October 6th, 2010 
Admin Question by John R: what does it mean if you have a ILS frequency and course, but the message “Loc Crs offset 2°”?
I saw this: 110.7/043, and next to it: rwy centerline 045°. So, of course, a 2° change. But is the ILS and the needles (actually just the LOC) in the plane then accurate?
Or which course do if have to fly in the end?
Best answer:
Answer by a1an
It just means that the LOC is not exactly aligned with the runway. If you keep the needles exactly centered, you will be off 2 degrees when you reach the runway. There is a limit of how far off the LOC can be from the runway center line and still be a LOC approach. I don’t remember the specs. If it too far off, the approach will be changed to a LDA approach which is just a LOC approach but not aligned with the runway.
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In addition to a1an’s answer, which is great, many larger airports intentionally offset the localizer on parallel runways as an added safety buffer on the approach corridor. Many class B airports will stack parallel runways with offset approaching traffic, so it gives them a little extra horizontal separation between the arrivals for the two runways
Dont forget that runways headings change over yearly periods. Maybe where you are the runway heading changes 1degree every 5-10 years. Runway’s headings (magnetic fields/Magnetic variations) change naturally, but not ILS and VOR radials/courses. This could also account for the error, as the Approach plate are updated every (158 days i think). Anyways it is still within safe limitations of the ILS as it is still an ILS as long as the localizer is less than 3 degrees off the runway heading. More than this is expected for a SDF(6-12 degrees, but not more than 30) or LDA (more than 3 but not more than 6 degrees).
In the end just follow the approach as published. If you follow the runway heading the ILS will start to deviate. A 2 degree error over a 7-10 miles or one dot LOC travel will only put you less than 100 feet to correct for when runway is in sight. Remember a full scale LOC is 700 feet.
At the DH or MDA(LOC APP) about a .5 NM away at 90 knots I dont imagine you will be banking over 15 degrees to make a safe approach while still on the Glide slope.
The localizer is offset from the runway, but changing the runway number does not affect this. The magnetic variation does change over time, but that would change the loc direction as well. You see, the loc antenna is attatched to the ground, the same ground that the runway is attatched to. So, if the magnetic variation of the runway changes, so would the loc direction.