A320 Direct Law
Joined: Oct 2007
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From: Greece
I guess because pitch flare mode is not available in alternate law. So as soon as gear is down pitch goes to direct law, since load factor demand stick input does not produce a conventional feeling for landing. Roll law (direct) is not affected when selecting gear down anyway.
Joined: Sep 1999
Posts: 311
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From: UK
Aristoclis' reasoning is on the money. That's correct. Alternate law (with or without protections) can not give FLARE mode. It would feel very strange and unconventional attempting to land with pitch being given as a load factor. The gear being selected down generally means you are going to be landing which is why Direct Law is activated at this point.
Incidentally, the only (to my knowledge) occasion of going from Normal straight into Direct law on gear down selection is a dual radalt fault. For similar reasoning, the 'system' won't know when to blend in FLARE mode due to no radalts, that's why direct law comes in.
Hope this helps.
Incidentally, the only (to my knowledge) occasion of going from Normal straight into Direct law on gear down selection is a dual radalt fault. For similar reasoning, the 'system' won't know when to blend in FLARE mode due to no radalts, that's why direct law comes in.
Hope this helps.
Joined: Mar 2005
Posts: 1,338
Likes: 1
From: Uh... Where was I?
By the way
It's an annoyance that the double radalt fault ECAM does not mention the landing distance procedure. So if, on the spur of the moment, you don't remember that direct law always has one associated, you will find out in final approach when selecting gear down.
I think the ECAM should include the approach procedure in the Status, as it does in all the failures leading to alternate law. I mean just the speed increment and the config 3 so you can calculate the landing distance required.
It's an annoyance that the double radalt fault ECAM does not mention the landing distance procedure. So if, on the spur of the moment, you don't remember that direct law always has one associated, you will find out in final approach when selecting gear down.
I think the ECAM should include the approach procedure in the Status, as it does in all the failures leading to alternate law. I mean just the speed increment and the config 3 so you can calculate the landing distance required.
Joined: Oct 2007
Posts: 98
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From: Greece
From FCTM NO-160.P2/12
When reaching 50 ft, auto-trim ceases and the pitch law is modified to flare law. Indeed, the normal pitch law, which provides trajectory stability, is not the best adapted to the flare manoeuvre. The
system memorizes the attitude at 50 ft, and that attitude becomes the initial reference for pitch attitude control. As the aircraft descends through 30 ft, the system begins to reduce the pitch attitude
at a predetermined rate of 2 ° down in 8 s. Consequently, as the speed reduces, the pilot will have to move the stick rearwards to maintain a constant path. The flare technique is thus very conventional.
system memorizes the attitude at 50 ft, and that attitude becomes the initial reference for pitch attitude control. As the aircraft descends through 30 ft, the system begins to reduce the pitch attitude
at a predetermined rate of 2 ° down in 8 s. Consequently, as the speed reduces, the pilot will have to move the stick rearwards to maintain a constant path. The flare technique is thus very conventional.
Joined: Mar 2005
Posts: 1,338
Likes: 1
From: Uh... Where was I?
Pensador, thank you
A TRE told about that to me. I wouldn't have noticed myself!
I wonder if it is something the airbus guys missed or there is a good reason to omit the
FOR LDG.................USE FLAP 3
APPR SPD................VREF + 10 KT
LDG DIST PROC........APPLY
A TRE told about that to me. I wouldn't have noticed myself!
I wonder if it is something the airbus guys missed or there is a good reason to omit the
FOR LDG.................USE FLAP 3
APPR SPD................VREF + 10 KT
LDG DIST PROC........APPLY
Joined: Jan 2007
Posts: 46
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From: VXXX
In support of what aristoclis has said, this image shows that it has to transition from Normal - (failure) - Alternate - (g/down) - Direct.
This is from DSC 27-20-20 P3/8
Apart from this, does anyone know if it is documented anywhere that the aircraft cannot transition from alternate law to the flare law? Or is this an answer we leave to "by design"
This is from DSC 27-20-20 P3/8
Apart from this, does anyone know if it is documented anywhere that the aircraft cannot transition from alternate law to the flare law? Or is this an answer we leave to "by design"
Joined: Jun 2007
Posts: 3,443
Likes: 39
From: Wanderlust
radical 100 and aristoclis
What you have stated is absolutely correct. In normal and alternate law as long as stick is out of neutral it is a constant load factor demand. Without the flare mode you will have to flare and neutralise the stick, flare and neutralise the stick. Which is not a good way to land. Flare mode allows continuous back pressure as the auto trim stops and aircraft starts pitching down. This mode not being available in alternate law, a direct stick to elevator relationship is given as flare mode where aircraft pitches down as the thrust is reduced. Direct law causes concern as it is described as the lowest form of flight control laws( other than mechanical back up) but when you look at it as another flare mode to help you land it doesn't look so bad. In fact landing in alternate law will be an ordeal.
What you have stated is absolutely correct. In normal and alternate law as long as stick is out of neutral it is a constant load factor demand. Without the flare mode you will have to flare and neutralise the stick, flare and neutralise the stick. Which is not a good way to land. Flare mode allows continuous back pressure as the auto trim stops and aircraft starts pitching down. This mode not being available in alternate law, a direct stick to elevator relationship is given as flare mode where aircraft pitches down as the thrust is reduced. Direct law causes concern as it is described as the lowest form of flight control laws( other than mechanical back up) but when you look at it as another flare mode to help you land it doesn't look so bad. In fact landing in alternate law will be an ordeal.
Last edited by vilas; 12th June 2013 at 05:42.
