A320 Direct Law

[Flyman35]

why when we fly in Alternate law as soon as Gear is Down the A/c goes to direct law?

[aristoclis]

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.

[Localiser]

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.

[Microburst2002]

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.

[pensador]

Microburst2002, very reasonable notice!

[Capt Groper]

< 100' it's called Flare Law = direct input to the elevators for landing.
Roll still in Normal Law until touchdown.

[aristoclis]

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.

[Microburst2002]

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

[radical_100]

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

http://img560.imageshack.us/img560/9831/screenshot20130612at959.png

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" :)

[vilas]

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.

[vilas]

Capt Groper
I am afraid what you stated is incorrect. What aristoclis has stated reply 7 is correct.

[Airbus_a321]

yes it's indeed a bad trap in the ECAM design and should be changed ASAP