Using GPS to fly the missed

meritflyer

Well-Known Member
Seems to be a bit of a debate whether you can flip your OBS to GPS to fly a missed approach procedure on a non-GPS approach.
 

bike21

9-5 Ruins Lives
Seems to be a bit of a debate whether you can flip your OBS to GPS to fly a missed approach procedure on a non-GPS approach.
I wouldn't see why not. For instance, an ILS missed that has you holding at a waypoint/intersection. Nothing wrong with that. You are back in the terminal environment then and no longer flying an approach. GPS all the way.
 

tgrayson

New Member
Seems to be a bit of a debate whether you can flip your OBS to GPS to fly a missed approach procedure on a non-GPS approach.
Interesting question. Since the missed approach segment is a segment of a published IAP, and the IAP isn't in the database, I'd argue that the legal answer is "no".

On the practical side, you'd need to make sure you have terminal sensitivity; if you don't have an approach loaded, the default may be enroute, which isn't good enough. You'd have to change it manually. Even so, if the missed approach intercepts a localizer, terminal sensitivity isn't nearly that precise.
 

bike21

9-5 Ruins Lives
I think the idea was more so using GPS to fly radials and to VOR's.
Gotcha, hence you said 'OBS' mode. My bad! :buck: If memory serves in some Garmin units, didn't we hit the OBS key to sequence the missed?

Using OBS mode to fly radials seems like too much work during a missed. I would say either use the GPS to fly the missed as prescribed or use the VOR receivers if applicable. Sounds like someone is trying to get too fancy or making too much work for themselves.

So is it legal? I suspect so, just seems like too much work :)
 

bike21

9-5 Ruins Lives
tgrayson - good reference on ensuring someone should be in the terminal mode. I was assuming someone had loaded an approach for an overlay and had the waypoints ready to go.
 

MidlifeFlyer

Well-Known Member
Gotcha, hence you said 'OBS' mode. My bad! :buck: If memory serves in some Garmin units, didn't we hit the OBS key to sequence the missed?
Yes, but that's just Garmin's shortcut key for sequencing into the missed. It doesn't really put you into OBS mode.

I think that the only restriction in the use of GPS on a non-GPS approach is that you can't use it in place of the primary navaid (the one in the name of the approach) for lateral final approach course guidance. IOW, in the ILS 23, LOC 26, NDB 14 and VOR-A you can't use it in place of the LOC, VOR or NDB for lateral guidance from the FAF, but you =could= use it for other purposes such as step-down, etc. Since the missed approach fix does not involve lateral guidance on the FAC, I think you are okay using it to fly the missed.

That seems to be the overall thrust of the IFR discussion in AIM 1-1-19 (so long as the fix is retrievable from the database)
 

woutlaw

Well-Known Member
Gotcha, hence you said 'OBS' mode. My bad! :buck: If memory serves in some Garmin units, didn't we hit the OBS key to sequence the missed?

Using OBS mode to fly radials seems like too much work during a missed. I would say either use the GPS to fly the missed as prescribed or use the VOR receivers if applicable. Sounds like someone is trying to get too fancy or making too much work for themselves.

So is it legal? I suspect so, just seems like too much work :)
Yup, on the Garmin 4xx if an approach is loaded and activated it suspends sequencing at the MAP. Hitting OBS (which is really just toggling "SUSP" off and allowing sequencing for the missed, the OBS button is doing double duty) gets the unit giving you guidance for the missed.

As for flying a particular radial that's part of the published missed approach procedure, it should be in the GPS (at least that's the way it works using a Garmin 4xx.) and is a piece of cake.

Check out the ILS 14 at STP: http://204.108.4.16/d-tpp/0807/00263I14.PDF

Load and activate the approach, when you're within 1.2 miles of the localizer and more than 2 miles but less than 15 miles outside HILZZ the CDI will switch over from GPS to VLOC automagically.

When you go missed and take the unit out of SUSP, it draws the 042 radial for guidance to the hold. The only thing the pilot has to remember is to manually set the CDI back to GPS.

Once you cross the fix defining the hold it goes back into SUSP mode and won't sequence past the holding fix until you tell it to.

As for intercepting some other radial that's not part of the published missed, yeah, getting the thing set up, making sure you're in Terminal sensitivity and in OBS mode seems like more hassle than just using the plain ol' VOR.

But if intercepting a radial is part of the published procedure it should be in the database and legal to fly via the GPS.

Hopefully I understood the question correctly. If not, well, it's an answer to some question. :)

As a side note, OBS mode works great for holds in general. Just put the fix in your flight plan, toggle on OBS and set the inbound course. Stupid easy. Even I can do it.
 

SlantG

Well-Known Member
The primary navaid for the approach must be used from the final approach fix to the missed approach fix. All other segments of the non-GPS approach can be flown with the IFR certified GPS with a current database.

The above is distilled from the AIM and AC90-94.

For going missed I use 5 C's:
Cram - full power
Climb - put nose on up
Clean - gear, flaps, etc.
Clear - the GPS (setup missed navigation)
Communicate - Hello controllers. . . We're baaaaack.
 

ClearedToThe

Well-Known Member
Airlines fly the missed using the FMS (gps) data all the time on every approach (assuming the fms is working). The only time they use "raw data", i.e. the actual localizer frequency, is from the time of final approach course interception to the start of the missed. Even with a non-precision approach the PF is on FMS data with the PM on raw data (monitoring the actual VOR data). Everything else is on the FMS.

This is covered in the AIM under the part where it discusses the "gps in lieu of...".
 
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