Use of 'BC' or Backcourse in full

**MikePotten** · 12-30-2002, 12:32 PM

Dear Simmers, I have been asked to explain the functionality and use of the autopilot 'backcourse'. I thought I understood it but have just tested my knowledge in my RAE Comet out of Boscombe Down it didn't work as expected. I would be obliged is anybody who fully understands this would correct my understanding, which is as follows:-

1) if flying towards a beacon from the west heading east i.e. 090 the aircraft will track towards the beacon, if I then wanted to fly away from the beacon on the same track I understood that this was one use for it i.e. I would continue to fly directly over the beacon and then select the backcourse button to fly 090 away from the beacon?

2) if making an ILS approach to an airfield that has only one ILS frequency from the 'other end' of the runway you use the backcourse in conjunction with the APR to track the ILS from the other direction so to speak?

Which one is correct or are neither of them? Would appreciate your kind assistance.

Thanks in advance and Merry XMAS

Mike P

**Falcon_20_Freight_Dog** · 12-30-2002, 01:19 PM

[font size="1" color="#FF0000"]LAST EDITED ON Dec-30-02 AT 01:24PM (EST)[/font][p]Here are 2 examples of useing a back crs.

A back crs approach
http://edj.net/Plates/EastCentral/CWA_lBCr26.GIF

Tracking a back crs on a missed approach
http://edj.net/Plates/SouthWest/ASE_vd_gC.GIF

http://www.flightsim.com/dcforum/Use...fb348bcf93.gif

**lesage** · 12-30-2002, 01:27 PM

[font size="1" color="#FF0000"]LAST EDITED ON Dec-30-02 AT 01:33PM (EST)[/font][p]Back course has nothing to do with a beacon, or VOR. Itis for a localizer approach.. the backcourse does not glideslope info, just the localizer beam. so, if say runway 27 had an ILS approach , runway 9 may have a backcourse approach,with localizer only, no glidslope info.. bob L

**lnuss** · 12-30-2002, 01:56 PM

Unlike a VOR, a localizer transmits essentially a left/right signal (and it is four times as sensitive as a VOR signal). Coming in on the localizer on the normal, front side course, if you are left of course, the needle will move to the right, telling you to fly to the right. Even after passing the localizer transmitter on the same course as the inbound front course, you still get the correct left/right information.

Of course this means that if you are going in the opposite direction (approaching from the back side), that the left and right indications are reversed, thus if you are left of course, the needle moves to the left and you'd need to fly to the right. Obviously, this makes your task more difficult, though it is certainly doable. Thus this approach from the rear is referred to as a back course. But this "reverse sensing" would make it impossible for an autopilot to track inbound on a back course, so they have a back course switch, which merely reverses the sensing. More sophisticated avionics may also have the ability to switch the VOR/Localizer needle to react properly when on a back course (the OBS is useless on a localizer).

Understand that there are not published back course approaches to ALL localizers, but many do have published back course approaches. As with any published approach, the FAA (in the U. S.) charts and tests the approach before it is allowed to be used as an instrument approach. And, as with all published approaches, it is periodically test flown by the FAA, to ensure the everything is working correctly and that the published approach plates (charts) accurately reflect the conditions to be found.

And, as mentioned here by others, back courses do not generally have a glideslope associated with them. And you most certainly can NOT use the glide slope for the front course when flying the back course. So while there are back course localizer approaches, there generally are not any back course ILSs.

Larry N.

http://www.flightsim.com/dcforum/Use...170941bddf.jpg

**KenG01** · 12-30-2002, 02:11 PM

On a couple BC approaches I did during my IR training, I actually did get an intermittent glideslope indication. Of course, it was garbage and I paid it no mind. My CFII told me it happens at some places more than others, but neither of us knew exactly why. Could it be part of the front course signal being sent out from the glideslope antenna? Or might it be reflective signals from the glideslope coming back at us? I don't know, but that's about all I could come up with. I only saw it when within the backcourse localizer "cone" and it would come on and then go away after a few seconds. This would be repeated throughout the approach and was never the same if flew the approach repeatedly.

Anyway, just asking if anyone has seen this (in real life, not FS) and knows the reason. I'm simply a little curious.

And for the record, I hate BC approaches. It would never fail that when I'd fly it for the first time in a while, I'd screw up the reverse sensing at some point during the approach.

-----
Ken G:-wave

**flyinggriffin** · 12-30-2002, 02:57 PM

You cannot use the backcourse function for a localizer or ILS safely unless there is a published approach. The reason is that you have no minimums set! You cannot assume that just because there is an ILS on one end of the runway that the opposite approach will be clear.
A perfect example is the ILS 1 into Glens Falls NY. If you were to fly a backcourse and try to use the glideslope, if you could pick it up at all, it would be the last time you would do it. There is a good size mountain to the North that really ruins your day. In fact, if you see a missed approach that starts by a climbing turn there is usually a pretty good reason why they do that instead of a straight ahead climb.

**lnuss** · 12-30-2002, 03:24 PM

It's not surprising that you might get some occasional indications on your glideslope indicator when flying a back course, but they are not safely usable. Few antennas are completely uni-directional (until you get into dishes, anyhow), although the signals off the back side will usually be attenuated considerably, unless, like a dipole, the antenna is designed as multi-directional. And since the glide slope signals are in the vicinity of 330 MHz, a dish is way too large to be practical. So there will be some radiation to the rear -- there is also the possibility of reflections of the signal being received on the back side. Since there are several different types of glideslope antennas, it is likely that the leakage to the rear will vary between antenna types.

For pictures of several different types of glideslope transmitting antennas, see: http://www.ngs.noaa.gov/AERO/navimages/cmanual_gs.html

Larry N.

http://www.flightsim.com/dcforum/Use...170941bddf.jpg

hoag · 12-30-2002, 03:53 PM

> You cannot use the
>backcourse function for a localizer
>or ILS safely unless there
>is a published approach. The
>reason is that you have
>no minimums set! You cannot
>assume that just because there
>is an ILS on one
>end of the runway that
>the opposite approach will be
>clear.
> A perfect example is
>the ILS 1 into Glens
>Falls NY. If you were
>to fly a backcourse and
>try to use the glideslope,
>if you could pick it
>up at all, it would
>be the last time you
>would do it. There is
>a good size mountain to
>the North that really ruins
>your day. In fact, if
>you see a missed approach
>that starts by a climbing
>turn there is usually a
>pretty good reason why they
>do that instead of a
>straight ahead climb.

Ah, the Warren County Airport. Flew out of there many times. I grew up in Fort Edward, NY.

John Hogan