Direct intercept course; VNAV glide path; FMC
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Joined: Oct 2011
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From: world
Direct intercept course; VNAV glide path; FMC
Hi,
Just a question concerning the B737NG "direct intercept" feature. I tried to find my answer in books, ask advises to peoples, but nobody have a clear answer for me, so I ask you through this forum guys.
Let's say you are approaching your final airport, and the IAF is called XXX.
You are in a LNAV roll mode, direct to XXX, so you have a magenta line from your present position, to the XXX point, your current track is 090, and you are on VNAV PATH (so on the proper glide path calculated by the FMC to XXX).
Now, you are on a radar vectoring; you press the LEGS page on the FMC, select the XXX point, and set the final approach course to 030, press EXEC, and then you have a magenta runway extension line, and suddently your VNAV glide path on the ND, says you are too high by hundred of foot.
My question, is, how is this "direct intercept" glide path calculated (from which point it calculates that you are too high now from before), is that reliable?
Many thanks,
Just a question concerning the B737NG "direct intercept" feature. I tried to find my answer in books, ask advises to peoples, but nobody have a clear answer for me, so I ask you through this forum guys.
Let's say you are approaching your final airport, and the IAF is called XXX.
You are in a LNAV roll mode, direct to XXX, so you have a magenta line from your present position, to the XXX point, your current track is 090, and you are on VNAV PATH (so on the proper glide path calculated by the FMC to XXX).
Now, you are on a radar vectoring; you press the LEGS page on the FMC, select the XXX point, and set the final approach course to 030, press EXEC, and then you have a magenta runway extension line, and suddently your VNAV glide path on the ND, says you are too high by hundred of foot.
My question, is, how is this "direct intercept" glide path calculated (from which point it calculates that you are too high now from before), is that reliable?
Many thanks,
Joined: Oct 2000
Posts: 114
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From: uk
On the classic when you enter an extended r/w centreline the path is calculated as if you are on the centreline. Hence if you had a few miles to run to centreline your profile will be correct.
However the FMC thinks your on the centreline and therefore high on the profile.
However the FMC thinks your on the centreline and therefore high on the profile.
Joined: Sep 2008
Aviation Qualifications: ATPL
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From: 41S174E
So on the NG is it the same? Ie does the FMC calculate based on a line from present position direct to the centre line and then to the threshold? I don't think it does, maybe it factors the turn onto the line direct to the centre line as well?
Joined: Dec 2012
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From: On SBY next to my phone
Framer I know for sure it doesn't because when you are on a high angle interception it sometimes shows 2000-3000' high even though track miles vs height is good. When you approach the centreline on profile the bug goes to the middle.
Joined: Jun 2003
Posts: 246
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From: The Netherlands
Direct intercept course; VNAV glide path; FMC
When in HGG SEL on intercept (and before next waypoint) VNAV calculates as if you were on the magenta line abeam your present position.
When in LNAV on the same intercept it calculates with your estimated track.
Do a search, has been explained before here.
When in LNAV on the same intercept it calculates with your estimated track.
Do a search, has been explained before here.
