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

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Today I encountered the following strange behaviour (tried two times,both same bahaviour, one time with AS4, second time without AS4):

approach to LEMG to ILS Z 12 via PIMOS1DCQ arrival

 - inbound D011S everything normal, LNAV, VNAV PTH and HOLD
35922246122_b382a14a63_c.jpg

 

 

- as soon as reaching D011S (why does she not turn before reaching -is it a must overfly wpt?) suddenly AT announces THR and the vertical path jumps to almost 2000 ft above..

36091244705_3e452a73f7_c.jpg

 

 

 

- ca.20 seconds later again HOLD is announced (but I have to pull back the throttles of course) and vertical path  is 450 ft above
35922240732_6c49b1a262_c.jpg

 

- then for ca. 2 minutes AT stays in HOLD letting rise the verical path without attemptimg to recover. then finally SPD announced and VP recovered
35922238492_8bca9e1f67_c.jpg

 

 

Later on the vertical Path jumped up and done significantly two times more....

Also the other day approaching LGAV via KEA I had the vertical path jumping 8000 FT down suddenly, so decent was obviously started way too late....(But I cant get details for that right now as it takes too much time)

So whats going on here...??  Is it me doing something wrong?
Hopefully but  I honestly dont think so...

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You didn't provide the identity of the location where this arrival and approach are located.  Not knowing that I cannot figure what went wrong since the vertical path is clearly not right (inbound to D011S is definitely not normal, you are at FL200!) and the 90 deg turn looks wrong.  Gotta provide enough information.

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  1. Aviate,
  2. Navigate,
  3. Communicate.

These rules apply. You should've reconciled the STAR and APP loaded into the FMC's with the Navigation Charts earlier. After you've checked the two correlate you may fly those procedures. 

 

Once you are on track, it is YOUR job to fly the plane, not the autopilots. This failure is entirely yours.

 

 

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so in the real world the pro-pilots would have aviated, navigated,comunicated ...and landed safely (I also did by the way) and after landing they would have gone for a beer without filling a report to their company or boeing about that incident with a probable mulfunction of the autopilot.......cool !! (or if they would have filed a report boeing would have responded: "This failure is entirely yours" Cheers)

Guys, whats going on with you? This topic is not about how to react when something like that occurs. I started this topic only to try to understand why the AP behaves like that....(THR on VNAV ?? ..., VNAV starting a decent way too early so that VP jumps 8000 feet?????,.....????)

 

and another thing:

Quote

You should've reconciled the STAR and APP loaded into the FMC's with the Navigation Charts earlier. After you've checked the two correlate you may fly those procedures.

How do you know I didnt ????

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I'll answer your last question with some questions and hope that you can put two and two together.

  1. On your nav chart, what are the name's of the two Initial Approach Fixe's (IAF) for the ILS Z 12?
  2. What are the 2 IAF's speed limits in knots?
  3. What is the Final Approach Fix's (FAF) name?
  4. On your nav chart, what is the FAF crossing altitude?

Once you have those answers, were they reflected in your CDU?

There is a FMC technique pilots use colloquially called "firming up the altitude/s". You might also benefit from learning it.

 

Knowing the answers to the things above, I managed to manipulate the FMC and autopilot to fly your procedures in IDLE/SPD | LNAV | VNAV PTH from TOD the whole way down. Since I don't have LEMG scenery, I had to do a Missed Approach and circled to land on the default (outdated) airport's runway.

 

There doesn't seem to be anything wrong with the 747.

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and hope that you can put two and two together.

Captain, you are really giving me a hard time.. I am not sure if am prepared enough for a checkride with you. And maths never have been my strength (that might be one reason why I am sitting here in front of a virtual cockpit on a monitor, in an armchair, in underwear and eating icecream...instead of wearing uniform with some stripes on the shoulders and being obligated to pass checkrides from time to time)

But seriously:

If you want to be helpful (what I suppose is your intention and what I really appreciate) please be so kind and just answer my following questions, because I really dont get your point (for example: why are you asking me the name of the second IAF if you can clearly see that I choseTOLSU ? etc....)


You can find my complete route setup in the screenshot sequence and the charts I used below: (AIRAC 0217, also in the FMC)

1) What exactly did I wrong on setting up the route ?

2) Is it expected behaviour that FMC is filing that faulty route data (as you suppose) if I choose ILS Z 12 approach, PIMOS 1DCQ Arrival and TOLSU transition on the CDU? Or is there maybe something wrong with the database or the fmc-calculations (at least on my PC)?

3) Is approaching a fix with high speed (which 266 Kt obviously is for that segment) and then being unable to get the 90° turn right with the Vp jumping up 2000 ft the reason for the AT to change to THR-Mode while on VNAV descent ? Why not SPD Mode ?

4)Why did the vertical path jump up 2000 feet suddenly ?

5) Why did the AP not try to recover the rising VP by appliying thust in SPD-Mode instead of staying at IDLE/HOLD during 2 minutes afterwards

thanks a lot for your help

35938397862_003d78b1e5_c.jpg
36107317645_60d53f1641_b.jpg

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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.

PS - your Charts - they are from 2015!

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7 hours ago, Suniram said:

You can find my complete route setup in the screenshot sequence and the charts I used below: (AIRAC 0217, also in the FMC)

What's throwing me off is you say you're using AIRAC 0217, and you've shown us the charts you used. Your charts are dated August 13th, 2015, so that would really be AIRAC 1508 or possibly 1509, as these things get updated every 28 days, so you'll have 13 cycles in a year. AIRAC 0217 would therefore be impossible, as that would indicate the 17th cycle of the year '02, or 2002. The AIRAC that came with the plane when it was released was I believe 1702, so maybe that's what you meant, but I'm not entirely sure.

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Note that Jeppesen does only incremental chart updates, not bulk as AirNav does. So, a chart from 2015 may be perfectly valid in the current cycle, unless it was actually changed.

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1 hour ago, icemarkom said:

Note that Jeppesen does only incremental chart updates, not bulk as AirNav does. So, a chart from 2015 may be perfectly valid in the current cycle, unless it was actually changed.

All right, fair enough. I wasn't aware of that.

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1 hour ago, icemarkom said:

Note that Jeppesen does only incremental chart updates, not bulk as AirNav does. So, a chart from 2015 may be perfectly valid in the current cycle, unless it was actually changed.

They're not Jeppesen charts

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3 hours ago, PMDG777 said:

They're not Jeppesen charts

Ah, yes, sorry. This is that other thing, I keep forgetting the name of... Lufthansa thing. Anyhow, they do the same thing :-)

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41 minutes ago, icemarkom said:

Ah, yes, sorry. This is that other thing, I keep forgetting the name of... Lufthansa thing. Anyhow, they do the same thing :-)

Yeah the Lufthansa Systems charts are LIDO I think? (I don't know what that stands for). They're formatted differently and I don't really know how they are updated, I used the Jeppesen charts.

But off topic here.

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The AIRAC that came with the plane when it was released was I believe 1702, so maybe that's what you meant, but I'm not entirely sure

yes, of course, that is what I meant, 1702, sorry for the mistake. The only thing I can say is that I got those charts from Navigraph while 1702 was current. So I have to assume is was the current chart at that time or Navigraph didnt update it.

 

 

 

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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 !

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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.

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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.

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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.....

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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 ......

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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?

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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.

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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.

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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

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