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14 Answers

Substituting GPS for localizer DME distances

Asked by: 31938 views Instrument Rating

In this KMFR LOC/DME rwy 14 approach I need DME to establish positions of step-down fixes, for example the location of the OSSAJ fix. http://204.108.4.16/d-tpp/1010/00251ILD14.PDF If my aircraft is not DME equipped can I use IFR certified GPS?. To be more specific I am talking about something as sophisticated as Garmin G1000 which when you load and fly this approach it counts down distance to runway treshold during the final approach. Can I use those distances in lieu of the localizer DME distances? I understand that localizer antennas can be displaced thousands of feet from the runway threshold so there would be some difference.

14 Answers



  1. Kent Shook on Oct 06, 2010

    Short answer: Yes, you can use your IFR GPS in lieu of ADF or DME, provided you have a current database.

    You can find this information in AIM 1-1-19, Table 1-1-6.

    Also, be sure that your GPS is set to the correct waypoint for the distance you’re trying to find. On the approach you linked to, the distance is measured from a localizer DME. If your GPS is set to “direct KMFR” for example, your distance will not be correct. IFR Approach certified GPS units should properly identify the fixes if you activate the correct approach. However, if you’re using an IFR GPS that is only Enroute and Terminal certified (such as the popular Apollo GX55 and similar units), flying this approach might not be quite so straightforward.

    For the math geeks among us, if you calculate the difference between what a DME will display (slant range) versus what a GPS will display for typical instrument approach altitudes, distances, and speeds, you’ll find that the difference will be on the order of fractions of a second, which is why the GPS makes an acceptable substitute for DME.

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  2. Chris Findley on Oct 06, 2010

    Yep… not much to add, except that I had this very question on my CFII checkride…yes is the correct answer.

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  3. Michael Jastrzebski on Oct 07, 2010

    Ok, so again going to the above approach and the OSSAJ fix which has localizer DME distance of 8.6 nm, I can see from the chart that this fix is really 2.8 + 4.2 = 7 nm from the runway threshold and since my G1000 gives me this distance then the 7 nm is what I have to use to find the fix. No need to say ‘yes’ if everyone agrees.

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  4. Curtis Ide on Oct 07, 2010

    1-2-3 (c) of the AIM which refers to allowable operations with RNAV devices states you may use the RNAV(GPS) system to substitute for the following.
    c. Allowable Operations. Operators may use a suitable RNAV system in the following ways.
    1. Determine aircraft position over or distance from a VOR (see NOTE 4 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.
    These operations are allowable even when a facility is explicitly identified as required on a procedure (e.g., “Note ADF required”).
    These operations do not include lateral navigation on localizer-based courses (including localizer back-course guidance) without reference to raw localizer data.
    It is my opinion that you can not use GPS to substitute for DME distance on an approach unless it is for a DME arc.  You may however, determine your position in reference to a DME waypoint.  Example: If you load the ILS/LOC/DME RWY 14 into yor G1000, you may determine your position over the OSSAJ intersection and that is why you are able to substitute the GPS for DME.  It is not allowable to program in I-MFR and use that distance.
    I know this is a contradiction to what was said above so I am interested to see what other people may say.
     

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  5. Michael Jastrzebski on Oct 07, 2010

    Curtis,
    You seem to contradict yourself. First you say I can only use GPS in lieu of DME arc distances but then you say I can determine my position in reference to a DME waypoint. Unless I am missing something these two sentences contradict each other. OSSAJ fix is not in the G1000’s database so the only easy way for me to determine its position in G1000 is looking for 7 nm distance from the runway threshold since the counted down distance from rwy14 is prominently displayed when I load/activate this approach and when I am passed FAF. By the way if we agree that I can do that then I don’t see disagreement with previous answers.

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  6. Matthew Waugh on Oct 09, 2010

    You can’t invent a DME fix by picking some random location and “calculating” the distance. So this idea of using the runway threshold and adding up distances from the chart doesn’t work.
    I’m not familiar with the G1000, but it looks like for the approach referenced you must either be able to recall OSSAJ from the database or be able to recall the localizer I-MFR from the database and use the published DME distance. If you cannot do either of those then you aren’t equipped for the approach and you can’t shoot it.

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  7. Curtis Ide on Oct 09, 2010

    Michael,
    I will try to explain my answer clearly.
    You may use the GPS (G1000 we are calling it) to fly a DME Arc.  Example – The Franklin Airport (FKN) near my homebase has a DME Arc for runway 28.  It requires that I fly a 10nm arc to an Intermediate Fix (IF) which is aligned with the runways final approach course.  I must fly the approach using conventional navigation (the vor) but if I do not have DME then I am allowed to use 10nm from the FKN vor on the GPS distance to substitute for the DME distance.
    Now as for your approach –  If OSSAJ is not in your database then you CAN NOT fly this approach.  You are not allowed to use 8.6 nm from I-MFR to identify OSSAJ.  You could only use the GPS to substitute for DME on this approach by using the GPS to identify a waypoint that is in the database and not from the GPS distance.  I hope this is clear that you can not use GPS distance to substitute for DME distance.  You are using the fact that the waypoint is in a known position that is published into the Garmin Database and identifying the waypoint when the GPS says you are passing the waypoint.  As Matthew said though, OSSAJ should be in your database and you should be able to load/activate this approach.
    Curtis

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  8. Curtis Ide on Oct 10, 2010

    Did a little more research – Looked in the 2008 Instrument Flying Handbook and Ch. 7 Page 29 covers all of the GPS/DME issues.  You can use the distance as you Kent stated.  The link for the flying handbook is below.
    Sorry for any confusion everyone!
    http://www.faa.gov/library/manuals/aviation/instrument_flying_handbook/media/FAA-H-8083-15A%20-%20Chapter%2007.pdf

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  9. Curtis Ide on Oct 10, 2010

    Another interesting website I found along the way in  reference to the OSSAJ intersection problem.
    http://www.pilotsofamerica.com/forum/archive/index.php/t-2939.html

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  10. Andrew K on Feb 06, 2011

    Another thing to consider is using the DME to GPS substitutions regarding selection of alternate airport’s alternate minimums.
    If one is selecting an alternate airport the alternate minimums cannot be based on an GPS approach or an approach requiring a GPS substition.  I.E. if a LOC/DME approach exists at the alternate then the aircraft must have LOC and DME equipment (no subs) to consider that approach for alternate mins.
    The only exception is if the aircraft is also equipped with WAAS.  In this case alternate mins can be based on a GPS approach.
     
    Further reading:
    http://expertaviator.com/2011/01/14/can-i-use-my-gps-when-an-arrival-requires-dme/
    http://expertaviator.com/2010/12/12/alternate-filing-with-a-waas-certified-gps/
    http://www.faa.gov/air_traffic/publications/atpubs/aim/Chap1/aim0101.html

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  11. John D. Collins on Feb 07, 2011

    I am not yet convinced that an aircraft without a DME but with a WAAS IFR GPS is restricted on the selection of an alternate to not use an approach for which a DME is a requirement.  There is a major difference between a TSO C129a IFR GPS (GPS for short) and a TSO C146a WAAS IFR GPS (WAAS GPS for short). The GPS is only approved for supplementary means of navigation and with 91.205, does not meet the requirement for “navigation equipment suitable for the route to be flown”, therefore, a DME is required to be on board and working in order to file an alternate whose approach has a DME requirement.  The WAAS GPS is approved as “sole means of IFR navigation”, which means that it satisfies the 91.205 requirement for “suitable equipment for the route to be flown”. Since both a GPS and a WAAS GPS can be used to substitute for DME, the main difference is whether or not you can plan on being able to use the capability when you choose an alternate. In the case of a GPS, you can’t, in the case of a WAAS GPS, you can. The aim discusses GPS in section 1-1-19 and WAAS GPS in section 1-1-20. In the GPS section 1.1.19 (g), you find the following note: “Any required alternate airport must have an approved instrument approach procedure other than GPS that is anticipated to be operational and available at the estimated time of arrival, and which the aircraft is equipped to fly.”
    In the GPS WAAS section of the AIM, under c.7. it reads in part (emphasis is mine):
     
    “Unlike TSO − C129 avionics, which were certified as a supplement to other means of navigation, WAAS avionics are evaluated without reliance on other navigation systems. As such, installation of WAAS avionics does not require the aircraft to have other equipment  appropriate to the route to be flown.
     
    (a) Pilots with WAAS receivers may flight plan to use any instrument approach procedure authorized for use with their WAAS avionics as the planned approach at a required alternate, with the following restrictions. When using WAAS at an alternate airport, flight planning must be based on flying the RNAV (GPS) LNAV minima line, or minima on a GPS approach procedure, or conventional approach procedure with “or GPS” in the title. Code of Federal Regulation (CFR) Part 91 non-precision weather requirements must be used for planning.” My argument is that using the WAAS GPS is authorized to substitute for DME and meets the requirement while the limitation stated would not apply to an approach where DME was a requirement. My belief is not set in stone and I can be convinced with an appropriate quotation from the AIM, FAR, or other authoritative document.

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  12. John D. Collins on Feb 25, 2011

    If an aircraft is equipped with a WAAS GPS but does not have either an ADF or DME installed and operational, can the pilot plan to use an alternate airport whose only approaches require(s) a DME or ADF.  The answer is a maybe.  The deciding factor is what the AFMS says.  In all non WAAS and in the early GNS430W/530W/GNS480 GPS units, they did not fully comply with TSO C146a and the AFMS had a restriction that reads:

    “For flight planning purposes, if an alternate airport is required it must have an approved instrument approach procedure other than GPS or RNAV that is anticipated to be operational and available at the estimated time of arrival. All equipment required for this procedure must be installed and operational.”

     

    Later, an updated antenna and software (Version 3.X) was made available for the WAAS GPS units mentioned, and any any aircraft that had this update installed, received an updated AFMS where the quoted text was eliminated.  So these units can plan on using the GPS in lieu of the DME or ADF.

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  13. Luis A. LLambias on Jul 17, 2013

    Flying under IFR rules, if a DME is required for an approach, does the DME NEEDS to be TSO certified?

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  14. Luis A. LLambias on Jul 17, 2013

    Can a NON TSO DME be used to fly an approach requiring a DME?

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