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low n' slow
15th Feb 2007, 11:10
Hi all!

On my last flight I was questioned about my navaid setup when flying an NDB approach with a minima based on 2 NDB's.
The approach in questions is the EFKK rwy 01 NDB and is based on two NDB's, the first (ESS) being the outer marker and the second (V) shortly before threshold.

I was planning to fly with both side ADF's towards the ESS beacon and once overhead or slightly before, switching over my side to V. My other crewmember then insisted I fly the intermediate leg woth both ADF's pointíng to V as he would then be able to monitor the approach better. In this case, the approach would not be based on 2 NDB's and thus the minima would not be valid. By flying the intermediate leg on one ADF with QDR and the other ADF with QDM, tracking is much more precise over the cone of confusion as one may revert to the more stable reading. However, I also understand that the splay of the NDB obstacle clearance calculation requires V to be the active frequency on whilst flying the intermediate leg. But does this override the possibility of using the ESS frequency on the second ADF (non flying side) to gain better tracking?

How should this be done and what are the PANS OPS requirements when the minima states "2 NDB". My logical reasoning says the minima requires 2 NDB's to be in use....

/ LnS

Old Smokey
15th Feb 2007, 15:06
When the minima states "2 NDB", that's exactly what you need, 2 ADFs.
Lateral obstacle clearance is based upon the splay from the applicable navaid/s, and, when 2 NDBs are specified, the splays overlap, and the protected area is within the "diamond" created. If you use only 1 NDB, the splay emanates from that one NDB, but the protected area after passing the first initially expands, and then contracts at the mid-point between the 2 NDBs. With 1 of the 2 NDBs required in use only, you have no assurance of remaining within the protected area.
Sorry that I don't have the facility to post the diagram, but the RWY 33 twin locator approach to Kuala Lumpur (Subang / WMSA / SZB) is a good example, if you stayed tuned to the first NDB only, there's a 600 foot+ obstacle within the splay from that NDB, which isn't considered for the twin locator approach, as the splay in the region of the offending obstacle is quite small, being based upon the second NDB. In this case, the MDA is actually BELOW the offending obstacle.:eek:
Regards,
Old Smokey