AIM Change - Use of GPS inside the FAF of a conventional approach

[MidlifeFlyer]

Especially in aircraft equipped with an HSI or FMS, many pilots have flown (and some CFIIs have advocated flying) the final approach segment for a non-GPS approach with the GPS source feeding Nav 1, while monitoring the VOR "raw data" in Nav 2. Now, in a May 26, 2016 update to the AIM the FAA specifically approves the practice, at least for VOR, NDB and TACAN approaches.

As the new Note 5 to AIM 1-2-3 puts it:

Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.​

I don't know for sure but have heard this was a concern for some Part 121 an 135 operators which rely heavily on FMS — a concern over a possible grey area over which nav display is "primary," especially when GPS is feeding the FMS and autopilot. Seems to be a clarification more than anything else, but it's always nice to see a possible grey area disappear - in a good way.

This and other changes in the May 26 update may be viewed at http://1.usa.gov/1TESEeB

 

[z987k]

Just this morning I had the other crew member want to do the LOC approach using the FMS(GPS) as the only guidance since we were not receiving the LOC.
Uh... no. How about we call it in sight and you can do whatever you want.

 

[MidlifeFlyer]

Just this morning I had the other crew member want to do the LOC approach using the FMS(GPS) as the only guidance since we were not receiving the LOC.
Uh... no. How about we call it in sight and you can do whatever you want.

Notice that the new note does not mention LOC-based approaches.

 

[thevideographer]

The good FMSs are already approved for use on ground based navaids. For example Q400s, with the right opspecs, are flying VOR approaches in FMS nav with no backup tuned at all. The only requirement is that Loc and ILS approaches be flown with the actual navaid tuned.

 

[mikecweb]

Just this morning I had the other crew member want to do the LOC approach using the FMS(GPS) as the only guidance since we were not receiving the LOC.
Uh... no. How about we call it in sight and you can do whatever you want.

Really?

 

[z987k]

Really?

Yup.

 

[mikecweb]

Yup.

Surprising with that crew makeup.

 

[xdashdriver]

Notice that the new note does not mention LOC-based approaches.

AC 90-108 specifically prohibits doing this with LOC approaches.

 

[djh1007]

At my past airline, we flew VOR approaches in white needles (FMS), while having raw data tuned and displayed using the on-side bearing pointer. Same for NDB approaches, although we only did those in the sim. We also shot non precession approaches using continuous angle descent paths (CANPA.)

 

[mshunter]

Just this morning I had the other crew member want to do the LOC approach using the FMS(GPS) as the only guidance since we were not receiving the LOC.
Uh... no. How about we call it in sight and you can do whatever you want.

Seems like a setup for a eff up.

 

[RamRise]

What if you just load the approach waypoints into the gps and it doesn't provide the RNP scaling to .2 or .1 full scale deflection, but instead enroute or terminal sensitivity? Or if you load the approach and it says "for guidance only" and the sensitivity doesn't change because the FMS doesn't register it as an RNAV or overlay approach? Now you're flying an approach with possibly lower accuracy than a VOR and the TERPS criteria are met ?

 

[rausda27]

What if you just load the approach waypoints into the gps and it doesn't provide the RNP scaling to .2 or .1 full scale deflection, but instead enroute or terminal sensitivity? Or if you load the approach and it says "for guidance only" and the sensitivity doesn't change because the FMS doesn't register it as an RNAV or overlay approach? Now you're flying an approach with possibly lower accuracy than a VOR and the TERPS criteria are met ?

I have flown a lot of RNAV approaches and I can reasonably say that they were all more accurate than a VOR. In fact I have been steered off course way more times navigating off a VOR than if I had loaded the segment in the FMS. This guidance is certainly a step in the right direction.

 

[rausda27]

What if you just load the approach waypoints into the gps and it doesn't provide the RNP scaling to .2 or .1 full scale deflection, but instead enroute or terminal sensitivity? Or if you load the approach and it says "for guidance only" and the sensitivity doesn't change because the FMS doesn't register it as an RNAV or overlay approach? Now you're flying an approach with possibly lower accuracy than a VOR and the TERPS criteria are met ?

Another factoid. Our FMS will scale to 0.3 scaling on every approach as long as it's in the database. I'll take that anyway over a VOR radial.

 

[gotWXdagain]

What if you just load the approach waypoints into the gps and it doesn't provide the RNP scaling to .2 or .1 full scale deflection, but instead enroute or terminal sensitivity? Or if you load the approach and it says "for guidance only" and the sensitivity doesn't change because the FMS doesn't register it as an RNAV or overlay approach? Now you're flying an approach with possibly lower accuracy than a VOR and the TERPS criteria are met ?

That's why the guidance advises showing the primary navaid on a secondary display. If you can load an RMI to a VOR underneath your CDI, that'll do it.

 

[RamRise]

Wait, so can you use the FMS for lateral guidance on a loc approach as long as you are monitoring loc raw data? It doesn't specifically prohibit this, and the advisory circular above just says it cannot be used as an 'substitute' means (aka when the localizer is inoperative)' or without reference to the localizer data.
Seems like by expressly omitting mention of using gps on the final approach segment for a loc, but allowing it for VOR, ndb and tacan, the intent is that you should only use loc data for the final approach segment. But I have colleagues who argue otherwise and I can see how they'd conclude that, upon reviewing the literature for second time.

 

[Zapphod Beblebrox]

Especially in aircraft equipped with an HSI or FMS, many pilots have flown (and some CFIIs have advocated flying) the final approach segment for a non-GPS approach with the GPS source feeding Nav 1, while monitoring the VOR "raw data" in Nav 2. Now, in a May 26, 2016 update to the AIM the FAA specifically approves the practice, at least for VOR, NDB and TACAN approaches.

As the new Note 5 to AIM 1-2-3 puts it:

Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID monitored for final segment course alignment.​

I don't know for sure but have heard this was a concern for some Part 121 an 135 operators which rely heavily on FMS — a concern over a possible grey area over which nav display is "primary," especially when GPS is feeding the FMS and autopilot. Seems to be a clarification more than anything else, but it's always nice to see a possible grey area disappear - in a good way.

This and other changes in the May 26 update may be viewed at http://1.usa.gov/1TESEeB

This is standard practice at American. On an RNAV non precision approach with overlays an older non precision, like a VOR or NDB, RNAV course guidance is used with raw data back up on the non pilot flying side.

 

[Zapphod Beblebrox]

This is standard practice at American. On an RNAV non precision approach with overlays an older non precision, like a VOR or NDB, RNAV course guidance is used with raw data back up on the non pilot flying side.

Legacy US is still non RNAV-RNP. Legacy US, for the Airbus fleet, is RNP.3. They say it's only a matter of money for the software update from Airbus. It will get done sometime soon, I think.