ILS crossing altitudes 2

28.10.2004

Submission:

The ILS OM check altitude, as defined in the OZ AIP, is primarily used as a gross error check satisfying the requirements of PANS-OPS III-21.4.3. The check altitude correction is not to determine where the source of error comes from, but rather the conservative corrective approach to unknown errors. The best I can ascertain, this correction (adding a value to DA when altimeter reads high on the glide path at OM) was introduced for those aerodromes which get days significantly colder than ISA.

On an ISA - 25 day your height above station is about 8% lower than the height read from the altimeter.

In Australia, the OM is placed approximately 3.9nm from threshold. Using this, the nominal check height will be about 1290'' for a 3° GS. On the cold day mentioned the over read error will be around 100'' (a bit of a penalty). The DA will typically be 220 - 250 ft, and the expected error on the same approach should be around 20'' on the glide path. The correction is probably overkill but it is conservative.

Ray ROMANO

Procedure Design Specialist - Australia