Strange AP/AT behaviour on approach (VNAV/LNAV/AT) - P3DV4

[Sekkha 15]

Quote

Read the FCOM, fly the tutorials Kyle and others painfully wrote and watch some YouTube videos.

Keywords are VNAV, VNAV PATH, VNAV DESCENT and VNAV PROFILE.

Here endeth the lesson.

maybe I should  rename the topic to "strange behaviour of simmers on forums"...but honestly I also understand it a little bit. It must be hard for the Pros to have to share the virtual skys with such unknowing armchair pilots like me.I keep fingers crossed that not one day there will be a virtual-sky-police to take away my permission to fly around there...or worse, that PMDG or other devs come up with a mandatory checkride prior to be able to purchase their products....

But again jokes aside:

Is there anybody else who would be so friendly to explain to me what I did wrong as I still dont understand it (without trying to make me pass an exam please!)

And I would be highly  interested in the following as well:

-Why did Copper ask me data of both IAFs, when only TOLSU seems to be part of the procedure? Why should OMIGO be reflected in the CDU as he suggests ?

-Where can I see the IAF speed limit for TOLSU? Why would the FMC not automatically use that speed limit if it is coded in the database?

- What does he mean with "firming up the altitude/s" which he says I should learn but is not willing to explain

 

THANKS A LOT IN ADVANCE !

[PMDG777 231]

2 minutes ago, Suniram said:

maybe I should  rename the topic to "strange behaviour of simmers on forums"...but honestly I also understand it a little bit. It must be hard for the Pros to have to share the virtual skys with such word not allowed armchair pilots like me.I keep fingers crossed that not one day there will be a virtual-sky-police to take away my permission to fly around there...or worse, that PMDG or other devs come up with a mandatory checkride prior to be able to purchase their products....

But again jokes aside:

Is there anybody else who would be so friendly to explain to me what I did wrong as I still dont understand it (without trying to make me pass an exam please!)

And I would be highly  interested in the following as well:

-Why did Copper ask me data of both IAFs, when only TOLSU seems to be part of the procedure? Why should OMIGO be reflected in the CDU as he suggests ?

-Where can I see the IAF speed limit for TOLSU? Why would the FMC not automatically use that speed limit if it is coded in the database?

- What does he mean with "firming up the altitude/s" which he says I should learn but is not willing to explain

 

THANKS A LOT IN ADVANCE !

I think your problem lies within the fact you are too high and too fast. The hold at TOLSU is at 7000 ft. You're doing mach 0.577 and 20,000 ft. Really you should be at about 200-230 kts and 7-10,000 ft tops. You're trying to make the 744 do something that just isn't possible, so it should be little surprise that she's overrun the corner and had to correct back. Think about a race car, if a race car takes a tight corner way too fast, they end up taking the corner too wide and end up off the track.

I'm no airline pilot, but I think "firming up the altitude/s" refers to starting at the landing runway and working backwards through your decent to ensure that it's actually possible without nose-diving your aircraft to reach a target. In my opinion, 20,000 ft at around 30 nm from landing is way too high, regardless of what VNAV thinks is possible.

[skelsey 765]

32 minutes ago, PMDG777 said:

The hold at TOLSU is at 7000 ft. You're doing mach 0.577 and 20,000 ft. Really you should be at about 200-230 kts and 7-10,000 ft tops.

TOLSU is still c. 56NM from touchdown; add another 11NM to run to TOLSU from the first screenshot and actually FL200/260 kt with 66 NM to run, whilst slightly high, is not excessively so: 6.6 x 3 = 19,800 ft. The constraint is not below FL70, because of the terrain.

42 minutes ago, Suniram said:

Is there anybody else who would be so friendly to explain to me what I did wrong as I still dont understand it (without trying to make me pass an exam please!)

And I would be highly  interested in the following as well:

-Why did Copper ask me data of both IAFs, when only TOLSU seems to be part of the procedure? Why should OMIGO be reflected in the CDU as he suggests ?

-Where can I see the IAF speed limit for TOLSU? Why would the FMC not automatically use that speed limit if it is coded in the database?

- What does he mean with "firming up the altitude/s" which he says I should learn but is not willing to explain

You didn't really do anything wrong as such: as some others have alluded, albeit in the main thing to remember at all times is that VNAV is dumb, and does dumb things, especially in the descent. It does dumb things in real life as well, because it is dealing with a highly dynamic situation and computers lack the foresight and ability to project ahead that a human has, and the algorithms are simply not capable of adequately dealing with every possible procedure or situation. Sometimes you can massage it in to doing what you want, other times you may simply need to ditch it and use more basic modes like FLCH or V/S.

The problems with this particular arrival are all the "at or above" constraints and the tight turns which, because of the length of the arrival, take place at high level and therefore high TAS (and, therefore, result in large turn radii) unless you take some action.

There is no published speed limit at TOLSU, therefore the FMC will just use whatever descent speed is appropriate for the CI. However, because of the above good airmanship dictates that it may be wise to restrict your speed in order to reduce the radius of turn and give the aeroplane a better chance. This holds generally true any time you see a tight turn in an instrument procedure, whether departure or arrival; indeed, there are times when HDG SEL with a speed intervention is the best way to get the aeroplane round a turn in a sensible radius.

To get a better path you can try putting in some "hard" altitudes (i.e. FL70 rather than FL70A) at suitable points which can sometimes help give the aeroplane a better target to aim at compared to the "at or above" constraints (provided that you still comply with all the relevant minimum altitudes). In some cases this will work, but not always.

[PMDG777 231]

3 minutes ago, skelsey said:

TOLSU is still c. 56NM from touchdown; add another 11NM to run to TOLSU from the first screenshot and actually FL200/260 kt with 66 NM to run, whilst slightly high, is not excessively so: 6.6 x 3 = 19,800 ft. The constraint is not below FL70, because of the terrain.

Oops turns out I can't count.....

[Sekkha 15]

 

Thanks Chris,

I completely understand that. But what I dont understand is why the FMC is giving me that vertical profile. Did I wrongly select the route on the CDU?

I ask that mainly because as I stated before, the other day I came into LGAV via KEA and LNAV took a turn way before the drawn magenta line and the suddenly the VP jumped to 8000 ft (!!) below the aircraft. So obviously VNAV started that decent way too late (which is kind of similar to that case her, although more logic because the VP jumped down, here at LEMG it jumped up! why that?). But I beliefe I did enter the route correctly. I never had such issues with the Queen before the update to P3DV4...I wil try to make that approach again and take some shots.

So I just have the feeling that I am having a systematic issue with VNAV calculations ......

[Sekkha 15]

and I forgot:

Any idea why AT switched to THR ? Is that normal ? I mean on my first attempt when I was surprised by that situation speed was going up quickly on THR until I took back the throttles to idle (not reflected on the screenshots as they were taken on my second atttempt)....

Why does LNAV draw that turn before TOLSU like an 90° angle and not like a curve?

[Sekkha 15]

Thanks Simon !

[PMDG777 231]

2 minutes ago, Suniram said:

and I forgot:

Any idea why AT switched to THR ? Is that normal ? I mean on my first attempt when I was surprised by that situation speed was going up quickly on THR until I took back the throttles to idle (not reflected on the screenshots as they were taken on my second atttempt)....

Why does LNAV draw that turn before TOLSU like an 90° angle and not like a curve?

In regards to the throttles, you need to make sure your hardware throttles are at idle during descent so that when it goes into hold mode, your hardware throttle position won't cause the sim throttles to increase past idle.

[skelsey 765]

14 minutes ago, Suniram said:

Any idea why AT switched to THR ? Is that normal ?

IIRC, yes, because the mode has changed from VNAV PTH to VNAV SPD. As such you will (without having checked the manuals) get THR whilst the throttles are moving, then HOLD.

You would get SPD if you were in VNAV PTH.

Because the box has (erroneously) calculated that you are below the profile, it goes "oops -- better give you some thrust". This is a silly thing for it to do, but it is absolutely typical of how VNAV can behave.

[Sekkha 15]

Quote

In regards to the throttles, you need to make sure your hardware throttles are at idle during descent so that when it goes into hold mode, your hardware throttle position won't cause the sim throttles to increase past idle.

I always have them at IDLE during descent. But the problem was THR mode. Speed build up until I touched the throttles slightly to get it back to IDLE

[Copper. 78]

Chris and Simon are exact. You did nothing wrong except that you didn't FLY the procedure. You trusted the FMC and Autopilot to do everything for you which is the ONLY thing you did wrong. YOU are the mouse clicker; THE virtual Pilot.

To quote Simon, "VNAV is dumb, and does dumb things, especially in the descent.". 

I think there is a lot to be learned in Simon's statement. Also in Chris' leading statement, "I think your problem lies within the fact you are too high and too fast."

If left alone, the FMC will try to turn 112 degrees left at approximately 260IAS kts, 350GS kts when sequencing to TOLSU. Like Chris said, "Think about a race car, if a race car takes a tight corner way too fast, they end up taking the corner too wide and end up off the track".

And finally; descent planning. You'll need to have a mental plan on how your descent is going to work according to the FMC, and according to physics. If the two do not correlate, then you need to check where you went wrong, where the computers will, or might go wrong, and create a plan.

For your information - by "firming up the altitude/s", my VNAV PTH went well, however the QotS II has problems with LNAV tracking and cornering didn't go so well using autopilot,despite my best efforts although I smoothed out the LNAV errors to get it back on track using HDG SEL.

I spent 78 minutes flying the procedures - once without modification to the FMC procedures and once WITH modification. The former still worked with little intervention, the latter worked with the only intervention being HDG SEL to get the aircraft back on the lateral track.

The moral of this lesson for you is, think not what the software isn't doing, and think more about what you could do to make the software perform for you.

PS - the latest state charts look like they are 2016. By the looks, Navigraph have changed to Jeppesen and the Lido charts MIGHT not be the latest. This channel has some useful videos from a real world pilot https://www.youtube.com/channel/UCZ_X7kk3XrHbl6ZWQ4wpfkQ

[Sekkha 15]

Thanks Brian for coming back and taking your time to check that all out..Really appreciate that

And I completly agree with you. But I probably didnt point out my problem clearly enough yet:

I didnt start that topic to get that "YOU must fly the aircraft - lesson" because - believe me or not - I know that well. I am far from being a Pro-Pilot but around 21 years of simming left some knowledge also with me ( and I learned a lot from you guys out there over the years)...I was  not surprised at all that she wouldnt make that turn before TOLSU at 266kts, but I indeed was surprised that LNAV was drawing that turn as an angle and not as a curve, and that she wouldnt start the turn before reaching that waypoint, and that overshooting it would cause the VP to  jump up 2000 ft and as a consequence THR to engage (searched that in the manual but really couldnt find anything saying that THR could engage in VNAV DESCENT, but of course I didnt do a really extensive search, so maybe its somewhere)

My point is that I have some indications that either something with my installation is wrong or the queen is really buggy on LNAV and VNAV. The purpose of that topic is to rule that out. The only thing which I started to doubt is if I had entered the lateral route correctly into the CDU. Therefore I asked you what exactly did I wrong there....

But to make my point a bit clearer : I got back to LGAV and flew the approach onto ILS Z 21R via the ABLON 1B arrival and IAF KEA again.  Without doing any modifications to the FMC-route data the result was the following (sorry, by mistake I chose the wrong popup CDU with the progress page instead of the legs page for the screenshot, but as I said, I didnt modify the FMC-filed route at all)

approaching KEA

 

a few seconds later VP jumped down  nearly 8000 ft (!!) . And this time she was getting the turn perfectly even with high speed and she didnt start the turn too early as she did the first time I tried the approach

 

I personally find a misscalculation of 2000 ft (by the way, did that happen on your attempt also Brian? Would be interesting to know..)  already excessive even allowing the VNAV to be dumb.

But 8000 ft ?????  I dont know you, but for me that seems to be a clear malfunction. So I will probably submit a ticket if after the announced updated and clean reinstall that behaviour persists.......

I never had such issues while on P3DV3, and I made dozens and dozens of flights there with the queen

 

 

 

 

 

[Sekkha 15]

Maybe I have to correct myself. KEA ist still 22NM to go on the 1. screenshot. So she did take that turn early.....maybe 10NM+ before reaching KEA...but that seems correct as it is prob the only way to get that turn right...and she is on the magenta line (what she didnt the last time)

[downscc 1,327]

All helpful feedback points back to technique.  The QOTSII has the most realistic VNAV of all PMDG products and in my opinion your problem is relying on VNAV.  A large change in heading on any terminal procedure is a big red flag for me, best example is the 90 deg turn arriving RJAA from the East.  I always step through the arrival and approach making sure I have the constraints I want and I handle large heading changes with care, usually in FLCHG mode rather than VNAV controlling my speed MCP SPD and rate of descent with thrust.  You cannot plan on a steeper descent than about 250 ft/nm in the terminal area.  Compare this to the 318 ft/nm angle for ILS GS and maybe it will start to make sense.  You gotta have a plan before starting your descent.

[PMDG777 231]

I'm with Dan on this, you're problems seem to extend from your technique, not from the aircraft. There's been no other reports of this happening that I'm aware of so I think we can safely rule out the 747 being the issue here.