ILS Start/goaround RNAV
To commence the ILSILS —Instrument Landing System approach, you must be within half full scale deflection on the localiser. You normally start the approach as the glidepath indicator becomes centralised.
If you are unable to maintain within half full scale deflection on either localiser or glideslope you must immediately go around as you are outside the area where terrain avoidance can be guaranteed.
This is not just a theoretical question: for those of you that will ultimately fly an instrument rating test, if you fail to initiate a go around in this circumstance, you will fail the test; conversely, the examiner cannot penalise you if you take the correct actions and then attempt the approach again.