Orion 1
While I agree that this subject is a big black hole it is really very simple.
Alt checks at the OM/DME distance occurr for the following reasons:
1/. Gross error check in case of incorrect subscale setting,
2/. To catch any instrument errors that have not been allowed for,
3/. Yes if you try and capture GS from above you may get a false GS...never seen it myself but there you go.
1/. is fairly rare in two crew ops as most have sufficient altimeter cross checks...but it can happen.
3/. is again very rare
The biggest uncorrected 'error' you will see in day to day ops is caused by ISA Deviation. The reason the old rule stated if the altimiter was undereading ignore it was because that is safe and essentially, in ISA+ Australia, going to be the case virtually every ILS you ever fly...with the possible exception of Melbourne/Hobart in winter.
Pressure sensitive altimiters are not corrected for air temperatures which deviate from ISA. This correction must be done manually and the correction applied to all IAL heights/altitudes including MSA/LSA but not to assigned altitudes, where ATC should make the allowance for you
I'm not going to pull out the ISA correction tables in my Dassault Manuals tonight but the rule of thumb I use is
+/- 4' per degree of ISA Devn x (Alt/1000') and that agrees with the tables to within a few feet until the temperatures get antartic cold and the height above the datum gets very high indeed.
Alt = height above the temperature datum, i.e. the airfield.
On an ISA+20 day in Australia sliding down an ILS past a check height at say 5DME/1300' to a MSL runway you would get;
-4x20x(1.3)= -104'. All other things being equal you would expect to see 1200 odd feet a 5 DME if you were on GS. Of course altimeters aren't perfect so there may also be some position error, which you should know and therefore be able to allow for, and perhaps some minor/in tolerance mechanical errors too....essentially though if you were indicating 1200 odd feet at the OM you could confidantly say "Outer Marker check altitude OK" or whatever your SOPS call for.
At the minima...say 200' the error would be -4x20 x (.2) = -16'. Hardy worth worrying about in any real way given the other errors, including parallax, but in an otherwise perfect world at 200' indicated your radalt might say 216'...no biggy as I say.
It is all opposite in temperatures colder than ISA...in the above example using ISA-20 you would expect to see approx 1400' at the OM but you can also see the mistake in just 'adding the difference' as the old rule stipulated. At the DA 200' your radalt migt say 184' which is a bust....so for a checkride I'd be adding the 16'...pedantic I know but if you didn't at least discuss it I'd fail you!!
Where it gets real interesting is high LSA/MSAs...say for instance MSA 10000', airfield elevation 1000', ISA-20. +4x20x(9)=+720'...that's how much you altimeter will be in error...putting you possibly only a few hundred feet above the highest obstacle within that sector.
Not being able to intelligently discuss the above for an Instrument Rating Oral in places like Canada would end the Test.
All the above does NOT mean getting paranoid about 20' here or there...it means prior to TOPD you have a good idea of what altitude you expect to see at the OM/DME GS Check and if the altimeters are close then that's good enough...have a coffee and slide onto to the minima
Chuck