En route, you can check RAIM if you’re planning to fly to LNAV minimums. But RAIM can’t tell you if you’ll have sufficient signal quality to fly to LPV minimums, so there’s no need to check it if you plan to fly to LPV or LNAV/VNAV minimums. Incidentally, if you do a RAIM check and it fails, you may want to play the lottery.
The current GPS availability for nonprecision approaches in the continental United States is more than 99.99 percent, meaning on average, if you check RAIM 10,000 times, you’ll fail to get RAIM only once!
But don’t be surprised if there’s insufficient signal quality to fly to LPV minimums. Along the East and West Coasts of the contiguous 48 states, availability can be as low as 99 percent, meaning that for one out of every 100 attempts, you won’t be able to fly to LPV minimums. But the good news is those outages are very short, usually no more than 15 seconds. So, if you do arrive during one of those outages, you may want to get vectored around to try the approach again. In Alaska, signal availability to fly to LPV minimums is significantly lower.
En route, brief the approach by reviewing the chart. Verify that GPS is in the title of the approach and not RNP (required navigation performance), because those approaches cannot be flown with a WAAS-capable GPS. Then set all frequencies including the one for weather at your destination.
Before loading the approach in the GPS, determine whether you’ll fly the approach using vectors, during which the controller issues a series of turns onto the approach, or via pilot navigation, where you fly the approach from a starting point without guidance from the controller.
Note that with some GPSs, loading an approach with vectors deletes all but the last few waypoints on the approach. In the flatlands that won’t matter, but in hill country I recommend that you not load an approach with vectors, even when being vectored onto the approach. Instead, load the approach with an initial approach fix (IAF) that produces a straight-in approach without a course reversal. Then activate the leg of the approach the controller is vectoring you toward.
One common mistake I see is failure to check and set the “kill switch.” That is the switch that determines whether the CDI or HSI course needle is displaying course guidance from a VOR receiver or a GPS. Leave it in the VOR position when flying a GPS approach and you could get killed.
Once you’ve properly set up your equipment, flying to LPV minimums is like flying an ILS. As you intercept the glidepath, reduce power and slightly lower the nose to descend. Keep the needles centered as you would for an ILS, and when you reach the decision altitude listed on your chart for LPV minimums, decide whether to land or to fly the missed approach. Don’t continue flying level at minimums while looking for the airport. If you don’t see the airport upon reaching LPV minimums, you must immediately initiate a climb.
Before flying an approach to LPV minimums in poor weather, practice doing it in clear skies. After all, the life you save may be your own!
Max Trescott is the 2008 National Certificated Flight Instructor of the Year. Based in Palo Alto, California, he publishes aviation books and software, including Max Trescott's GPS and WAAS Instrument Flying Handbook
