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A320 autoland : what’s the significance of ILS course check at 350ft RA?

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A320 autoland : what’s the significance of ILS course check at 350ft RA?

Old 20th Jan 2018, 02:03
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Thanks C.M
We don't do autoland so had not dug that deep.
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Old 20th Jan 2018, 13:08
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In my previous 320 job, I have never seen the track/course differ by more than 1degree.
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Old 2nd Feb 2018, 10:47
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Just to confirm this as I have checked on a simulator. The aircraft will change the localizer dagger on the PFD to the front course loaded into the MCDU (so long as this is changed before the MCDU data lock at 700AGL, anything below 700AGL has no change).
The aircraft will continue to track the localizer like normal showing the lozenge central on the PFD (still tracking the correct localizer beam) but the dagger will be whatever is loaded in the MCDU. Then in the flare the aircraft will align with the course entered into the MCDU. The 5 degree limit (before go around is required) between ILS course and course on the PFD is mentioned elsewhere in the FCTM where in a crosswind you can land with up to 5 degrees of drift. Anything greater than this then you may risk damage to the gear due to torsional forces.
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Old 2nd Feb 2018, 20:59
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Thanks for that, mighty good work!
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Old 3rd Feb 2018, 10:20
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Hi Guys. If I’m not mistaken, the FMS data is frozen at 700ft but the FCU is only frozen at 400ft hence the check at 350ft and not earlier.
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Old 4th Feb 2018, 20:11
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That's quite a nice find! Ok, now, we check it for a reason. If the course it's wrong, then what? You can't change it because the mcdu is locked. So, manual landing? Revert to fail passive if in CAT III ops?
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Old 5th Feb 2018, 05:37
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If it differs by more than 5 degrees its a mandatory go around.
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Old 6th Feb 2018, 23:21
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Thanks Denti. Do you have a reference?
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Old 7th Feb 2018, 05:42
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Well, the basic reference has been given in post #20, however the specification to 5 degrees i found only in the OM B of my current airline.
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Old 8th Feb 2018, 03:57
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Ah thanks for that one. I sort of missed it
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Old 8th Feb 2018, 06:42
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applecrumble

That deepens the mystery .... Its commonly said and understood that the roll-out mode simply tracks the LOC beam ...... in the same way it does during the approach... now it would appear that a difference course in the FMGS acts as a track command on ground .
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Old 8th Feb 2018, 09:37
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The track is used to align the flare down the correct course (remember you can lose the localiser below 15’ and the aircraft will still autoland). You won’t get the yaw bar if you don’t have a LOC signal at this stage.
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Old 8th Feb 2018, 09:54
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Hi C.M
now it would appear that a difference course in the FMGS acts as a track command on ground
No. The course in the FMGS will be the aircraft's touch down heading.
The LOC signal will guide the aircraft along the centre line. That's why there is a 5 degree error limit.
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Old 8th Feb 2018, 11:38
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C.M
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Hi goldenrivett
You are right , in the flare mode it should be heading so a de-crab and align the nose with the runway.
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Old 8th Feb 2018, 11:51
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Hi goldenrivett
You are right , in the flare mode it should be heading so to decrab and align the nose with the runway. And once ROLL OUT mode engages , whatever is in the FMGS should be irrelevant since after touch-down it is the LOC signal that guides the aircaft . To sum -up the whole issue should be for correct decrabbing - runway alignment to avoid main wheel torsional stresses.
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