Terrain

08.12.2009

Submission:

I am interested in finding out how ICAO addresses precipitous terrain. Here in the U.S. the FAA has software that makes an assessment of the terrain model and then if necessary adds an additive to the ROC/MOC. I’d be interested to know what factors your readers use to first determine if terrain is precipitous or not and then how they determine how much of an adjustment, if necessary, needs to be made.

Bill Frenz, Procedure Designer, Mitre, USA

Answer or Commentary:

(I. W.)

I do not think that ICAO specifically covers terrain where it rains (precipitation) a lot.

I just could not resist that.

Seriously,
Precipitous (steep) terrain is covered in the “mountainous terrain” section (Part 1, Section 1, Chapter 1, paragraph 1.7).

“1.7 INCREASED ALTITUDES/HEIGHTS FOR MOUNTAINOUS AREAS
1.7.1 When procedures are designed for use in mountainous areas, consideration must be given to induced altimeter error and pilot control problems which result when winds of 37 km/h (20 kt) or more move over such areas. Where these conditions are known to exist, MOC should be increased by as much as 100 per cent.

1.7.2 Procedures specialists and approving authorities should be aware of the hazards involved and make proper addition, based on their experience and judgement, to limit the time in which an aircraft is exposed to lee-side turbulence and other weather phenomena associated with mountainous areas. This may be done by increasing the minimum altitude/height over the intermediate and final approach fixes so as to preclude prolonged flight at a low height above the ground. The operator’s comments should also be solicited to obtain the best local information.”

Good information and as one can see it is up to the procedure designer as to what increased MOC is used.

However what exactly is considered as “mountainous terrain”? For that, you have to look in the most obvious place, the “Abbreviations” which states:

“Mountainous area. An area of changing terrain profile where the changes of terrain elevation exceed 900 m (3 000 ft) within a distance of 18.5 km (10.0 NM).”