WAAS G1000 GPS Approach question..

GreenDayPilot

GreenDayPilot

Well-Known Member
1) How do you know if the airplane is equipped with WAAS? Please be specific. I have looked everything in the POH, specifically the Equipment List and the G1000 Reference Guide. What exactly will it say? "WAAS Antenna Part ########?"

2) When you do a GPS approach, what are the limitations on the course discrepancy between the GPS and the approach plate? Is there a reference to this in the FAR/AIM? Example: GPS shows a course of 302 while the approach plate shows a final approach course of 301.. I'll still fly it, but what are the discrepancy limitations on that and how do you know?


THANKS a million in advance!! :-)
 
1) On the MFD, use the FMS knob and select the AUX page. From there, scroll to the "GPS Status Page". One of the bottom softkeys should say "SBAS". Select SBAS and you should see a
"WAAS" with a checkbox next to it just up and to the right of the screen. If you don't have it, it probably won't be there.
 
GreenDayPilot said:
1) How do you know if the airplane is equipped with WAAS? Please be specific. I have looked everything in the POH, specifically the Equipment List and the G1000 Reference Guide. What exactly will it say? "WAAS Antenna Part ########?"

2) When you do a GPS approach, what are the limitations on the course discrepancy between the GPS and the approach plate? Is there a reference to this in the FAR/AIM? Example: GPS shows a course of 302 while the approach plate shows a final approach course of 301.. I'll still fly it, but what are the discrepancy limitations on that and how do you know?

THANKS a million in advance!! :-)
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First, there are a few ways to figure out if your airplane has WAAS, but the easiest way would be to look in the GPS status page on the aux page group. If you don't see the letter "D" on any of the satellite signal bars, that's your first clue--your GPS isn't receiving a differential correction from WAAS. Also, there will be no SBAS softkey on that page, since non-WAAS aircraft have no SBAS equipment. In addition, in the POH you should see in the airplane weight and balance equipment list a GIA-63. WAAS equipped aircraft have the GIA-63W i. Also, you could try loading a GPS approach with LPV or LNAV/VNAV minima. If it shows just RNAV/GPS LNAV in the flight plan, that's another clue (if some idiot hasn't disabled SBAS in your airplane :p ). If your plane has the Garmin GFC-700 autopilot, then it will have WAAS--if your NAV III Cessna has a GFC-700, it has GIA-63W integrated avionics units with WAAS.

As far as the difference between the courses: I'm going to make a long story short--the FAA and Jeppesen interpret magnetic variation slightly differently. Approach plates use data from the FAA in the US. The G1000's navigation database comes from Jeppesen. In normal operation, you should notice less than a 4 degree error between the G1000 and the plate. If you notice anything more, you can submit a correction to Jeppesen through their website. There's nothing in the FAR/AIM about this--this comes from Garmin's engineering team. If you're consistently noticing a large (>4 degree) difference, you might have an outdated magnetic variation database. When necessary, a normal navigation database update will include an update to the magnetic variation database. If I remember right, there's no way for the user to know if the update includes variation data. Also, if I recall, they include that variation database update in quite a few of the update cycles--not just one, so it's pretty hard to get out of date on that.

I'll double-check these answers if I have time at work tomorrow, but off the top of my head that should help a bit.
 
GreenDayPilot said:
1) How do you know if the airplane is equipped with WAAS? Please be specific. I have looked everything in the POH, specifically the Equipment List and the G1000 Reference Guide. What exactly will it say? "WAAS Antenna Part ########?"

2) When you do a GPS approach, what are the limitations on the course discrepancy between the GPS and the approach plate? Is there a reference to this in the FAR/AIM? Example: GPS shows a course of 302 while the approach plate shows a final approach course of 301.. I'll still fly it, but what are the discrepancy limitations on that and how do you know?


THANKS a million in advance!! :)
Click to expand...

#1 has already been answered adequately, but the installation approval should also tell you.

Here is the answer to #2, from the AIM:

There may be slight differences between the course information portrayed on navigational charts and a GPS navigation display when flying authorized GPS instrument procedures or along an airway. All magnetic tracks defined by any conventional navigation aids are determined by the application of the station magnetic variation. In contrast, GPS RNAV systems may use an algorithm, which applies the local magnetic variation and may produce small differences in the displayed course. However, both methods of navigation should produce the same desired ground track when using approved, IFR navigation system. Should significant differences between the approach chart and the GPS avionics' application of the navigation database arise, the published approach chart, supplemented by NOTAMs, holds precedence.
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