We live in a day and age of amazing aviation technology. 20 years ago, GPS was brand new to the GA marketplace and Avidyne had a cutting edge full glass panel running on Windows ’98 technology. Garmin was still a year out from debuting the G1000.
Fast forward to 2023. We have GPS approaches that give pilots the same precision as an ILS approach, so much so that GA pilots will almost always pick the RNAV approach over the ILS approach at any given airport. NDB approaches have (thankfully) gone the way of the buffalo, while VOR and LOC only approaches are usually only performed on Instrument Proficiency Checks and check rides. We even have glide paths on all straight in GPS approaches.
Correction, almost all straight in GPS approaches.
And there’s the catch with WAAS (Wide Area Augmentation System) GPS approaches. In certain Garmin G1000 equipped airplanes, pilots have to be very careful in the approach briefing to identify if an approach is a WAAS LPV (Localizer Performance plus Vertical) approach or a WAAS LP (Localizer Performance) approach.
In most pre-G1000 NXi GA airplanes (Cessna, Columbia, Beechcraft, Mooney, Piper, there may be a few others, but Cirrus is definitely exempt from this list), when flying an LP approach, the G1000 does not give an advisory glide path. In fact, the G1000 doesn’t even recognize the WAAS signal. When the pilot loads an LP approach, a careful study of the flight plan will show that the G1000 is receiving the LNAV (Lateral Navigation) only signal, not the LP signal.
What does this mean? Pilots are used to flying GPS approaches that have glide paths, whether an LPV glide path down to a DA (Decision Altitude) or a non-WAAS LNAV + V advisory glide path down to an MDA (Minimum Descent Altitude). The pilot brain then becomes trained to assume a glide path will be on any GPS approach. Hence, when the GFC 700 autopilot turns the airplane onto the final approach course and the pilot has already pressed the Approach button, the pilot is assuming the autopilot will follow the glide path.
But there isn’t a glide path. Then, there is a flurry of movement very close to the Final Approach Fix (FAF) when the pilot realizes the airplane didn’t descend, is 900 feet too high, attempts a steep dive (with or without the autopilot) to reach the FAF altitude by the FAF. The pilot arrives at the FAF not configured properly, traveling at too high of a speed, and may or may not be at the proper altitude, very unstabilized. Sound familiar?
How to avoid this? This is where the approach briefing becomes very important. On the approach plate below (RNAV 15 at the West Houston Airport, KIWS), two things should stand out. First, this is a WAAS approach. It shows that in the upper left hand corner of the approach plate. But, what type of WAAS approach? This is the second item, probably the most important item. In the minimums section, the most precise minimum is an LP minimum (note: not LPV!). Alarms should now be going off in the pilots head. “Alert, Alert, possibly no glide path, Alert, Alert.”
How to confirm there will be no glide path, even though this is a WAAS GPS approach? When loading the approach into the Garmin G1000, take special care to note what GPS signal the G1000 is receiving. In the picture below, the approach options for LAX are being displayed on the Load Approach page. As you can see, the GPS signal that the G1000 is receiving is displayed next to each RNAV approach. For example sake, let’s pretend the RNAV GPS Y 06R is an LP approach instead of an LPV approach.
When loading an LP approach in the aforementioned G1000 equipped aircraft, instead of seeing LP next to the RNAV GPS Y 06R, the G1000 would actually display LNAV (note the absence of the +V). This is the final indicator that there will not be a glide path available for the approach. The G1000 will give VNAV guidance (that’s for another Glass Panel Minute) down to the FAF, but that will disappear after the FAF and the pilot will have to resort to VS mode on the autopilot.
Pro tip: make sure that you are using the correct minimums for the GPS signal that the airplane is receiving. On the above RNAV 15 at KIWS, if the G1000 is using the LNAV signal, the minimums are 580 on the approach, not 540.
Why this was programmed this way and never updated on these airplanes? I have no idea. The odd thing is, on a non-WAAS LNAV approach, the same G1000 airplane will give a +V advisory glide path, just not on a WAAS LP approach. Also, Garmin 430/530 WAAS units, Garmin GTN 750/650 units, and Avidyne IFD units all receive the LP signal and give a + V advisory glide path. Just not the early G1000.
Frustrating? Absolutely. Now, though, you’ll be prepared for the next time you see LP minimums at the bottom of an approach plate. Go fly with a CFII and request a couple of LP approaches to get some practice.
This is one of many gotchas on the Garmin G1000. At The Aviator’s Academy, we understand the depth of the Garmin G1000 and strive to give pilot’s the proficiency they want to fly with confidence. Take our Garmin G1000 Proficiency Course today to become proficient with your Garmin G1000.