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3 Answers

Non Standard IFR Alternate

Asked by: 6742 views Airspace, FAA Regulations

  An ILS gets you lower than a VOR approach. Since the VOR approach will not get you lower than the ILS,  the VOR approach is more restrictive  than a ILS. Logically, if the weather (visibility low and low clouds---say 300 feet) the VOR approach would not work but the ILS would (typical ILS DA 200 feet AGL) Reference KRDU:  ILS 23R has the nonstandard alternate symbol. VOR 23L uses standard alternate criteria. VOR 23L  MDA brings you to 625 AGL.   ILS23R brings you to 200 AGL but the nonstandard alt. is 800-2. So to file RDU as an alternate, the non-precision VOR approach allows me a lower ceiling(625 and 2400 RVR)  than the ILS 800 ceiling and 2 miles vis. yet the precision ILS ironically calls for a ceiling 125 feet higher compared to the 625 foot ceiling of the VOR approach.   My question: Why have nonstandard alternate  (higher mins.) for the ILS 23 R but standard alternate (lower mins.) for the VOR approach?

3 Answers

  1. Best Answer

    John D Collins on Aug 28, 2013

    Alternate minimums are based on the standard 600-2 for an ILS and 800-2 for a NPA. They are designed so that the forecast weather is conservative and that one can reasonably count on making a successful approach at the alternate, even with system failures of components that are not absolutely required for the approach to be conducted. As part of the conservative planning a few other assumptions are made. First, the approach lights may fail and these can increase the approach minimums to be above the 2 mile visibility. So on any approach where this might happen, alternate minimums are increased in order to account for this possibility. Second, in the case of the ILS, the GS could fail and the approach could still be accomplished with the localizer minimums. This is accounted for by making sure that the localizer minimums on the same chart can be used.

    In the case of the ILS or Loc RWY 23R, some of these factors force the alternate minimums to be higher than standard and therefore they override the standard values. In the case of the VOR RWY 23L the standard values for the alternate minimums are high enough and don’t need to be changed.

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  2. Dan Chitty on Aug 29, 2013

    Thank you John. Very helpful and much appreciated.

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  3. scott on Jan 25, 2017

    why are t.o. alternate based on number of engines

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