Terrain - 3

28.12.2009

Submission:

Unfortunately the ICAO mountainous terrain is not the same as TERPS precipitous terrain. If you search for precipitous terrain on Google and you will find the history. Some 15 or more years ago, ALPA asked the FAA to apply its own rules and increase ROC (MOC) at 5 airports which ALPA considered as having precipitous terrain. At the time this depended on subjective judgement. It took the FAA another ten years to develop the current computer model which assesses surface roughness and gives an increment to ROC, and in the case of SAAR - prohibit the procedure altogether. Unfortunately when the ICAO RNP/AR was written, the significance of this was not appreciated and it was assumed (as have you) that precipitous terrain was the same as mountainous terrain. A procedure designer from New Zealand noticed this and asked why the ICAO RNP/AR criteria was less restrictive than the FAA SAAR criteria. The FAA criteria prohibited SAAR in mountainous terrain and the RNP-AR criteria did not. As a result a patch had to be rapidly added to the ICAO criteria - some words added that indicated Bernoulli effect on altimeters had to be considered in approval of the procedure. This has by now probably filtered down into the approval material, although the last time I looked it was not very specific, and was certainly not as clear a prohibition as in the TERPS criteria. Now regarding wind adjustments to DA/H for wind effect in PANS-OPS; Vol I gives very specific adjustments for the pilot as a function of wind speed. However Vol II does not tell the designer to advise pilots that the effect can be present at a particular aerodrome. Instead it confuses the issue by including wind effect in the mountainous terrain paragraph. This is clearly illogical as both designer and pilot could both be applying a correction - or neither! Also the designer is the one who should know that the wind effect is significant at a particular aerodrome, and provide a warning on the chart. This is on the long list of PANS-OPS errors and anomalies that is currently being worked through by ICAO. So the moral of this story is always ask - but you may not get the answer you expect!

Robert Woodhouse - ICAO IFPP advisor - Canada