Can someone explain to me how RNAV approaches actually work?

User Support Hub Aircraft & Systems
41

So I am not a real pilot but I have done many RNAV approaches in Zibo Mod and Toliss Airbus.
An RNAV approach does not use or intercept any signal from the ground like ILS. RNAV approach is all calculated using GPS way points. So basically you fly can in “Nav” mode and pass the Initial approach Fix. Before you get to the Final Approach Fix you should Press the APPR button. That will give you the vertical and lateral guidance (purple diamonds) on the PFD for the final approach. The Garmin computers calculate based off your aircraft position and the position relative to Approach way points. It knows if you are high or low, right or left of the runway. Then you only need to follow the guidance, lower your speed and drop the landing gear etc.

Greazer.

42

Actually WAAS is not necessary for vertical guidance during an RNAV approach. The minimum will be better, but even if MSFS doesn’t simulate WAAS, vertical guidance should be possible.

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You mean not all planes in MSFS with autopilot have autoland???

They sure do. Disable collision damage, cut the engine, done.

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44

If you have a GS, you are tuned into ILS. RNAV have a GP.

Does that runway maybe have it’s ILS on the default NAV frequency? Ok, checked, it is not default.

Not sure why it is showing GS. Did ATC maybe assign you that?

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Ok, tried it once more, but now with the RNAV 26L at EDDM. Works nearly perfect.
What i maybe forgott yesterday, to deselct the ALT Holding at the AP. Maybe thats the reason why it didn´t worked at KORD, even if i don´t need it for ILS appoaches.
BUT…short before touchdown, aprpoximatly 3 or 4 nm away from the threshold, the TBM suddenly rolled violently to the left. I had to disengage the AP. I stabilized the A/C and brought it back to the right position. It came without any prewarnings. GP was fine, direction was fine, speed was fine and i was fully configured for landing for a minimum of 20 or 30 seconds or maybe a minute before the event.
Strange behavior. :crazy_face:

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Sounds like your heading bug was not set for the runway bearing.

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the easyest way is:: first install a flightplan then look at a chart for your destination:: on the chart you will see what altitude you should be for your last waypoint:: usually there is an asterix for your last waypoint on the chart ,if your last waypoint should be at 1600 about half the way to your last waypoint descend your altitude to 1600 ,when you are at 1600 you will see the blue diamond going up and position to the center : if the diamond is down youre to high if if is high your to low.
no need to put any frequencies like the ILS. let it go and plane will begin to go down by itself if you are in AP auto pilot. let it go to the runway or disconnect the AP at about 500 and land manually. on the PFD you will see GS glide slope, that mean that youre on track to the runway. so in RNAV no need for a frequency like the ILS, fly the route , be at the exact altitude for your last waypoint and land, thats it dont follow the ATC because there is many issues with the atc and it will ask you to go up at 12000 about 3 miles from runway. do it without atc. simply with a flightplan and a chart. on the PFD let it to FMS dont change it to NAV or werever, by FMS youll go by GPS so let it like that. for RNAV APPROCHES the trick is:: be at the exact altitude for your last waypoint, and follow the blue diamond to the center, thats it.

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48

But why did it flown correctly up to the last few miles? I didn’t touched the heading bug because I guess I just need the purple gps arrow?!

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it flown correctly to the last few miles because maybe youre in AP mode.if you loaded the right flightplan you dont have to touch the heading bug if you are in AP autopilot mode. YES just the purple gps arrow, after you adjust your last waypoint altitude.
all that counts is :: AP mode, last waypoint altitude, and GPS mode.
if you look in the PFD were is the altitude, youll see a little diamond going up to the center, if you adjust your last waypoint altitude. then it will land by itself
or if you prefer in manual by disconnecting the AP at around 500 altitude.
DONT TOUCH THE HEADING BUG STAY IT IN AP MODE.

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Thats what i thougth and why i was totally surprised from the runaway.
I´m just at the sim. I will fly that RNAV once again. Will see if i can find out what went wrong the first time.

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if yoou clic on the heading bug all its doing is its going in the middle of the blue ilne and SYNC with the heading, even if you dont touch it it will go still straight because youre in AP.

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So, it´s done. I´ve just landed safely…my first completed RNAV. :grin:
Same RNAV as earlier today and even i was prepared now, no AP runaway. Straight and smooth all the way down.
After my last turn to FAF i switched ALT mode off and AP hold it on level anyway, because the purple diamond was in the middle and the APR Mode just control the Altitude with the diamond. Over FAF it started the desecent and i took over just 10 seconds before touchdown.
And i never touched the heading bug.
I don´t know what happend at the first time or at KORD yesterday. No clue. :man_shrugging:t2:

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yesss sir very happy for you, at first when we dont know how to do it it seems very difficult but its very easy. i have friends who are real pilots and they explain me how to do it.and since then never had any problems . and this apply also to all airplanes even in FSX OR MSFS2020, have a nice day

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if we clic on the heading bug it will synchronize with the heading going with the AP , but we dont touch the HDG button because we will go left or right and will not be on the route anymore.

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Why would you deselect ALT exactly? Moving the heading bug has no effect as long as you are in NAV (GPS /FMS green) mode.

Assuming the RNAV approach is activated in the Garmin, the correct way is to fly direct to the final approach fix in NAV mode (GPS / FMS green, CDI magenta) and in ALT mode (ALT green). You could elect to descent and pick-up the GP without level off if you want as long as you hit the correct altitude at or before the FAF.

Using Approach (APR) mode:

When inbound the FAF, select APR, this will arm the GP mode (GP in white on the Garmin if I’m not mistaken). When established on the final track its good practice to select final approach track on the heading.

At the FAF the GP mode becomes green and starts following the magenta GP. As soon as this happens you can select go-around altitude in preparation for a go-around.

  • Established on final approach track -> select runway heading.
  • Established on GS or GP -> select missed approach altitude.

And thats all, you don’t have to deselect ALT, moving the heading bug does not do anything as long as you are not in HDG mode.

Using Vertical Speed (VS) mode:

If you want to fly an RNAV approach in VS mode (manually adjusting the vertical speed to maintain the magenta GP), the autopilot won’t allow you to descent below your selected altitude. You should first “unlock” the altitude by turning the selected altitude up by 300 ft.

So lets say altitude at FAF is 2000 ft, as soon as you reach 2000 ft and ALT mode is green, select 2300 ft, this will “unlock” the altitude. The autopilot will still maintain 2000 ft, but it will now allow descent.

At FAF (or slightly before), select VS, select the correct rate of descent. As soon as you are 300 ft below your missed approach altitude you can safely select missed approach altitude.

Selected Altitude

Just remember this, the autopilot (and flight director) will NOT allow you to climb or descent passed your selected altitude except:

  • When using approach mode so GS or GP mode will trigger a descent from the selected altitude down to the runway.
  • When “unlocking” the altitude by dialing the selected altitude through the current altitude. Just try to dial up the altitude during a descent or down during a climb through the current altitude and you will see the aircraft won’t level-off.

Knowing this consider this case: if you are high on the approach and you end up high at the final approach fix the autopilot won’t intercept the RNAV glidepath before reaching the selected altitude and will instead level off! Same is true for an ILS.

In other words, make sure you intercept the GS or GP before reaching the selected altitude.

Hope it helps.

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57

If you dive-n-drive, (e.g., if there is no FAF), you would set MDA as selected altitude at that point. With CDFA, do you see a value in still setting MDA as selected altitude to avoid accidentally ducking below?

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Why would you use a MDA on a RNAV approach? It makes sense in situations where there aren’t adequate navaids to built an approach, since you don’t need anything ground based you won’t need to time, descent to an MDA until a MAPt etc. For CDFA you convert a LNAV MDA into a DA.

That said, in Europe almost all approaches are CDFA (Continuous Descent Final Approach) level-off segments are removed, there might be some exceptions still like an airport with only a locator and no DME, the so called singled beacon approach. In this case its normally a timed base turn, when established on an inbound track descent to MDA, level off and fly level until the MAPt.

With a commercial aircraft you won’t fly such an approach, I have flown for different operators and none of them allow non-CDFA approaches. if there is such approach its normal to calculate a descent point which gives you a 3 degree glidepath to the runway and use the MDA + correction as a DA instead.

The only scenario you could still end up in the situation you are describing is a circling approach. That is always a discussion. Some operators say set MDA and when leveled off with altitude capture set go-around altitude, on some planes you can’t use the VNAV / GP or whatever it is called in such a case as an approach using LNAV / VNAV is treated just as an ILS GS and the aircraft won’t level off anyway.

On the Embraer we use VS to follow the VNAV glidepath in such a case, might be type specific. Some say set go-around altitude and hit ALT at (or just before) MDA. There is something to say for both techniques, I would tend towards setting the MDA in such a case but be aware that in case of a go-around before reaching MDA you have to reset the altitude.

Also using the MDA procedure, should you forget to set go-around altitude during the circling, you can’t go-around using FLC or VS as the autopilot will not allow you to without resetting the altitude. Not normally a problem since the proper procedure is to use the go-around mode which will allow you to do this as its a basic pitch mode.

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59

If you can only fly LNAV minima for some reason there would be an MDA right? But yeah if you fly LNAV/VNAV or LPV minima to a DA I see your point.

I went looking for single beacon approaches to find some good examples of where it is still needed in places that airliners frequent but it looks like they are mostly gone. The smaller Greek islands had lots of them, like Karpathos, but even that place has a VOR/DME now and the NDB approach is withdrawn.

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Yeah I knew a place but that one is also replaced by RNAV approaches now. This is an old chart:

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The thing is that all minimas on LNAV approaches are basically MDA’s so you need to add a correction per aircraft type. We use 30 ft for speed category B and 50 ft for category C. But the approaches are CDFA nowadays, there is no level-off segment.

So basically if MDA is 300 ft, we would use 350 ft as DA. We decide to go-around at 350 ft before we achieve positive climb we would have descended to 300 ft and then climb away. Thats why it is possible sometimes to have a lower LNAV minima on the approach chart compared to LNAV / VNAV. That sounds weird, but if you think about it the LNAV / VNAV minima on the charts are DA’s while the LNAV minima are MDA’s. (it is exceptional though to have lower LNAV minima).

Using LNAV / VNAV minima, should you decide to go-around, you lose some altitude before achieving positive rate and then climb away while you are not allowed to descent below your LNAV MDA so the decision point should be 30 or 50 ft higher depending on aircraft type. So the MDA + correction for LNAV is higher then the LNAV / VNAV minima, it just looks funny on the chart.

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In this example the LNAV / VNAV DA = 563 ft while the LNAV “DA” = 560 ft + 50 ft = 610 ft

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