Hey guys,

what exactly is a 'half scale / full scale deflection' in a LOC or GP?

Thanks

If the CDI is exactly in centre of the centre circle, you are precisely on the set track. If the CDI is on or beyond the last dot on the scale i.e. the extreme left or right hand dot, it is said to be on full scale deflection. If the CDI is half way between those two positions, it is at half scale deflection.

If you count the edge of the centre circle as the first dot [which is normal] then half scale deflection is two and one half dots away from the exact centre of the scale. In the case of a VOR, each dot is 2 degrees so half scale deflection is actually 5 degrees off the centre of the scale.

In the case of a LOC half scale deflection is still two and a half dots from the centre of the scale, but each dot is only one half of a degree, so half scale deflection for a LOC is one and one quarter of a degree off the centre of the scale.

In the case of the ILS glide slope, the same rule applies except that each dot is only one tenth of a degree so half scale deflection in the vertical is only one quarter of a degree.

In every case, if you are within half scale deflection, you are considered to be doing an acceptable job of maintaining the required flight path.

Bob

Thanks a lot Bob - very detailed answer!