We all have our standard routines when it comes to flying an instrument approach. These vary from airplane to airplane in regards to power settings, speeds, when to gear and flaps down, and when the pilot expects the glide path to go active on the Final Approach Course (FAC). Typically, on an RNAV approach, once the airplane turns onto the Final Approach Course, the glide path indicator will display and the WAAS glide path will display at the top of the indicator. As the airplane moves closer to the Final Approach Fix (FAF), the glide path moves toward center. Once the glide path centers, then the autopilot GP/GS mode goes active, if the autopilot is engaged, and the airplane begins to follow the glide path. If the pilot is hand flying, once the glide path centers, then the pilot pitches down to follow it. Simple, right?
This is how most RNAV approaches around the US are set up. One thing to note, which will become relevant below: on an RNAV approach, the WAAS glide path is programmed to display/go active only when the FAF is the active waypoint.
On a typical RNAV approach in the US, there is an Initial Approach Fix (IAF), an Intermediate Fix (IF; this starts the FAC), and a Final Approach Fix (FAF). Take the RNAV 17 approach into Sugar Land, TX (KSGR; see below). The approach has the familiar T-shape to it, with BASGE and DIZGY as the IAFs, EBOKE as the IF, and CASOB as the FAF. Once an airplane turns onto the FAC at EBOKE, the RNAV GP indicator will display and the glide path will become active (not alive, meaning it’s not moving yet, only active in the fact that the avionics are getting the signal). As the airplane moves closer to CASOB, the glide path will start moving toward center (come alive) before centering, then the autopilot (if the proper buttons are pressed!) will capture the glide path and the airplane will start descending (exception for G1000NXi and GTN 750Xi/650Xi units. On these units, the VNAV is prioritized, so always follow the VNAV on the FAC).
There are a number of RNAV approaches around the US that are set up a little bit different. It takes an observant pilot who carefully briefs each approach to catch the difference. Let’s take the RNAV 03 at the Sedona Airport (KSEZ) for example. There are two IAFs (COTSA and DRRTY), the IF (HUKIN), and the FAF (BARME), but there is another fix in between HUKIN and BARME, EXUTY. Is that relevant? It most certainly is!
Why is it relevant? I’m so glad you asked! Remember above where I said that the FAF has to be the active waypoint in order for the glide path to become active? On the SEZ approach, if the pilot elects to stay at 9500 feet cross HUKIN, waiting for the autopilot to capture the glide path, it won’t happen and the airplane will be 1500 feet high at EXUTY (the altitude at EXUTY is 8,000). Because of the WAAS programing, the glide path does not become active until after crossing EXUTY because the final approach fix is not the active waypoint until then.
Let’s look at another approach, the RNAV Y 13R at the San Antonio Airport (KSAT). All the approaches at SAT were updated in January 2024 and have taken a little getting used to. Looking at the approach plate, there are a lot of different fixes, but I want to focus in on the IF and the FAF. The IF is NCLDA (if you’re from San Antonio, you know this is pronounced Enchilada) and the FAF is ALAMO. Looking closer, NUGIT pops out as an extra fix between the IF and the FAF. What does this mean? The glide path won’t become active until cross NUGIT!
One more example before I go into the solution, the RNAV 31 at the New Braunfels National Airport (KBAZ). NUMMO is the IF, TIVYO is the FAF, and sneaking in between the two is the FEKNI intersection at 2,500 feet. Pop quiz, will the glide path become active after crossing NUMMO? Answer, no! Not until crossing FEKNI.
The main reason these extra fixes are present is that there is something below that fix that the designers of the approach want airplanes to stay a certain altitude above. It could be a tower, it could be hills or mountains, it could be a variety of different things. Pilots cannot descent below the extra fix altitude until the fix is crossed.
So how are these approaches flown? Great question! If the pilot has a VNAV equipped airplane, make it a habit to always follow the VNAV on an RNAV approach. The altitude at that extra fix will be in the flight plan when the approach is loaded, causing the system to calculate the VNAV and display a Top of Descent (TOD).
If the airplane isn’t equipped with VNAV, then simply descend to the published altitude of that extra fix. In the case of the SEZ approach, that’s a pretty steep descent rate to go from 9,500 to 8,000 feet in 4.5 miles, which means the pilot has to carefully brief the approach. On all of these approaches, if a descent isn’t initiated to the extra fix, when the airplane crosses that fix, the glide path will activate, but it’ll be way below the airplane.
There are not many approaches like this in the US, but there are certainly enough to keep pilots on their toes. The main takeaways are: if the airplane is equipped with VNAV, always follow the VNAV on an RNAV approach. If it’s not, carefully brief each approach so that you will catch the extra fix and know that you will need to descend to the altitude at that extra fix, or risk being high when the glide path activates.