Since the advent of WAAS the rules and acceptable use of the panel mounted WAAS certified GPS has been constantly evolving. The panel mounted WAAS certified GPS allows us to file an IFR flight plan with an alternate airport with only a GPS approach, provided you comply with the defined requirements for filing an alternate.
First let us assume the weather at your destination is forecast to be 200 feet overcast and 2 miles visibility. Let us also assume our aircraft has a panel mounted TSO-145c WAAS certified GPS, no DME, and no ADF.
Per 91.169 the weather forecast requires an alternate because the ceiling is forecast to be less than 2000 feet. We look for an alternate and decide to use an airport with one GPS approach and the weather forecast of 1000 feet and 2 miles visibility.
Per AIM 1-1-20 the RNAV approach in our example is authorized as an alternate. We may file the airport with only a GPS approach as an alternate provided the weather minimums meet the non precision alternate minimums of not less than a ceiling of 800 feet and visibility of 2 miles.
Example 1 :
Lets plan an IFR flight from KORL to KLAL in an aircraft with a WAAS certified GPS, no ADF and no DME. The weather forecast for KLAL is ceiling 200 overcast with 1 mile visibility. We decide to use KTPF as our alternate because the weather forecast is 1000 overcast and 2 miles visibility. Since KTPF has a GPS approach and we have a WAAS GPS, we can use the alternate minimums off 800 foot ceiling and 2 miles visibility.
Instead of filing the alternate in Example 1 we decide to use KBOW as our alternate. Lets assume the weather at KBOW is forecast to be ceiling 800 and 2 miles visibility. This is one case where we can file KBOW only if we have a WAAS GPS since if you look at the alternate minimums only the GPS approaches have minimums defined and the VOR Approach is not authorized as an alternate.
Using the same flight as above we decide to use KGIF as our alternate. Weather at KGIF Is forecast ceiling 800 visibility 2 miles. Again we can use this as an alternate because we have a WAAS GPS.
If we did have a VOR and DME in the aircraft we could have used KGIF as an alternate if our GPS was inoperative, but since we don't have DME we can only file as an alternate based on the published GPS approaches.
A WAAS certified GPS has enabled us to get into airports we could never have been able to fly to in the past. Legacy navigation systems including non directional beacons were the only way to fly into small and remote airports for many years. With higher minimums than WAAS we were many times unable to get into our destination because of the low ceilings thus diminishing the utility of many airports. Only 10 years ago I remember having to divert on many occasions because I could not get into my home airport. With WAAS GPS I am able to make it in more often.
The WAAS GPS also gives us more options for filing alternates especially if the airplane we are flying has no DME either because the equipment is inoperative or removed from the aircraft. With that said we must remember to comply with the rules concerning filing alternates and we must also comply with additional requirements concerning RAIM, which is a topic of an upcoming article. Also remember if you do not have a WAAS GPS different rules apply for finding an alternate, which I will discuss in a future article.
If you have invested in a WAAS GPS you have increased the utility and safety of your aircraft. With WAAS you will be able to land at more airports and choose more airports as alternates.
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