You can set the OBS to a northerly
course, but not to a northerly
heading. Mixing up the terms course, track, and heading is the path to confusing the problem. Here's a reminder:
- Course is the direction over the ground you want to fly.
- Heading is the direction the nose is pointed.
- Track is the direction over the ground you are actually flying.
There is no such thing as "reverse sensing". When a VOR is selected, the needle will always go to the side of the indicator corresponding to the cardinal direction of the selected course line from the aircraft's position. "Reverse sensing" is a misconception associated with thinking left/right instead of east/west or north/south and
does not exist.
So let's take the scenario discussed above -- heading 010, 180 course selected, currently on the 185 radial. For orientation, with a 180 course selected, the top of the CDI corresponds to south, the bottom to north, the left side to east, and the right side to west. In this situation, you are located west of the 180/360 course line, meaning the course line is east of you. Therefore, the needle will be deflected to the east side of the CDI, which happens to be the left side. If you look at that side of the case, you'll see a big E or 9, meaning you have to fly east or 090 to go straight-line/shortest distance to that course line.
In addition, you'll see a FROM flag, indicating you are south of the perpendicular line across the course line through the station. You can then see that to join that course line headed south (outbound), you have to turn southeast. Now you look at your heading indicator (HI). With 010 at the top, you see that a southeasterly heading like 135 is to the right of the nose, so you turn right to join that course line southbound
even though the needle is deflected to the left.
Now, let's say you want to join that same 180/360 course line headed north instead of south, but for whatever reason (say you're training for LOC BC approaches), you leave the OBS set to 180. Everything in the second paragraph above is still true -- the needle is still deflected to the east (left) side of the CDI. However, if you are east of that course line and south of the perpendicular, you can easily visualize that you need a northeast heading to join that course line northbound -- and you can help yourself by looking at the CDI to do that. Since the needle is deflected east, and you want to go north, pick a heading off that CDI OBS ring between the side to which the needle is pointing (E/9) and the desired course (N/36) on that OBS, such as 045. Now look at the heading indicator, and you'll see that once again a right turn is needed to get to the desired heading despite the needle being to the left.
The key is using the CDI to tell you the direction you want to fly to get to the desired course line, and then using the HI to tell you which way to turn.
Let's say you want to join the 180 radial inbound from south of the station, i.e., a 360 course TO the station. You set the OBS to 360, see that it is deflected 5 degrees to the east side of the case (in this case, the right side). That means you are 5 degrees west of the 180 radial, or on the 185 radial. Your heading indicator is on 010. Do you need to turn left, right, or not at all to intercept the 180 inbound?
The answer is "insufficient information". The initial answer might be that since we are west of the course line, but our heading is to the east of it, we can just stay on heading and intercept. That would be true if and only if you see the needle moving from right to left showing your track is angled towards the selected course line.
However, say you have a strong wind from the east such that your
course is 355 even though your
heading is 010. In that case, you'll see that not only is the needle 5 degrees to the east of 360, it's moving further east, meaning you are moving further away from the course line. You will have to turn further east to counter the wind, and that means looking at the HI to see you need to turn right of 010. As you turn, the needle will stop moving as you exactly counter the wind, and then start to move back the other way towards the west side of the CDI once your track is moving towards the 180 radial.
OTOH, let's say the needle is stationary, showing you are tracking directly to the station, but on the 185 radial, not the 180 radial. In that case, you'll have to make a small heading change to the east to get your track more easterly towards the desired course line. Of course, if you're flying a holding pattern at a VOR station, you probably don't want to make much of an adjustment at all, maybe none. Inside one minute from the station, being 5 degrees off the radial is very well within the protected airspace. If you do nothing and the needle stays where it is, you'll fly right over the station only 5 degrees off the desired course, but if you start trying to catch it inside 2 miles, you're more likely to get yourself in an S-turning chase than to cross the station wings-level on or close to the desired inbound course.
Now, let's go back to that upside-down case, where you want to track backwards, like going out the front course or in the back course of a LOC. Remember that the LOC signal is different from VOR, and it doesn't matter what you spin on the OBS, it will act like you have set the forward direction (the front course inbound course or back course outbound course). If you set the CDI on the forwards direction, then you can use the technique I described above to track it. Let's say we're trying to go inbound on the front course of a LOC 27. You put the 27 at the top. Say you are south of the course -- the needle deflects to the right (north) side of the CDI. If you fly a heading to the north side of 270 (e.g., 300), that will take you to the localizer in the direction you want to go (wind factors ignored for the moment).
What about going backwards on that 27 localizer, i.e., out the inbound or in the outbound? If you set the desired course (090) on the CDI, that will have no effect on needle movement. If you are south of the localizer, the needle will still deflect to the right side of the case even though you have now caused the S/18 to be on that side. Now you have to ignore the numbers on the OBS ring, and think to yourself "with a 27 localizer, the right side is north, so I need a heading north of the 090 course such as 060 to intercept." And if you are on 090 at this point, that is a left turn, not right, even though the needle is deflected right. That's what people mistakenly call "reverse sensing" even though it is still responding the same based on the position of the aircraft.
You can help yourself in this case by putting the OBS on 27 instead of 9, which will give you 27 at the top and 9 at the bottom. Now the needle is deflected towards the S/18, and if you look between the 18
and the desired 9 course at the bottom of the CDI, you will see that a northeasterly heading such as 060 is needed to get you onto the localizer. If you're currently headed 090, your heading indicator tells you that you need a left turn to alter your track so it will intercept the localizer.
IOW, to fly a localizer , you set the OBS to the forwards direction
regardless of which way you want to fly it, see what the needle points to, pick a heading between the desired course (i.e., top for forwards, bottom for backwards) and the direction to which the needle points, and then look at the HI to decide which way to turn.
Whew! My fingers are tired.
Yup.
over this issue -- it's a systemic problem of bad training, not anything for which the poor PP never taught any better is responsible.
