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737 Vertical Profile with Extended Centreline

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737 Vertical Profile with Extended Centreline

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Old 16th Aug 2001, 00:04
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Question 737 Vertical Profile with Extended Centreline

What does the vertical profile on the EHSI actually show when you have an extended centreline displayed?

eg) You're downwind (or base) 5 miles abeam the centrefix. Does the profile assume you are at the centefix heading the right way? Or, does it allow for the track miles required to turn and cover the 5 miles + between you and the centrefix?
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Old 19th Aug 2001, 14:52
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Anyone know the answer to this? Still can't find it in any of the manuals.
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Old 19th Aug 2001, 15:30
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Hi BM21, I will go out on a limb here at least partially. The profile in your eg. will not assume you at at the CF going the right way. Lets say CF06 157/2000, that as you know is your next descent restriction and thats what you path/profile will be based on. If you are flying away from it on a downwind leg the ROD required to meet this restriction will reduce.

Next bit, I believe the path is calculated on a direct line from your FMC computed present position, however I will do some more work on this and get back to you.

Best rgds
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Old 20th Sep 2001, 20:09
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Hi bmi21,

I can’t find a reference for it, but from experience with an extended centreline your profile is based on you being on the centreline, abeam your present position, heading towards the runway.

The way to demonstrate it is to construct your predicted route when you are on an intercept heading (not good operational practice but will prove the point) and note your vertical deviation from the profile, then make your extended centreline and you will see that you are now higher on the profile than you were.

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Old 20th Sep 2001, 22:42
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Cool

Thanks CaptainSandL, ----------------------
----------------------------

Must try it sometime.

[ 21 September 2001: Message edited by: BmPilot21 ]
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Old 21st Sep 2001, 03:53
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Alas my cover is blown. Rgds to all at Midland.
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Old 21st Sep 2001, 12:31
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Sorry S&L, this is supposed to be an
anony-mouse forum!
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Old 22nd Sep 2001, 07:11
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hi Bm,

for my experience on the B737, B777 & B744(B737's FMC being Smith and B777 & B744's FMC being Honeywell) they behave quite differently when you select extended centerline (which is, i believe, what most of us do when we are being radar-vectored).

for the B737, i noticed the vertical profile on your EHSI will be in gross error if you are heading AWAY from the CF. the FMC seems to calculate your current profile based on your current track (straight ahead) till the point of intercepting the inbound track, thus giving you a false sense of being low on profile. it would be quite accurate if you are heading TOWARDS the CF. to overcome this problem, i dont usually select extended centerline until i am approx on an inbound course.

whereas on the B777 & B744, when extended centerline is selected, the FMC assumes a great circle track from present position to the next fix (in this case, would be the CF) thus give you a rather accurate descent profile.

while i am writing this response, an idea pops into my head. i remember the B737 FMC also assumes a great circle track to the NEXT waypoint IF there is a DISCONTINUITY. as i am not on the B737 anymore, BmPilot, could you try this out and see if it give a more accurate profile as compared to an extended centerline??

any other B737 guys have any say?

HS
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Old 26th Sep 2001, 03:46
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Talking

Had a good look at this the other day. The FMS on our 733's definitely assumes you are on the magenta track (abeam your current pos'n), moving towards the R/W, regardless of your current track.

This was indicated by us seemingly us intercepting our profile from above, by being downwind & in level flight. The altitude the profile wanted us to be at corresponded perfectly with the ILS/DME distances on the plate.
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Old 30th Sep 2001, 14:03
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Downwind abeam the CF the other day, I brought the CF to the top (DCT CF), and was 1700' high. Then selected extended centreline and it jumped to 3000 feet high. I guess this proves that it does assume you are on the extended centreline.

High speed, don't quite understand your last bit. If you select DCT CF (CF to top) then it will give you an accurate profile as the magenta line does a direct track to the CF. However, it does not update your current position as you move, so you'd have to keep bringing it to the top. Can't find anything about great cirlce tracks / discontinuities.

I might be able to give it a try if you describe how to do it. Do you mean create a discontinuity AFTER the CF?
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Old 1st Oct 2001, 18:56
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propeller,

what i meant was a discontinuity BEFORE CF. to accomplish this, you select a waypoint before CF to the top then DELETE that waypoint. the result will be a discontinuity from present position to CF. come to think of it, i'm now not too sure if you are able to do that with the Smith's FMC (B737), it had been awhile since...

BmP, looks like from the experiment of Phoenix & Propeller, it seems like my assumption was wrong.

HS
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Old 1st Oct 2001, 22:52
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HS, I'll give it a go and see what it does.
Regards.
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