HSI accuracy using GPS for NDB during approach

[ahramin]

There is a conflict in your question. If you are using the GPS to identify a fix on the approach, as Hedley has shown that is perfectly legal. However if the GPS is not approved for approaches why are you asking about the HSI and the CDI scale on the GPS? The HSI would be on the VOR or the ILS for the approach no?

Yes you're right, the question was badly worded. My GPS is approved for approaches and I shouldn't have asked about a non-approach approved GPS.

The situation I have in mind is using the GPS to locate the NDB-based IAF and missed approach hold for an ILS approach. Practically speaking I'm wondering if I should be changing the sensitivity. That's 5nm fsd as it stands; If I loaded the NDB approach based on the same navaid, I'd have 1.0nm fsd to the IAF and from the MAP to the hold.

You are correct that you can use the GPS to locate the IAF. Hedley has quoted the applicable regulation. I would manually change the CDI scale to 1 if you have that option, if not don't worry about it.

Lots of aircraft being built nowadays without ADFs, but make sure the procedure name doesn't have NDB in it.

Also remember, for commercial or POC ops, IFR GPSs must have a current database for any use.

[photofly]

You are correct that you can use the GPS to locate the IAF.

But not, apparently, for missed approach guidance.

[nbinont]

More research...
TC AIM 3.16.9 says

Note that for NDB or VOR approaches that are not part of
the GPS overlay program described in Section 3.16.5.2.2,
pilots shall use ADF or VOR as the primary source for
final approach track guidance. For these approaches, and for
approaches based on a localizer (LOC) for lateral guidance,
pilots shall not use GNSS as the primary source for missed
approach guidance when the missed approach procedure
requires flying a published track to or from an NDB or VOR.

You are correct that you can use the GPS to locate the IAF.

But not, apparently, for missed approach guidance.

Sorry for dragging up this old thread, but this seems to be the only spot where Instrument flying with a GPS but no ADF is discussed. By my interpretation we're not reading this right, but we're very close. 3.16.9 says we can't use the GPS as the primary guidance for the missed approach with a "published track" to or from a NDB/VOR. For example (all non-overlaid approaches):

Climb to 2200 on track of 261. (The final track from the FAF (a NDB) was 261) Left turn to "7B" NDB.

Clearly the 261 track is based off the FAF NDB. We must have a ADF here.


Now it gets a bit more fuzzy, for CYSB VOR/DME 04:

Climb to 3500 on R-035. LEFT turn direct to "SB" NDB.

The only track specified here is R-035, so we require a VOR and can't use GPS for this track. But there's no specific track to the NDB specified - just "direct" which could be many different tracks depending on your performance to 3500. Can we use the GPS to fly direct to the NDB where the track is not specified (after 3500)? A reasonable person will probably tell you that you can, but 3.16.9 says "when the missed approach procedure requires flying a published track to or from a NDB or VOR", and the VOR part of the missed approach procedure certainly qualifies, so a strict interpretation probably means you're out of luck.


However, CYOO LOC(BC)/DME 30:

Climb to 1500 on track of 302 (localizer track). LEFT climbing turn to 3200 to "SN" NDB.

Ok, so we've got a published track of 302 to 1500, but it's not to the NDB - it's following the localizer. Then there's a turn "to" the NDB without a track specified. Thus there's no published track to or from a VOR or NDB in this missed approach, and the prohibition against using GPS as the primary missed approach guidance does not apply. Just track the localizer to 1500 and then you're legal to navigate using GPS to the NDB.


And this one is pretty clear, CYQG ILS or NDB 25:

Climbing Left turn to "QG" NDB at 2400

No published track - just get to the NDB. We can use GPS for this missed approach.

[CID]

You know, a LOT can be cleared up by checking your flight manual and supplements.

[CID]

You know, a LOT can be cleared up by checking your flight manual and supplements.