# Why do some back course operations use normal sensing?

So the LOC/DME-E approach to KASE lists for the missed approach course to use the I-PKN localizer, but then there's a note 'course outbound is normal sensing'. Does that mean that in effect, this particular LOC is installed 'backwards' so when flying away from it you get normal indications? How common is this? Only for LOC-defined missed approaches?

• Flying another localizer as a missed approach is fairly uncommon. I imagine that the reason this is being used as a backcourse is that it's only purpose (unless there is an airport on the top of that 11,000ft mountain or something) is to guide you back to LINDZ keeping you clear of the mountains. Therefore it would make it easier for the pilot if it just gives correct sensing. Commented Jan 16, 2014 at 19:13
• @p1l0t I agree! In this particular case (I've flown this approach a lot) the only purpose is indeed only for the missed approach. Commented Jan 16, 2014 at 19:17
• Hehe, it's a very simple trick ;) What do you do to achive normal sensing when flying a LOC BC? You twist the needle by 180°. Now take a look in which direction you track this back beam on the missed approach. (Watch the little word 'outbound')
– Falk
Commented Jan 16, 2014 at 22:14
• @Falk I'm not sure what you are trying to say here. You will be tracking away from the station (outbound) but in this case you don't need to twist the needle 180 deg. because they switched it at the source. Commented Jan 16, 2014 at 23:01
• Let imagine HPKN would be located an an airport. Than it would most probably be a LOC for a runway 30. The portion of it present on this chart is it's back beam that's why "BACK COURSE" is written next to it and the left side is blue (schadet). If you would like to approach RWY 12 of this fictional airport via this LOC BC and have 123 set on your HSI (or what else) you would have 'non normal' sensing. But if you would set 303 on this fictional approach the sensing would be correct. Now take a look what you do flying this missed approach procedure.
– Falk
Commented Jan 16, 2014 at 23:12

Notice that in the plan view, the shading of the localizers are on the opposite sides. If you are flying south-west, the actual localizer used for the approach has the shading on the right, while the one used for the missed approach has the shading on the left. This can also be used to tell you what sensing to expect. The note is also there to make it more obvious that the sensing will be backwards from what you might expect when flying outbound on a "normal" localizer.

So yes, this particular localizer is designed to transmit backwards and you will get indications that would be normal while flying inbound when flying outbound on the missed.

I'm guessing that because of the high terrain/mountains in the area that they wanted to make it as easy as possible to fly without getting confused as to which way you are supposed to turn. Combined with the fact that it is never used in a situation where you would be flying inbound makes it a perfect candidate to reverse the sensing. Unfortunately, in my mind it actually makes it harder to understand. We are taught from the beginning of our instrument training how to handle flying a localizer outbound, and now we have to do it the opposite way. Rather confusing if you ask me, but you get used to it.

As far as "how common is it", this is the only one that I've seen (but I'm guessing that there are others)....

• Aspen is odd because the I-PKN localizer is used only for the missed approach. That missed approach is "interesting" to fly.
– xpda
Commented Jan 17, 2014 at 6:55
• @xpda I've only flown it once in IMC, but all the time as part of the departure. Many people won't even attempt the approach unless they can expect to see the airport at KICER. Commented Jan 17, 2014 at 12:25
• I-PKN localizer is not designed to "transmit backwards." It's actually the normal backcourse of the I-PKN localizer. When you fly a localizer backcourse in the opposite direction (like some missed approach procedures require) you are getting the same normal sensing you get on any localizer approach when flying opposite direction on the backcourse.
– user22445
Commented Nov 26, 2021 at 23:09
• "So yes, this particular localizer is designed to transmit backwards": This may be incorrect. Here is the array, and the front lobes seem to be used to transmit the signal. Source.
– mins
Commented Dec 13, 2021 at 1:00

This procedure is unusual mainly because it uses a remotely installed localizer for a missed procedure. A backcourse used in a localizer approach is not unusual, the uniqueness here is that that the localizer back course being used is not the one which was used to approach the runway. This procedure calls for flight away from the localizer transmitter during the missed approach, just as many other missed approaches would do. Contrast this with a typical final approach segment of a conventional localizer approach in which the aircraft on approach flies toward the transmitter.

I would not say that this localizer is designed to transmit backward any more than any other localizer with a back course is designed to transmit backwards. It is simply a matter that it is the back of I-PKN that is in use for the procedure. Since the course is outbound from the transmitter on the back course, sensing is not reversed, and the chart has been designed with a note to remind the pilot of that fact.

To further convince ourselves that this localizer is a run-of-the-mill navaid, picture an airplane on the missed approach procedure, outbound on a course of 303°. Now slew that aircraft backward to a point Southeast of the transmitter, still on a heading of 303°. It is now inbound toward the transmitter and sensing is in the same direction, not reversed. There is simply not a runway to be found directly prior to the transmitter. Passing over the transmitter, course does not change and sensing does not change.

• I agree, you are correct. It's normal sensing when flying opposite direction on any loc backcourse. Good answer.
– user22445
Commented Nov 26, 2021 at 23:10

The key issue here seems to me how to set up the instruments and what the indications will be. For an HSI you set the CDI to the front course, i.e. in this case to 303 and you will get correct sensing. For the old CDI/OBS because you are going “backwards” on a backcourse, you will get correct sensing (as the chart states). The reason they appear to have used the otherwise odd idea of flying outbound on a backcourse is so you do get correct sensing (and because there is not a mountain high enough and far enough away to use a localizer front course!).

• Welcome to aviation.stackexchange! Commented Mar 15, 2021 at 15:56

I-PKN localizer is not designed to "transmit backwards." In my opinion, it's actually the normal backcourse of the I-PKN localizer antenna that sits on a 11000 ft mountain. Its only purpose is to serve as part of the missed approach procedure at KASE (and a departure procedure from KASE) . The front-course is not used because there is no airport, runway etc. associated with this localizer antenna's front-course. (It is essentially just a navaid sitting on a mountain)

When you fly a localizer backcourse signal in the opposite direction (like some missed approach procedures require after flying the front-course and then executing a missed approach) you are getting normal sensing.

The reason why the note on the plate states that I-PKN is "normal sensing" when flying its backcourse is to reinforce the fact that it is "normal sensing" and eliminate any confusion (which apparently is not always the case). Having a separate localizer antenna installed by itself away from an airport and serving solely as a missed approach navaid is unusual.