So I think I have a fairly good understanding of how ILS approaches. For RNAV, I thought I’d know at least the basics, but maybe not so any more…
I always assumed that, contrary to ILS where you have the localizer at or near the runway sending out a signal to the airplane avionics that guides the plane down along the glidepath, with RNAV there is no such thing. My understanding was that it is solely the GPS unit in your airplane that, given you entered a specific RNAV approach into the unit, it will calculate the glidepath to the runway based on the altitudes defined for the approach and guide the plane down.
Well, yesterday I did an RNAV approach with the TBM where I actually created the flight plan by including the waypoints/fixes of the published RNAV procedure as part of the flight plan itself (Pilot2ATC has this export feature). So to be clear, the FMS in the TBM showed all the approach fixes as Enroute waypoints, and no approach was selected. I flew the whole flight with autopilot on, and it proceeded nicely from waypoint to waypoint. When I got to the IAF waypoint I manually configured an appropriate descent rate using VS, but also switched the autopilot from NAV to APPR just out of habit.
And a few miles in all of a sudden the GP indicator switched green, the plane captured a glidepath and the autopilot brought me all the way down to the runway, as if I had actually configured the approach in the FMS (fwiw, I did not see any glide path ball indicator in the PFD)…
I was baffled. How can that be?? Was I wrong all along and there is actually a signal that the avionics pick up for vertical guidance for RNAV, or is this just MSFS doing some weird thing and this would not happen in the real world?
In the real world, you can have something called “LPV”, which is a category of RNAV that’s precise enough that the computer provides an imaginary glideslope to follow. This glideslope is 100% calculated based on GPS position and altimeter reading, and not based on ground signals. The prerequisite for that, however, is WAAS capability which allows the GPS signal to be more accurate.
In the sim, I suspect it’s possible the ILS frequency was conveniently already tuned.
The short answer is modern GPS units have a feature called WAAS that gives them 2-3 meter accuracy. On an RNAV approach the autopilot uses the GPS to get both lateral and vertical guidance.
I have never been able to capture the GS on a GPS approach, and had concluded weeks ago that WAAS is NOT simulated in any of the (stock) Garmins in this sim - ie. that vertical guidance will never be provided to the AP for a RNAV approach. Sounds like the OP’s experience (tentatively) disputes that. Can anyone here definitively dispute it?
I just cutoff my engine at an Airport witch has RNAV approach only. Got a visual approach from ATC. Not for the first time, I never completed a RNAV with the TBM so I believe is not implemented so far.
Thank you for asking this, because I have also been able to lock onto a GP while doing an RNAV approach.
For those of you who have more knowledge about this, is the way the sim treats these RNAV approaches realistic? That is, should you be able to expect to use WAAS as long as you have a fancy Garmin unit in your plane?
If you are on final and using WAAS to decend toward touchdown, is this considered a precision approach? Should you still switch off Autopilot around minimum to do a manual landing?
Thanks guys, that makes it very clear and confirms that my initial understanding was indeed correct
Actually, now that you mention that - I’m not sure any more. As I said I did not see the usual glide slope ball indicator in the PFD, but I did not pay attention whether the ILS frequency was dialed in and the AP automatically switched the nav mode from FMS to LOC1. I’ll repeat that flight tonight and see if I can make that happen again.
So just to confirm - in the real world, if you do not enter the RNAV approach into the GPS, it would never ‘automagically’ figure out that the enroute waypoints in the flight plan actually map exactly to an RNAV approach in its database, and then use that approach for lateral/vertical guidance when autopilot is switched to APP, correct? IF this happened to me in MSFS (as said above it might have
just picked up the ILS frequency, I have to re-test this), it’s a purely fictional / unrealistic MSFS behavior?
I absolutely have flown several RNAV approaches before (the ‘right way’, by choosing the specific RNAV approach in the GPS/FMS) successfully, so vertical guidance is definitely provided. Not saying it always works, sometimes it doesn’t capture at all, sometime very late, but it definitely is somewhat implemented in MSFS.
Yes in the real world at least with the 530/430 navigators you must put in the RNAV approach. And then you have to make sure CDI output is using GPS and not VLOC. If you left it / put it on VLOC (the real GPS changes it for you) and you happened to have the correct ILS frequency dialed in - and that runway had an ILS you could be confused and actually fly the ILS when you meant to be flying the RNAV. Worse, if you had a wrong ILS frequency dialed in you would have a mess on your hands.
It might be helpful to download the Garmin manuals for the navigator you are using. They are full of helpful examples.
RNAV Approaches work. This is RWY 7 into Westerly State (KWST). The approach takes you all the way out over Montauk Point on Long Island NY. Note the source is set to GPS and GS slope indicator is magenta. I’ve captured RNAV GS from as far away as 6NM from runway threshold.
It is. RNAV in this game can be inaccurate in terms of GS to the threshold. But so is ILS in some instances. As in real life, you have to watch the descent rate and judge if you’re on track. At times, you may have to disconnect AP and take over above MDA. For the most part, I’ve had one to two good slopes where I brazenly let it ride me down to 50’ AGL before I took over, cut idle and flared. Otherwise, I’m about at 150’ or so (nothing scientific or MDA enforced for that matter) when I feel the itching need to just take over.
Vertical guidance definitely does work but there are a couple of things to keep in mind. It has to be an LPV approach, you should see that category on the approach plate.
Also, from what I’ve encountered, the sim is a bit buggy and it will only follow the glideslope from the last waypoint before the runway. Some RNAV approaches have a final waypoint very near the end of the runway so it doesn’t appear to work if you were expecting it to follow from the normal intercept point on the plate.
Ok, did you flown with AP?
I just tested an RNAV Approach at ORD. It worked so far that i had LNAV and VNAV on my PFD.
My problem was, that the AP didn´t followed the VNAV aka. Glidepath. No chance to flown the apporach automatic.
Is it a bug or did i forgott something?
Thanks for that and thanks to other responders. Back to testing for me. Guess I’ll have to be more meticulous in observing specified minima. (I’m a novice.)
One thing that’s still not clear to me is whether an RNAV approach chosen in the world map translates properly to the approach you see in the Garmin procedures menu.
Just to confirm - you did switch from NAV to APR after passing over the Initial Approach Fix? I’ve had RNAV GS picked up from as far away as 6NM, but usually at around 3-3.5 NM, GS is captured and you’ll see descent rate kick in.
All - This fellow does a very good RNAV approach on this link. I have watched it several times to make sure I got it all down pat, which I don’t. I tried on into KMGM, and it did not work at all. I am creating my FP in the World Map, and therein may be the problem. I should learn about the FMC and do it in there like the video shows. Enjoy: https://www.youtube.com/watch?v=lrWAiXUfK9k&t=23s
Yes, I switched TBMs AP to Approach and with an ILS approach it start the descent right after the diamond come to the middle.
I had the flightplan added in the system and got the RNAV chart from Skyvector. I would guess, that the AP will fly the altitudes as the in the FPL which are 100% corresponding with the RNAV chart. But nothing happened. Just flown straight ahead.
So i descent on my own and as far as the purple diamond came near the middle position, activated the AP again and nothing. Straight flight, no descend.
I broke up because had something else to do. Maybe it’s Pilot error, because I have no experience with RNAV approaches. I guess, need to fly it again.
Anyhow, for a correct capture is it also necessary to to catch the diamond from below like ILS?
But but but…MSFS 2020 can’t do IFR, RNAV etc at all. Your video is fake news. MSFS is a video game, a glorified Google Earth Viewer, screenshot generator designed from the bottom up to attract the X-boxers. Only (insert names of some ancient flight sims here) can do IFR. MSFS is only for VFR! Also a reminder: this is only my opinion and it’s very very valid. Please don’t criticize me. (Edit: # sarcasm )
Hashtag sarcasm has now been added. Sorry for the confusion.
