1. When would you pilots press ALT HOLD?
......3. It says an ILS overlay! But if you select ILS 15 in the FMC why is it an overlay?
Not sure what equipment you are using (simulating?) ...and not familiar with Salzsberg but until someone familiar with ops there comes along I’ll offer a generic answer in line with our SOPs and equipment.
1.I’ll start with the “overlay” query first.
with an ILS overlay Approach selected in the FMC.
That looks slightly clumsily worded, as I read it it means “select ILS, fly it as an overlay” - in this case you are going to select the ILS from the FMC but are not going to use Approach (APP) mode, you are going to use LOC and e.g. VNAV to fly the profile...more on that in a minute.
2. You are probably not going to press ALT HOLD. More likely you’ll set the circling Minima in the Mode Control Panel (MCP) alt window when cleared for the approach so that you will eventually get
automatic altitude capture at circling minima....but...
3. If you do for some reason fly the ILS using APP mode (i.e. coupled to the both localiser and the glide) there is a serious trap. The MCP altitude will not be automatically captured as you reach it, so there is a risk that you’ll descend through the circling minima ...which brings us back to why the overlay...
3. By flying ILS as an overlay you can use a vertical mode which
will capture the MCP alt in the descent e.g. VNAV, vertical speed or flight path angle....
There are other advantages to not trying to transition from a full blown coupled approach into a circling manoeuvre, usually ergonomic, but the busting of the circling minima is the main safety reason.