A320 Flight control law Landing Mode

[Reverb_SR71]

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

[Fursty Ferret]

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.

[vilas]

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.

[Dont Hang Up]

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?

[MD83FO]

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

[vilas]

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.

[Reverb_SR71]

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.

[Denti]

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

[vilas]

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

[Reverb_SR71]

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 ?

[vilas]

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.

[Crosswind Limits]

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

[Reverb_SR71]

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 ??

[Amadis of Gaul]

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.

[Amadis of Gaul]

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

[Amadis of Gaul]

You must have him on speed dial. Nice try.

[Amadis of Gaul]

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?

[autoflight]

I understand that if, for example, aircraft was 1 degree aircraft nose up at 30 ft, the target becomes 1 degrees aircraft nose down, requiring side stick back pressure to re-establish 1 deg ANU plus required extra ANU for flare.

From day one on A320 (my first fbw) attitude control during flare seemed natural and similar to other swept wing jet airliners. I think Airbus did a great job with flight control laws during the flare.

[Amadis of Gaul]

I agree, it doesn't seem like any big deal to me either.

[vilas]

Reverb_SR71
Normal aircraft is other than yoke, also responsive to changes of thrust weight couple. AB FBW being flight path stable resists changes due T/W couple. So it will try to hold the flight path even during flare which is not what the pilot is looking for. So it is made to pitch down and pilot controls it through elevator to his requirement.
As far actual handling is concerned you just land like any other aircraft no calculation are needed.

[Reverb_SR71]

Amadis of Gaul


I read in another forum that when it says " at 50ft the attitude is memorized and at 30ft it is reduce to 2deg down in 8 sec " what it actually means is

The attitude at 50ft is the reference. For eg if the pilot pulls the stick then the system pitches up from the reference , vice versa and if the pilot lets go then the aircraft returns to that reference attitude.

This reference is then reduced to -2deg from 30ft over a period of 8 secs.


autoflight

Yes landing the bird is natural , which increases my doubts is there a difference when landing in direct law ?

[vilas]

autoflight, Reverb_SR71
The target attitude is not two degrees from the present attitude but minus 2degrees below the horizon, that means if you are at +4 degrees as you cross 50 ft. then from 30 ft. it will start pitching down to achieve -2 degrees in next 8 seconds. So in effect it will try to pitch down 6 degrees. This is clearly explained in a Airbus presentation exactly as what jon_smith has stated.

[Reverb_SR71]

Vilas

Yes -2degrees from the horizon.

That explains a lot , so in conclusion the flare mode is essentially tweaking the controls in a way that makes the aircraft feel more " like other aircraft" .

It is at its heart just there to make the pilot feel more comfortable landing the 320.

[Amadis of Gaul]

In that case, I hereby withdraw any and all objections I might have had now and forever more. Krishna knows, anything you might have read on another forum must be Osiris' own truth, beyond all doubt whatsoever.

[tcasblue]

autoflight, Reverb_SR71
The target attitude is not two degrees from the present attitude but minus 2degrees below the horizon, that means if you are at +4 degrees as you cross 50 ft. then from 30 ft. it will start pitching down to achieve -2 degrees in next 8 seconds. So in effect it will try to pitch down 6 degrees. This is clearly explained in a Airbus presentation exactly as what jon_smith has stated.
Thanks,

If this is true that the target a pitch attitude of -2 degrees(nose below the horizon), then it would mean that the amount of nosedown pitch input(and rate of input) by the automatics varies based on whatever the actual pitch is at the start of the nose-down trimming. Obviously, that can vary depending on flap setting, approach speed, weight, etc.

Is this correct?

[tcasblue]

In addition to the above question, I would like to post a quote from an AAIB report on a heavy landing in and Airbus. According to the report....

"When the aircraft entered the Flare Mode at 50 ft the pitch attitude was 3.2° nose-up. The system would have ordered a nose-down pitch to reach 2° nose-up over a period of 8 seconds. However, this would have been a relatively gradual change and was not considered to be a significant factor in this event."

I thought that the trimming was to -2 degrees. Is this an error?

https://assets.publishing.service.gov.uk/media/5423034de5274a1314000b7f/Airbus_A319-111_G-EZFV_01-13.pdf

[vilas]

Tcasblue
in the report while describing the flare mode it mentions 2°nose down which correct then further in conclusion it say 2°nose up which is obviously an error. The normal attitude at 50ft during flare mode with all the variables you mentioned is between 3°to 5°. From 30ft the system will lower it to 2° nose down. I clarify again that without flare mode the stick out of neutral is load factor demand so as long as the stick is out of neutral the aircraft will keep on pitching at commanded G. So to prevent ballooning you will have to neutral the stick. This process has to continue till touchdown which is not a good way to do consistent satisfactory landings. So flare mode keeps pitching down you control the ROD to touchdown by adjusting back pressure on SS.

[pineteam]

On that topic. Did you guys notice the flare law is different on the A321 Neo?! The THS stops at 100 feet RA instead of 50 feet RA and there is no mention about the pitch reduction to -2 degree nose down. Does the 321 neo really does not pitch down by itself or they just removed it from the FCOM??

[tcasblue]

Tcasblue
in the report while describing the flare mode it mentions 2°nose down which correct then further in conclusion it say 2°nose up which is obviously an error. The normal attitude at 50ft during flare mode with all the variables you mentioned is between 3°to 5°. From 30ft the system will lower it to 2° nose down. I clarify again that without flare mode the stick out of neutral is load factor demand so as long as the stick is out of neutral the aircraft will keep on pitching at commanded G. So to prevent ballooning you will have to neutral the stick. This process has to continue till touchdown which is not a good way to do consistent satisfactory landings. So flare mode keeps pitching down you control the ROD to touchdown by adjusting back pressure on SS.

Thanks as always Vilas,

This would ean that the rate of nose-down input would vary based on the pitch attitude when it starts. Has anybody actually noticed this as a difference in feel?

[hikoushi]

On that topic. Did you guys notice the flare law is different on the A321 Neo?! The THS stops at 100 feet RA instead of 50 feet RA and there is no mention about the pitch reduction to -2 degree nose down. Does the 321 neo really does not pitch down by itself or they just removed it from the FCOM??

It simply goes into the equivalent of direct law in pitch at 100 feet, and auto trim stops. Essentially natural longitudinal stability and pitch/power couple take care of the same thing the original flare mode was trying to accomplish. Some of the earlier NEOs without the most current ELAC mod still have the old flare law. You can definitely feel the difference going from one to the other. Advantages of the newer software are much less susceptibility to excessive float if someone gets spooked and starts to flare above 50 feet, and a more natural feel in the landing, especially if using manual thrust (“just hold it till it hits, Hoss!”).

[sonicbum]

It simply goes into the equivalent of direct law in pitch at 100 feet, and auto trim stops. Essentially natural longitudinal stability and pitch/power couple take care of the same thing the original flare mode was trying to accomplish. Some of the earlier NEOs without the most current ELAC mod still have the old flare law. You can definitely feel the difference going from one to the other. Advantages of the newer software are much less susceptibility to excessive float if someone gets spooked and starts to flare above 50 feet, and a more natural feel in the landing, especially if using manual thrust (“just hold it till it hits, Hoss!”).

Hi hikoushi, thanks for the background info, always highly appreciated. Quick question on the A330 it is very similar but with a small difference regarding the last part of the statement : "At 50 ft, a slight pitch down elevator order is applied. Consequently, to flare the aircraft, a gentle nose-up action by the pilot is required."

I'm just curious about the magnitude of this "slight pitch down elevator" and this has changed with NEOs/800/ or the A350 ?

Thanks !

[FlightDetent]

On that topic. Did you guys notice the flare law is different on the A321 Neo?! The THS stops at 100 feet RA instead of 50 feet RA and there is no mention about the pitch reduction to -2 degree nose down. Does the 321 neo really does not pitch down by itself or they just removed it from the FCOM??When the aircraft passes 100 ft RA, the THS is frozen and the normal flight mode changes to flare mode as the aircraft descends to land. Flare mode is essentially a direct stick-to-elevator relationship
(with some damping provided by the load factor and the pitch rate feedbacks). At 50 ft, a slight pitch down elevator order is applied. Consequently, to flare the aircraft, a gentle nose-up action by the pilot is required.

That is A330 DSC-27-20-10-20 "C". Is the text for NEOs you see any different?

EDIT (cross-posted with sonic)

[hikoushi]

When the aircraft passes 100 ft RA, the THS is frozen and the normal flight mode changes to flare mode as the aircraft descends to land. Flare mode is essentially a direct stick-to-elevator relationship
(with some damping provided by the load factor and the pitch rate feedbacks). At 50 ft, a slight pitch down elevator order is applied. Consequently, to flare the aircraft, a gentle nose-up action by the pilot is required.

That is A330 DSC-27-20-10-20 "C". Is the text for NEOs you see any different?

EDIT (cross-posted with sonic)

The A321 NEO with the new ELAC is very similar, and states:

”When the aircraft passes 100 ft RA, the THS is frozen and the normal flight mode changes to flare mode as the aircraft descends to land. Flare mode is essentially a direct stick-to-elevator relationship, with some damping provided by the load factor and the pitch rate feedbacks. The flare law provides full elevator authority. The flare law has no compensation for the ground effect or thrust effect.”

Basically the earlier model A320 series entered flare mode (autotrim stops, etc) at 50 feet and put in a pitch down moment at 30 feet for artificial feel.

The new ELAC version enters flare mode at 100 feet and does not give a pitch down moment, letting natural stability take care of the same thing on the way down.

[tubby linton]

The A320ceo with sharklet is horrible in the flare. It is incredibly twitchy and a normal ceo flare demand seems to induce a rapid pitch up.I don’t know anybody who likes it. The A320neo with sharklet seems different again to the basic model.

[Uplinker]

To answer the OP: in flight the FBW continually changes pitch when the side-stick is held out of neutral - the rate of change is determined by how far and for how long the side-stick is held out of neutral, so the pitch is not directly related to the side-stick angle but to the time and distance the side-stick is held out of neutral. The FBW also holds the new attitude after the side-stick has been centred.

During the flare this would make fine control awkward - pilots would have to nudge-centre-nudge-centre etc., which would make fine adjustments difficult and delayed. The flare mode is not about pitching the aircraft down, but it uses that to force pilots to pull the side-stick out of neutral and to hold it out of neutral against the centring spring to maintain pitch. Now the pitch is effectively directly related to side-stick angle: pull harder = higher pitch, pull less hard = lower pitch. This is more conventional and more instinctive when small, rapid and accurate adjustments are required.