GPS Flight Planning – WAAS Going On?

Robert Chapin
2015-04-23T23:16:08+00:00

Symbol for Alternate Minimums Not AuthorizedIt’s time to review the rules for planning an IFR flight with GPS navigation.  Maybe the airplane has an old receiver without WAAS capability.  Or maybe the rules have changed too many times to remember the current limitations.  Where to find the answers?

Destination Alternates – Without WAAS

I’m starting with destination rules, because most flight plans begin with that basic question:  Where to go and how to get there?

2013 – If you were aware of these rules a couple years ago, you knew to look them up in Published NOTAM No. GEN13000.  That notice expired in 2014.

2014 – AIM 1-1-18(g)(1) superseded the obscure notice, making the alternate airport rules somewhat easier to find.

2015 – The published 2015 FAR/AIM is already obsolete because the rules changed again on 8 January 2015.  Now, one must look for AIM 1-1-18(b)(5)(c).

I found four basic rules for flying GPS without WAAS:

  1. Pilots “may file based on a GPS-based IAP at either the destination or the alternate airport, but not at both locations.”
  2. Pilots may plan for LNAV or CIRCLING minimums only, unless equipped for baro-VNAV.
  3. A preflight RAIM prediction for the destination or the alternate airport is required.
  4. Language left over from AIM 1-2-3(d) and Notice N 8900.218 indicate the non-GPS approach at the other location is required to “be flown without reliance on GPS.”

Destination Alternates – With WAAS

The GPS rules for operating with WAAS are not in the GPS section of the AIM.  There is a separate section under AIM 1-1-19 governing GPS with WAAS capability.  The WAAS alternate airport rules, as of 8 January 2015, can be found under AIM 1-1-19(c)(9)(a):

  1. Pilots “may flight plan to use any instrument approach procedure authorized for use with their WAAS avionics as the planned approach at a required alternate.”
  2. Pilots may plan for LNAV or CIRCLING minimums only, unless equipped for baro-VNAV.

En Route – Without WAAS

AIM 1-1-18(d)(1)(b) requires that the aircraft “must be equipped with an approved and operational alternate means of navigation appropriate to the flight.”  Also, “active monitoring of an alternate means of navigation is required when the RAIM capability of the GPS equipment is lost.”

GPS Flight Planning Scenario #1

Your destination is Longview, Texas, KGGG.  The forecast ceiling is 1,000 feet AGL.  You have calculated that the minimum fuel requirements will prevent you from using Shreveport, Louisiana as an alternate destination on this flight.  Your possible alternates are:

  • Gilmer Muni, KJXI
  • Gladewater Muni, 07F
  • Harrison Co, KASL
  • Panola Co, 4F2
  • Rusk Co, KRFI
  • Tyler Pounds Rgnl, KTYR

Which of these airports can be used as a required alternate if your airplane is WAAS equipped?  Not WAAS equipped?

Analysis:  07F, 4F2, and KASL are not authorized for alternate airport IAPs.  KJXI and KRFI have authorized IAPs that require GPS.  These two airports can be used as alternates only when the airplane is WAAS equipped or when planning a conventional approach at KGGG without reliance on GPS (this may be unlikely).  Of the six airports in the area, KTYR is the only airport with authorized IAPs that do not require GPS.  However, of the four IAPs that do not require GPS, two require DME, and the other two require DME or dual VOR receivers.  In this scenario, if the airplane is not WAAS equipped, then it is reasonable to plan to use the GPS at KGGG as substitute for ADF or DME, and plan to use the ILS at KTYR with two VOR receivers.

GPS Flight Planning Scenario #2

While flying out of Denton, Texas, KDTO, or Grayson Co, Texas, KGYI, for instrument flight training, it is often convenient to list the other airport as the alternate because it is the best place to get company fuel or maintenance without a hassle.  However, there is usually no way to know, in advance of flight planning, which airplane will be used or whether it will be WAAS equipped.

Can you always use KDTO or KGYI as a required alternate?  If not, what would be the closest suitable alternate?

Analysis:  When using KDTO or KGYI as a destination, all of the IAPs require ADF, DME, or an RNAV substitute.  Unfortunately, none of the available airplanes are equipped with ADF or DME.  In this scenario, a WAAS receiver is required in order to use either airport as a required alternate for the other.

When not WAAS equipped, KDTO may not be used as an alternate for KGYI because it offers only two authorized RNAV (GPS) procedures for alternate planning.  A suitable alternate in this area is Fort Worth Alliance, KAFW.  The ILS procedures at that airport can be flown using RADAR and dual VOR receivers.

KGYI has three IAPs authorized for alternate planning.  Two of them are the RNAV (GPS) type, and the third is a VOR/DME type.  Unfortunately, this means use of GPS is required for all three procedures.

In the area of KGYI, you will have some difficulty finding any IAP that does not require either GPS or DME.  The closest authorized alternate is almost 40 NM away at Ardmore, Oklahoma, KADM.

23 Apr 2015

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Aeronautical Knowledge

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  • Gary Thompson says:

    HI ROBERT,THANKS, QUESTION: DESTINATION ALTERNATES WITH WAAS AND WITHOUT WAAS SAY THIS.
    (Pilots may plan for LNAV or CIRCLING minimums only, unless equipped for baro-VNAV) IT SEEMS TO SAY THAT RNAV PRECISION APPROACHES ARE NOT ALLOWED WITHOUT BARO-VNAV WITH WAAS OR WITHOUT WAAS OR IS IT SAYING YOU CAN PLAN A NON PRECISION APPROACH UNLESS EQUIPPED FOR BAR-NAV. PLEASE HELP ME INTERPRET IT CORRECTLY. GARY

    • Robert Chapin says:

      Hi Gary, for the selection of an alternate airport without a conventional approach, you must be capable of (plan for) the non-precision GPS approach. This means if the weather is below 800-2 or any nonstandard minimums or the minimums for LNAV guidance, you cannot file a flight plan to that airport.

      If you divert to the alternate airport, then fly the procedure as published, and you can use LPV minimums when available.

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