ILS crossing altitudes

28.10.2004

Submission:

I have a query regarding outer marker check altitudes, from the pilot's perspective. The following paragraph is extracted from the Australian documentation issued to pilots; AIP ENROUTE.

"7.3 Altimeter Checks

7.3.1 The final approach area contains a fix or facility that permits verification of the glide path/altimeter relationship. The specified check altitude will have a tolerance due to atmospheric conditions, fix accuracy, altimeter calibration and static error. On final approach the aircraft altimeter indication must be noted at the fix or facility. If the aircraft altimeter indicates a higher altitude than the check altitude, the difference between the two altitudes must be added to the approach minima. If the aircraft altimeter indicates a lower altitude than the check altitude, corrective action is not required."

The term,

"The specified check altitude will have a tolerance due to atmospheric conditions, fix accuracy, altimeter calibration and static error",

seems at odds with the term,

"If the aircraft altimeter indicates a higher altitude than the check altitude, the difference between the two altitudes must be added to the approach minima".

My current employer has a policy of adding the exact difference (if high) to the minima. For example, if the aircraft crosses the OM 30'' high (as judged by the peak of the aural tone), the minima is raised by 30''. My previous employer was reputed to have advice from the regulator to the effect that the OM check altitude is a gross error check of QNH settings or false glide-slopes rather than a "fine-tuning" of the minima. I am currently writing pilot procedures and would appreciate any comment.

Greg Scholes Australia

Answer or Commentary:

(I.W.

I personally tend to think that these check altitudes are a gross error check. In the following diagrams one can see the altitude difference when only the ICAO overhead beacon tolerance (OM defined as NDB) is taken into account. Two distances from threshold are shown 3nm and 5 nm. Do the check pilots reading this have any comments?)