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So, why is the FAP at 2500 ft?
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Combination of;
Glideslope coverage
Lowest level at which flights will be at on the intermediate segment (on the loc before GS intercept)
Ensuring that the above mentioned intermediate segment is an appropriate length for stabilisation - 2nm normally between LOC established and GS intercept. - Pre continuous descent noise abatement!!
Keeping the above flights within the relevant airspace.
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cleared ILS approach 27R" can you guarantee me that the traffic won't then descend to 2,500ft which is the stated start altitude on the approach plate for a radar vectored ILS/DME approach to 27R.
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Is it? Where is that on the chart?
My reading of the chart shows an intermediate approach level of 2500ft altitude but there is no indication of the initial approach level which is what the level assigned by ATC while vectoring is and there is no procedure for descending from the initial approach level to the intermediate approach level during vectors unless cleared by ATC.
If the procedure is completed without radar then the appropriate chart details when the aircraft can descend from the initial approach level but that does not apply when being vectored.
The problem with clearing flights to descend with the ILS from say 3000ft and using that as a basis to keep vertical separation from a helicopter on the extended centerline, is that it is doing in the vertical sense what UK ATCOs refuse to do in a horizontal sense
i.e. Just because the flight is told to descend with the glideslope after establishing on the loc it does not guarantee that the flight will not descend early. Even being told "maintain 3000ft until the glideslope" does not guarantee that descent will not start at say 12 miles from touchdown due to the inherent errors that can be in the system.
The only way to ensure separation is to treat the traffic as mobile obstacles, build in a buffer and apply the appropriate procedure design criteria. having done that the chart can show the area(s) within which the mobile airborne obstacles are and everyone knows (just like they do when there is terrain) why they have to respect the appropriate levels.
Anything else is only tipe toeing round the edges of the problem.