A320 Flight control law Landing Mode
 

Hey guys

Fcom extract " At 50ft the attitude is memorized as reference pitch attitude .

At 30 ft this value is progressively reduced to 2degrees nose down to induce a gentle positive action for a conventional flare "

Please help me understand the logic behind this.

How is this supposed to help the pilot make a conventional flare ? do non fly by wire aircraft pitch down on their own in final descent ?

wont the pitching down increase the rate of descent close to the ground ?

also when in direct law i am assuming this wont happen so is there a marked difference in landing the 320 in direct law when compared to normal law ??

Thanks

In normal law if you put continuous pressure on the sidestick you'll get a constantly increasing pitch attitude. This doesn't work well for landing, as you'd need to apply pressure, release to neutral, then fine tune.


By introducing a nose-down command in the flare, you can apply back-pressure and hold it, as you would when flaring a traditional aircraft. Airbus didn't word it well in the FCOM. In direct law the landing is almost identical provided you're stabilised in pitch before beginning the flare.

Reverb_SR71.
While landing a conventional aircraft you pull the yoke back close thrust and as the nose drops also as the speed is dropping you need to keep pulling on the yoke to control the descent till touch down. In Airbus FBW stick out of neutral is some load factor demand. So when you flare by moving the stick back you ask for certain load factor and the aircraft starts pitching to give you that and will continue to pitch as long as the stick is out of neutral. So you will have to keep releasing the stick to neutral after each backward flare movement. This will not work well all the time so they have devised the flare mode where the auto trim stops and from 30 feet the aircraft starts pitching down. The pilot can now maintain steady backward pressure like conventional aircraft and continue to land.
In direct law pilot directly moves the elevators and computers do not modify control out put so aircraft behaviour is like a conventional aircraft from the time it goes in direct law. In pitch Alternate law is same like normal law and due to multiple failures flare mode is not available so aircraft is put in direct law for landing. Direct law is flare mode of alternate law.

Does this logic ensure that Normal Law and Direct/Alternate Law have approximately the same "feel" for the pilot during the flare? Or does the pilot need to be practised in two different landing techniques?

I also read that conventional aircraft have a pitch down tendency due to ground effect dynamics.

Direct law is a transition from alternate law with multiple failure when the gear is put down. Landing technique doesn't change but handling in direct law does. You need to trim the aircraft in pitch yourself and the aircraft is more sensitive. Ground effect differs with wing design. A300B4 had a marked lack of ground effect while B747 had noticeable ground effect which could be used for very smooth landings.

I understand it now , in normal law the pitch is a g factor demand which does not give a natural feel to the pilot while flaring so the system introduces nose down trim to compensate. :ok:

thanks a lot guys.

The FCTM says that flare law is actually a pitch demand law, not a g load demand law.

Denti, Reverb_SR71
Yes, I would like to correct my post. It is direct stick to elevator relationship and not load factor demand.

Vilas

yeah i was going through the fcom flare mode is direct stick to elevator , so we are back to square one .

why is there a need to introduce a pitch down after 30 ft ?

Reverb_SR71
No, we are not back to square one. My explanation is what would happen if flare mode was not there. Flare mode consists of no auto trim, change to pitch demand from load factor and nose pitching down. You need nose dropping down to have steady backward pressure during landing.

vilas has it correctly. It is simply the introduction of artificial feel in Normal Law to simulate landing a conventional aircraft in the conventional manner! :p

Vilas, crosswind limits


I get that this is there to make the aircraft feel conventional . What I want to know is how does it work ??

I've been reading online that the 777 has something similar called flare compensation.

If in flare mode the elevator moves directly according to the side stick then isn't it like a conventional aircraft ?

What am I missing here ??

ReverbSR, one particular area of what I call "Airbus Mythology" is landing mode. Many people misunderstand it, and it doesn't help that the FCOM is a translation from the French, and often times (like in this case) not a very good translation. This statement:

"...At 30 ft this value is progressively reduced to 2degrees nose down to induce a gentle positive action for a conventional flare..." is downright incorrect. It's not that at 30' you "progressively go to 2 deg nose down", it's that at 30' the ELAC start introducing elevator down input at 1/4deg per second UP TO a total of 2deg (so a total of 8 seconds). Not nose-down trim, actual elevator input. So, THAT is what you're counteracting and THAT is what gives you a normal flare feel.

It also bears mentioning that both the FCOM and popular literature make landing a 'Bus a lot more complicated than it is. When I'm landing, I'm not exactly thinking "gee, I wonder what law/mode I might be in", I just look outside and fly the airplane. Works every time.

I'm afraid what I wrote IS correct. You just regurgitated the FCOM.

You must have him on speed dial. Nice try.

So, that would mean that the ELACs put in nose down moment at different rates, depending on where we were at 30' since the goal is 2 deg AND in 8 sec, correct? Like if we were at 5deg ANU at 30' it would be roughly 1deg/sec, while if we were at 1 deg ANU it would be .37deg/sec? Do you see the problem here?