Tauno Voipio wrote:

There should be a co-located (and associated) DME if
there are no markers, and in can be used instead of the markers.

In the USA, the MM used to be a required part of the ILS system. Now,
nothing is required to replace the MM (not even DME), so most have
been removed.



It's a weird interpretation of the ICAO rules, but that would
not be the first time FAA does not respect the internationally
accepted rules.

I'll check from the relevant ICAO docs as soon as I get to
the office.

AFAIK, there should be at least either operative outer
and middle marker or a co-located DME. The markers are
positioned so that outer marker is roughly at the crossing
of the intermediate approach altitude and glideslope
(the point where glide starts) and middle marker at
the crossing of glideslope and MDA.


When DME is used as a substitute for the Outer Marker (OM) in the USA,
it doesn't even have to be co-located. As long as the DME source is
within 6 degrees of the final course, it can be used as a replacement
for the OM (Even more if the FAA provides a waiver).
For the military, they only require that the DME be within 23 degrees of
the final course. Many airports use VORTACs or VOR/DME facilities to
provide the DME for an ILS localizer FAF. The FAA's policy is that if
DME is available, it will be used to provide a localizer FAF at the same
point as the glideslope intercept point (called a PFAF), and the OM or
LOM will no longer be used if the DME is available (most do not allow a
common PFAF, because they are not perfectly placed at the point where an
even 100' altitude increment and the glideslope are overhead. In other
words, the glideslope altitude at the OM might be 2367, but the PFAF
will be placed at either the 2400 or 2300 point, and DME will be used
instead of the OM.
The downside is that there are a lot more approaches getting the title
"ILS or LOC/DME" instead of "ILS or LOC" because of this, requiring DME
in order to fly the localizer procedure.
The FAA policy is if the DME is out of service, the ILS (precision) can
still be flown, because the glideslope intercept defines where the final
approach begins, and DH defines where missed approach begins.
This is part of the reason for this current discussion; many procedures
have no way of marking the LOC missed approach point in the USA other
than the time-distance table, because no MM or DME is available.

John