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4 Answers

Substitute GPS for out of service VOR enroute and terminal phases

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FAA Regulations

Regarding a TSO C-129 RNAV system…. Let’s assume the RAIM check, database, AFM, etc. are ok with it and the aircraft has the navigation equipment suitable for the route to be flown (i.e. has a functioning VOR receiver). For enroute and terminal operations (other than the final approach segment) both AC 90-108 1(a)(1) and AIM 1-2-3 (a)(1) allow the use of a “suitable” RNAV system as a substitute for an out of service ground based navaid (VOR,DME, etc). In part 6 of AC 90-108 and in AIM 1-2-3 (b)(1), a TSO-129 RNAV system is included in the definition of a suitable RNAV system. I interpret this to imply that, if a VOR is out of service, I could still fly a victor airway route using a suitable TSO-129 RNAV system. This seems to conflict with AIM 1-1-17 (3)(b)…….or am I not understanding something? “When using TSO-C129() or TSO-C196() receivers, the avionics necessary to receive all of the ground−based facilities appropriate for the route to the destination airport and any required alternate airport must be installed and operational. Ground−based facilities necessary for these routes must be operational. Thanks, Craig

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4 Answers



  1. John D Collins on Sep 23, 2019

    Going back to the 2007-3-15 edition of the AIM, the following change notes were listed:

    “a.1−1−19.Global Positioning System (GPS) Subparagraph f has been consolidated into the new paragraph 1−2−3, Use of Area Navigation (RNAV) Equipment on Conventional Procedures and Routes.”

    “c.1−2−3.Use of Area Navigation (RNAV) Equipment on Conventional Procedures and Routes. Since the original allowances for the use of GPS in lieu of ADF and DME were developed and published, a number of questions have arisen regarding different technologies and potential situations for substitution. In response to these inquiries and data gained from operational experience, the Federal Aviation Administration has updated these allowances and incorporated the changes into this paragraph. Two changes from previous guidance on this subject are the allowance for “substitution” of VOR facilities and the use of RNAV systems described in Advisory Circular (AC) 90−100, U.S. Terminal and En-Route Area Navigation (RNAV) Operations.”

    It appears that the paragraph you cited originally was part of the 2007-03-07 edition of the AIM: 1−1−19 Global Positioning System (GPS), e. Use of GPS for IFR Oceanic, Domestic En-Route, and Terminal Area Operations, paragraph 2, which read as:

    “GPS domestic en route and terminal IFR operations can be conducted as soon as proper avionics systems are installed, provided all general requirements are met. The avionics necessary to receive all of the ground−based facilities appropriate for the route to the destination airport and any required alternate airport must be installed and operational. Ground−based facilities necessary for these routes must also be operational.”

    The paragraph has now migrated to: 1-1-17 Global Positioning System (GPS), 3. Oceanic, Domestic, En Route, and Terminal Area Operations, paragraph (b) and now reads as:

    “Conduct GPS domestic, en route, and terminal IFR operations only when approved avionics systems are installed. Pilots may use GPS via TSO−C129() authorized for Class A1, B1, B3,
    C1, or C3 operations GPS via TSO-C196(); or GPS/WAAS with either TSO-C145() or TSO-C146(). When using TSO-C129() or TSO-C196() receivers, the avionics necessary to receive all of the ground−based facilities appropriate for the route to the destination airport and any required alternate airport must be installed and operational. Ground−based facilities necessary for these routes must be operational”

    It is my belief that it did not get properly edited to reflect the change allowing the ground facility to be out of service. So I have a strong suspicion it is wrong and you should report it to the FAA for review and or correction. The AIM has a a reference in it as to where to send questions and request corrections.

    One final note, AC 90-100A indicates that the GPS AFM may override the guidance if it has a limitation statement. Also, the AC points to a compliance document that lists GPS systems and some of them have a note in the column titled “Eligible to use RNAV Substitution or RNAV Alternate means of navigation (US only)” that reads as “Yes, but not for
    VOR substitution”. Many of the TSO C129 GPS systems have the note or have limitations listed in their AFM, examples KLN90B, KLN94, GNC300XL and many more.

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  2. Craig on Sep 24, 2019

    Thank you for the response. I’ll send them an email and see what happens.

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  3. Craig on Oct 04, 2019

    I emailed an inquiry to the AIM gurus. So far, they responded that “this control is in KSN 2552”.

    I will provide an update when I receive more info.

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  4. Craig on Dec 12, 2019

    Mr. Collins was correct. Below is the feedback I received….

    Assuming the RAIM check, database currency, AFM, etc. are ok with it and the aircraft has the navigation equipment suitable for the route to be flown (i.e. has a functioning VOR receiver), could I interpret this to imply that, if the VOR RMG on victor route V333 is out of service, I could, for example, still fly that victor airway route V333 using a suitable TSO-129 RNAV system as long as that VOR RMG is retrievable from the GPS database? Yes. A TSO 129 RNAV system qualifies as suitable for use as a substitute means of navigation per AC 90-108.

    If so, does this conflict with AIM 1-1-17 (3)(b) that reads “Ground−based facilities necessary for these routes must be operational.” Yes

    The apparent conflicting guidance is because the different documents are updated on different timelines. When the guidance on PBN operations in the NAS is next updated (currently in progress), there will be harmonized terminology between applicable Advisory Circulars and the AIM. In the meantime, operations in full compliance with the requirements in the AIM, or in full compliance with the requirements in AC 90-108, and at all times in compliance with the CFRs, may be considered safe.

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