VNAV transition to RNAV(GPS) Approach Demo

A short video of VNAV and approach capabilities of the Garmin GFC-500 autopilot coupled with a GTN650Xi navigator, displayed on a G3XTouch.

[@locustvalleystring]

Busy environment. You did a good job finding the words to describe events as they unfolded. Thanks.

[@ericpauley5246]

Great video. One note: even though the GTN is giving an advisory glidepath it is still fundamentally an LNAV approach. This means that the airplane must not descend below the MDA when going missed, so if you follow the glidepath the missed needs to be executed before the MDA to account for the ~50ft of altitude overshoot.

[@AV8BS]

Good point. I just went back now and watched as we reached minimums, and you’re absolutely correct: we busted minimums! Either the TO/GA button should have been pushed earlier or we should have manually intervened with ALT prior to mins, and then executed the go around. Thanks!

[@TGraysChannels]

Good job. Thanks

[@simonhughes-king8493]

Excellent. Rare to see the G3X and GTN used in such a comprehensive way. You defined the alt of 1000ft as a "do something" trigger? ie At 1000ft , then go next wpt (MOSSE)? And I see that you can also sequence the hold at MOSSE... Comprehensive indeed.

[@AV8BS]

1000’ is not in the profile. I referred to the “bottom altitude”, which is the lowest altitude to which we use VNAV. In this case, it’s 1800’ at PAGED. Prior to that we armed APR, Which engaged and followed the advisory glide path down to mins. If you select the bottom altitude and the intermediate altitudes are all hard altitudes (“at”), then VNAV will honor each one.

[@ebrinn2008]

I have a G3X, GNX 375 , and 507 auto pilot and have looked for simple explanations on how to use this system correctly. You by far do the best job!!! Any ideas for more training like this. Do you have a simple sequenced step by step check list or procedures such as approaches VNAV and the like. Thanks

[@AV8BS]

Hi Danny, Thanks for the kind words. I too have been frustrated by the lack of decent training material available. Especially frustrated by Garmin for not having quality training stuff. So, sorry - I can't recommend anything more. As to step-by-step stuff... for VNAV, there are three very important components prior to starting a VNAV descent: 1. Make sure that each desired arrival altitude (at each fix) is a hard "AT" altitude i.e. not "at or above" or "at or below". I recommend you open the altitude window for each fix and verify. 2. Set the "bottom" altitude, so... regardless of the number of step-down altitudes that you wish to make, put the lowest altitude (for the last fix - probably FAF) in the altitude select window (right above your altimeter). 3. Don't forget to arm VNAV....and then, of course: MONITOR. And as to approaches, other than your usual preparation, you now need to set baro mins and set the Missed Approach Procedure altitude - (ideally after you have armed APR) but prior to actually starting down. One other general note is to ALWAYS verify on the "Scoreboard" - mode annunciator - that you got what you asked for with your A/P selections. Something as simple as selecting a VS. Look up there and make sure it's actually in VS at the desired rate. It's just a habit that you MUST develop. I'm a big fan of talking out loud - even when alone.

[@CliffordStaley]

After a comment on agreeing with the poor Garmin of garmin, I have a question. When loading up your flight plan are you loading and then using vnav? When do you activate the approach. I didn’t see that? Are baro minums required in all instances. I have flown a step down to the faf but then nothing happens even when the alt select is lower. I’ve been assuming if the approach is in and you see the runway as a final waypoint, the approach is active, but it may be Garmin’s stupid logic. I’ve technically flown several legs on the approach with step downs, but it won’t activate the decent to runway because I didn’t activate it? Am I missing something?

[@kirbyclone3293]

Interesting. Pretty advanced avionics, yet you are doing the old dive and drive rather than just descending continuously on a VNAV path to DH? Will the system not draw out a continuous descent to honor all the crossing restrictions and arrive at DH (and then the runway) in the correct place?

[@AV8BS]

VNAV only takes you to the FAF, no further. There’s no dive and drive here, a term used for hurrying down from FAF to MDA. (Not DH). On this non-precision approach, the A/P followed the advisory glide path down very evenly. The VNAV predicates its descents from one fix to the next on a set vertical speed, not on a continuous descent. Some fixes are far enough apart that you could be descending very slowly, say 200 fpm. So instead, it waits until you are at a point to descend at that preset VS, then starts the descent to arrive at the programmed altitude.

[@kirbyclone3293]

Yes, you are right, dive and drive usually refers to from the FAF inbound. But, what I was getting at is all of the level offs outside the FAF. Also, no VNAV available inside the FAF, did I understand that right?

[@AV8BS]

@@kirbyclone3293 RE: the level offs outside the FAF, the VNAV system uses a preset vertical speed and then waits to begin descent using that VS to arrive at the next fix at the proper altitude. VNAV is not used on an approach to LNAV minimums. APR mode on the autopilot armed then captured the GP - advisory glide path - and flew it down at a constant rate.

[@glendavis1266]

For the uninitiated it’s all Greek. I have not see a full video that throughly explains how these pilot avionics work.

[@AV8BS]

VNAV computes a top-of-descent point, based on distance to waypoint, groundspeed, and predetermined rate of descent. Then, provided you have armed and enabled it, VNAV will descend at the appropriate point so as to arrive at the waypoint at the altitude. That’s it in a nutshell.