Legal Localizers and Such

[Talan2000]

Pilots,
I will confess that heretofore in many PE Instrument approaches I flew in GPS equipped aircraft I used the GPS to fly the approach while also using the VOR heads. To some, including (the old) me, that would seem like a no brainer - surely the GPS is more accurate. But apparently it isn't so as in my ongoing home study I've come across references to AC 90-108 which indicates that one is not allowed to use GPS as the "primary" means of lateral navigation for a VOR, VOR/DME, or Localizer approach. The key, as I understand it from reading about it elsewhere, seems to be what is the "primary" instrument (GPS or VOR) and what is secondary. AC 90-108 also gets more specific, so that I think that one IS allowed to use the GPS for lateral NAV but NOT on the Final Approach Segment (following the FAF). Phew. That's a lot to wrap one's brain around. At least for me.

Soooo, this leads to my questions on the KSBP LOC RWY 11 Approach:

Lot's of work

KSBP LOC 11.JPG (153.03 KiB) Viewed 4373 times

My airplane: Skyhawk /G with dual VOR/ILS heads, Garmin 430, No DME.

1. Using VOR Head 1 for the localizer course, Can I use the GPS to LEGALLY identify the intersections/fixes? CREPE, ZEGAX, DOBRA (OM)? (I think AC 90-108 tells me YES)

2. Can I legally use the GPS to identify HASBY? I think Ac-90-108 tells me - NO

3. What's the BEST way to fly this approach (from a checkride perspective?) Setup LOC on VOR1, Setup MCQ on VOR2 and use it to ID CREPE, ZEGAX, DOBRA, and HASBY? This is how I would do it in a pre-GPS world.

But it is awful busy dialing those radials, and surely the GPS would provide me with better accuracy (which is what matters in the real world no?)

It would seem that a Localizer approach such as this would in NO way be worth the trouble of flying if you had any other option - such as the ILS 11.

Thanks,

Todd

[Peter Grey]

Hello Todd,

This one is actually pretty straight forward.

You are refering to the following portion of AC 90-108 I presume:

Substitution on a Final Approach Segment (FAS). [GPS] Substitution for the NAVAID (for
example, a VOR or NDB) providing lateral guidance for the FAS. [Is not allowed]

Note that AIM 1-2-3 is a summart of the relevant portion of AC90-108 and states the same thing.

Note in the same AIM section there is the following note:

Determine aircraft position relative to, or
distance from a VOR (see NOTE 5 below), TACAN,
NDB, compass locator, DME fix; or a named fix
defined by a VOR radial, TACAN course, NDB
bearing, or compass locator bearing intersecting a
VOR or localizer course.

Key word in those statements is "lateral".

Identifying a fix along the final approach course is not lateral (it's longitudinal).

So to answer your questions.

1. Yes

2. You can as it's not a lateral deviation you are measuring.

3. There are a lot of answers, your proposal is fine. The "simplest" is to simply plug the approach in the GPS and the localizer in Nav 1. The GPS will force you over to the localizer at the proper point (note flight sims generally don't do this). Baring that auto switch, switch from GPS to LOC prior to glide slope intercept (I'd do it joining the localizer inbound).

[Keith Smith]

You might fly this approach rather than the ILS if:

a) you didn't have a glideslope receiver, or
b) the glideslope was out of service, or
c) by request for training

[Ryan B]

What is the difference between lateral and longitudinal then?

Can one use GPS as DME? example I want to fly a VOR/DME approach, using VLOC as the source but I select the VORTAC on my GPS to distance.

[Keith Smith]

PG and I were just talking about that recently with DME arc approaches. You can use GPS to replace DME except where DME is providing lateral course guidance on the final approach segment. About the only example of that would be the VOR/DME into Martin State.

[Keith Smith]

Note: moved to Pilot forum. This isn't really related to the Pilot Training Program.

[Talan2000]

Peter,

I'm with Ryan here also - I don't understand the difference between a "lateral" and a "longitudinal" fix here.

ZEGAX and DOBRA would seem to be defined in 3D (they have altitudes associated) just as Hasby is, they just aren't on the Final Approach Segment ?? (It seems the AC makes a big deal about the Final Approach Segment & GPS, and I'm not really sure why...can you elaborate?

And Ryan's DME question is also my next one -- can I use GPS DME while on the the Localizer (and accordingly forego using my stopwatch to identify the missed approach point?) I had thought that i could NOT use GPS DME (at least on the Final Approach Segment) but I could on an ARC for example.


Thanks

Todd

[RyanK]

Peter,

I'm with Ryan here also - I don't understand the difference between a "lateral" and a "longitudinal" fix here.

ZEGAX and DOBRA would seem to be defined in 3D (they have altitudes associated) just as Hasby is, they just aren't on the Final Approach Segment ?? (It seems the AC makes a big deal about the Final Approach Segment & GPS, and I'm not really sure why...can you elaborate?

And Ryan's DME question is also my next one -- can I use GPS DME while on the the Localizer (and accordingly forego using my stopwatch to identify the missed approach point?) I had thought that i could NOT use GPS DME (at least on the Final Approach Segment) but I could on an ARC for example.


Thanks

Todd

You are over complicating this. Simply, you can substitute GPS for everything except left-right course guidance inside the FAF.

[Peter Grey]

Lateral = Left/Right

Longitudinal = Forward/Backwards

There is no such thing as a lateral fix in the sense that a fix doesn't tell you if you are off course. Getting to a fix only tells you how far along the approach you are.

The intent of that section is that you cannot use the GPS to determine how far off course you are in the final approach segment. You must use the navaid defining the final approach segment.

So on this approach you must use a localizer to get I-SBP once at DOBRA. All other information can come from a GPS.

ZEGAX and DOBRA would seem to be defined in 3D (they have altitudes associated) just as Hasby is, they just aren't on the Final Approach Segment ?? (It seems the AC makes a big deal about the Final Approach Segment & GPS, and I'm not really sure why...can you elaborate?

I don't understand this question. Getting to a fix does not directly tell you if you are or are not on course. That's what the localizer does (ignore the argument that if you aren't on the localizer then you aren't at the fix).

Can one use GPS as DME? example I want to fly a VOR/DME approach, using VLOC as the source but I select the VORTAC on my GPS to distance.

Yes, this is ok (with 1 exception that I won't bog the discussion down with, let's say it's an exemption that applies to 1 approach in the country).

From AIM 1-2-3 under allowable GPS uses:

Determine aircraft position relative to, or
distance from a VOR (see NOTE 5 below), TACAN,
NDB, compass locator, DME fix; or a named fix
defined by a VOR radial, TACAN course, NDB
bearing, or compass locator bearing intersecting a
VOR or localizer course.

[Talan2000]

Ryan

Great. Simple is good. I like simple. I think i understand now what the FAA is saying with that AC. You can use gps to identify the fixes/intersections but can't use the magenta line (GPS) to legally steer you back on course.

Peter

Thank you for the clarification. I misunderstood "longitudinal" in this context to be referring to 3d position vs lateral. Disregard.

I think I have it now. In sum. Use the GPS in (vloc) mode and tune vor head 1 to the localizer and you can do no wrong. No need to use the stopwatch for the MAP since GPS identifies the Rwy (though this scares me for some reason for fear of examiners disagreement and I'd likely start the clock anyway )

Thanks

Todd