Identifying MAP via GPS

Chief Captain

Chief Captain

Well-Known Member
Hello all,

Question on the use of GPS. Let's say we're doing the LOC 36R at KPIE, in an airplane without DME. Most GPS units will load the FAF (sones) and the runway as waypoints. Here's my question. Is it legal to use the database waypoint (RW36R) to identify the MAP, or must we have PIE as the active waypoint, and use 0.5nm from the fix as the MAP?

http://www.airnav.com/depart?http://155.178.201.160/d-tpp/1412/00625IL36R.PDF

References would be appreciated.

Thanks
 
Been a while since I used an actual GPS, but I'd say if it's IFR certified, it's totally legal. I remember something vaguely being able to use an IFR GPS in lieu of actual DME.

Edit to add: check out AC 91-108.
 
Given it is called PIE 0.5 and DME is required, this is the only way to shoot this approach. If the GPS approach has RW36R then you would use that, even though they appear in the same place they are different waypoints for different approaches.

It's like saying the MAP is the runway threshold - it may appear in the same place, but those waypoints based on the navigation systems required for each approach are the only MAPs.
 
yes you could use it in place of DME but you would have to be in NAV mode flying the LOC and using the GPS to verify the distance as 0.5 from PIE.
 
as long as the GPS is approach certified, current database (28 day requirement) - from AIM section 5 as I remember.
 
ah ok, I might have to start digging in the AIM

I think confusing waypoints between a LOC and GPS approach (ie flying the LOC while having the GPS loaded into the unit) could be troublesome but not sure if the OP meant that.
 
ahw01 said:
http://www.faa.gov/documentLibrary/media/Advisory_Circular/90-108.pdf
Click to expand...
I was just having a discussion today about when you HAD to switch from GPS to NAV mode on an approach(pink/white needles to green), and from what I understand this AC to read, you still need to use the VOR or NDB on the final approach segment - inside the FAF. Except for 8c. It seems like it wants you on the LOC at all times when on the LOC course, even outside the FAF(but inside of the LOC service volume)?
 
Chief Captain said:
Hello all,

Question on the use of GPS. Let's say we're doing the LOC 36R at KPIE, in an airplane without DME. Most GPS units will load the FAF (sones) and the runway as waypoints. Here's my question. Is it legal to use the database waypoint (RW36R) to identify the MAP, or must we have PIE as the active waypoint, and use 0.5nm from the fix as the MAP?

http://www.airnav.com/depart?http://155.178.201.160/d-tpp/1412/00625IL36R.PDF

References would be appreciated.

Thanks
Click to expand...
Any Garmin product, and really any GPS/FMS worth a crap, will load SONES, SAWGA and RWY36R as points on the approach. On the pile of garbage FMS in the CRJ you would have had to do DME hold off PIE to get the DME because it was a pile of crap and didn't load fixes inside the FAF. CRT screens and a big welcome to 1960. :rolleyes:
 
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On an approach at OU, we would load the approach (ILS17) like normal, tune/identify, and then do direct to the localizer (I-BWM) on the GPS. This keeps all the fixes on the map view, but it's direct to the localizer antenna (or whatever the DME is based off of). We'd then set the OBS on the GPS to whatever the final approach course is to draw our final. I had never done it like that before, but it was helpful. It would leave the fixes in the active flight plan view so if you had to go missed then you'd just do direct to the start of the hold. I hope that makes sense. I've been up all day and I'm half asleep.
 
z987k said:
I was just having a discussion today about when you HAD to switch from GPS to NAV mode on an approach(pink/white needles to green), and from what I understand this AC to read, you still need to use the VOR or NDB on the final approach segment - inside the FAF. Except for 8c. It seems like it wants you on the LOC at all times when on the LOC course, even outside the FAF(but inside of the LOC service volume)?
Click to expand...
You can get Ops Spec C300 and get approval for GPS only on a NDB or VOR approach. All the Garmin products I have seen make the switch from GPS to the ground station on a ILS/LOC/VOR approach.
 
Be mindful of where the waypoint is predicated from. Like a stepdown fix on a 6000-ft runway from the LOC at 3.4, but the GPS might measure from the beginning of the runway so that fix might be at 2.4 dme.
 
Capt. Chaos said:
You can get Ops Spec C300 and get approval for GPS only on a NDB or VOR approach. All the Garmin products I have seen make the switch from GPS to the ground station on a ILS/LOC/VOR approach.
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If they are installed correctly. Ask me how I know about that.

Thanks for the heads up on C300, didn't know about that one. We should probably get it.

I can't find it at the moment, and it's also new to me, but we have the one that allows an airport with only a GPS approach to be used as an alternate.
 
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Ajax said:
Be mindful of where the waypoint is predicated from. Like a stepdown fix on a 6000-ft runway from the LOC at 3.4, but the GPS might measure from the beginning of the runway so that fix might be at 2.4 dme.
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The fix is at 2.6PIE. The GPS will put SAWGA exactly where SAWGA is via LAT/LONG, not at 2.6 RWY36R or even 2.6DME from PIE.
 
Ajax said:
Be mindful of where the waypoint is predicated from. Like a stepdown fix on a 6000-ft runway from the LOC at 3.4, but the GPS might measure from the beginning of the runway so that fix might be at 2.4 dme.
Click to expand...
This is kind of where the discussion started.

We had one person interpret "GPS as a substitute for DME" to mean that that's literally all we can do. That is, we have to load PIE as the active waypoint, and use the distance to define the FAF and MAP.

The other school of thought is to load the approach via the database, and identify the waypoints (including the MAP) just as you would on a GPS approach.

My question is, is there any official guidance saying either method is or isn't correct?

Thanks for all the replies so far.
 
Wow I think we're making things way too hard. If you've got the GPS in-lieu of DME, use the RWY waypoint and reference your distance to that- there is no reason for you to, after having loaded an approach, set a D-> PIE so you can see your distance relative to the FAF. The only thing the GPS is doing is determining its position relative to lines of latitude and longitude, and then using the pre-programmed LAT/LON of whatever waypoint you've loaded into the box to compute your relation to said waypoint. Whether that waypoint is where it thinks the VOR is or whether that waypoint is where it thinks the runway threshold is are going to have the exact same amount of error either way, and it really isn't going to be much.

Sheesh, sometimes I wonder if some of you are forgetting to raise the flaps in the flare.
 
Wouldn't an IFR approved GPS have the approach loaded?

Or are there still enroute-only GPS systems?

You have to forgive me as the last GPS unit I used was a KLN-90 when it was new. Now it's just airline FMS and I figure for at least the last 10 years GA units have far surpassed what we have.
 
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