Hello, I'm getting slightly confused about the theoretical 'width' of an ILS localiser beam but hoping an expert here can possibly explain what I'm missing.
It's often quoted that a full scale deflection of the Course Deviation Indicator (e.g. 2 dot deviation on a typical instrument) is equivalent to 2.5 degrees either side of the ILS beam centre.
But then you also hear about the "Course Width" of the ILS, the angle of which varies as a function of runway length, but is set such that it provides a FSD of the indicator at 350ft either side of beam centre when you pass over the landing threshold. So (presumably) for a very long runway FSD would be equivalent to a smaller angle from centre, compared to FSD when approaching a shorter runway.
So my question - is the 'often quoted' figure of 2.5 degrees just an approximate/average value of what a CDI's FSD represents? I hope my question makes sense.
many thanks