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Cat 3 single vs Cat 2 on A320

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Cat 3 single vs Cat 2 on A320

Old 2nd Apr 2018, 01:32
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Cat 3 single vs Cat 2 on A320

Ladies and Gentlemen, once again, I am in need of your technical expertise.

Looking at FCOM SRP 01-70 for the definitions of Fail passive system, I am a bit perplexed by the following:

An auto. landing system is fail passive if, in the event of a failure, there is no significant out of trim condition etc, but the landing is not completed automatically.

What if that failure results in a capability degradation to CAT 2? The landing could still be completed automatically.
Let's assume we are executing the approach and above 1000 feet, FMA goes from CAT 3 single to CAT 2. If the RVR allows it and we have time to change the minimums in the MCDU, we could continue to a CAT 2 Autoland.
So why is Airbus stating that "the landing is not completed automatically" after a failure from a fail passive system?

What am I missing?


Thank you
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Old 2nd Apr 2018, 08:06
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I `m no expert, but could the degradation to CAT 2 trigger an autopilot disconnect in some scenarios?
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Old 2nd Apr 2018, 08:06
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Loss of auto thrust would be the first failure that comes to mind.
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Old 2nd Apr 2018, 09:16
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saviboy

Just consider where the alert height is and a failure at that point. Is the autopilot staying in or coming out? Failures prior to that point you have (potentially small!) amounts of thinking time and so aren't really as critical.
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Old 2nd Apr 2018, 09:24
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Fail passive/operational is more to do with what happens later on the approach, eg for fail operational, a single AP failure when cat 3 Dual below Alert height will still result in an automatic landing. In a fail passive system (cat 3 single shown), this same loss of AP will result in the autoland warning light and a manual go around is required.
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Old 2nd Apr 2018, 10:49
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Originally Posted by saviboy
Ladies and Gentlemen, once again, I am in need of your technical expertise.

Looking at FCOM SRP 01-70 for the definitions of Fail passive system, I am a bit perplexed by the following:

An auto. landing system is fail passive if, in the event of a failure, there is no significant out of trim condition etc, but the landing is not completed automatically.

What if that failure results in a capability degradation to CAT 2? The landing could still be completed automatically.
Let's assume we are executing the approach and above 1000 feet, FMA goes from CAT 3 single to CAT 2. If the RVR allows it and we have time to change the minimums in the MCDU, we could continue to a CAT 2 Autoland.
So why is Airbus stating that "the landing is not completed automatically" after a failure from a fail passive system?

What am I missing?


Thank you
Fail Passive (a failure after reverting to CAT 3 SINGLE) means that if this is triggered, the autopilot will no longer be flying the aircraft but it will continue on the same trajectory it had before the failure (until disturbed by your or turbulence).

Fail operational means the autopilot is still flying the aircraft and will continue to land automatically (e.g. CAT 3 DUAL reverting to CAT 3 SINGLE).

Both of these conditions refer to the auto land capability.

A reversion to CAT 2 simply means you could not satisfy the requirements for a CAT 3 and you need to use higher minima. It does not affect the autopilot behaviour. You can continue with an autoland or revert to a manual landing before 160ft AGL.

In your approach scenario it depends on the failure as to what you get, if you have failures that cause the loss of CAT 3 DUAL and CAT 3 SINGLE you will be left with CAT 2 or CAT 1 capability because Airbus say the reliability of the landing feature isn't as good. If there are further failures that may also mean the AP will fail, in which case you will get the auto land warning light and you will see the AP trip off.

I think of it as two parts:

Who is flying? DUAL (two AP's), SINGLE (one AP) or Manual (no AP/passive)
What minima?
CAT IIIB =NO
CAT II = DH100 or 200
CAT 1 = regular Decision Altitude
I hope that helps.

TSIO
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Old 2nd Apr 2018, 22:41
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As usual, thank you very much everyone for the assistance.
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