course Selector D.G." (heading bug) must be set to the inbound course.  I suspect that most times LOC tracking isn’t engaged until within +/- 15 deg of the inbound course (especially with GPS) so the heading bug has already been removed from the circuit and no longer necessary for tracking the LOC.  Proper tracking beyond that 30 deg window must have the heading bug set to the inbound course.

Bob
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