A320 LOC intercept with FMGC

[Check Airman]

Hi all,

I remember hearing some time ago that the A320 uses the FMC to help intercept the localiser. Can't find that information in my company's manuals. Can anyone provide details?

[vilas]

Hi CA
There are two functions that help capture the Localiser. One is LOC CONVERGENCE FUNCTION and the other is ENHANCED LOC CAPTURE FUNCTION. They have different requirements. They are given in Aircraft Systems, Auto Flight- Fight Guidance- AP/FD Common modes- Approach. Enhanced Loc Capture enables the capture to start earlier to prevent overshoot. The salient features below:
The LOC* mode engagement in the pre-capture zone is possible, when :
‐ The LOC deviation is more than 2.3 dots
‐ The FMS is in GPS PRIMARY
‐ The difference between the track and the QFU is between 25 ° and 115 °
‐ The guidance roll order is such that LOC* will capture the LOC beam without overshoot.
When the LOC deviation becomes lower than 2.3 dots, LOC* mode no longer uses the FMS
position data for guidance, but the actual LOC beam deviation to complete the capture of the
axis.

[IFLY_INDIGO]

Hi all,

A320 uses the FMC to help intercept the localiser.

Also note that Without FMGCs, pilot has to fly ILS raw data manually. That itself answers your question. :)

[Check Airman]

Thanks,

That's exactly what I was looking for.

[vilas]

INDIGO
The question is not about loss of FMGC. It is about use of FMS in APP mode for the precapture of localiser. You may have FMGC but if GPS primary is not available the enhanced loc capture is not available. But you still have FD/AP. You don't need to fly raw data. Similarly the Loc Convergence Function requires NAV mode. You can be flying in HDG mode with FD or with AP on but this function will not be available.

[Check Airman]

How does the Loc Convergence function work?

[vilas]

Check in the FCOM the reference I have given. Explained nicely with a diagram.

[pineteam]

FCOM_Aircraft Systems_ Auto Flight- Flight Guidance_AP/FD Common Modes_ Precision Approach_ Precision Approach Modes.

One of the condition is: ''The guidance roll order is such that LOC* will capture the LOC beam without overshoot.''
Does it means like if we come like 90 degres from the final approach course at high speed the LOC convergence function won't be triggered?

[FlightDetent]

And then there's this:

FMS1 HONEYWELL TEMPORARY ABNORMAL BEHAVIORS
FMS2 HONEYWELL TEMPORARY ABNORMAL BEHAVIORS
FMS2 THALES TEMPORARY ABNORMAL BEHAVIORS
ERRONEOUS LATERAL GUIDANCE IN NAV MODE WITH LOC MODE ARMED DURING APPROACH


https://cimg9.ibsrv.net/gimg/www.gmforum.com-vbulletin/592x447/nzke_14ca4034a7a1b56c3b77c237955c2bddbefac975.png

[pineteam]

Yeah I saw this one many times when flying straight in approach in Hangzhou. Many guys were unaware of this function and thought there was something wrong with the guidance system as the aircraft is drifting away from the green line on the ND. :}

[FlightDetent]

"It's not a bug, it's a feature".

[pineteam]

"It's not a bug, it's a feature".

Yeah, I know. But looks like many skippers are unaware of it.:}


One of the condition is: ''The guidance roll order is such that LOC* will capture the LOC beam without overshoot.''
Does it means like if we come like 90 degres from the final approach course at high speed the LOC convergence function won't be triggered?

Anyone to clarify this?

[Check Airman]

Check in the FCOM the reference I have given. Explained nicely with a diagram.

Thanks Vilas. My current company has customized the manuals, so there's no mention whatsoever of the enhanced function. I was able to find the information in an old manual though. I also saw the information on the convergence function, but no diagram.

[pineteam]

You won't see any diagram in the FCOM for the Convergence Function as far as I know; There is only one graph for the Enhanced Loc Capture Function ( see below). In our fleet, all the Airbuses, had the Convergence Function feature but the oldest models did not have the Enhanced Loc Capture Function ( A321-100). You could see clearly the difference during the LOC*.
One of the old A 321-100 was famous for banking suddenly up to 33 degrees while doing a tight localizer interception.https://cimg6.ibsrv.net/gimg/www.gmforum.com-vbulletin/489x421/img_5341_5596__57ed9e674d58e67400c5bdff9bcda94ad83eea89.jpg

[vilas]

True! The graph is only about ELC function.

[Check Airman]

Thanks for the info guys