OCA/H adjustments 1

28.10.2004

Submission:

Since the nominal track altitude can be off by as much as the RAS adjustment, then the RAS adjustment should be applied to the formula for obstacle clearance in the missed approach segment as well as the final approach segment.


R. P. Myers - HQ Air Force Flight Standards Agency - USA

Answer or Commentary:

(I.W.

Firstly I think this will not occur with an ILS approach as any airport that I know of that has an ILS does not have a requirement for a Remote Altimeter Setting (RAS) adjustment. However with a non-precision approach (NPA) it is quite possible and this is especially true now with the introduction of GPS. Small airports that were previously only VFR now could possibly have a NPA capability with no setup investment (i.e. VOR/DME) or yearly maintenance costs.


I think Mr Frenz is right that the adjustment of the OCA/H continues through into the missed approach. Mr Myers is also right that the RAS should be applied in the missed approach as well and in fact it does depending on when the RAS adjustment is made. In the following diagram one can see how the RAS continues through from the final OCA/H into the missed approach segment.

These conditions will only occur when one designs the procedure Without including the RAS adjustment and then add the OCA/H RAS adjustment at the end of the design of the procedure. If one designs the procedure up to the MAPt and then adds the RAS adjustment to the OCA/H then no allowance for the RAS; as I can see it, is allowed for in the missed approach. See following diagram.

One can see in the above diagram that the obstacle in the final approach has a RAS adjustment while the obstacle in the missed approach does not. However in both diagrams the RAS adjustment was added as per ICAO Pans-Ops and it is my opinion that if an aircraft has an altimeter error on final approach it will also have an altimeter error in the missed approach.

In the very unlikely ILS case, it seems to me that the RAS value is accounted for within the precision segment no matter when you add it but is only accounted for after the precision segment when it is added last.

As a general rule, I personally add OCA/H adjustments at the end of the procedure design process while MOC adjustments (i.e. excessive length of final) I add at the time when they are required.)