ILS Approach, when can you descend on the glide path?

Question

The following quote is from G. Bristol’s book, Ace the Technical Piloting Interview:

You can descend on the glide path when

1. You have been cleared for the ILS procedure.
2. 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.

Thank you.

Answer 1

You’re making this more complicated than it needs to be.

Once you are cleared for an approach and established on any segment of that approach, then you may descend to the altitude published for that segment. If the segment has a glide slope/path rather than a fixed altitude, you may follow it down.

That raises the question of exactly when you are “established”. The obstacle clearance surface extends out 50% wider than full-scale deflection. However, since the HSI needle pegs at full scale, you must be less than full-scale deflection (i.e. off the peg) to be certain you are within the protected area.

A localizer beam is 5° wide, so full-scale deflection is a mere 2.5° off course. I suspect the quoted author meant (less than) full-scale deflection but forgot to halve the width.

Answer 2

You've asked what ICAO PANS-OPS say:

From Volume III − Aircraft Operating Procedures First Edition, 2018:

... the pilot should not be required to complete a turn on to final approach at distances less than will [...] in the case of instrument approaches, permit the aircraft to be established on final approach prior to interception of the glide path, as detailed in PANS-OPS, Volume I, Section 4, Chapter 5, 5.2.4, "FAF crossing". (p. 9-3-3)

and

... when vectoring to intercept the final approach course or track, the final vector will meet the following conditions [...] enable the aircraft to be established on the final approach course or track, in level flight for at least 3.7 km (2.0 NM) prior to intercepting the glide path or vertical path for the selected instrument approach procedure [...] (pp. 3-1-3 and -4)

Meaning the turn to final should permit LOC capture before G/S capture.

And from Volume I − Flight Procedures Sixth Edition, 2018 (p. II-5-3-4):

... An aircraft is considered established when it is:

• a) within half full scale deflection for the ILS and VOR; or
• b) within ±5° of the required bearing for the NDB.

➤ As a summary to all the above: be established (half full scale deflection) 2 NM prior to G/S interception.

I'd say the author confused it with the NDB's ±5° quoted above, also note that a full scale localizer deflection is not a fixed angle; it depends on the runway length and ILS category, i.e. the angle varies so that a suitable width is achieved at the threshold.

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Regarding deviations by EASA/FAA: member states are not required to list differences from PANS-OPS 8168 as they do to SARPs (I checked USA AIP, 8168 is not mentioned):

The PANS do not carry the status afforded to Standards adopted by the Council as Annexes to the Convention and, therefore, do not come within the obligation imposed by Article 38 of the Convention to notify differences in the event of non-implementation (Volume III, p. (xi))