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a busy cat

For the RNAV (GPS) RWY 17 approach at KSMN, why does it say "Helicopter visibility reduction below 3/4 SM NA."?

Looking at the LNAV, there is a 5 mile visibility requirement and if we cut that in half per AIM 10-1-2, wouldn't there be a 2 1/2 mile visibility requirement? If so, then a helicopter couldn't fly the approach with 3/4 mile visibility anyway.

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  • $\begingroup$ I'm not entirely sure what your question is. $\endgroup$ – Carlo Felicione Jun 8 '17 at 0:53
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    $\begingroup$ @CarloFelicione -- his question's fairly simple actually: "why did the FAA put a note on the plate that has no effect in practice?" $\endgroup$ – UnrecognizedFallingObject Jun 8 '17 at 1:23
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    $\begingroup$ It does seem odd. My only thought is that perhaps some §135 operations could have authorizations that could allow them to use lower visibility minimums, apart from a prohibition like this. $\endgroup$ – J Walters Jun 8 '17 at 4:18
  • $\begingroup$ Can't an helicopter approach on a 350° course and then circle to land to RWY 17. In this hypothetical explanation, the 5 mile requirement wouldn't apply. $\endgroup$ – mins Jun 8 '17 at 18:16
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    $\begingroup$ AFAIK, the 3/4 SM restriction is there because of obstacles (presumably the tower between the MAP and the runway) but I have no idea why they printed it when the minimums are much higher anyway. My guess is it was included by default but the best way to get an answer might be to ask the FAA. We've done that before :-) $\endgroup$ – Pondlife Jun 8 '17 at 19:15
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My understanding is that similar to the notation on the chart that "DME/DME RNP-0.3 NA, the helicopter notation (Helicopter Visibility Reduction less than 3/4 SM NA) is just a charting convention.

Even though it's obvious from the (non-helicopter) 5 mile min vis (reduced to 2.5 miles for helicopters), the issue of 3/4 SM is never relevant. The same goes for the "DME/DME RNP-0.3 NA, which is irrelevant also since in order to fly a RNP -0.3 approach (which an LNAV approach is), you must use a properly certified GPS system to achieve the RNP -0.3 performance accuracy (DME/DME RNP can never be used). I think both are just a charting convention.

This question has been asked before on another forum (can't remember which one) and this seemed to be the accepted most reasonable answer.

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Helicopters may operate under special VFR when in flight visibility is less than 1 SM. Therefore, the approach can be flown at lower visibility than that published on the approach plate. However, the note must be published to further restrict these types of operations: "The procedure is annotated with 'Visibility Reduction by Helicopters NA.' This annotation means that there are penetrations of the final approach obstacle identification surface (OIS) and that the 14 CFR Section 97.3 visibility reduction rule does not apply and you must take precaution to avoid any obstacles in the visual segment."1

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