Flying WAAS GPS Approaches
When flying a WAAS GPS approach, there are several different levels of WAAS signal that a GPS receiver can get. The most precise is an LPV signal. LPV stands for Localizer Performance with Vertical guidance. An LPV approach has the lowest minimums of all the WAAS GPS approaches, typically in the range of 200-300 feet AGL. A GPS glide path (GP) is guaranteed with an LPV approach and the minimum altitude is a decision altitude (DA).
Just like a localizer, an LPV course width get’s tighter and the CDI becomes more sensitive the closer the plane get’s to the runway. Even though the LPV approach minimums are so low and the approach is down to a DA, they still aren’t considered precision approaches by the FAA (which leads to some extra planning when selecting an airport as an alternate that only has GPS approaches, since the AIM specifies only the LNAV minimum are to be considered if an alternate airport only has GPS approaches, bringing the 800 foot ceiling requirement to bear)
An LNAV/VNAV approach is still a WAAS approach that has a GPS glidepath, but is slightly different than an LPV approach. An LNAV/VNAV final approach course does not get more sensitive the closer the plane gets to the runway. The smallest course width on an LNAV/VNAV approach is 0.3 miles either side of center. LNAV/VNAV approaches will, most of the time, have higher minimums than LPV approaches and can have minimums no lower than 250′ AGL.
The third type of WAAS approach is strictly a non-precision approach with a Minimum Descent Altitude (MDA). These are designated LP approaches, which stands for Localizer Performance. These are like old school Localizer only approaches that, similar to the lateral portion of an LPV approach, the course width tightens the closer to the runway that a pilot is. There is no glide path by definition of an LP approach, though there is a caveat.
Now, by looking at an approach plate that is a WAAS approach, but only has LP minimums listed, a pilot would assume there would be no glide path. Depending on what type of GPS unit the airplane has, that pilot could be wrong. Garmin Perspective units (Cirrus G1000), all GTN 750s and GTN 650s, All G1000 NXi units, most Garmin 430W and 530W, and all Avidyne IFD 550/540 and 440 units will display an advisory glide path on an LP approach, designated LP+V.
What does LP+V indicate? An advisory glide path is just advisory, but it is totally legal to follow down on a non-precision LP approach. The kicker is obstacle clearance is not guaranteed and the pilot needs to keep an eye on minimum altitudes at the different waypoints on the approach. The big thing I tell people is, when you arrive at the MDA and the runway is in sight, following the advisory glide path below the MDA could get you in trouble with obstacles. Don’t just hone in on your instruments when you break out of the clouds. Look out the windscreen and make sure you won’t hit anything.
If you arrive at the MDA on the advisory glide path and the runway isn’t in sight, DON’T GO BELOW THE MDA! Most autopilots won’t level off at the MDA, even if that altitude is set in the altitude pre-select, so this will involve turning off the autopilot before the MDA and manually leveling off, or engaging the altitude mode of your autopilot at the MDA.
One other type of GPS approach that you will encounter is an LNAV approach. This is a non-WAAS approach down to an MDA, but your GPS unit may still give you a +V. Most modern ones will.
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