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5 Answers

Filing alternate with GPS only approach

Asked by: 2616 views Instrument Rating

1. As 91.169 telling us, when choosing alternate we have to take 2sm/800ft as minimum for non precision approach. Let’s consider that we haven’t negative “A” in triangle on plate.

2. At the same time, in AIM 1-1-17.b.5.(c) we can read that “For flight planning purposes”... “pilots may plan for: (1) Lateral navigation (LNAV) or circling minimum descend altitude (MDA);”, which in most cases is lower than 800ft.  

Does it mean that statement “2” is just exception for statement “1”?

5 Answers



  1. John D Collins on Feb 29, 2020

    No.

    The actual MDA or circling minimums apply when you fly the approach and are typically with an MDH and visibility less than 600-1.5. The 800-2 is intended to be a forecast minimum that will conservatively still allow you to fly the approach. Take an example where the MDH on the approach is already 800-2 or higher, then you are going to see one of two things, a) either the approach may not be used for the purposes of meeting the flight planning alternate requirement or b) there will be special alternate minimums assigned for the procedure. So the 800-2 is a standard alternate minimum for a non precision approach procedure and in the case of LPV, because it is not considered a precision approach, the alternate minimums are based on the associated LNAV procedure on the same chart, it there is one. There are cases of LPV that do not have an associated LNAV option on the chart and these procedures will not be authorized to be used to satisfy the alternate planning requirements.

    Finally, a Category I ILS procedure has a DH of 200 and 1/2, yet the standard alternate minimums for these procedures is 600 -2. This is because in a worse case condition, if the GS and approach lights are out of service, the localizer procedure may be used and it typically has a MDH of 400- 1/2.

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  2. Rkon on Feb 29, 2020

    Thank you John. I got the idea that “planning” minimums are always more conservative than “executive” minimums.
    But does statement “2” from my question related with topic (selecting alternate), and if does- which way?

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  3. John D Collins on Feb 29, 2020

    It is merely a statement of fact. LNAV procedures typically have an MDH lower than 800 feet. The worst case situation is that the integrity needed to use LPV minimums is not available and the only alternative is the less precise LNAV option, which is a non precision approach and therefore needs to use the associated standard minimums of 800-2 when the standard minimums apply. Non standard minimums will apply when the LNAV MDH and visibility requirements dictate a higher value than standard ones.

    One could make a similar statement regarding an ILS or VOR procedure.

    On an ILS, for flight planning purposes”… “pilots may plan for : Lateral navigation using the Localizer or circling minimum descent height (MDH);”, which in most cases is lower than 600ft.

    On a VOR approach, for flight planning purposes”… “pilots may plan for : Lateral navigation using the straight in or circling minimum descent height (MDH);”, which in most cases is lower than 800ft.

    It is not stated, that is it goes without saying because that is how and why the alternate standards are determined in the first place for NPA and ILS procedures. In the case of an RNAV, the guidance is there so that the pilot uses the appropriate standard alternate minimums for a NPA of 800-2 and not 600-2, which the pilot may be tempted to do. Remember the standard alternate values are only used when higher values are not specified. The ILS can use the lower value because the lateral guidance is narrower with a localizer and the obstacle clearance allows a lower MDH. Pretty much everything can fail with an ILS except the localizer and the procedure may still be flown. The lateral path accuracy is not changed. With a downgrade of LPV to LNAV, the lateral path is quite different in terms of its width, although they are the same at the FAF, at the threshold, the LPV full scale CDI deviation is +/- 350 feet, whereas the LNAV is +/- 1823 feet.

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  4. Rkon on Feb 29, 2020

    So my mistake was that I assumed that AIM allows to decrease minimums for planning, while AIM telling just the opposite: that we have to increase minimums above 2-800 for LNAV when particular airport has GPS approach only. Which by the way is quite logical.

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  5. Mark Kolber on Mar 03, 2020

    Whoa. I don’t understand what you mean by that last comment, but it sounds like you may be mixing apples and oranges. As you did, I will stick with standard alternate minimums. And I am really saying the same thing as John, but in a slightly different way.

    91.169 is a flight plan/flight planning rule. The part dealing with destination alternates says two things: (1) when you must file and alternate and (2) what the forecast weather at your alternate must be in order to use it. With respect to (2) if your are basing your filing alternate on the availability of a precision approach, the weather must be at least 600 and 2. If you are choosing your filing alternate based on a nonprecision approach being available, the forecast weather needs to be at least 800 and 2. This is a rule of flight planning intended to ensure you planned for where better weather will be in case you an’t get into your destination – to have a good “out” if the flight can’t be completed as planned..

    The AIM section you quoted is not telling us anything about what minimums to use when filing an alternate. The “for flight planning purposes” is only telling you what you expect in general from the equipment “at the airport where the RNAV (GPS) approach will be flown” which may be “at either the destination or the alternate airport.”

    I think you are reading the AIM’s information about things to consider about GPS equipment when you plan a flight – generally plan a flight – as a rule about the minimums required for a 91.169 alternate airport. It’s not. It has absolutely nothing to do with it.

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