This is a vital question, but incredibly there's a big black hole in the aviation culture on this subject.
I have personally noticed that the majority of the colleagues I have flown with didn't know how to check the glide slope correctly.
Even some directives from the manufacturer or other authorities lead in the wrong direction.
The problem is: how to check I'm on a valid glideslope.
To my knowledge, nobody has an official rule to refer to for doing an
altimeter accuracy test at the outer marker in order to make a correction at Decision Altitude.
The tolerances and variables in real life are too many to specify exactly what is the maximum acceptable altimeter reading error.
Everybody uses good common sense for accepting/refusing the altitude value they read.
If it's colder than standard, you can expect to read higher than published, for example.
Anyway, altimeter accuracy is not the point during the approach (you have checked it before departure, you've completed a whole flight without complaints from ATC, etc...).
The point is
checking the glideslope.
You can leave your safety altitude only if you make sure the lobe is correct.
Various fatal accidents have happened because people failed to do this check
correctly .
What most people think is the correct method is: "When I overfly the outer marker, I must be at 1300 ft."
BANG! DEAD!
The truth is exactly the opposite!
"When I descend through 1300ft, I must check the outer marker is there".
A simple point like that seems to be so hard to understand, sometimes!
If you are on a wrong low lobe, or if your instrument gets stuck, with the first incorrect method you are gonna wait for the outer marker to do the altimeter check.
You you are gonna find your a§§ on the ground before you ever reach the marker.
Instead, if you do it properly, at 1300ft you expect to hear the marker: if not, at 1200ft you are gonna start worry seriously, at 1100 even more, at 1000ft you get the hell out of there!
It's amazing to see people calculating exactly, with their own formula, hom many feet deviation they will get with ISA minus 7°C, waiting for the outer marker in order to do a meticolous check, instead of waiting for the altitude to do it!!
The above is true, of course, also in case the official glide check is by the DME instead of the outer marker.
"At 1300ft I
must read 5 DME" and not "At 5 DME I must read 1300ft".
Got it? I hope so!
LEM