AIM 

3/21/24 

When using WAAS at an alternate airport, flight planning must be based on flying the RNAV (GPS) LNAV or 

circling minima line, or minima on a GPS approach procedure, or conventional approach procedure with “or 

GPS” in the title. Code of Federal Regulation (CFR) Part 91 non

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precision weather requirements must be used 

for planning. Upon arrival at an alternate, when the WAAS navigation system indicates that LNAV/VNAV or 

LPV service is available, then vertical guidance may be used to complete the approach using the displayed level 
of service. The FAA has begun removing the 

NA

 (Alternate Minimums Not Authorized) symbol from select 

RNAV (GPS) and GPS approach procedures so they may be used by approach approved WAAS receivers at 
alternate airports. Some approach procedures will still require the 

NA

 for other reasons, such as no weather 

reporting, so it cannot be removed from all procedures. Since every procedure must be individually evaluated, 
removal of the 

NA

 from RNAV (GPS) and GPS procedures will take some time. 

NOTE

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Properly trained and approved, as required, TSO-C145() and TSO-C146() equipped users (WAAS users) with and using 

approved baro-VNAV equipment  may plan for LNAV/VNAV DA at an alternate airport. Specifically authorized WAAS users 

with and using approved baro-VNAV equipment may also plan for RNP 0.3 DA at the alternate airport as long as the pilot 

has verified RNP availability through an approved prediction program. 

d.  Flying Procedures with WAAS 

1. 

WAAS receivers support all basic GPS approach functions and provide additional capabilities. One of 

the major improvements is the ability to generate glide path guidance, independent of ground equipment or 

barometric aiding. This eliminates several problems such as hot and cold temperature effects, incorrect altimeter 

setting, or lack of a local altimeter source. It also allows approach procedures to be built without the cost of 

installing ground stations at each airport or runway. Some approach certified receivers may only generate a glide 

path with performance similar to Baro

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VNAV and are only approved to fly the LNAV/VNAV line of minima 

on the RNAV (GPS) approach charts. Receivers with additional capability (including faster update rates and 

smaller integrity limits) are approved to fly the LPV line of minima. The lateral integrity changes dramatically 

from the 0.3 NM (556 meter) limit for GPS, LNAV, and LNAV/VNAV approach mode, to 40 meters for LPV. 

It also provides vertical integrity monitoring, which bounds the vertical error to 50 meters for LNAV/VNAV and 

LPVs with minima of 250’ or above, and bounds the vertical error to 35 meters for LPVs with minima below 

250’. 

2. 

When an approach procedure is selected and active, the receiver will notify the pilot of the most accurate 

level of service supported by the combination of the WAAS signal, the receiver, and the selected approach, using 

the naming conventions on the minima lines of the selected approach procedure. For example, if an approach 

is published with LPV minima and the receiver is only certified for LNAV/VNAV, the equipment would indicate 

“LNAV/VNAV available,” even though the WAAS signal would support LPV. If flying an existing 

LNAV/VNAV procedure with no LPV minima, the receiver will notify the pilot “LNAV/VNAV available,” even 

if the receiver is certified for LPV and the signal supports LPV. If the signal does not support vertical guidance 

on procedures with LPV and/or LNAV/VNAV minima, the receiver annunciation will read “LNAV available.” 

On lateral only procedures with LP and LNAV minima the receiver will indicate “LP available” or “LNAV 

available” based on the level of lateral service available. Once the level of service notification has been given, 

the receiver will operate in this mode for the duration of the approach procedure, unless that level of service 

becomes unavailable. The receiver cannot change back to a more accurate level of service until the next time an 

approach is activated. 

NOTE

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Receivers do not “fail down” to lower levels of service once the approach has been activated. If only the vertical off flag 

appears, the pilot may elect to use the LNAV minima if the rules under which the flight is operating allow changing the type 

of approach being flown after commencing the procedure. If the lateral integrity limit is exceeded on an LP approach, a 

missed approach will be necessary since there is no way to reset the lateral alarm limit while the approach is active. 

3. 

Another additional feature of WAAS receivers is the ability to exclude a bad GPS signal and continue 

operating normally. This is normally accomplished by the WAAS correction information. Outside WAAS 

coverage or when WAAS is not available, it is accomplished through a receiver algorithm called FDE. In most 

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Navigation Aids