It's beginning to reach that time of year where we transition into Spring, meaning one really important thing:
Not that we don't experience low IFR throughout other seasons, but as we transition into warmer temperatures, the temperature/dewpoint spread likes to stay close. This is especially true for nighttime and early mornings until the sun comes out and burns everything off. So now is the perfect time to read up on approaches and make sure you're ready for it!
Specifically, RNAV/GPS approaches. Thanks to the invention of WAAS (wide area augmentation system), these GPS approaches are becoming more common.
GPS approaches are also highly accurate because they require something called RNP-required navigational performance. RNP means that the needle when centered for the course is within .3nm of runway centerline 95% of the time. So next to an ILS approach, GPS approaches can get you some pretty low minimums.
When it comes to understanding GPS approaches, there are a lot of terms and acronyms to know that can be confusing at first. Understanding them, however, makes for a better IFR pilot! So let's discuss and break these down:
- DME: you've probably already heard and remember this term but if not here's a refresher. DME stands for distance measuring equipment. Notice at the bottom of the KHOU chart above categories you see numbers in nautical miles, there's your DME! So you can also identify each fix by their distance.
- LP: Localizer performance. Remember earlier we talked about WAAS? Well, this requires WAAS and is a mode independent of LNAV AND LPV. The above plate doesn't depict it but it would be the equal counterpart of having localizer only on an ILS approach. Higher minimums but still more sensitivity as you reach closer to the runway area.
- LNAV/VNAV: This is more commonly seen than LP. It has higher minimums than LPV but can still bring you in pretty low on an approach with great accuracy (you won't break out 30 degrees off the centerline). What you should understand about this is it is horizontal and vertical guidance down to minimums. They however are not flown down to an MDA, but a decision altitude. Meaning look outside at this altitude and decide if you're landing or not! The quicker you can reach this (while still being stable), the better. Don't forget to also take a look at baro-VNAV temperature notes. This can raise minimums and get you in a pickle if not adhered to as your airplane has to abide by these corrections (I say get in a pickle...the possibilities of what can happen can truly be unsafe).
- LPV: my favorite type of GPS approach minimums. LPV stands for localizer performance with vertical guidance. In reality how I picture this is it is the next best thing to an ILS approach, but still NONPRECISION. But why is it nonprecision? Well LPV minimums are the lowest of all GPS mins and you must have WAAS onboard. Unlike a localizer, the sensitivity does not increase as you become closer to the threshold. Instead, it caps to linear scaling 700 feet wide AT the threshold but will not become any narrower. They are very operationally similar to an ILS and are flown to a DA just like LNAV/VNAV, but are far more economical because no navigation infrastructure is required at the airport.
- Baro-VNAV: and here's one of my least favorite types of GPS approaches. This stands for barometric vertical navigation (that's a mouthful). From what we mentioned earlier, it can constitute for sticky situations if not compensated for. Think of your pitot-static system when you think of baro-VNAV, because that's exactly what it relates to. It uses approach-certified baro-altitude information from the pitot-static system and air data computer to register the vertical guidance.
- Pictured is a circled noted area on the RNAV 35L Z approach into KOKC. Notice how it gives you temperature restrictions that make the procedure NA? That means don't even try it.
- GBAS: last but not least is ground based augmentation system. GBAS does what is called "augment" the GPS, meaning it provides corrections and improves navigation. This is very much a precision approach. You will also see this termed as GLS, which the FAA uses as GBAS landing system. You likely have never heard of GBAS or shot a GBAS approach, and this is because it is only in use by several airlines around the world. The way it works is by using a 5-digit channel (similar but don't get it confused with WAAS) to tune into the FMS for better accuracy.
There are a few other terms this post does not cover, examples including LNAV+V or APV type procedures. There is a lot to know about RNAV/GPS approaches but this covers the most commonly used and also the most commonly covered on instrument check rides. Stick around for a post in the future digging into the rest of this information. The more educated we are, the safer instrument pilots we become.
Note: all information here was derived from the AIM and FAA published instrument handbooks. These are subject to change over time so please ensure you keep your materials updated!