Friday, February 19, 2010

Reverse Sensing? No Such Thing

I often hear comments from pilots about confusion over normal sensing and reverse sensing. To some it seems no big deal, but to others it is a source of worry and confusion. I am going to make what may seem to be a rather startling comment to some of you.



If you truly understand VOR navigation, there is no such thing as reverse sensing.


Okay, bear with me. The confusion arises because the majority of pilots have been taught to “fly towards the needle” when learning VOR navigation. That in turn gets interpreted to mean fly either right or left in order to get on the desired course. And therein lies the problem and the source of the confusion.


You do not fly towards the needle. Instead you fly towards a heading, and that may mean fly either right or left. It depends on where the nose of the aircraft is pointing.




In the above figure, the OBS is set to 360, same direction as the nose of the aircraft. So students have been taught to fly towards the needle, or to the left. But instead of right or left, think heading. Looking at the VOR display, a heading of 270° will take you directly to the selected course. In this case that does happen to be a turn towards the left. But now let’s spin the aircraft around, so it is flying south.




Notice the VOR display has not changed. The VOR head (as the VOR display is technically called) neither knows nor cares where the nose of the aircraft is pointed. The CDI is still deflected to the left. It is still saying that to get directly to the selected course, you need to fly a heading of 270 degrees. But now look at the DG (or heading indicator). A heading of 270° is towards your right – in the opposite direction of the needle. This, then, is the so-called reverse sensing. But if you think heading rather than right or left, there is no reverse sensing. Instead there is just a heading to fly, and that heading may be to your left or to your right.


This works for localizer as well as VOR courses. Let’s take a look at the ILS-18 into Lebanon, NH (KLEB). Here is what the instruments look like as the pilot is being vectored for the ILS, flying a heading of north. Look at the approach plate first and see where the aircraft is.





Now let’s look at the aircraft instruments. Note the aircraft is flying north, on the east side of the localizer. The VOR display says the pilot needs to fly towards the west to get to the localizer course. The DG (heading indicator) says this means a turn towards the left. Think heading, not right or left!






Now let’s look at tracking inbound on the localizer. Again, first look at the approach plate to see where the aircraft is.



Now let’s look at the instruments. Note that the aircraft is slightly west of the localizer course. The instruments show exactly the same thing – the aircraft is slightly west of course. Also note in both cases, the OBS for the VOR display is set to the final approach course. If you set the OBS to the final approach course, it results in less knob twisting, and in a busy cockpit, this is an advantage.




So you see that if you disavow yourself of the “fly right or left or towards or away from the needle” way of thinking, the entire concept of reverse sensing simply disappears.

I’ve shown it here for a localizer, since this notion of reverse sensing is very common when either flying a back course or tracking outbound on a localizer to do a procedure turn. But it works exactly the same way for tracking on a VOR approach.


Let’s look at the VOR-25/LEB. Below is a portion of the approach plate, showing the position of the aircraft being vectored for the approach.






Now let’s look at the instruments. The aircraft is being vectored on the east side of the final approach course. The OBS is set to the final approach course. The needle is deflected to the right and a heading of 330° would take us directly to the final approach course. But 330°is to our left, away from the needle. Think heading, not right or left.



Now let’s look at the picture as the aircraft has been given a heading that provides an intercept for the final approach course. First look at the approach and see where the aircraft is.







The aircraft is on a heading of 280°, just about ready to join the final approach course. This is what the instruments look like.




If you think heading rather than right or left, you can set the OBS to the final approach course, even if you are flying outbound to do a procedure turn. By the way, with an HSI you always set the course to the final approach course, sometimes called the front course, even if you are flying a Localizer-Back-Course approach.

If you are used to always setting the OBS to your heading, as students have been traditionally taught, then continue with the procedure if you are comfortable with it. However you decide to do it, though, think heading – not right or left.


I close this discussion with the following acronym.


IBOT


I have used it for a long time to help students remember what radial they are on.


I = inbound or “TO” flag on VOR head

B = radial is at the bottom of the VOR head

O = outbound or “FROM” flag on the VOR head

T = radial is at the top of the VOR head


So think “inbound bottom” and “outbound top.”


Try out some of the ideas I’ve presented here yourself. You will find out they work, and they can significantly reduce the amount of confusion that occurs when doing VOR navigation.










9 comments:

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Javalí Rubio said...

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phaques said...

This is an excellent article, and a great blog. I just started instructing instrument students, and even for my private candidates, this deals with a sticky, complicated issue in VOR navigation.

Anonymous said...

Thanks for the feedback. It's always gratifying to learn that the articles are helpful.

Linda

Anonymous said...

Finally, I get it! Thank you! I am working on my instrument, and only wish I was taught this way from the very start (and not "turn towards the needle").

Unknown said...

This is some of the best information on basic navigation I have ever read. Never thought of reverse sensing like this.

Anonymous said...

Great way of teaching. Helps me think of it in a way I've never thought of. Also the acronym you ended with, I've learned it as (from top to bottom) and that seemed to soak in a lot better

Anonymous said...

What if you have reverse sensing on an HSI? Need explanation please!

Anonymous said...

You don't get reverse sensing on a HSI since the needle follows you while turning.