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4 Answers

Filing Alternate airport on IFR flight plan when using WAAS – HELP!

Asked by: 13537 views Commercial Pilot, FAA Regulations, Flight Instructor, General Aviation, Instrument Rating

I was going through the AIM in Chapter 1 to study about the suitable RNAV equipments and WAAS to understand the alternate airport selection when flying IFR flight.

I have listed the questions as below, and I would appreciate your comments, references and interpretations on what is correct according to FAR/AIM.

Some explanations were really vague and there were grey areas where two or more information were overlapping each other.

 

1)So, our school is thankfully equipped with WAAS/GPS and hope it is TSO-C145, or 146 certified. Now, when filing an alternate aiport, what I understand from the text in the AIM is that, we can shoot both GPS or file based upon GPS approaches at both destination and alternate airport?

2) If not WAAS equipped or TSO-129, 196 certified, then alternate airport must have an available instrument approach procedure that does not require the use of GPS. For example, shooting GPS or substituting with DME or VOR?

Also, in this case, can we shoot GPS approach at destination and shoot conventional approach at alternate? is this what text means?

3) Under AIM1-2-3(d)(1) states that for flight planning purposes, TSO-C129, 196 equipped aircraft, may only file alternate based on a GPS based IAP at either the destination or the alternate airport.

Does this mean that we are not using the standard weather minimums 800-2, 600-2 at destination and it must be greater than minimums for published LNAV or circling MDA or LNAV/VNAV DA?

What does either destination or the alternate airport mean? Aren't we caring only filing alternate airport?

4) Under AIM1-2-3 (d)(3) states that this restriction does not apply to TSO-145, 146(my school). So, we can shoot both GPS approaches at both destination and alternate airports using just 800-2(GPS approaches are non precision?!) and whenever wx at destination is below 1-2-3 rule, just file the alternate anyway?

5) Under AIM1-1-18(d)(1)(b) , aircraft using TSO-C129 or 196 must have operational alternate means of navigation and active monitoring of alternate navigation(let us say VOR ro NDB) is not required if the GPS receiver uses RAIM monitoring. 

From the information above, can I fly IFR XC with TSO-145 or 146 (WAAS)  without monitoring alternate means of navigation and do not have to monitor VOR or ADF ?

6)In continuance from previous question, under AIM1-1-18(e)(2)(a), it states that GPS/WAAS receiver(TSO-145, 146 certified), though not required, operators should consider retaining backup navigation equipment in their aircraft to guard against potential outages. From the classic memory GRABCARD, we need to have equipment for radio navigation and communication, and if we are flying Victor airways with GPS/WAAS, can we still fly without the VOR receiver and DME?

Steve.

4 Answers



  1. John D Collins on Jul 31, 2016

    Steve,

    First, filing an alternate is primarily a fuel planning exercise. There is no requirement that you actually use your alternate or that you restrict yourself to approaches that qualify to be used as an alternate when you arrive at the airport. The alternate is not even included in the information sent to ATC, so they don\’t have a clue what your plans are.

    The point of the special rules using GPS as an alternate has to do with the fact that non WAAS GPS is supplementary navigation. That means, you must be equipped with other navigation systems suitable for the route being flown that may be used in the event that GPS is not available. This includes considerations where you plan to use GPS in lieu of other required equipment such as a DME or ADF.

    91.205 requires that your aircraft be equipped with navigation equipment suitable for the route being flown. Without a WAAS GPS, this is a VOR and any specific equipment required for conducting an approach. So if the only approach at an airport requires an ADF or DME, you need to also have that equipment or you will be in violation of this regulation. Having a GPS installed can add the possibility of substituting for a DME or ADF on a ground based approach, except for the lateral guidance on the final approach segment. It can also be used to fly an approach approved for GPS, but if GPS ends up not being available during the flight, you need a backup plan. The backup plan is that an approach at the destination or alternate may have a GPS dependency, but not at both locations. This is considered as adequate for planning an alternate.

    In the case of a WAAS GPS, it meets the requirements of 91.205 without any other equipment being installed, so there are no restrictions on using suitable approaches with GPS. However, the WAAS integrity may or may not always be sufficient to support LP, LPV, or LNAV/VNAV vertical guidance. So the planning must be based on using the LNAV minimums, which the standard alternate weather is 800-2.

    So with the above as background,

    1) yes
    2) No, the alternate or the destination may be based on GPS, but not both
    3) Use LNAV 800/2
    4) Yes, both may be GPS, use LNAV 800-2 (or as specified) for weather planning
    5) Yes, with GPS you must carry both, WAAS you don\’t need to carry both
    6) Is this really a question, so you have two fully satisfactory means of navigating on board and you think there might be a restriction on using one or the other on an airway?

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  2. Steve on Jul 31, 2016

    so, for the Baro-VNAV, the decision height is DA which sounds like a precision approach, then the minimum weather requirement would go down as low as 600-2 ?

    Sir, thank you very much for the help and appreciate your positive support !

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  3. Steve on Jul 31, 2016

    Also, 91.205 requires us to have VOR or DME and AIM states they can be substituted with GPS, then why AIM states that we do not need to have those if equipped with WAAS TSO-145, 146 equipped?
    Sir, I think those two are conflicting each other sir…

    Steve

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  4. Best Answer


    John D Collins on Aug 01, 2016

    Steve,

    91.205 does not require VOR or DME. You won\’t find either of those navigation systems mentioned in 91.205. 91.205(d)(2) is actually a very simple requirement:

    \”91.205 (d) Instrument flight rules. For IFR flight, the following instruments and equipment are required:
    (2) Two-way radio communication and navigation equipment suitable for the route to be flown.\”

    You can read more about uses of suitable RNAV equipment as defined in CFR 14, section 1.1 and AC 90-108. Read the specific limitations that you find in the AFMS associated with any specific RNAV equipment. The AFMS for TSO C129/C196 will include a limitation that requires that the equipment suitable for the route (typically VOR) is required to be installed and operable or IOW meets 91.205 (d)(2), whereas the TSO C145/146 WAAS GPS systems AFMS does not have this restriction. The AFMS section 2, limitations is a regulatory limitation.

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