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Take RNAV (GPS) Y RWY 36L and RNAV (GPS) Z RWY 36L at APC for examples, enter image description here enter image description here

All fixes and routes in the two approaches are similar. The noticeable differences that I observe are minimums and WAAS CH number.

So some questions:

(1) How may I know which approach I am able to shoot? (or which WAAS channel is available?)

(2) Is the WAAS channel number automatically selected when I load the corresponding flight plan in G1000?

(3) ILS vs LPV: For an ILS approach, I may tune the localizer frequency and get both horizontal(CDI) and vertical (Glide Slope) information, without loading the flight plan. Can I get something similar with an LPV? I mean to get the LPV glide path guidance without loading any particular approach (set the WAAS CH number for instance?).

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    $\begingroup$ The WAAS channel number is unique to the approach plate. Just as the name of the approach is unique to that approach, so is the channel number. It does not refer to a radio channel or freq. Some databases allow you to look up an approach by airport -> type of approch -> runway -> etc..., some other databases allow you to type the approach channel number into the FMS to pull up the approach plate. See AIM 1-1-19 (d) (6) for a more complete description. $\endgroup$
    – Steve H
    Dec 20, 2015 at 17:50
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    $\begingroup$ One of the primary differences between the two approaches you reference is the required climb gradient. The Z approach requires a higher level of aircraft climb performance. See this sentence from the notes section: "Missed approach requires minimum climb of 480 feet per NM to 2200. If unable to meet climb gradient see RNAV (GPS) Y RWY 36L." $\endgroup$
    – J Walters
    Dec 20, 2015 at 23:00
  • $\begingroup$ I found the WAAS channel number is verifiable and selectable from both PFD and MFD while in the select flight plan box. Meanwhile, the WAAS id is verifiable but not selectable. $\endgroup$ Jan 13, 2016 at 2:13

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I don't know if you can check or set the WAAS channel number directly (and I couldn't find anything about it in the C172 G1000 manual) but even if you can there's really no point. When you load the approach, the G1000 gets the WAAS channel number from the database as part of the general approach definition and configures everything automatically.

I heard or read somewhere that the original concept was that pilots would 'tune' the WAAS channel in the same way that they tune an ILS frequency, but that idea was dropped because an RNAV approach is totally dependent on the database anyway: if you trust the G1000 to load all the waypoints, altitudes and other information correctly from the database then there's no reason to think it will load the wrong WAAS channel.

As for your specific questions:

(1) How may I know which approach I am able to shoot?

You read your G1000 manual, check the approach database, and review the approach plates to make sure that the approach is available and you meet all the requirements. If the approach is in the database that doesn't automatically mean that you can fly it, there can also be climb performance or altimeter setting requirements (for example) that are unrelated to the G1000.

(2) Is the WAAS channel number automatically selected when I load the corresponding flight plan in G1000?

Yes. The G1000 sets up the approach completely for you, the only input required (usually!) is to load the approach and select how you initiate it, i.e. via an IAF or radar vectors.

(3) [Can I] get the LPV glide path guidance without loading any particular approach (set the WAAS CH number for instance?)?

No, this wouldn't make any sense. The LPV glideslope is constructed virtually by the G1000 for each approach and the WAAS input just provides correction to an existing GPS position, it doesn't give you a position directly. So even if you could receive the WAAS corrections directly (and maybe you can, for all I know), the G1000 wouldn't be able to generate a glideslope without all the other approach information about the runway, elevation etc. It would be like asking, "the magnetic declination here is 4°W, what's the correct magnetic heading to fly from A to B?" Unless you know the uncorrected heading, the correction by itself isn't useful.

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  • $\begingroup$ for (3), I am comparing ILS and LPV, in this case. Do you think we may deploy an ILS system for an aircraft carrier, but there is no way to develop an LPV procedure for the aircraft carrier? $\endgroup$ Dec 29, 2015 at 8:29
  • $\begingroup$ @spacepure I have no idea, that could be a good question. We already have a question about autolanding on carriers, some of the links might give you some information but it isn't about ILS or LPV specifically. $\endgroup$
    – Pondlife
    Dec 29, 2015 at 13:52
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The channel number does not mean what you think. The channel is a legacy of how the LPV is encoded in the nav database. The format used is the same as a GBAS approach where a local VHF transmitter was used to broadcast local diff GPS corrections for precision GPS approaches. The avionic used to fly these approaches had a code selection window where you entered a channel number - the channel number being generated based in the VHF number. When LPV came along they used the same format in the nav database as the GBAS approach but still needed a "channel" number even though there was no longer an associated VHF transmitter. ICAO manages allocation of these codes (used to be the FAA). So basically they are helpful to identify you have the correct approach but otherwise mean absolutely nothing.

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