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15 Answers

Challenging approach’s – LOC/DME incompatible with RNAV

Asked by: 10235 views , , ,
FAA Regulations, Instrument Rating

Look at this approach. I thought I understood everything but I admit I am a little GPS handicapped. I hope you find this challenging as well.

I have read the following:

 LOC/DME Approach Incompatible with RNAV

On the example LOC, the missed approach point may be identified only by using a DME receiver. Because general aviation aircraft are increasingly dependent on GPS distance information and do not have a DME receiver, many aircraft are not equipped for this type of procedure. LOC/DME procedures rarely include timed legs, and rarely include a named fix or intersection to identify the missed approach point. Unless the DME antenna is based at a VOR (as in Scenario #3) or other suitable navaid described in AIM 1-2-3(c), the procedure is unusable without the DME receiver. This affects procedures titled “ILS or LOC/DME” which are relatively common. In most cases, only the ILS portion remains compatible with non-DME aircraft.

I can not say for sure that I understand why GPS CAN NOT be used as DME for this approach. I'm assuming that if the KORK LOC/DME RWY 5 approach is loaded then GPS is going to give the DME information.

I believe the key is that AIM 1-2-3 (c) "OMITS" that a GPS can be used to determine relative position or navigate to a LOCALIZER. It states that a GPS can be used to navigate to a VOR, TACAN, NDB, compass locators, or DME fix.

 

 

 

15 Answers



  1. Mark Kolber on Jun 19, 2014

    The key for pilot understanding that GPS may not be use in lieu of DME to identify the MAP on this approach is much simpler than trying to outguess or parse the AIM language you quoted.

    The key is that DME is in the title of the approach.

    Whether it’s a LOC, ILS, NDB or VOR (or GPS for that matter), the title of the approach always tells us what equipment is required for the final approach course, and no automatic substitution is allowed.

    If you want to ask why the approach developers precluded use of DME to identify fixes on the FAC for the Little Rock appraoch, that’s one you’d have to ask someone with approach design/TERPS knowledge.

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  2. John D Collins on Jun 19, 2014

    I don’t agree with this analysis. I believe a GPS may substitute for the DME on a LOC/DME approach. AC 90-108:

    “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.

    The provision that states “Determine aircraft position relative to or distance from a …DME fix …” permits using ATD (Along Track Distance) to locate almost any fix. Just because DME is in the title, does not mean that DME substitution can’t be used as it is not used for the lateral guidance on the procedure. The most challenging situation is with a localizer based DME and only a single DME location to mark the MAP. In this case, one may select the localizer DME as the active GPS waypoint, for example IUZA, and use the displayed value on the GPS to determine the distance. On a ground based approach, the GPS may not always be able to be used by to determine a fix location such as the MAP by loading and activating the approach from the GPS database, but it is not required in order to fly the procedure.

    Give me an example approach procedure where the assertion applies that the GPS can’t be substituted for LOC – DME approach.

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  3. tommytom on Jun 19, 2014

    I thought a “IFR” certified GPS could be used in place of DME anytime. For example KBOS VOR/DME RWY 15R, DME ARC approaches or even a LOC/DME approach.

    Example:

    Flying a LOC/DME approach using the localizer freq but using the garmin 430 to get the DME information.

    AIM 1-2-3 (c)

    c. Uses of Suitable RNAV Systems. Subject to the operating requirements, operators may use a suitable RNAV system in the following ways.

    1. 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.

    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.

    Notes

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

    2. These operations do not include lateral navigation on localizer-based courses (including localizer back-course guidance) without reference to raw localizer data.

    3. Unless otherwise specified, a suitable RNAV system cannot be used for navigation on procedures that are identified as not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently decommissioned NAVAID.

    4. Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach segment. This restriction does not refer to instrument approach procedures with “or GPS” in the title when using GPS or WAAS. These allowances do not apply to procedures that are identified as not authorized (NA) without exception by a NOTAM, as other conditions may still exist and result in a procedure not being available. For example, these allowances do not apply to a procedure associated with an expired or unsatisfactory flight inspection, or is based upon a recently decommissioned NAVAID.

    5. For the purpose of paragraph c, “VOR” includes VOR, VOR/DME, and VORTAC facilities and “compass locator” includes locator outer marker and locator middle marker.

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  4. Mark Kolber on Jun 19, 2014

    >>Give me an example approach procedure where the assertion applies that the GPS can’t be substituted for LOC – DME approach.

    The one he linked to in the original post. It can be used in place of DME everywhere but on the FAC.

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  5. Mark Kolber on Jun 19, 2014

    …maybe…

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  6. John D Collins on Jun 20, 2014

    Mark,

    I don’t see a link to an approach in the initial post. I assume it is not a secret, what is the airport identifier and procedure?

    Edit: I see the second post has KBOS ILS or LOC/DME RWY 15. That one is a no brain-er. You could load the approach and when the GPS indicated 0 at the MAP, you are at I-MDC D1.4. You could also use the GPS waypoint of I-MDC as the current direct-to waypoint and when the GPS distance was at 1.4, you would also be at the same place. You could also use MALDY as the direct-to waypoint, and when the distance equals 5.0, you are at the same MAP point. IMHO, all these methods are permitted by AC 90-108.

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  7. Mark Kolber on Jun 20, 2014

    It’s the KORK LOC/DME RWY 5 mentioned in TommyTom’s initial post (he didn’t link to it but the current chart is at http://155.178.201.160/d-tpp/1406/05897LD5.PDF) The “LOC/DME Approach Incompatible with RNAV” analysis he refers to is at http://captainslog.aero/2013/missed-approach-points/#comments

    The more I think about it, the more I think Chapin’s (CaptainsLog) analysis (and my initial one) is incorrect. What is precluded for substitution purposes is GPS in place of the non-GPS navaid that provides lateral guidance for the final approach course. Otherwise it would be fair game absent a specific note in the approach.

    AFAIK, there is no basis for a distinction. DME is DME, no matter what other navaid it is associated with – IOW, there is no difference between a VOR DME and a LOC DME except for the navaid the DME transmitter happens to be located.

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  8. tommytom on Jun 20, 2014

    Sorry I would have put a link in the post but I didn’t know how to.

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  9. tommytom on Jun 20, 2014

    Thanks for the clarification. I thought I understood correctly until I found his scenario’s as a teachling tool.

    I alway’s appreciate you two for your comments and clarifcations.

    Thanks

    Tom

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  10. Mark Kolber on Jun 21, 2014

    My guess is that Chapin is looking at the Profile View note about using I-ORK DME when on the localizer course and interpreting that as a limitation on GPS substitution.

    It’s not. That note is standard on IAPs that (1) use more than one DME facility, (2) in front of the aircraft during the approach, (3) one of which is associated with a LOC or LDA. [See para 8-2-5(f)(3) in the FAA’s Order 8260.19F dealing with Flight Procedures. If you want to take a look, search for I-XXX DME in the PDF).

    It’s just a reminder to use the correct DME source when there is more than one DME around, not a limit of GPS substitution.

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  11. Robert Chapin on Jun 21, 2014

    Interesting debate. I see AC 90-108 was mentioned already.

    “Pilots must extract waypoints, NAVAIDs, and fixes by name from the onboard navigation database and comply with the charted procedure or route.”

    In this case, unless the avionics can display the I-ORK waypoint by name, I feel it would not be a suitable use of RNAV. The charted MAP does not offer any alternate means of identification. I will add that note in the next update. I’m open to other interpretations, but would not risk my certificate on this detail.

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  12. tommytom on Jun 21, 2014

    Robert,

    Per the AC 90-108 and the AIM 1-2-3 (c) I believe RNAV can be used to give DME information.

    With that being said I believe the following to be acceptable.

    The “primary” Navigation source is tuned to the LOC frequency for the I-ORK approach.

    Secondary NAV source is an “IFR Certified” GPS unit with I-ORK as a “named” waypoint.

    The GPS is now giving DME information from I-ORK. 1.0 from I-ORK can now be identified as the MAP.

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  13. Mark Kolber on Jun 22, 2014

    The “retrieve by name” requirement appears multiple places in the AIM as well. But AFAIK, all that means for “pure” DME distance is that the co-located NAVAID must be in the database so one can determine distance from it, not that the DME distance must also be a named waypoint that is in the database. One cannot, for example, enter a bunch of lat/longs to place it.

    If distance from the co-located primary navaid could not be used, one could not fly a DME arc using GPS and that’s clearly permitted.

    BTW, the MAP for the KORK LOC 5 is in the database – at least the one in the GTN600/700 simulator.

    Bu there’s another issue with reliance on AC 90-108 – its age. At this point that guidance document is 3 years old. The AIM, OTOH, is amended twice a year and updated between those times by NOTAM. In the case of a discrepancy between the two (and in this case there is not a discrepancy) I would place heavier reliance on the more current AIM discussion.

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  14. Robert Chapin on Jun 22, 2014

    > Secondary NAV source is an “IFR Certified” GPS unit with I-ORK as a “named” waypoint.

    I need to research this more. I know my avionics wont do this after loading an ILS procedure. It will say “RWY” or “MA” and count down to zero distance at the MAP. I Googled up some very unofficial looking pages that say this type of NAVAID can only be used in “Direct to” mode. The problem then is how many more buttons will I have to push to comply with the terminal sensitivity requirement? What if I need to sequence to a missed approach fix? Are these things always in the database?

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  15. John D Collins on Jun 23, 2014

    Robert,

    There is no requirement that the ILS procedure be loaded by the GPS in order to fly the ILS procedure. This is done only for pilot convenience and situation awareness as one can tune the NAV radio and fly the procedure with anything else what so ever being displayed on the GPS. On the G1000 system or other PFD based system, loading a ground based approach from the database, although not requied is very helpful and will generally auto slew / set the appropriate final approach course for the CDI/HSI, so if the pilot does the approach by manually loading the frequency and switching to VLOC, they have to remember to set the CRS pointer to the appropriate value.

    The statement in AC 90-108 “Pilots must extract waypoints, NAVAIDs, and fixes by name from the onboard navigation database and comply with the charted procedure or route.” is a general consideration as regards the database.

    Specifically, the section dealing with what is authorized lists the following:

    “(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.”

    Parsing this, one may “Determine aircraft position relative to or distance from a … DME fix;”
    permits using the distance from a DME fix to determine aircraft position relative to or distance from.

    The statement goes on to state, after the semi colon “or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass locator bearing intersecting a VOR or Localizer (LOC) course.”

    This phrase adds another option to how one may “Determine aircraft position relative to or distance from a… DME fix”. If they wanted to restrict the latter to only identifying the named fix and exclude determining a distance from it, the sentence would have to be restructured and include this as part of the phrase as a limitation. The author chose not to do so.

    My main point is that AC 90-108 is not as restrictive as some pilots read it. Using the GPS in lieu of the DME is fairly liberal. That said, using the MAP as identified in the GPS approach procedure using a ground based Navaid can not always be relied on as anytime the MAP is located a distance from the runway threshold, the GPS MAP will be coded at a different location. In most cases, they are the same, but not always.

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