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FEDEX061

777 and STAR altitude restrictions

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Still honing my skills with the 777 amd love it so far. My question regards altitude crossing restrictions on STARs. It seems that the 777 comes close to meeting the restrictions but not quite. For example, if I'm on a STAR with a requirement to cross a waypoint at 11,000 feet it seems I'm usually at 11,200 feet or so. I know, close, but I like to be at exactly 11,000' feet before the waypoint. Am I doing something wrong or is there an adjustment that I'm overlooking?

Thanks,

Jerry Carroll

FEDEX 061

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The real 777 does that as well sometimes.

The FMC will assume the winds are as entered on the Descend/Forcast page.

If the winds are different (less headwind/more tailwind) then the aircraft might not be able to reach programmed restrictions.

It will try by speeding up (diving down) but at some point it just cant speed up enough (limit speed).

The message "drag required" will then appear, letting you know that speedbrakes are required.

 

200ft is not a big deal in real life though and I think it is not enough for the "drag required" message to be triggered.

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Am I doing something wrong or is there an adjustment that I'm overlooking?

 

As Rob mentioned, filling out the DESCENT (VNAV Page 3) > FORECAST information will vastly improve the plane's descent performance.  I'll add a video below that explains the background of descent forecasting.

 

Additionally, I know you'd like it to be exact, but to be honest, it's not a huge deal.  The altitude restrictions are there for a couple reasons:

  1. To get the aircraft down to an agreed altitude between facilities (XYZ Center controls 12,000 and above, while TUV TRACON controls 12,000 and below - they agree to have aircraft handed off at 10,000 prior to the TRACON lateral boundary).
  2. To keep various streams separate (aircraft going into DCA cross OJAAY at 10,000, while aircraft going into BWI cross SABBI - a few miles east of OJAAY - at 15,000)

While some TRACONs do have more closely-spaced streams than the above BWI/DCA example, a controller likely won't notice (not to say that they're not paying attention - just not nitpicking your altitude, because there are more important things to pay attention to), and won't get on your case for being 200' off.

 

EDIT - here's that video:

Edited by scandinavian13

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Just to add: If you're using real-world weather, you can download wind data from SimBrief that's automatically generated for your flight, so that you don't have to enter all the data manually.

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Goes back to the fact that aviation is far less precise and rigid as some people think. Apart from when in RVSM airspace. Then you better not be 200 ft out...

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Then you better not be 200 ft out...

 

For what it's worth, even in RVSM areas, ATC won't say anything unless they notice it's 300+.  It's on them to ensure you're separated by 5mi* or 1000'*.

*changes due to a few different factors, but that's the basic min.

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Goes back to the fact that aviation is far less precise and rigid as some people think. Apart from when in RVSM airspace. Then you better not be 200 ft out...

I disagree with this notion. Over the years I have learned that flying can be as precise as you choose to make it or at least as precise as your equipment will allow. I'm talking real world aviation. Everyone sets their own allowances for holding heading, altitude, airspeed and so on. This even applies to how you taxi. Some just want to stay on the paved surface while others, including myself, want to straddle the centerline.

I said I wouldn't nitpick but I take that back. I'm disappointed that the PMDG777 doesn't do a better job being at required altitudes at or before the waypoint. Why not fail on the safe side and be at the published altitude slightly before reaching the waypoint? Not a violation of procedures if you're at the required altitude a mile before the waypoint.

To me if you are consistently late on altitudes that's just sloppy flying. I can live with it and will find ways to adjust for it but just think some part of the programming isn't exactly right.

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I disagree with this notion.

 

You can disagree all you want, but it's rather true: aviation isn't as rigid as many would believe.

 

 

 


I'm disappointed that the PMDG777 doesn't do a better job being at required altitudes at or before the waypoint.

 

Airplanes don't fly on rails like you might think.  The very matter you're flying through is dynamic.  It does not exist in a perfect, stable, state.  You cannot, therefore, expect perfection from the automation.  It does a very good job for the job it's been tasked with.

 

Disagree all you want, but it's just life.


 

 


I can live with it and will find ways to adjust for it but just think some part of the programming isn't exactly right.

 

Have proof for that somehow?

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You and I are two completely different pilots. As a real world pilot I simply do not accept the "close enough" attitude when flying. If I have a crossing altitude of 12,000' then I will be at 12,000' at or before the waypoint. If I am not then I will be reviewing what went wrong because in my opinion that was a mistake and I do not take mistakes made while flying lightly no matter how small they may seem. If your criteria is that anything close to 12,000' is good enough then so be it. All I'm saying is that is not the way I choose to fly.

I plan on continuing to fly the 777 because I love it. I accept the fact that this is sim flying and airplanes do what they are told to do whether by control input from the pilot or by commands from the autopilot. My only point was I thought the FMC was not as accurate controlling descent to a crossing restriction as I wish it was. No need to posture up and get defensive from someone's opinion even though it may differ from yours. And if you want to paraphrase my comments please keep them in the same ballpark. I never stated I thought airplanes flew on rails.

Have a good one.

Jerry

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I never stated I thought airplanes flew on rails.

 

...but you really did.

 

You implied that the coding was wrong because the automation didn't put the aircraft exactly at 12,000', anecdotally citing further flights where it's slightly off, and therefore a bug in code.  As all of this is natural behavior, this implies that you think aircraft are capable of this perfection.  The term for flying with perfection is flying on rails.

 

 

 


As a real world pilot I simply do not accept the "close enough" attitude when flying.

 

Based on your posting, I'm guessing you're not a real world pilot of a large aircraft (evidenced by your "at or before" comment - VNAV descents try to remain high as late a possible to save fuel, so they're programmed to be there at the altitude, not level off before if it can be avoided).  If you were, you'd understand the limitations of automation.  Automation is never perfect in aviation, as we fly in an imperfect environment (evidenced in multiple ways, if not only by the fact that we have an adjustment knob for the altimeter to adjust for pressure changes).  Sometimes you get above path or below path - even when you properly forecast - because the wind somehow changes from what the automation predicted.  If you don't compensate manually by adjusting the descent speed, then it's going to remain off path.

 

As someone who's been on teams to help design approaches and optimal profile descents (OPDs), I can tell you that it's a fact that certain margins of error are built into them for both human factors and errors/imprecisions in automation.

 

It has nothing to do with being different pilots, and everything to do with not understanding the tools at your disposal.

 

...evidenced further by:

Thanks AAL125.  I find myself using these more and more.

 

Using them is not at all wrong/incorrect/etc, and not using VNAV because it's not perfect enough for you is your own choice, but striving for perfection will actually end up causing more headaches (and diminishing returns than it's worth).

 

VS and FLCH are used for 99.99% descents.

 

You're gonna need to source that.

 

FLCH sure.  VS not at all anywhere near where the frequency of VNAV.

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You and I are two completely different pilots. As a real world pilot I simply do not accept the "close enough" attitude when flying. If I have a crossing altitude of 12,000' then I will be at 12,000' at or before the waypoint. If I am not then I will be reviewing what went wrong because in my opinion that was a mistake and I do not take mistakes made while flying lightly no matter how small they may seem. If your criteria is that anything close to 12,000' is good enough then so be it. All I'm saying is that is not the way I choose to fly.

 

What is more efficient, having to increase power on the engines because you are at 12000 well before the waypoint, or leaving them at idle and continuing descent even if you are slightly above the waypoint? In most cases the FMC calculated path would take into account at or above or at or below and the end result is if flying a STAR for example, you might be slightly off on the initial waypoints but you will be on profile at the FAF.

 

Also, if you are off by 300ft that's not much more than the length of a 777-300, perfectly acceptable. I don't think anyone would class it as a mistake. If anyone tried to write me up for a violation because I was 300ft above an "at or above" waypoint, I would laugh, and then lodge a complaint.

 

I realize you have your own opinions however and at the end of the day the more precise we are the better overall. Just don't let the quest for precision distract you from actually flying the airplane.

For what it's worth, even in RVSM areas, ATC won't say anything unless they notice it's 300+.  It's on them to ensure you're separated by 5mi* or 1000'*.

*changes due to a few different factors, but that's the basic min.

Yeah I was being sarcastic... Still, if your 300 above and the guy on the track above you is 300 below, you got a nice 400ft separation. Close enough for the FA's to throw you another drinks trolley. :)

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VS and FLCH are used for 99.99% descents.

By you maybe...In real life Vnav and Lnav are used...I would say 80% of the time!

 

The way this discussion has turned is totally unnecesarry.....as most often!

 

 

Facts:

1)In real life 200ft off target altitude is not a big deal....ATC will not say anything.

2)That does not mean you yourself should be ok with that.

(A pilot should allways try to do his utmost best to meet restrictions ofcourse....and I dont think anyone replying here has meant to imply any different!)

3)If you cant get the PMDG777 at 12000ft at a certain waypoint then you need to read up on the auto flight systems.

(you can use speed intervention with higher speed for example to stay in Vnav and still meet your restriction.....no need to change to FLCH or VS, but you can do that as well if you are uncomfortable with Vnav or if you feel it is the better/less distracting mode)

 

Now to the OPs problem;

If you are saying that the PMDG777 cant get to its target altitude at any time....even with correct winds (forcast winds=real FSX winds)....and is ALLWAYS 200ft high...then yes I can see why you suspect a bug.

And maybe you are right....

 

But like I said before....I have to intervene quite a lot (in real life) to make "at" restrictions if they are much lower than the optimal (about 3 degrees) path of the FMC.

Or sometimes it will make the "at 12000ft" but instead of the required 250kt you will find yourself reducing through 270kt right there....also not perfect, not a big deal either.

 

As was mentioned already....the FMC of the 777 does not waste energy....it does not want nor plan to reach an "at" altitude prior to where it has to!

Now that does not mean It should not do everything it can to reach that "at" restriction!

It should, but it cannot do more in Vnav than pulling throttles to IDLE and increasing the Vnav descend target speed (to dive down so to say).

Your PMDG does not do that?

Have you checked you are still in Vnav PATH mode?

because you do know it reverts to Vnav SPD mode if it cant stay within certain parameters of its planned descend speed right?

And if you plan all your descends with 320kt then there is no more room for Vnav to increase speed if it gets too high due to unfavourable winds.

Which is why I allways use a descend speed of 280kt to 300kt.

Keep a little buffer....try that!

Anyway, once the FMC (or FMA to be precise) reverts from Vnav PATH to Vnav SPD....it is all up to YOU again to make the restriction.

It is telling you with that (and with the drag required message) that it cant make it and you have to fix it.

Thus use speed brakes or increase speed more or both!

 

If you are saying that you are in Vnav PATH and NOT in an idle descend (partial thrust set by the automation for a shallow descend between two restrictions) and you THEN dont make your "at" target,... then that would be a bug yes.

  • Upvote 2

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My last post on this thread. Always helpful to get ideas from others and I appreciate you all taking the time to reply. A shame that not all responses wanted to address the topic as opposed to addressing other interpersonal matters. Internet life I suppose.

Over and out.

Jerry

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A shame that not all responses wanted to address the topic as opposed to addressing other interpersonal matters. Internet life I suppose.

 

Yes, because facts of the limits and precision of automation are interpersonal matters...

 

I came with facts.  Just because you don't like said facts does not mean it's an interpersonal matter.  In fact, it's quite the opposite.  Refusal to accept facts is actually rather personal.  See: Resolution (Reduction) of Cognitive Dissonance.

 

For your convenience:

A classic illustration of cognitive dissonance is expressed in the fable "The Fox and the Grapes" by Aesop (ca. 620–564 BCE). In the story, a fox sees some high-hanging grapes and wishes to eat them. When the fox is unable to think of a way to reach them, he decides that the grapes are probably not worth eating, with the justification the grapes probably are not ripe or that they are sour (hence the common phrase "sour grapes"). The Moral that accompanies the story is "Any fool can despise what he can not get". This example follows a pattern: one desires something, finds it unattainable, and reduces one's dissonance by criticizing it. Jon Elster calls this pattern "adaptive preference formation".[17]

Further reading: http://en.wikipedia.org/wiki/Cognitive_dissonance

 

The metaphoric connection (in case you don't see it):

One desires something - you want VNAV to be perfect

One finds it unattainable - it isn't (realistically) perfect

One reduces one's dissonance by criticizing it - you assert that it's a bug, and that the facts of the matter aren't really facts, but are differences in pilot opinion regarding acceptable precision

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Kyle when did you study Psychology? Can you diagnose me as well I feel left out...

 

Anyway an altitude delta of 200ft wouldn't bother me in the sim much. Life ain't perfect.

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Kyle when did you study Psychology? Can you diagnose me as well I feel left out...

 

haha - not diagnosing as much as observing psychological factors at play in the discussion.

 

...and Virginia Tech 04-08 as somewhat of a minor and prereq to my outside-of-aviation (intentionally) studies of Public and Urban Affairs (city planning from a statistical, psychological, political, design and engineering point of view).  Honestly should've done computer science or engineering, but what's done is done.

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Kyle has been studying psychology for 7 years, 3 months and 19 days  - at least ...   :rolleyes:

 

Yes, the RW T7 is a non-perfect plane and ... yes, PMDG modelled that accurately to a large extent. And ... yes, it's even mentioned in the intro doc ...   B)

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Sometimes I think this forum is a psychological experiment in itself, set up by Kyle, disguised as a sub forum about a payware aircraft. The aim is to answer the question "how mad can one be driven by observing the behaviors of those incapable of reading supplied information". Sub sections and theses comprise of "will a subject back down once pinned in a corner of ignorance or will they go on the attack".

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