I wish I was a little more artistically inclined and had the tools to put together a really good presentation for you but alas, I am not and I don't so you're just going to have to bear with me and follow along in the cockpit if you want to know what that instrument is telling you.
First off, you have a nice compass rose, which always points to where you are headed. On the outside of that ring is a little rectangle with a centerline in it and this represents the desired course your mission planner has set up between two waypoints - the one you have selected to go to (shown up in the HUD) and the one you came from - whether you did or not. It portrays the course between those two waypoints. In RL you have a knob that allows you to select what course you want to fly between two points. The little triangle on the inside of the ring always points towards the selected waypoint, the one shown up in the HUD. The long line with the arrow head on it is the "course deviation indicator", which tells you where the course line is relative to you right now.
Just for giggles, lets pretend that we are right on course, with no wind, and right in the middle of the course line - right at the mid point between the two waypoints, the one you're going to and the one you left behind. Your HUD will tell you which Nav waypoint you are going to on the top line (this is down in the lower right hand corner of the HUD). The middle line gives you the distance to that waypoint and the bottom line gives you an estimated time enroute to that waypoint. The HUD will have a little vertical line in the center of the flight path marker indicating that you are right on course and you will see up on the top of the HUD a heading tape with a little carrot (that's an inverted V) showing your heading and a little tick mark where the waypoint is. This little tick mark is the same as the little triangle down on the HSI and the little vertical line through the flight path marker is the same as the long line with the arrow head on the HSI.
Now, if you were to drift off to the right, the little vertical line on the HUD is going to move over to the left, as will the long line on the HSI. These are telling you that you are RIGHT of course or that the course line you need to be on is off to the left. This is a no brainer and I am quite sure that all of you understand this concept. But there are a few little things that might be not so obvious and can really screw you up if you don't have a good handle on how this navigation equipment works.
Always remember that the little triangle will point to the waypoint. The long line (the course deviation indicator) will tell you where the course line is relative to your position in space and the selected course (the little box on the outside of the compass ring). It has nothing to do with your heading. So, when you are way out there in La La land, having chased down a tanker for some of the life giving juice, and you need to get back to the course line, first put the little triangle on the top (and you will see the little tic mark up on the HUD heading tape) and then look at the heading selector to try to imagine where you are in relation to the course line that the mission builder made and what direction you need to go in order to get back on it. Of course, this is only important if you need to follow a course line over the ground that the mission planner created, say in order to avoid a AAA threat or something like that. Otherwise, just fly to the waypoint. I have succeeded in confusing myself so I can only imagine what you all are thinking! I'll try to do this in a training mission online with you.
