Make sure that Lubbock channel 29 is set in so you can receive navigation information from the TACAN.  Also ensure the navigation radio has 109.2 set in so you can get directional information from the VORTAC.

Figure 2 - DME

Figure 3 - Navaid tuning popup panel

The theory behind using the RMI to do a fix to fix is the scale.  This makes the geometry work out.  The method of using a geometric principle to accomplish the fix to fix is called the "pencil method". 

Start by figuring out if either the point you are going to or your present position is further away.  Which DME is bigger?  The farthest DME determines the scale.   In our case, we are at 20 DME, and we want to go to 30 DME.  So 30 DME becomes the scale, which becomes the outer area of the RMI as seen in Figure 4.

Figure 4

Now you plot an imaginary line from the center of the instrument to the ticker mark on the compass card representing the radial of the fix you are going to.  In our case, we were told to go to the 010 radial, and so the yellow line in Figure 5 points to 010°.

Now a good initial action is to start heading in the general direction.  Do this by turning the aircraft to a heading between the head of the bearing pointer and the tip of the imaginary line representing the radial of the fix (also called a head), so, between 257° and 010°.   310° looks like a good initial heading that will get us started in the right direction, so make a left turn to 310°.  Note:  we are already between the bearing pointer head and the tip of the imaginary line, so you can maintain the current heading or go to another arbitrary heading in between. 

Figure 5

Now we need to further refine the heading to take us directly to the fix.  This is accomplished using a geometric method referred to as the "pencil method".  Generally it's done by mentally forming a picture, but students are typically taught to pull out their pencil and use it to "draw" the lines on the instrument.

1. Imaginary fix line :  begin by mentally forming a line between the center of the instrument and the radial that you are fixing to.  We want the 010 Radial, so the line extends out to there.  Since we are going from 20 DME to 30 DME, this also corresponds to the farthest scale on the concentric scale.  This is shown as Dot 2.
2. Plot your position on the tail of the RMI's needle.  Since the scale of the instrument is 30 DME, and we're at 20 DME, the location of our aircraft on the needle is 2/3rds of the way out (20/30 DME).  This is shown as Dot 1.
3. Connecting Dot 1 to Dot 2, we get an Uncorrected Solution Line.
4. To properly correct this, translate (or move) that line so that it begins at the center of the instrument.  Where it now points to is the heading you need to complete a fix-to-fix to the location you were given a clearance to. 
   
   The solution is not wind corrected.

Figure 6

• Be sure to update the fix to fix heading continually because wind varies and will skew you off course.  The best update points are:
  • when the DME of the destination and the DME of your current position are the same
  • some other combination of DME's that are easy to visualize (i.e. 10 DME to 30 DME).  Just don't choose something like 17 DME to 30 DME because that's a harder point.
• It is important to turn to the initial heading between the two heads as quickly as possible because you will often fly at 3 or 4 miles per minute, so the potential to become mis-positioned can happen rather quickly.   Turning between the two heads will start you in the right direction at least.
• In the example above, your current position determined the scale.  This may not always be so.  We were going from a larger DME to a smaller DME.  There may be times when you will go from smaller DME's to larger DME's.  If this is the case, then your position along the tail of the bearing pointer will no longer be at the very tip of the tail.  It will be some percentage of the total scale (the reverse situation of the example).  Just trace the line the right way, and you will still end up at your destination.
• If you are going from the same DME as the destination DME, i.e. 30 to 30 DME, it is called "cutting the arc".  Both points are then 100% the distance from the center to the edges of the instrument circle. 
• While the RMI only method of doing a fix to fix works well, you can refine that last fraction of a mile using the Course Indicator (CI).   This is why you initially put the fix's radial into the CI.  As you reach the fix, the Course Deviation Needle on the CI will begin to deflect and center.  Using the bearing pointer (toilet bowl) on the CI, you can determine your relative angle between the fix's radial and your current heading, which you can use to control your angular rate of closure to the fix.  You can "play" the last part of the fix to fix as a course intercept, so your fix to fix becomes as accurate as 1/10th of a mile.  The best way to see this, is practicing this firsthand on the simulator.  Get a feel for this technique as it seems to work rather well.  The USAF teaches this method but what is written here is all technique.  The USAF has procedures, which must be followed to the letter.  Techniques are just that, methods which can be altered to suit your need and are not graded.