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When the active leg is VECTORS

Started by Hardy Heinlin, Thu, 9 Feb 2017 07:30

Hardy Heinlin

Thanks to Peter's and Jon's tests in the big sims, we learned some new features that are not documented in official manuals, and it seems most Boeing pilots are not aware of them. I think these features are very useful; they are now implemented in PSX version 10.1.1.

The new main features are:

• VECTORS need not be sequenced by doing a direct-to on the LEGS page anymore; you can also keep your heads up and just engage HDG SEL on the MCP.

• If the next intercept leg course is not defined in the database, that next leg will be floating such that it always has a direct-to geometry suitable to the present position.

• When LNAV engages, that floating leg automatically becomes a static direct-to leg. No CDU typing required anymore.


Step 1

VECTORS will become active when CUGBA is sequenced. The leg to NATHN is dynamic and has a floating starting point; the FMC is assuming the pilot will fly directly to NATHN as soon as CUGBA is sequenced.

Step 2

VECTORS is now active. On the LEGS page the line title above VECTORS reads 210°HDG. "HDG" indicates that there is no fixed geographic route to be flown but a heading, so LNAV will now maintain heading 210°. The vector line on the ND previously had the same course as the leg to CUGBA, but due to some crosswind a different heading was used on the leg to CUGBA (that leg was a fixed geographic leg). This is why the aircraft is now making a slight turn as the crosswind can now be ignored; the command is just "maintain heading 210°", nothing else. The crosstrack error is zero as long as a heading based vector is active.

Step 3

If the vector is heading based and not tied to a fixed geographic leg, the vector line on the ND will always start at the aircraft symbol, no matter where the aircraft is. (If the line title above VECTORS had no "HDG" suffix, the vector would not float, i.e. its starting point would stay at the last waypoint.)

Step 4

ATC just instructed "fly heading 290". So, select 290 on the MCP and press HDG SEL. As soon as HDG SEL is engaged, the FMC will automatically sequence the VECTORS leg. This trick is also possible before passing CUGBA (has been proven in the BA sim). This is possible because, I think, CUGBA is the anchor of the expected vector, and there is no reason to keep both VECTORS and CUGBA in the route if HDG SEL is engaged shortly before.

Step 5

In the meantime ATC instructed "fly heading 340", so 340 has been selected and HDG SEL is now maintaining heading 340°. The compass rose on the ND is indicating a current track (TRK) of 338° due to some crosswind; the heading pointer is sitting exactly on 340°. As the aircraft flies along, the starting point of the leg to NATHN will be repositioned every second so that the line can anytime be used as a direct-to; the calculation of the starting point always takes an inital turn into account, based on the current TAS. However, the turn is not shown yet. -- Now arm LNAV.

Step 6

When LNAV is armed in such a situation, LNAV will engage just a second later. When engaged, the turn segment appears and the direct-to leg becomes static. LNAV will guide along this leg.

Here's the situation file -- Vectors01.situ -- that I used for this example. If you like, just right-click on the link and save the link directly to your Aerowinx/Situations folder.




This is very helpful, Hardy. Thanks.
Will /Chicago /USA



Hi Hardy, quick question about "Step 5".

You wrote: "340 has been selected and LNAV is now maintaining heading 340°"

Forgive my ignorance: Can you please elaborate why LNAV is maintaining 340 if you're flying HDG SEL according to the active roll mode on the PFD? I understand it is later in the leg that you arm LNAV to intercept NATHN.

Thanks :-)

Enrique Vaamonde

Robert Staudinger

A very clear explanation, thank you very much.

Hardy Heinlin

Thanks! That was a typo. It now reads: "340 has been selected and HDG SEL is now maintaining heading 340°".



Thanks again, Hardy. I noticed on my demo I just posted that there was a TRK to GMN after VECTORS. Is there a reason for this, versus just a course to GMN like my course to VTU on the video I did in the sim a few weeks ago?


Hardy Heinlin

They coded it like this in the database; for Rwy 25 and 24 the leg to GMN has a "CF" path terminator, and from Rwy 06 and 07 a "TF". It's not wrong anyway. And if it was a course instead of a track, it would have no effect on such a short distance either. (Great circle leg with changing course indication versus spiral course around the globe with static course indication.)




Is the information in this tutorial, and the accompanying situ file, still accurate?

I'm asking because when I load the situ, the magenta line from NATHN does not angle left towards CUGBA, but continues on away from the final approach path along the extended centerline of the runway. The magenta line continues outwards without updating. In other words, I do not see this behavior: "As the aircraft flies along, the starting point of the leg to NATHN will be repositioned every second so that the line can anytime be used as a direct-to."


I know there has been a lot of work with the VECTORS logic since 2017, so I'm wondering if something has changed since the creation of this tutorial and this situ, or maybe I'm doing something wrong.

Will /Chicago /USA

Hardy Heinlin

Thanks for the hint. Indeed, since version 10.61 (19 Feb 2019) the dynamic leg function is inhibited if the leg belongs to an approach procedure:

QuoteVersion 10.61 - 19 February 2019

61.12. Intercept leg after VECTORS can no longer be dynamic if it is an approach leg.

You'll get it on a SID. The tests in the big sims were done on SID legs. There it worked. On approaches it didn't work.




Got it. I just tried it on the SEATTLE SEVEN departure from KSEA, and the VECTORS + dynamic intercept leg works well.
Will /Chicago /USA


Very nice analysis, Hardy!

Much better than "Why is it doing that?" !