Failed / Downgraded Equiptment and the OM

[ooizcalling]

Some ILS proceedures have Outer Markers, others dont. For those that do, a failed OM without a published equivalent position or SRE/PAR fix means the proceedure is 'Not Allowed'. At least thats my reading of the books.

Why is this ? What criteria determines the necessity of the OM in the first place.

It seems strange that in places like Holland, Belgium, Northern Germany for example, where the terrain is as flat as a pancake, for those approaches with an OM specified, the lack of it (ie. U/S) eliminates an otherwise useable ILS approach. In contrast, in other places with quite rugged terrain in the vicinity there are ILS's without OM's.

Just curious, as it seems inconsistent.:hmm:

[Clandestino]

Every ILS approach must have at least one point at which one can cross-check altitude vs. distance to runway. Historically it was outer marker but nowadays more often it's DME fix (hence the proliferation of no-OM ILSes). This crosscheck is done to assure that your altimeters are working correctly, that you have set the correct QNH, and, most importantly, that the you have the correct glideslope guidance.

Discusssion about false glideslopes and lot of useful links here. (http://www.pprune.org/forums/showthread.php?t=80062)