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News: The latest PSX update (version 10.40 from 18 July 2018) is available at: http://aerowinx.com/board/index.php?topic=4191.0

Author Topic: FMC offset route  (Read 434 times)

emerydc8

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Re: FMC offset route
« Reply #20 on: Sat, 14 Jul 2018 02:56 »
His reply:

Quote
When you’re on a star or a sid and select the fix in front of you and place it upon itself and execute you are essentially getting rid of the coding in the box that makes it a Sid or a star. It will offset all the way to the end of your flight plan until you cancel offset.

Hardy Heinlin

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Re: FMC offset route
« Reply #21 on: Sat, 14 Jul 2018 04:06 »
And what if the SID contains conditional waypoints, procedure turns, open hooks, holds, vectors, arcs? There is simply no reasonable offset headroom for such tiny curves. Will it at least, as usal, discontinue at course changes greater than 135°?

When they apply offsets on the real ship on certain SIDs, is it always applied to normal, non-conditional SID waypoints? Have they ever tried it on conditional SID waypoints?

emerydc8

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Re: FMC offset route
« Reply #22 on: Sat, 14 Jul 2018 04:21 »
It's on the departure out of CGO, so I'll have to check and see if there are conditional waypoints. I am almost certain that no one has ever flown a whole oceanic trip on an offset route because when you coast out you're going to want to be in a non-offset position at the coast-out fix, even if you did want to offset for the oceanic part. And coming into PANC on an offset star is not normally done.

These are tough question and ones that they will probably never be able to test in real life. And in the sim there's no time to test a whole trip like this. I know it's impossible to program the unknown into PSX, but my friend is certain that it offsets the SID and continues after the SID. At what point beyond that where it stops the offset feature is a mystery. Maybe you can come to some middle-ground position like it would allow it to offset the SID and the enroute part, buy go back to the END OF OFFSET as it approaches the first fix in the STAR?

Markus Vitzethum

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Re: FMC offset route
« Reply #23 on: Sat, 14 Jul 2018 10:38 »
I just gave it a try in one of the 747 Freeplay FMSTs (FMC Systems Trainer) I have access to ... (which surely has it's own limitations and might not be a complete replica of the 747-400 FMC). Here it works as I expected. On a EGLL departure via a Dover SID, I get the the offset prompt on 6R only once past DVR VOR. In the FMST, selecting direct to a waypoint/navaid (which is part of the SID) did not bring up the offset prompt.

Speaking of the CGO departures, there are conditional waypoints, but it surely depends on the database supplier (we know that different NDB manufacturers having different coding styles).

Anyway, looking into the database there are some conditional waypoints but mostly early in the procedures (like CA - Course to Altitude) and some FD (Fix to Distance; I think that one is fixed in space) ones. The ones which are applicable for a direct are mostly TF fixes.

Markus
« Last edit: Sat, 14 Jul 2018 15:06 by Markus Vitzethum »

emerydc8

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Re: FMC offset route
« Reply #24 on: Sat, 14 Jul 2018 11:01 »
Thanks for checking this, Markus. I talked to my buddy again and he said it is also SOP to get offsets out of PVG. When Hardy's conditional waypoint question came up, he said that some of these SIDs are very long -- like 100 miles long -- and they don't ask you to offset prior to departure; so the conditional waypoints are not an issue -- They are always past them before they are asked to offset anyway.

Another friend of mine at the FAA is a nav specialist and he issues oceanic ops specs for US carriers. I remember a conversation with him years ago about the Chinese controllers having UPS offset on SIDs out of PVG and he was not happy about it because he believed the only way to do it would be in HDG SEL (which is how they did it at the time). So maybe the solution by Boeing/Honeywell was to "trick" the system into allowing an offset using the technique that is now being used by the same carrier on virtually every departure from that airport.

I'll see if the United 767 sim will allow this when I go to Denver for CQ later in the month. I guess this is one of those "not in any book" techniques, just like the deletion of the VECTORS condition in 1L on a vector SID when you press HDG SEL.

Cheers,
Jon


Hardy Heinlin

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Re: FMC offset route
« Reply #25 on: Sat, 14 Jul 2018 16:26 »
Here's a theory that might simplify the background. The theory is probably wrong, but anyway:

The offset feature may be enabled whenever the active waypoint is no conditional waypoint (and no hold, no arc, or similar un-offset-able waypoint); the texts in the manuals which say the offset is disabled on SIDs, STARs, approaches, may be simplified texts; perhaps they just refer to the fact that such conditional waypoints are usually in SIDs, STARs, approaches. So you may get the offset feature not only by activating a direct-to but also by autosequencing to a non-conditional waypoint, even if it's a SID leg. Perhaps pilots don't notice it because they don't look at the RTE page when flying a normal SID. -- End of theory.


Regards,

|-|ardy

emerydc8

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Re: FMC offset route
« Reply #26 on: Sat, 14 Jul 2018 22:46 »
I'm pretty sure the offset doesn't appear until the end of the SID, regardless of the waypoint type. On a video I did earlier, I was heading to the first fix on a STAR and that waypoint was  not conditional. I was getting a message END OF OFFSET. So if your theory is correct, wouldn't it still allow an offset at this waypoint?

Hardy Heinlin

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Re: FMC offset route
« Reply #27 on: Sat, 14 Jul 2018 23:00 »
Yes, my theory is probably wrong (or incomplete).

jcomm

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Re: FMC offset route
« Reply #28 on: Sun, 15 Jul 2018 09:12 »
Yes, my theory is probably wrong (or incomplete).

Don't worry HH, if it's worth it's better being  incomplete anyway... Godel is your friend :-)

Hardy Heinlin

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Re: FMC offset route
« Reply #29 on: Sun, 15 Jul 2018 16:09 »
Theory No. 2:

The direct-to obviously destroys the SID, so to make the rest consistent, the remaining SID legs become enroute legs (if there are no conditional waypoints). As a side effect, these legs will allow an offset. This destruction trick only works on the SID (and perhaps on the missed approach). Why? The SID guides us away from an airport, whereas the arrival guides us to an airport. Nobody wants to end up abeam the airport, but there's no problem to end up abeam a cruise leg (in most cases). So if you make a direct to an arrival fix, the remaining arrival legs will not be changed to enroute legs. -- End of theory No. 2.


|-|ardy

emerydc8

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Re: FMC offset route
« Reply #30 on: Sun, 15 Jul 2018 22:36 »
I'll have to check this one out when I get back to the airplane. I'm not so sure it's so much a question of whether anyone "wants" to end up abeam the airport as it is some trick that Honeywell added afterwards when they found out that China was making crews offset on SIDs and they looked for a way to get the box to do it instead of using HDG SEL. But I think your theory No. 2 is probably more correct.

Jon

Hardy Heinlin

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Re: FMC offset route
« Reply #31 on: Sun, 15 Jul 2018 22:59 »
I'm not so sure it's so much a question of whether anyone "wants" to end up abeam the airport ...

I was just mentioning that because I think the trick only works on SIDs and not on arrivals.


|-|ardy

emerydc8

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Re: FMC offset route
« Reply #32 on: Mon, 16 Jul 2018 01:24 »
I'll check that. I think it works on arrivals too, but we'll see.

Jon

emerydc8

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Re: FMC offset route
« Reply #33 on: Mon, 16 Jul 2018 07:45 »
Just got an email from my friend who confirms that they get offsets on the STAR going into PVG all the time too.

Hardy Heinlin

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Re: FMC offset route
« Reply #34 on: Mon, 16 Jul 2018 07:49 »
And where does it end?

emerydc8

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Re: FMC offset route
« Reply #35 on: Mon, 16 Jul 2018 10:22 »
I'll ask, but I doubt he will remember. What happens is you will eventually get vectored off the STAR for the approach and at that point you are just going to extend from the CF or FF and the STAR is flushed out.

Markus Vitzethum

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Re: FMC offset route
« Reply #36 on: Mon, 16 Jul 2018 12:04 »
> The direct-to obviously destroys the SID

Has anybody (ever) checked what's happening on the route page when choosing a direct-to on a SID? Does it keep the SID identifier or does it say DIRECT on 1L?

The question "what happens when you choose direct-to?" with two possibilities is probably the same, just in different words:
1) does it pull the direct-to-fix freshly out of the waypoint/VOR/NDB/... database inside the box?
2) does it just up-select the existing fix from the LEGS page?

(Keeping the route page should also answer when the offset prompt appears.)

There is one difference with offsets on STARs - it's well documented in CBTs that route offsets will end at the last route waypoint and the airplane will keep its present heading and say END OF OFFSET. I learn this from this random 767 CBT from youtube:
https://youtu.be/-5zYseI6ddk?t=588

Markus

jcomm

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Re: FMC offset route
« Reply #37 on: Mon, 16 Jul 2018 15:25 »
I didn't know it was possible to set the start and end fixes for the offset...

https://youtu.be/OOfOneUP-dE?t=5201

Also got this from a 737 driver: "You can offset up to  the approach or approach transition on the 737."

And this @ PPrune regarding 777: https://www.pprune.org/tech-log/527957-lateral-offset-fmc-777-a.html
« Last edit: Mon, 16 Jul 2018 16:02 by jcomm »

Hardy Heinlin

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Re: FMC offset route
« Reply #38 on: Mon, 16 Jul 2018 17:26 »
1) does it pull the direct-to-fix freshly out of the waypoint/VOR/NDB/... database inside the box?
2) does it just up-select the existing fix from the LEGS page?

It must be (2) as the direct-to also works for pilot created fixes like PBDs or Lat/Lons. Why should the FMC pull it from the nav database if the pilot refers to an existing route fix?


|-|ardy


jcomm, the 737 and 777 FMCs are different to the 744 FMC.

Hardy Heinlin

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Re: FMC offset route
« Reply #39 on: Tue, 17 Jul 2018 06:22 »
I'll ask, but I doubt he will remember. What happens is you will eventually get vectored off the STAR for the approach and at that point you are just going to extend from the CF or FF and the STAR is flushed out.

I just implemented the same trick to STAR legs. Most STARs contain normal legs anyway.

It works very well. The offset ends at the approach transition.

(The VIA identifiers of the STAR legs remain intact.)

The trick has a desired side effect: Once a direct-to has "destroyed" the STAR procedure, the procedure becomes a pilot created route segment, and therefore it will not be autodeleted when another STAR is selected. As we know, the selection of another procedure normally deletes the previously selected procedure.


|-|ardy