The following quote is from G. Bristol’s book, Ace the Technical Piloting Interview:
You can descend on the glide path when
- You have been cleared for the ILS procedure.
- You have captured the localizer, within ±5°C. [sic]
Regarding point 2. There is plenty of discussion about what it means to capture the localiser and/or be established on the localiser (e.g. see airliners.net discussion). Can someone please clarify, from an ICAO PANS-OPS perspective, the point that a pilot can descend on a glide path on an ILS approach with regards to the localiser, following a clearance for the ILS (and EASA & FAA differences to ICAO).
Regarding point 1. Would the word approach would be better than procedure? For example, an ILS published procedure approach might involve an IAF, perhaps an outbound leg to a platform before the start of the glideslope descent. Whereas you can be cleared for an ILS approach following radar vectoring at a altitude above the published platform altitude.