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I'm reading AIM 1-1-17 section 5. (c). I understand that the current regulation allows un-augmented GPS (non-WAAS GPS) to be used at either destination or alternate, but not both. But the chapter goes on saying that

At the alternate airport, pilots may plan for:

(1) Lateral navigation (LNAV) or circling minimum descent altitude (MDA);

(2) LNAV/vertical navigation (LNAV/VNAV) DA, if equipped with and using approved barometric vertical navigation (baro-VNAV) equipment;

(3) RNP 0.3 DA on an RNAV (RNP) IAP, if they are specifically authorized users using approved baro-VNAV equipment and the pilot has verified required navigation performance (RNP) availability through an approved prediction program.

What do the extra items here signify? The alternate weather minimum is still 800-2, so these don't have anything to do with that, right? If so, are these items trying to exclude certain types of GPS approaches that aren't available as alternate?

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    $\begingroup$ For operations conducted under the authority/requirements of Operations Specifications (Op Specs), Management Specifications, LOA's, etc., alternate airport weather minimums may not be 800-2 (for non-precision appchs for example). They can be lower. Look up info on op specs paragraph C055 for info on this. Applies to various operations such as FAR Part 121, 135, 125, etc. $\endgroup$
    – user22445
    Aug 11, 2022 at 5:18
  • $\begingroup$ The 800-2 is for the airport to qualify as an alternate. If you divert there, you fly whatever approach makes the most sense, to its published mins. The planning requirement is separate from flying the actual approach. $\endgroup$
    – Ralph J
    Aug 11, 2022 at 18:02
  • $\begingroup$ @RalphJ 800-2 qualifies as a non-precision alternate for most part 91 ops. However, Operations subject to OP Specs (e.g. Part 121/135/125, Operators using MSpecs, LOAs etc) base their Alternate Minimums on para C055 of the Op Specs/ MSpecs/LOA and use a specific adjustment to the Minimums section of the IAP they plan on using. (Not the Part 91 800-2). Check out op specs C055 online for the details. $\endgroup$
    – user22445
    Aug 11, 2022 at 18:27
  • $\begingroup$ @757toga I get all that; my point above is that 800-2 (or whatever other number one gets from those various sources) applies preflight when considering using the airport as an alternate, and not when actually flying an approach at the alternate. I know you understand that already; some readers may not. $\endgroup$
    – Ralph J
    Aug 11, 2022 at 19:40
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    $\begingroup$ @RalphJ. Agreed $\endgroup$
    – user22445
    Aug 11, 2022 at 20:50

2 Answers 2

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Offhand, it looks like they exclude LPV minima and minima that use an RNP below 0.3 NM. Those would typically be well below 800-2, but there would usually be an option with higher minima for the same approach (i.e. most RNP approaches have a 0.3 minima in addition to lower minima for tighter RNP values, and most approaches with LPV minima have higher mins for methods like LNAV/VNAV or LNAV-only).

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While it’s common for approaches to have circling minima, not all do for various reasons. Likewise, while it’s common for RNAV approaches to have LNAV minima, not all do.

#1 says that you can use an RNAV approach if it does have either LNAV or circling minima.

#2 says that if you have Baro-VNAV, you can use an RNAV approach if it has LNAV/VNAV minima. This could come into play if the approach has neither LNAV nor circling minima.

#3 is similar but for RNP.

If approach doesn’t pass (at least) one of these three tests, you can’t use it to qualify an alternate.

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