In the article on how to read back a final approach clearance, I said, at 10 miles from the runway, a pilot on the VOR Runway 18 final approach course would be 1.83 nautical mile (NM) west of the ILS final for the same runway. Here’s how I came up with that figure.
The 60:1 Rule says at 60 miles from a VOR, each radial will be separated by 1 NM.
At 10 miles from the Madison VORTAC, radials would be separated by (10 divided by 60 = .166) .166 NM.
The ILS Runway 18 final approach course is 182 degrees. The VOR Runway 18 final approach course is 171 degrees. Obviously, the Madison VORTAC is not on the centerline of the runway, but offset slightly to one side. If we can agree that offset creates a minor error in our calculation, then the difference between the VOR final and the ILS final is 182 – 171 = 11 degrees. (If you want to be picky about it, the VOR final is on the 351-degree radial and the ILS final is on–approximately–the 002-degree radial. The math works out the same.)
An aircraft on a 10-mile final for the VOR Runway 18 approach is 11 radials x .166 radials per NM = 1.83 NM west of the ILS final approach.