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When we use this awesome instrument for navigation (not only for ILS/LOC), it is said that there is no reverse sensing (except for back course thing). I kind of understand why pilots are confused when we use conventional VOR, and now I want to know why there is no reverse sensing? How does it work? Can anyone help me out?

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For conventional VORs, Positive sensing occurs when the headng currently being flown is on the top half of the VOR. Reverse sensing occurs when the current heading is on the bottom half of the VOR.

Picture of conventional VOR with positive / reverse sensing

Used wth permission from allaboutairplanes.wordpress.com

Taking this theory to the HSI where the currently flown heading is always on the top half and you can never have reverse sensing.

It is also proper technique to set in the front course when flying a LOC/BC approach. This will negate the effects of reverse sensing for HSIs. To get all my students into the habit of setting in the front course for HSIs I teach to always set in the front course even when flying on conventional instruments.


I don't believe in positive and reverse sensing anymore. A discussion for another day.
Tracking and intercepting VORs

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Essentially because you turn the instrument upside down and read it normally.

On the back-course, the signals are reversed. That is a property of the ILS signal. When using traditional ILS/LOC instrument, the needle moves right when it senses more of the right signal, but since the signal is now on the left, you get reverse sensing.

On the HSI, the needle still moves right when it senses more of the right relative to the arrow when it senses more of the right signal. However, the needle points in the selected heading and is rotated with compass. So when you fly a back-course, you dial the normal course of the ILS and since you actually fly opposite direction, the arrow points down and right relative to it is left. And you get normal sensing.

For VOR, the situation is similar. In VOR, the needle moves normally if you are flying the heading selected and it moves in reverse if you are flying the opposite heading. And when you are flying the opposite heading, the HSI arrow points down, reverting the sensing back to normal.

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  • $\begingroup$ This is true for VOR sensing as well, which is what I understand the question to be primarily in reference to. The question indicates that the OP understands that HSI reverse sensing is possible for ILS back course, but is inquiring why it is not possible for VOR sensing, as opposed to a conventional CDI navhead. This answer, while already good, could be made even better by explaining why this is true for VOR sensing. $\endgroup$ – J Walters Feb 22 '16 at 13:22
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I guess what the OP means is that why there is no "reverse sensing" switch on the instrument. The answer being rather simple - to avoid the pilots making a mistake when they forget to flip the switch to the correct position.

You can always flip the OBS 180 degrees to make that change. I like to think of the HSI as always sensing "positively". You're just tracking the wrong side.

The instrument always tells you the relative position to the VOR course / localizer, it is up to the pilot to manipulate the plane to the desired position. It's reverse tracking, not reverse sensing. The instrument tells you your deviation from the selected course, it has been and will be doing that all the time. If your selected course is 180 degrees off, following the rules of geometry, you'll naturally end up switching left and right.

It is worth noting that the autopilots have an "reverse course tracking" switch: enter image description here

If you want an instrument which tells you how to fly to intercept the course, it is called a Flight Director.

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