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A320 Flight control law Landing Mode

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A320 Flight control law Landing Mode

Old 4th Oct 2014, 07:53
  #21 (permalink)  
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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 ?
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Old 4th Oct 2014, 11:27
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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.
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Old 4th Oct 2014, 14:47
  #23 (permalink)  
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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.
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Old 4th Oct 2014, 17:31
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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.
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Old 31st Mar 2020, 14:59
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Originally Posted by vilas View Post
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?
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Old 1st Apr 2020, 04:34
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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.go...EZFV_01-13.pdf

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Old 1st Apr 2020, 11:08
  #27 (permalink)  
 
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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.
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Old 1st Apr 2020, 13:31
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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??

Last edited by pineteam; 1st Apr 2020 at 14:18. Reason: typo
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Old 1st Apr 2020, 17:32
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Originally Posted by vilas View Post
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?
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Old 6th Apr 2020, 09:30
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Originally Posted by pineteam View Post
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!”).
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Old 6th Apr 2020, 10:22
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Originally Posted by hikoushi View Post
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 !


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Old 6th Apr 2020, 10:46
  #32 (permalink)  

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Originally Posted by pineteam View Post
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)
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Old 7th Apr 2020, 09:20
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Originally Posted by FlightDetent View Post
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.
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Old 7th Jan 2021, 21:55
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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.
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Old 8th Jan 2021, 11:00
  #35 (permalink)  
 
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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.
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