CRM vs. OAS

25.10.2004

Submission:

I have run into an incompatibility problem that I hope you will be able to help me with. Attached is an ILS procedure which, when I run the CRM gives the ground plane as the obstacle with the highest risk. When however, I reduce the one of the obstacles situated 3661m beyond the threshold I find that I need to increase the OCA/H by 11.69m. This obstacle penetrates both the OAS and Basic surfaces. Why does the CRM not consider this obstacle as the obstacle with the highest risk?



Gerhard Coetzee: Flight Procedure Specialist (South Africa)

Answer or Commentary:

(I.W:

Using your files, I also ran the CRM and found that the ground plane was the obstacle with the highest risk. Then I moved the obstacle that penetrated the OAS surfaces, at the edge of the Z surface, closer to the extended centreline and ran the CRM again. I did a few runs of the CRM each time moving the critical obstacle closer and closer to the nominal missed approach track. As I did so the risk of hitting the obstacle increased with each run until it became the obstacle with the highest risk.

What I think is occurring is that the OAS surfaces are an approximation of the overall risk distribution that is calculated by the CRM. The CRM risk distribution is approximately an oval in shape and the OAS surfaces are flat plane surfaces.

As can be seen in the diagram there is an area (marked in red) where penetration of the OAS surfaces would not lead to a penetration of the CRM risk probability area. The CRM risk probability area and the OAS surfaces move closer and closer together as you move into the centreline. This continues until the CRM and the OAS areas finally touch around the centreline.

I hope this explains the apparent anomaly between the CRM and the OAS surfaces. I would greatly appreciate any other comments on this subject.)