SI higher mins than Circ

[gprellwitz]

Can someone explain why a straight-in approach would have higher visibility requirements than a circling approach? On the NDB RWY 5 to KRYV http://204.108.4.16/d-tpp/0707/06179N5.PDF , the category C has visibility of 1.75 if straight in, but only 1.50 if circling. The VOR/DME or GPS 5 has the same mins (1.75) for both straight-in and circling.

BTW, the VOR/DME or GPS RWY 5 is on the list of approaches the FAA is considering eliminating as redundant.

[FONT=Arial,Helvetica,Geneva,Swiss,SunSans-Regular][FONT=Arial, Helvetica, Geneva, Swiss, SunSans-Regular]FAA PROPOSES TO ELIMINATE 'REDUNDANT' APPROACHES
For the third year in a row, the FAA is proposing to eliminate hundreds of instrument approaches that it feels are underutilized or redundant. The action would not decommission actual navaids, just eliminate the approaches. AOPA is conducting a detailed analysis to make sure the move would not eliminate all IFR or ground-based and satellite navigation access to airports, or result in higher instrument minimums. AOPA encourages members to download the list of approaches and send comments to the FAA postmarked by September 15. "Local knowledge is invaluable. If you rely on a particular approach, tell the FAA that, and how frequently you use it," said Randy Kenagy, AOPA senior director of strategic planning. Send comments to: National Flight Procedures Office, AJW-32, P.O. Box 25082, Oklahoma City, OK 73125. Please e-mail a copy of your comments to AOPA.[/FONT][/FONT]

 

[Areeda]

Can someone explain why a straight-in approach would have higher visibility requirements than a circling approach? On the NDB RWY 5 to KRYV (http://204.108.4.16/d-tpp/0707/06179N5.PDF), the category C has visibility of 1.75 if straight in, but only 1.50 if circling. The VOR/DME or GPS 5 has the same mins (1.75) for both straight-in and circling.

BTW, the VOR/DME or GPS RWY 5 is on the list of approaches the FAA is considering eliminating as redundant.

BTW I could not download the plate from your link??

I believe the difference in minimums is because the MAP is the on-field NDB and if you don't have the airport in sight well before that you won't be able to land straight in.

On the VOR/DME or GPS the MAP is before the threshold.

Does seem to me that eliminating the NDB would make more sense but maybe more people coming to Watertown have an ADF than have VOR/DME or GPS.

Joe

 

[poadeleted20]

My guess is that the lower circling vis min has to do with the descent gradient from MDA to the touchdown zone on the straight-in approach. If the straight-in MDA is high enough, the interception of the max gradient line with the MDA is so far out the vis min has to be raised to allow the airport to be sighted in time to start down and make the touchdown zone without exceeding the maximum descent gradient (400 feet/nm on the final segment for nonprecision approaches). OTOH, the circling vis min is driven more by the circling maneuver area radius, i.e., it's OK to spot the airport from overhead and then circle down from MDA.

 

[gprellwitz]

I fixed the link.

The MDA is 619' AGL, so 1.75NM gives a descent rate of 353ft/NM, or over 700fpm for a category C plane . Matches Ron's hypothesis. (no big surprise there!)

Thanks!