aterpster,
With all do respect,
RA is a Radio Altimeter height, associated with CAT II approaches. For a CAT III the DH acronym is used, although it is a Radio Altimeter reading as well.
As for the Alert Height (AH) it is defined as:
An Alert Height is a height above the runway, based on the characteristics of the aeroplane and its fail-operational automatic landing system, above which a Category III approach would be discontinued and a missed approach initiated if a failure occurred in one of the redundant parts of the automatic landing system, or in the relevant ground equipment (ICAO).
In other AH definitions, it is generally stated that if a failure occurred below the Alert Height, it would be ignored and the approach continued. (TP 1490)
The Alert Height will be set between 100' and 300' (if I remember well)
As for Category III A operations: a precision instrument approach and landing using ILS or MLS with:
(A) a decision height lower than 100 ft; and
(B) a runway visual range not less than 200 m
You are correct in stating that CAT III does not have have a DA. CAT III has a DH (lower than 100')
Although I agree that it is common to find CAT III approaches with no published DH, there are a a few airports that have CAT III with a DH. Oslo (ENGM) comes to mind.
Cheers
Last edited by Glonass; 12th Jun 2013 at 10:20.