The GTN 750Xi VNAV Conundrum

You have just pulled the trigger and upgraded your legacy Garmin GTN 750 and Garmin G500 to a brand spanking new Garmin GTN750Xi, Garmin G500TXi, and the Garmin GFC 600 Autopilot. You are amazed at the processing speeds, the bigger font, the better pixelation. All Garmin promised has come true in your airplane.

The chapters and pages of the Garmin GTN750Xi all appear to be the same as your old legacy Garmin GTN750, so you decide you don’t need any extra training. Everything appears to be in the same place, the autopilot seems pretty straight forward, so you feel comfortable.

A few weeks after you pick the plane up from your avionics shop, you have a business trip to go on and the ceilings are 900 feet. No big deal, as your personal minimums are 700 feet. You file your flight plan and, since the ceilings are forecast to stay below 2,000 feet for most of the day, you file your required alternate airport (remember the 1, 2, 3 rule?). The flight is about 1.5 hours long and appears to be relatively simple and straight forward. No convection or icing along the route.

As you get close to your destination, you get the ATIS and load the LPV approach to the runway you have selected just like you normally do. ATC clears you direct to the initial approach fix (IAF), so on your Garmin GTN750Xi, you tap the Direct To key on the side of the unit, tap the IAF, then tap Activate. . Your Garmin GFC 600 Autopilot is still in NAV mode, so the autopilot navigates the airplane to the IAF.

After you complete your approach and before landing checklists, ATC clears you for the approach, so you push the APR button on the Garmin GFC 600 to arm the autopilot approach mode, see the white GP symbol display in the autopilot scoreboard at the top of the Garmin G500TXi PFD, and sit back to watch the airplane fly the approach.

Then, you notice something a little different. The Glidepath indicator displays to the left of the altimeter, but the Glidepath diamond, which you remember seeing as magenta on your legacy Garmin G500 PFD, is now white. The VNAV indicator is also showing up, but it is magenta instead of the GP being magenta. You ignore the VNAV indicator and wait for the Glidepath to turn magenta. While you wait, the VNAV indicator goes through center and continues down while the Glidepath continues to stay white.

You begin to get a little nervous because this is different from your legacy Garmin G500 and Garmin GTN 750. On those units, the Glidepath diamond turned magenta immediately when it started to center, not white. The VNAV indicator also never showed up on the LPV final approach course.

The white Glidepath diamond centers on the indicator, at which point you expect the Garmin GFC 600 autopilot to capture the Glidepath, thus beginning the descent on the approach. Alas, this does not happen. The Glidepath indicator remains white, the autopilot remains in altitude hold, GP on the autopilot scoreboard didn’t capture, and the glidepath is quickly moving down, away from center.

By the time the glidepath gets to one dot below center (the airplane now being above the glidepath), you realize the autopilot won’t be following the glidepath. You are really sweating now, but your training kicks in. You disconnect the autopilot, configure the airplane for the approach, then push the nose over to hand fly the approach. You aren’t really stabilized, but you eventually manage to recenter the white Glidepath diamond. Eventually, it turns magenta, allowing you breath a small sigh of relief, but you don’t turn the autopilot back on because now you don’t trust the system.

Once on the ground, you pull out your cell phone to call your avionics shop. They aren’t much help as they said they installed all the wiring correctly. Your next call is to your trusty CFI, who has flown the Garmin GTN750Xi system extensively. Thankfully, he is familiar with this exact scenario and educates you over the phone with how to change your normal approach button pushing sequence now that you have the Garmin GTN750Xi.

For those of you who have experienced the above scenario, here’s a little insight into the Xi system. Garmin wants the pilot to follow the VNAV on the Final Approach Course (FAC). Because of this, if the VNAV is not tracked on the FAC, the Glidepath will not turn magenta and the autopilot will not capture the Glidepath.

So, the normal operating practice with a GTN750Xi and Garmin GFC 600 Autopilot while flying an approach is to engage VNV on the autopilot prior to crossing the IAF, then set your altitude selector for the Final Approach Fix (FAF) altitude. When you are cleared for the approach, still engage the APR mode of the autopilot. Then, the VNAV mode of the autopilot will step the airplane down to the FAF, where the Glidepath diamond will then turn magenta just before it centers, allowing the autopilot to capture the Glidepath and start descending.

If your airplane has the Garmin G1000NXi, most of the Garmin G1000NXi factory installs (not a retrofit upgrade to a legacy Garmin G1000) has the same programming, so follow the VNAV on the approach in those airplanes too.

If your airplane has a GTN750Xi but no Garmin Autopilot, then the 750Xi should have VCALC turned on, not VNAV. If you are seeing a VNAV indicator on your PFD and seeing VNAV on the Utilities page, have your avionics shop switch the unit to VCALC. Only Garmin autopilots can use the VNAV function of the 750Xi.

Finally, if you have a GTN650Xi and a Garmin autopilot (GFC 500 or 600), this scenario also applies.

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