Welcome Guest. Sign in or Signup

Just wondering what the implications are of the limitation in AC 90-108, para. 9 a. (2):

"Pilots may not use their RNAV system as a substitute or alternate means of navigation if their aircraft has an AFM or AFM supplement with a limitation to monitor the underlying NAVAIDs for the associated operation."

If this limitation was in the AFM (or supplement), would it not render all of the substitute/alternate operations otherwise allowed in AC 90-108 useless?

For "substitute" operations this limitation seems obvious, but for "alternate" operations it doesn't make sense to me.

For example, as of about 2016, it's been clarified that as long as the VOR final approach course raw data is monitored, a suitable RNAV system can be used to navigate that segment (I'm paraphrasing).

Can anyone help me understand this?

 

Thanks,

3 Answers



  1. John D Collins on May 13, 2017

    The comment regarding using using a suitable RNAV system to navigate on a VOR or NDB approach as long as the raw data is monitored (CDI or RMI) and the ground facility is operational has always been the FAA position. The AIM language was intended to clarify this because there were those that did and did not believe this to be the FAA policy. In the ACF-IPG meetings, the FAA agreed that it was OK, but several attendees wished it to be stated explicitly. It is not a policy change, but it is a clarification of the policy.

    The comment regarding the AFM Supplement has to do with the wording found in most pre-WAAS supplements and some early WAAS AFM supplements. The wording in the initial AFMS for the GNS500W series had this limitation:

    “Aircraft using the GPS or WAAS capability of the 500W series navigation equipment under IFR must be equipped with an approved and operational alternate means of navigation appropriate to the flight”

    This limitation was later removed when the software was updated and a new WAAS antenna was installed. In effect, the limitation means that 91.205 is not satisfied unless the aircraft is equipped with a VOR system.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  2. R. Anderson on May 13, 2017

    Thank you

    0 Votes Thumb up 0 Votes Thumb down 0 Votes



  3. R. Anderson on May 13, 2017

    John, I appreciate your comments regarding the limitation in AC 90-108, para. 9 a. (2), you are always a wealth of information on the subject of RNAV navigation requirements.

    But, in reading your last sentence:

    “In effect, the limitation means that 91.205 is not satisfied unless the aircraft is equipped with a VOR system.”

    I’m struggling to align this with the limitation in para. 9 a (2), which states:

    “Pilots may not use their RNAV system as a substitute or alternate means of navigation if their aircraft has an AFM or AFM supplement with a limitation to monitor the underlying NAVAIDs for the associated operation.”

    I’m fully comfortable with the essential interpretation of AC 90-108, as clarified and supported by the AIM. But the para. 9 a. (2) words do not seem to logically co-exist with the other parts of AC 90-108 or the AIM.

    Lastly, I am aware that it is not uncommon for a sentence to be inserted into an AC, Handbook, etc., that has a certain meaning for the author/authors, yet does not translate in the exact same way to some others.

    Thanks again for your insight.

    0 Votes Thumb up 0 Votes Thumb down 0 Votes


The following terms have been auto-detected the question above and any answers or discussion provided. Click on a term to see its definition from the Dauntless Aviation JargonBuster Glossary.

Answer Question

Our sincere thanks to all who contribute constructively to this forum in answering flight training questions. If you are a flight instructor or represent a flight school / FBO offering flight instruction, you are welcome to include links to your site and related contact information as it pertains to offering local flight instruction in a specific geographic area. Additionally, direct links to FAA and related official government sources of information are welcome. However we thank you for your understanding that links to other sites or text that may be construed as explicit or implicit advertising of other business, sites, or goods/services are not permitted even if such links nominally are relevant to the question asked.