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UKJim

Strange Transition

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So I programmed in the following route into the FMC of the PMDG 737 and when I got to the STAR it had the following...

 

EGNM POL DTY EGLL

 

STAR - BNN1C

TRANS - BNN

 

Do they really loop like this in real world? I doubt it and I do not know why it does this? Can anyone shine some light on this please as I am still slightly new to flight planning the tubes and would really like to know :)

 

2015-12-3009_59_50-MicrosoftFlightSimula

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Hi Jim!

 

There can be two reasons for that sort of presentation. One is that there's a flaw in the presentation or other logic of the FMC simulation in the aircraft you're using, but since it's PMDG I'm hopeful that it's not that.      :unsure:

 

The second is that, given the laws of physics, the aircraft is unable to fly the route you requested owing to the fact that the radius of the turn is too tight for the speed and weight of the aircraft at that point. Looking at your presentation, it seems likely that the only way it could make that right-angled turn would be to perform the loop that's shown (for obvious reasons, this is unlikely to happen in the Real World™, where the dispatcher would have inserted a Place-Bearing/Distance waypoint to ease the turn (unless it's inside the STAR, in which case the problem must lie elsewhere) — which I suspect is probably what you need to do to fix things — unless your speed is way too high, or something like that).     :cool:

 

Cheers,

 

bruce

a.k.a. brian747

 

P.S. Please don't refer to airliners as 'tubes' — unless, of course, for consistency you're going to refer to all GA aircraft as "little puddle-jumpers"?     ;)

 

B.

 

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Lmao puddle jumpers - love it! I must admit as I'm getting more and more into the VA flying and the NGX, I think I will have to get out of the habit of calling them tubes ;)

I cannot be 100% sure but I normally step the route before heading off - I don't believe it was like this when entered. Also my last waypoint was Daventry - so this means this is within the star.

I will have to take another look and see where it changes. Either that or tweak my route into Heathrow :)

Thanks for the help Bruce.

P.S. Is your name Bruce or Brian? We have both been here some years now and I have never asked lol.

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Hi Jim!

 

Well my real name is Brian (see below my avatar), but because there's another Brian in the Crew (who is usually known as Needles in fact — go figure) I became bruce.    :wacko2:

 

Obvious, really (not).    :P

 

Back OT — once you're within the STAR then it's not possible to change the waypoints (STARs are standard and immutable by definition, so are protected from being meddled with by pilots), hence there must be some other reason for the weirdness. Some of the EGLL approaches are pretty tight, though, and need me (in a heavily-loaded 744) to be at no more than about 175 kts with suitable flap settings to be able to negotiate the turns — not to mention the fact that to complete the STAR without being put into a hold at some point is pretty unusual at busy times.

 

OK, I've just checked out the BNN 1C STAR. From DTY your next interesting point is the SLP (Speed Limit Point) — at that point you must be at or below 250 kts. From there you head towards the BNN [potential hold] (limit 220 kts) and by the time you start your right turn onto the localizer at D19 BNN you must be at a maximum of 180 kts; also, during the turn you also need to be descending from 3500 to 3000 ft. Once on the localizer you pick up the glideslope when D7.5 from IRR, at which point you must be at 2500 ft.

 

BNN-appr-to-rwy-27R.jpg

 

All of which seems perfectly doable, so I'm baffled by what your FMC is trying to achieve (which is reminiscent of the old cry from those new to glass cockpits — "What's it doing now?").   :)

 

Obvious questions:

 

1. You did load the STAR correctly (from DEP ARR) and check your route in PLN mode?

 

2. Did you also check that all the speed and altitude constraints match those on the chart?

 

If I get the chance I'll put your route into my (PSX) 744 and see what turns up.

 

Cheers,

 

B.  [Which does for both]

 

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Hi Jim!

 

OK, I'm back — so further to the above I quickly sketched in a figure-of-8 round trip from EGLL and back again via DTY and the BNN transition to the BNN 1C STAR, and the results were boringly as expected.    :bored:

 

Here's the route on the ND (please ignore the outward leg of the "8"):

 

ND-presentation.jpg

 

And here are the constraints from the navdata (i.e. the selected STAR and transition) as seen in the CDU:

 

 

CDU-presentation-too.jpg

 

 

I usually then enhance those constraints by adding the speed constraints as well (which helps with the workload at a busy time).

 

If you compare the above with what you get, then you may be able to spot where the problem arises?

 

Good luck,

 

B.

 

 

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Wow thanks for looking at this - any excuse for a flight aye ;)

In my planner it also shows the route yours did too - unfortunately tonight is family night so cannot check it out but as soon as possible I'll check this. I do have a dated airac 1507 I believe but that still shouldn't happen.

I'll do screenshots of the step through on ground and then see what is happening.

Thanks once again

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Hi Jim!

 

FYI, the effectivity date of the chart extract above is 10th Sept 2015, so with cycle 1507 you should get a perfect match.

 

And just to confirm, once activated (here, just after pushback in T5) the route on my ND looks like this:

 

conf.jpg

 

Best of luck,

 

Cheers,

 

B.

 

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OK I have just taken a look - I entered my route into the FMC and stepped through the route again before doing anything else - this looked fne and exactly like your approach too. But then I put in a cruise altitude of FL190 and execute - this is where it all goes to pot.

 

I have updated my cycle to 1513 now too - problem still exists.

 

I'm going to run it through my 777 and see what it shows there too.

 

Thanks

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UPDATE: It just gets better lolol

 

So I run up the 777 - programme exactly the same and this time you can get the 777 FMC to 'request' the PERF INIT page where you select the cruise alt. It actually suggested FL190 lmao! And after activating all looked perfectly fine. 

 

It has to be a quirk with the NGX as both have the same airac and cruise alt set!

 

Damn - the search continues!

 

UPDATE 2: OK I have just found I entered everything apart from the cost index and it looked fine - then I put in cost index of 26 and it alll went funny again. I think this is doing it when it tries to sort out the altitudes on the legs page but I am not sure.

 

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I found out that the waypoint causing the issue was the (intc) waypoint shown above. Removing this caused the plan to be correct.

I did not have to modify altitudes or speeds for this to correct itself so that was good too.

Looks like a quirky feature of the NGX and that transition.

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