Jose wrote:
Well, maybe I'm thick, but I don't understand. True, you need to be
prepared to switch (i.e. have the chart handy, have the DME tuned
properly) but I see no reason why a properly prepared pilot could not
switch.
Or... set up for the LOC, but fly the GS if it's available.
I'm looking right now at the BFI ILS 13R and the LOC/DME for the same
runway.

Try looking at the BTM ILS 15 and LOC/DME for the same runway, and it
will be a lot clearer. You can't set up for the LOC and use GS if
available, because the procedures for setting up are not the same. On
the ILS, you intercept the GS at 10,600, at 14+ DME from I-BEY. On the
LOC/DME, you can be down to 9600 at that point. Even if you do pull
that off (and it can certainly be done - at least the intercept is from
below), there is another reason you can't do it. The missed approach
for the ILS has you climbing to 11,000. On the LOC/DME, it's only
9,200. Might be iffy if the controller has someone holding at CPN at
11,000 or something similar.

There certainly are situations (BFI ILS 13 and LOC/DME comes to mind)
where the approaches are sufficiently similar that you could do it
safely, but that's not how the regulations work. If you are cleared
for one approach, you can't just change your mind and do another,
because there is no requirement to design them such that the change is
safe - and BTM is one example where the change is NOT safe. Therefore,
you are cleared for one given approach (unless given a cruise clearance
or explicitly cleared for any approach) and you must obtain an amended
clearance if you want to change your mind.

In those situations where the approaches are on the same plate (say
ILS/LOC or VOR/GPS or some such) the approach clearance covers whatever
is on the plate for which you have the equipment, and thus you can make
the change midstream if you so choose since the design of the approach
is such that the change is safe.

Michael