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KORD - RWY 10L (LOCalizer and APP question)

Started by vnangli, Sat, 3 Jun 2023 22:48

vnangli

This question is coming out of curiosity. I was practicing my flight into KORD and on RWY10L. I noticed the GLIDESLOPE DIAMONDs were solid magenta at the Waypoint BURKE (7000A). At this point I armed LOC and APP. I was on LNAV and VNAV (VNAV PTH)

However on this RWY 10L approach, the LOCalizer doesnt become ACTIVE till crossing the WPT PRATT. Is this how this RWY 10L is supposed to be?
747 is not an airplane, it is a symbol of inspiration....

Magoo

Could it be a range issue, BURKE is about 20 NM out and PRATT is about 15 NM so the FMS is waiting for a good G/S  signal before capturing LOC/GS together?

Hardy Heinlin

A situ file that shows the problem is always the best basis for a discussion.


As far as I recall, the LOC capture condition is not just "deviation from center" but also deviation rate. I.e. when the LOC diamond is close to the center but moving very slowly, it may be not close enough for a capture yet. Otherwise, an early capture at a low closure rate would cause the aircraft to make an undesired S-turn instead of a smooth intercept.

Hardy Heinlin

#3
Just tested it. It's like I wrote in my previous comment.

Only at 12 nm the LOC diamond is close enough to the center for a capture. It would capture earlier if you were on a greater intercept angle. But on this LNAV route via BURKE on 093° the intercept angle is just 1°, so the LOC diamond will move very slowly. At that point the diamond is off-center by almost half a dot. After CARLE the route track changes to 094°.

I would say this is all normal. You don't want to track the 094° localizer while your STAR still instructs a 093° track to CARLE.

There may be rounding errors in the track numbers on the LEGS page due to locally differing magnetic variation, but the true-north track on the ND map shows the truth: There you can see the difference between the runway heading (white dashed line) and the LNAV route (magenta). There's a tiny little difference.

vnangli

Quote from: Hardy Heinlin on Tue,  6 Jun 2023 06:47...on this LNAV route via BURKE on 093° the intercept angle is just 1°...

Thank you...

I will try with HDG SEL and intercept the APPROACH at BURKE with a larger angle. Will report back.

Used AEROMAP for the first time, it is a good addon. Thanks to the Developer.
747 is not an airplane, it is a symbol of inspiration....

Hardy Heinlin

Why HDG SEL? Why do you want to leave the STAR? You're not supposed to capture the ILS that early.

The STAR is not wrong. That slight offset is intentional. Just follow it with LNAV.

vnangli

Quote from: Hardy Heinlin on Tue,  6 Jun 2023 14:03Why HDG SEL?

Wanted to check if LOCalizer would go active at BURKE, if I were to intercept at a larger angle.

This is what I was doing till I ran into this question.
1) Flight from KSDF to KORD.
2) STAR - WYNDE TWO arrival
3) After crossing POSSM, I had chosen HDG SEL (to replace Vectors) to intercept RWY 10L Approach at or before ARIST
4) Engage LNAV at around ARIST
5) After crossing ARIST, I would arm LOC. Thats when I would notice LOC not active till PRATT

After understanding from your response, I was going to switch from LNAV to HDG SEL (approximately 007 deg) at around POSSM to intercept BURKE and arm LOC to and see if Localizer would go ACTIVE without waiting to reach PRATT.

I wish I could share the situation file. I am at work and couldnt resist replying to the post :-) ::)
747 is not an airplane, it is a symbol of inspiration....

vnangli

Quote from: Hardy Heinlin on Tue,  6 Jun 2023 06:47It would capture earlier if you were on a greater intercept angle...

LOC was active at BURKE now...Thank you Hardy.....
747 is not an airplane, it is a symbol of inspiration....