What is "Half Scale Deflection" on a CDI?

[miss_heard]

Hi, on a CDI that has 2 dots on either side of the ILS beam centre, does "half scale deflection" equate to a 2 dot (~2.5 deg) deviation or does it refer to a 1 dot (~1.25 deg) deviation?

I'm getting very confused!

And 'generally', how much deviation would be classed as a missed approach? (1 dot or 2?)

Thanks in advance!

[DB6]

Depending on how the dots are spaced i.e. if one is halfway and the other is at full scale then the first is half scale, and beyond this (and not correcting) would be out of limits and theoretically a go-around. Most CDIs that I have seen have about 5 dots either side of centre though, and half scale is not so much counting dots as seeing whether the needle is halfway across or not.

[miss_heard]

So, broadly speaking:

Half scale = half way across from centre to one side
Full scale = ALL the way across from centre to one side

See this is where I was getting confused - to me "Full Scale Deflection" implies the *maximum* possible deflection of the indicator, i.e. all the way from one side of centre to the other. But what you are saying is that this would actually be TWICE the FSD - correct?

[DB6]

Deflection = deviation from the correct course (from a dictionary), so it can only be one way or the other from the centre. You are confusing movement with deflection.

[miss_heard]

ok, thanks for clearing it up.

I guess I was thinking back to my A-Level physics days when a full scale deflection on a moving coil meter meant that the needle moved all the way from one side of the scale to the other!

cheers

[Avtrician]

Full scale deflection is when the needle moves to the limit of travel away from the zero point, so in this case you could have full scale left or right deflection from the centre zero point.