Charles, a 2000+ hours ATP asked the following:
“Hi, I have a question about GPS approaches. Why are LNAV and LNAV/VNAV minima different on RNAV GPS Approaches?”
Max Trescott - Author and 2008 National CFI of the Year
Great question Charles. Things are changing so fast in the GPS world that it’s hard for instrument pilots to keep up with all of the new acronyms and just what they mean. But you want to understand them before you’re stuck in the clouds, furiously twisting knobs, while hoping to avoid an encounter with a cumulo-granite cloud (Ouch!).
I’m going to be self-serving–since it will save me a lot of typing–and quote directly from two of my books and CDs. Many readers may not be familiar with LNAV/VNAV minimums for a GPS approach, so let me quote from my new Max Trescott’s GPS and WAAS Instrument Flying Handbook (which I might add would be a great Christmas gift for pilots to buy now for themselves!):
“As previously mentioned, airliners with special equipment fly to LNAV/VNAV minimums and now you can too with a WAAS-capable receiver. From a practical standpoint, however, you’ll probably use these minimums only on the handful of approaches for which no LPV minimums are designated, since LNAV/VNAV minimums are almost always higher than LPV minimums. In very rare cases, LNAV/VNAV minimums can be lower than LPV minimums, due to differences in how obstacles are evaluated for these approaches. So surprisingly, a GA pilot’s WAAS-capable receiver that can fly to LPV minimums is far more versatile than many airliners’ equipment, which cannot go lower than LNAV/VNAV minimums. Like LPV minimums, LNAV/VNAV minimums are specified with a DA or decision altitude.”
Charles, you wanted me to contrast that with LNAV minimums. Let me briefly explain to readers that LNAV, which stands for lateral navigation, is the tradition minimums to which we’ve always flown GPS approaches. Originally there was only one minimums type for a GPS approach, so those minimums didn’t need a name. Now that there are 5 different minimum types available for GPS approaches, the original ones needed a name so they’re called LNAV. LNAV minimums are basic, non-precision minimums. Just fly to the MDA (minimum descent altitude) specified and don’t go an inch lower than that altitude!
Now we need to know how what pilots call the “protected area” differs for LNAV/VNAV and LNAV minimums. This time let me quote from the audio track of one of my computer courses, Max Trescott’s WAAS and GPS CD-ROM Course (another great gift idea for a pilot to get him or herself for Christmas).
“Let’s talk briefly about the protected areas for LNAV/VNAV approaches. First, the horizontal protected area is the same as that used for constructing a LNAV approach, so the equipment required to fly these approaches only has to meet the TSO-C129a, which is the standard for non-WAAS capable receivers. Since the navigation will therefore be less accurate, larger protected areas are needed than for an LPV approach.”
Now we know that the horizontal protection areas are the same for LNAV/VNAV and LNAV minimums. So essentially your question would be identical to “Why are the minimums different between an ILS and a localizer approach.” The answer is of course the glide path (or in the case of an ILS, the glide slope). Non-precision approaches resemble the steps in a flight of stairs and pilots can immediately descend down to the next altitude at each step. But the glide path of a RNAV (GPS) approach more close resembles the handrail for a flight of stairs. It provides a smooth, continuous descent path. The glide path can be designed to avoid obstacles that might force a higher MDA for a non-precision approach. So the answer to your question is that LNAV/VNAV minimums are usually lower than minimums LNAV minimums since it incorporates a glide path, helping it avoid some obstacles.