RNAV confusion - what don't I understand?

[TacomaSailor 191]

What don't I understand about RNAV or GPS approaches and how they are implemented in the G1000 as modeled in the DA62?

I was trying to fly the RNAV (GPS) Y Rwy 34 approach to KBLI.  I was on the 74° course from ZONUV (IAF), which is 8 NM WSW of TUBTY, which is the IF for the approach.  Jeppesen, as shown in Navigraph, shows a crossing of TUBTY at 3,000' with a heading of 334° to a straight in landing on Rwy 34 at KBLI.  That is what the FPL showed before I activated it. 

At 5 NM from TUBTY I activated the procedure in the MFD.  

At that point the autopilot turned the plane 180° and drew a magenta line in a large arc to the south and back around to the east, then north to cross IWANY.   That (IWANY) is not shown on the Jeppesen chart, but there is a small note at the bottom of the chart showing IWANY as the IAF 8 NM SSE of TUBTY.   But, the chart clearly shows ZONUV to also be an IAF, also at 8-miles. 

The PFD showed a distance of 9,875 NM to IWANY from my location just WSW of TUBTY, as the AP made it's 180 turn away from TUBTY.

Navigraph and AirNav both show no such fix or navaid when I search for IWANY.  

If Jeppesen shows the approach to TUBTY to be 74° and 8 NM from the ZONUV IAF, why is the G1000 trying to fly to IWANY when the plane is headed directly to the IF?

AND, why is this approach held so long at such a low altitude?  The chart shows a crossing of the IAF at 3,000' but from there it is 20 NM to the TDZ.  All of this distance is over open water of Skagit and Bellingham Bay.  Why 3,000' at a distance of 20 NM?

Thanks for any helping clearing up my misunderstanding of a seemingly simple RNAV approach.

Edited September 9, 2020 by TacomaSailor

[richjb2 195]

You didn't mention which airplane you were flying, but my experience is that RNAV in MSFS is not ready for primetime yet.  I have seen missing waypoints, strange behavior with the magenta line and LNAV, etc.  I would not get too worked up over it. 

Regarding the RNAV (GPS) Y Rwy 34 approach altitudes, those are minimum altitudes unless otherwise published.  They are generally established for obstacle clearance, and occasionally for airspace or procedure constraints. Once you cross the IAF, you can descend at your discretion on the approach, remaining above these minimum altitude unless ATC assigns a lower altitude.  For example, a typical clearance might be "cleared direct ZONUV, cross ZONUV at or above 4000', cleared RNAV Y runway 14 approach".  You may descend from 4000' at your discretion on the approach, typically using VNAV if available.  If not using VNAV, VS or pitch mode.  ATC will generally assign an altitude from which, down to the minimum published altitudes, you are clear of conflicting traffic. 

Hope this helps...

[JRBarrett 3,080]

The NOS chart for this approach does show both ZONUV and IWANY as IAFs. both are 8 nm from TUBTY. It appears IWANY would be used when ariving from the south, and ZONUV for arriving from the west. One current bug in the MSFS implementation appears to be that if there is more than one IAF for a given approach, the only one that will be used is the one that appears first (alphabetically) in the AIRAC file.

Some users on the MSFS forum with real world G-1000 experience have said that the unit will indeed turn around and fly back to the IAF if you first activate the approach after crossing the IAF, but before arriving at the IF.

[Atiti312 0]

Hi,

I haven't flown the G1000, but typically you only activate the approach if ATC vectors you to intercept the final approach course (it draws an extended magenta line on your mfd).

So to fly the approach from ZONUV you would stay in LNAV or GPS mode then arm vnav which will command the autopilot to follow the vnav profile at TUBTY. 

I haven't tried it in the game yet though so there may be bugs.

[JRBarrett 3,080]

9 minutes ago, Atiti312 said:

Hi,

I haven't flown the G1000, but typically you only activate the approach if ATC vectors you to intercept the final approach course (it draws an extended magenta line on your mfd).

So to fly the approach from ZONUV you would stay in LNAV or GPS mode then arm vnav which will command the autopilot to follow the vnav profile at TUBTY. 

I haven't tried it in the game yet though so there may be bugs.

That particular approach is WAAS-enabled with LPV minima, so if you activate APR mode at TUBTY, it should provide a glide path on the PFD, and could be flown like an ILS. I don’t think VNAV yet works in any MSFS airplane, but LPV does work.

[Bert Pieke 7,585]

There is some confusion between loading and activating an RNAV approach (in the real world as well..)

If you LOAD an approach, it gets added to the end of the current flightplan, conserving the in-between waypoints.

If you ACTIVATE an approach, the intermediate waypoints get erased, and the airplane will fly directly to the IAF.

If you make the mistake of activating the approach late (after passing the IAF), the airplane will indeed turn around.

[Atiti312 0]

6 minutes ago, JRBarrett said:

That particular approach is WAAS-enabled with LPV minima, so if you activate APR mode at TUBTY, it should provide a glide path on the PFD, and could be flown like an ILS. I don’t think VNAV yet works in any MSFS airplane, but LPV does work.

Good to know, on my aircraft (Q400) APR is only used on ILS (arms LOC and GS), all RNAV including RNP are flown by having vnav and lnav modes engaged.

[fppilot 4,385]

1 hour ago, TacomaSailor said:

At that point the autopilot turned the plane 180° and drew a magenta line in a large arc to the south and back around to the east

I am still working on determining what is happening but have not had time the past two days to do some trial and error.  Here is something I have noticed.  On the flight plan below, the active segment is the first one, as indicated by the magenta bracket:

 

If I activate the cursor, place it say on TTH, and click the ACT LEG key at the bottom right of the GTN then the active route becomes VLA → TTH (see below).  So it does not fly to TTH,  It flies to VLA first.

What may be happening when activating an approach AFTER the fix before the IAF of the approach may be that the DA62 G1000 is making that segment the active segment and turning back to that fix.  Only it does not then pick the approach back up.  At least once when this happened, I flew back beyond that fix prior to the IAF, then went back into the PROC screen and activated the approach a second time and it flew it correctly from there.  It may be that the approach needs to be activated before that prior fix.

By the way, I believe that behavior in the DA62 is different than what I experienced in the 172 (G1000).

 

Edited September 10, 2020 by fppilot

[Andreas Stangenes 1,628]

Often, with RNAV, I notice that the AP treats the lateral navigation like an ILS (swings to 30 degree intercept), but the vertical diamond often wont start moving until the last fix before the threshold. So if the FAF tells you to be at 3000 ft for intercept, but there is an intermediary fix between it and the runway at ie 1600ft, the glideslope diamond wont move until you pass that point. Until then it will show full deflection below or above. And then when it starts to move, usually the AP refuses to intercept the GS and I have to handfly it or adjust with VS. 

[JRBarrett 3,080]

12 hours ago, Atiti312 said:

Good to know, on my aircraft (Q400) APR is only used on ILS (arms LOC and GS), all RNAV including RNP are flown by having vnav and lnav modes engaged.

LPV is not common on airliners, but it is an option for some. Our company operates three Challenger-850 aircraft (corporate version of the CRJ-200), and all have LPV capability, which was added under an STC. It required installing new GPS receivers and antennas, and upgrading the FMS computers, plus adding some additional annunciators on the glare shield.

[richjb2 195]

On 9/9/2020 at 6:51 PM, Bert Pieke said:

There is some confusion between loading and activating an RNAV approach (in the real world as well..)

If you LOAD an approach, it gets added to the end of the current flightplan, conserving the in-between waypoints.

If you ACTIVATE an approach, the intermediate waypoints get erased, and the airplane will fly directly to the IAF.

If you make the mistake of activating the approach late (after passing the IAF), the airplane will indeed turn around.

and it does that in the real airplane too...   Don't ask me how I know... 

[TacomaSailor 191]

Just flew the approach coming from the SW of the IAF (IWANY) with the approach loaded and then activated about 5 NM (2-minutes prior to crossing IWANY).  The autopilot tracked the lateral path perfectly all the way to the runway threshold.

BUT - the approach provide NO vertical guidance - there was a G in the box next to the altitude tape and it set itself to 4,800' as we crossed the IAF.   The G gradually worked it's way down the tape to 2,200 feet at OHADO.  But, the G did not lower after that and the plane leveled out at 2,200'

I see a very small white 3 in a round  black circle at 1.3 NM from the TDZ on the Jeppesen chart.  The 3 note says "LNAV only."   The 3 in a circle is at the point on the glide slope where the LNAV MDA is reached so, maybe, that note is just referring to the MDA and missed approach?

SO I am so confused - does that mean this RNAV (GPS) Y approach must be hand flown from OHADO 6.2 NM to the TDZ? 

But the Y approach chart also provides Decision Altitudes for LNAV/VNAV and an MDA for LNAV alone. 

The RNAV (GPS) Z approach from the same IAF seems to provide Vertical Navigation from UPEKE (FAP) 3.4 NM to the TDZ but requires RNP 0.30.  Does the Z approach provide VNAV Only from UPEKE?  The approach chart only includes a vertical view from UPEKE to Rwy 34. 

Edited September 12, 2020 by TacomaSailor

[ryanbatc 12,862]

rnav34Y_BLI by Ryan Butterworth, on Flickr

If I was flying this approach in our little Musketeer with GNS430W, I'd activate approach from IWANY....and assume ATC gives me an approach clearance "cross IWANY at or above 3000, cleared RNAV Y RWY 34 approach."  We don't have an AP but if I did and say I was at 4000 SW of IWANY, I'd VS mode down to 3000, while still in NAV mode (CDI in GPS mode), and crossing TUBTY I'd press APR mode since the GNS430W supports LPV minimums.  

From TUBTY to OHADO it's 6.6 miles and you need to lose 800 ft.  At TUBTY you could easily have time to intercept the glide path (assuming you pressed APR mode at 3000 after crossing it).  I'll have to try this one in the sim.  But I don't usually press APR until the next fix is  the FAF (on this style of approach).

Oh, I don't have jepp plates but I'm guessing you're speaking of the missed approach point maybe?  1.3 miles is where you would transition to a visual landing if using LNAV mins I believe (don't quote me this is only from my experience as ATC - I am not instrument rated).

Edited September 12, 2020 by ryanbatcund

[Bert Pieke 7,585]

On 9/9/2020 at 5:24 PM, Andreas Stangenes said:

Often, with RNAV, I notice that the AP treats the lateral navigation like an ILS (swings to 30 degree intercept), but the vertical diamond often wont start moving until the last fix before the threshold. So if the FAF tells you to be at 3000 ft for intercept, but there is an intermediary fix between it and the runway at ie 1600ft, the glideslope diamond wont move until you pass that point. Until then it will show full deflection below or above. And then when it starts to move, usually the AP refuses to intercept the GS and I have to handfly it or adjust with VS. 

I see the same thing... that is clearly a bug in the current Asobo G1000 simulation.

[Adrian123 1,015]

Many bugs, Usable but barely at this point. Like a box of chocolates. Never know what your going to get.