Instrument / Equipment required for Private IFR flight?

[mooniac15u]

 

first of all, there are no approaches like "VOR/DME with no mention of GPS anywhere on the approach plate": all terrestrial navaid approaches that require DME have intersections co-located with the DME fixes. For example, this TCY VOR/DME 26 approach shows:

 

 

(source: http://155.178.201.160/d-tpp/1503/05815VD26.PDF)

and if you are using GPS-in-lieu-of-DME, you must, as per AC 90-108, use RW26, ZADBA, SIGBY, and WIBAG to identify the stepdowns and MDA. you cannot use MOD + DME distance

 

 

Yes, there are GPS waypoints, but I was referring to specific mention of GPS, particularly in the title which would result in an official GPS overlay.  Any approach that you find in your GPS that does not have an official GPS overlay is not an official approach and is for advisory purposes only.  You can check your GPS manual and when you load the approach it will give you a warning that it is for advisory purposes only.

[rbp]

It clearly says you can select either the named fix or the facility as a waypoint.

 

you can select the navaid as a WAYPOINT

 

but you cannot identify and navigate to a point in space on your route as "22.1 miles from MOD VOR." You have to use ZADBA.

[mooniac15u]

you can select the navaid as a WAYPOINT

 

but you cannot identify and navigate to a point in space on your route as "22.1 miles from MOD VOR." You have to use ZADBA.

 

From the IFH:

 

"select either the named fix as the active GPS WP or the facility establishing the DME fix as the active GPS WP"

 

" If selecting the DME providing facility as the active GPS WP, a pilot is over the fix when the GPS distance from the active WP equals the charted DME value"

[Bob - S50]

I'm a little late to the game here, and I realize you are all talking just academically (or at least I hope you are), but just to watch the food fight grow, let me throw a couple more scraps in here.

 

IF you were going to enter the DME based fix and do the mental math for other fixes:

 

1.  Unless you know something I don't, there is no way to enter an ILS as the DME source.  You cannot use the airport and assume it is the same as the DME on the ILS.  So for the ILS 28L to SFO example you were using, you could not enter the airport as the DME source.  Depending on the airport configuration, the location of the GPS 'airport' could be a significant distance from the ILS DME equipment.  I suppose you could enter a named fix like HEMAN or PONKE but do I really want to be doing the math while tracking the ILS in the soup solo?  Let me see, PONKE is 21.6 and WETOR is 18.3, so I have to at or above 5000 until I'm 3.3 miles past PONKE.  Wow, no thanks.

 

2.  True DME equipment is slant range.  GPS DME is not.  Although, during approaches, altitudes are low enough that the difference is probably insignificant, there is a difference in distances.  However, at significant AGL heights, the difference can be significant.

 

Carry on...

 

Bob

[mooniac15u]

then what's the purpose of WIBAG, SIGBY, ZADBA, and RW26? 

 

RW26 is not a waypoint and it does not have a DME reference distance.  The distance from the MAP to RW26 is informational only.

 

The purpose of these waypoints is that you may use them in lieu of DME.  That is allowed.  The whole point is that you are not required to load an entire approach.  You may separately load one (or all) of these waypoints from your database and use them in lieu of DME.

[rbp]

OK, i give up

 

but just as a reminder, those of you using GPS-in-lieu-of-DME on approaches requiring DME, but without loading and activating the procedure, please make sure you've set the GPS to approach mode, or at least CDI 0.3nm

[mooniac15u]

I'm a little late to the game here, and I realize you are all talking just academically (or at least I hope you are), but just to watch the food fight grow, let me throw a couple more scraps in here.

 

IF you were going to enter the DME based fix and do the mental math for other fixes:

 

1.  Unless you know something I don't, there is no way to enter an ILS as the DME source.  You cannot use the airport and assume it is the same as the DME on the ILS.  So for the ILS 28L to SFO example you were using, you could not enter the airport as the DME source.  Depending on the airport configuration, the location of the GPS 'airport' could be a significant distance from the ILS DME equipment.  I suppose you could enter a named fix like HEMAN or PONKE but do I really want to be doing the math while tracking the ILS in the soup solo?  Let me see, PONKE is 21.6 and WETOR is 18.3, so I have to at or above 5000 until I'm 3.3 miles past PONKE.  Wow, no thanks.

 

2.  True DME equipment is slant range.  GPS DME is not.  Although, during approaches, altitudes are low enough that the difference is probably insignificant, there is a difference in distances.  However, at significant AGL heights, the difference can be significant.

 

Carry on...

 

Bob

 

Agreed, that the SFO ILS is a very complex example.  Take a look at AOPA's discussion on this (http://www.aopa.org/Advocacy/Air-Traffic-Services-,-a-,-Technology/Air-Traffic-Services-Brief-Use-of-GPS-in-lieu-of-DME-ADF) particularly this part:

 

"Caution: Pilots should be extremely careful to ensure that correct distance measurements are used when utilizing this interim method. It is strongly recommended that pilots review distances for stepdown points during preflight preparation. (See Figure B; a GPS receiver set to the FAF, AASON, would be counting up to 8.1 at the missed approach point, where as a DME would have counted down to 0.9 at this same missed approach point.)"

 

The approach for Figure B is:

 

http://155.178.201.160/d-tpp/1503/00129LDAD23.PDF

 

You can see that you have to do exactly that kind of mental math when referencing AASON.

 

I would in no way advocate trying that SFO approach without a Garmin style overlay to help you find those waypoints.

[Super Dave]

I will say this a third time

 

If you want to locate a NAVAID, like a LOM, you can do that with the GPS.

 

If you're flying a VOR/DME,  ILS/DME, ADF/DME (yes, they exist), or an approach that says "DME required", and you are using the terrestrial NAVAID as your primary means of navigation, you must load the overlay procedure into the GPS unit, and use the co-located DME fixes. You CANNOT use "Direct To:  DME + distance to identify the DME fixes.

You could say it 100 times, but it would still be wrong. It it is perfectly legal and safe to fly a VOR/DME approach without loading anything into the GPS other than the VOR that the DME is based on. The approach is flown just as it would be without the GPS; tracking outbound and inbound is done with VOR equipment, but instead of identifying stepdowns, MAPs, etc... by the distance readout of the DME, the distance is simply read from the GPS.

 

OK, i give up

 

but just as a reminder, those of you using GPS-in-lieu-of-DME on approaches requiring DME, but without loading and activating the procedure, please make sure you've set the GPS to approach mode, or at least CDI 0.3nm

This isn't right either. If you are just using GPS-in-lieu-of-DME, CDI sensitivity has nothing to do with distance from the station.

[rbp]

sorry, let me rephrase: I have provided three separate references that specifically say that DME distances cannot be used for approach mode; the DME-based procedure must be loaded from the database, and the named 5-letter fixes must be used.

the reason you need to set the GPS to approach mode is because it increases the RAIM sensitivity and prediction appropriately. You're right about setting CDI, but not for the reason you said: only some GPSs will increase the RAIM prediction when the CDI is set to .3.

[midlifeflyer]

How about a practical experiment for those with dual GPS installations? Do an approach with DME fixes. In one, load the approach. In the other, load the DME source as a GPS waypoint (not using VLOC mode). Compare the two keeping in mind the greatest discrepancy will be when high and close.

Are the results significant enough to be concerned about when considering the .5 NM/3% DME tolerance?

I am treating this as a purely academic discussion. I personally can't imagine not loading the approach or, at the very least retrieving the waypoint from the database.

[Super Dave]

sorry, let me rephrase: I have provided three separate references that specifically say that DME distances cannot be used for approach mode; the DME-based procedure must be loaded from the database, and the named 5-letter fixes must be used.

the reason you need to set the GPS to approach mode is because it increases the RAIM sensitivity and prediction appropriately. You're right about setting CDI, but not for the reason you said: only some GPSs will increase the RAIM prediction when the CDI is set to .3.

I am familiar with the sources you referenced; I just disagree with your interpretation. I argue that AC 90-108 specifically states that you CAN use GPS distances in place of DME distances on approaches or other procedures.

7. USES OF SUITABLE RNAV SYSTEMS.

a. Usage of Suitable RNAV Systems. Subject to the operating requirements in this AC,

operators may use a suitable RNAV system in the following ways.

(1) Determine aircraft position relative to or distance from a VOR (see first note in subparagraph 7b), TACAN, NDB, compass locator (see second note in subparagraph 7b), DME fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator bearing intersecting a VOR or Localizer (LOC) course.

(2) Navigate to or from a VOR, TACAN, NDB, or compass locator. (3) Hold over a VOR, TACAN, NDB, compass locator, or DME fix. (4) Fly an arc based upon DME.

b. Specific Allowances. The allowances described in this section apply even when a facility is identified as required on a procedure (for example, “Note ADF required”).

[rbp]

there are a lot of things that will work that are illegal. for example, you could probably land safely using the GPS for a VOR approach, but its not legal without an overlay

[rbp]

it says "Subject to the operating requirements in this AC" and its in those exact operating requirements that you'll find the rules I quoted

[Marauder]

Ok, since this pony has been beaten into submission, let me put another one in the pasture for you guys to bounce on. I am based at an airport that has a VOR only approach. I have flown this approach for years and dealt with the scalloping of the VOR signal. When I purchased the GTN, I was pleasantly surprised to see the GPS overlay for the VOR approach was there. This made for flying the hold much easier and as long as I convert the CDI to VLOC before the FAF, I'm legal.

 

I have flown this approach using my Narco Nav displaying the VOR signal on my Aspen on Nav1 and let the GPS overlay display on my second HSI on the MFD running in reversion mode. From my dozen approaches flown this way, it is clear that although not legal, the frigging GPS overlay is much more accurate for guidance.

 

What prevents the FAA from converting these VOR approaches to GPS approaches?

[Marauder]

Sorry, forgot to add in the pony for you guys to beat. I know there are those pilots at my field who do fly it as a GPS approach. I just don't happen to be one of them. What's the risk?

[Super Dave]

rpb, please humor me and re-read the excerpt from from the AC that I posted in #61. Specifically, how do you interpret the subsections beginning at (1) and b.

[midlifeflyer]

.

 

What prevents the FAA from converting these VOR approaches to GPS approaches?

The overlay program was an interim measure and, as I recall, they are not being created. There'so nothing that prevents the FAA from creating a GPS approach to your airport, which may or may not follow the same course as your VOR approach, other than priorities to do the required layout (typically a T) and testing.

[Marauder]

The overlay program was an interim measure and, as I recall, they are not being created. There'so nothing that prevents the FAA from creating a GPS approach to your airport, which may or may not follow the same course as your VOR approach, other than priorities to do the required layout (typically a T) and testing.

Thanks for the explanation. The reason I asked is that my old airport did not have an approach initially (although one idiot did build his own NDB approach). After I moved here, I noticed that they went directly to a GPS approach even though a VOR was within 11 miles of the airport. I suspect it was a TERPs issue.

My current airport has only the VOR approach and it appears never attempted to go to a GPS approach.

Sent from my iPad using Tapatalk

[rbp]

rpb, please humor me and re-read the excerpt from from the AC that I posted in #61. Specifically, how do you interpret the subsections beginning at (1) and b.

 

exactly the same way you do.

but that subsection can't be read in isolation, the text preceding it says "Subject to the operating requirements in this AC". 

if all I read was subsection (1)( b ), then I wouldn't need to use an IFR-certified GPS, would I?, because the requirement to use an IFR-approved GPS appears elsewhere in the regs

You can't cherry pick one sentence and ignore the rest of the reguations.

 

[Super Dave]

I give up