flyinTLow
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A320 Winglet: Flight Control Law Question

Tue Apr 02, 2013 12:36 pm

Hey everyone!

This is one for the ultimate experts of you around here. I have a tricky question:

We all know the different flight control laws Airbus aircraft have to offer. And from what we all know so far, from the A320 at least:
Whenever the aircraft degraded to Alternate Law, with Gear down (except tripple SEC fault, and ok, Abnormal Attitude Law, but that's a whole different ball park there) it reverted to Direct Law.

Now as the A320 with winglets came out, Airbus decided with Yaw Damper Faults (resulting for example from FAC1+2 fault), the aircraft would remain in Alternate Law with gear down.

Now from what I understand from Alternate Law, it will not let the aircraft go into Flare Mode, hence making it impossible to land the A320 in alternate law.

Does anyone on here have any clue how to get me out of this clinch?  

Any help appreciated!

Thilo
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VC-10
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RE: A320 Winglet: Flight Control Law Question

Tue Apr 02, 2013 2:32 pm

According to my 320 notes:

' In pitch alternate law, there is no specific landing mode, the pitch and roll direct laws are used.

If pitch & roll axes are under the direct law, in flight mode, they remain in direct law when the landing mode is active'
 
chrisMUC
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RE: A320 Winglet: Flight Control Law Question

Tue Apr 02, 2013 3:02 pm

Quoting flyinTLow (Thread starter):
Now from what I understand from Alternate Law, it will not let the aircraft go into Flare Mode, hence making it impossible to land the A320 in alternate law.

Pitch three degrees up, power idle and the bird is landed  

What do you need Flare Mode for when you fly manually?
Or did I get your question completely wrong?
A340s also stay in altrenate law when the gear is down.
 
Pihero
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RE: A320 Winglet: Flight Control Law Question

Tue Apr 02, 2013 5:34 pm

Quoting flyinTLow (Thread starter):
Whenever the aircraft degraded to Alternate Law, with Gear down (except tripple SEC fault, and ok, Abnormal Attitude Law, but that's a whole different ball park there) it reverted to Direct Law....
...Now from what I understand from Alternate Law, it will not let the aircraft go into Flare Mode, hence making it impossible to land the A320 in alternate law.

and...

Quoting VC-10 (Reply 1):

According to my 320 notes:
' In pitch alternate law, there is no specific landing mode, the pitch and roll direct laws are used.

The secret is in the wording , from the FCOM :
"In Pitchl "Alternate lLaw", the flight mode changes to the fFlare Mode when the pilot selects landing gear down . the Flare Mode is a direct stick-to-elevator relationship"
Historically, the passage from Alternarte to Direct was followed by warnings one didn't need which anyway disappeared in phase 7 ( approach below 800 ft) and thought to bring some possible confusion. Therefore, the term *Flare* in Alternate was kept with the info above... it's a flare in alternate law... different from the normal *Flare* in normal.
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flyinTLow
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RE: A320 Winglet: Flight Control Law Question

Mon Apr 08, 2013 1:40 am

Quoting Pihero (Reply 3):

I dont think you understand me correctly:

When approaching in normal law, in the pitch channel there is something called "flare mode". The aircraft memorizes the pitch in 50' and then begins to trim nose down in order to result a pitch of -2 degrees nose down if no further input is made by the pilot. This flare mode, as far as I know, only exists in Normal Law.

When in alternate law, looking at the pitch channel only, it basically does not differ from the normal law too much (still g-load-demand). But when selecting the gear down, the alternate law reverts to Direct Law, which for the pitch channel means direct stick to elevator control with no more automatic trim. With this, there is no more necessity for a flare mode, as the aircraft can be landed just like any other conventional aircraft.

Now back to my problem:
As the new aircraft supposedly do not revert to direct law after selecting the gear down, the aircraft remains in g-load-demand in the pitch channel in alternate law. Now coming down to 50', with no flare mode available: how can the aircraft be landed then? Just pull back the stick will not be of use there. It's still no stick to elevator control, and no trim to help you land the aircraft either?
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Starlionblue
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RE: A320 Winglet: Flight Control Law Question

Mon Apr 08, 2013 2:07 am

Quoting flyinTLow (Thread starter):
Now from what I understand from Alternate Law, it will not let the aircraft go into Flare Mode, hence making it impossible to land the A320 in alternate law.

According to http://www.airbusdriver.net/airbus_fltlaws.htm, in Alternate Law "Pitch alternate law degrades to pitch direct law when the landing gear is extended to provide feel for flare and landing, since there is no flare mode when pitch normal law is lost."

Quoting chrisMUC (Reply 2):
What do you need Flare Mode for when you fly manually?

Flying manually, you are still (normally) in Normal Law, and thus inputs are filtered through the flight control system using Normal Law.

If you pull back on the stick to flare and there is no Flare Mode, you are demanding a g load (positive vertical g) and the flight path will change upwards. However in the flare you just want to pitch up, not change your flight path upwards.
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Pihero
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RE: A320 Winglet: Flight Control Law Question

Mon Apr 08, 2013 10:39 am

Quoting flyinTLow (Reply 4):
I dont think you understand me correctly:

Yes, I did : the introduction of a *Flare* mode in alternate law is to make it a normal event, not an abnormal situation with warnings and alarms . The result is, flight-control-wise, the same : you have, after lowering the gear a direct stick-to-elevator relationship.

As far as I know, nothing has changed with the NEO.

Quoting Starlionblue (Reply 5):
Quoting chrisMUC (Reply 2):
What do you need Flare Mode for when you fly manually?

Flying manually, you are still (normally) in Normal Law, and thus inputs are filtered through the flight control system using Normal Law.

If you pull back on the stick to flare and there is no Flare Mode, you are demanding a g load (positive vertical g) and the flight path will change upwards. However in the flare you just want to pitch up, not change your flight path upwards.

     
Boeing uses a similar set-up on the 777... for exactly the same reasons.
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flyinTLow
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RE: A320 Winglet: Flight Control Law Question

Mon Apr 08, 2013 11:35 pm

Quoting Pihero (Reply 6):
Yes, I did : the introduction of a *Flare* mode in alternate law is to make it a normal event, not an abnormal situation with warnings and alarms . The result is, flight-control-wise, the same : you have, after lowering the gear a direct stick-to-elevator relationship.

I'm sorry, I don't understand that.

Flare mode and direct law are not exactly the same thing control wise. And most importantly, they are not the same when it comes to timing. Normally, in Alternate Law, the degredation comes when lowering the gear.

Now with the new aircraft, this exact degredation does NOT happen when you have dual FAC or dual Yaw Damper fault.

Could you try it for me once more? Sorry if I seem a little rude, I am just trying to understand it.

Thilo
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Starlionblue
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RE: A320 Winglet: Flight Control Law Question

Tue Apr 09, 2013 12:01 am

Quoting flyinTLow (Reply 7):
Flare mode and direct law are not exactly the same thing control wise.

I'll let Pihero give the exact details. What I can glean from AirbusDriver is this:

If the flight controls degrade to Alternate Law, Direct Law automatically becomes active when the landing gear is extended if no autopilots are engaged. If an autopilot is engaged, the airplane will remain in Alternate Law until the autopilot is disconnected.

However in Normal LawFlare Mode
Transition to flare mode occurs at 50' RA during landing.
System memorizes pitch attitude at 50' and begins to progressively reduce pitch, forcing pilot to flare the aircraft
In the event of a go-around, transition to flight mode occurs again at 50' RA.


Quoting flyinTLow (Reply 7):
And most importantly, they are not the same when it comes to timing. Normally, in Alternate Law, the degredation comes when lowering the gear.

This is a question of timing as you say. In Alternate Law, this degrades to Direct Law at gear down. In Normal Law Flare mode comes into effect at 50 feet AGL (presumably on the RA).
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
flyinTLow
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RE: A320 Winglet: Flight Control Law Question

Thu Apr 11, 2013 11:56 pm

Quoting Starlionblue (Reply 8):

That is exactly what I was saying. That is how it is with the flight control mode laws ever since the A320 was introduced. Now with the new sharklet-A320s, when there is a FAC1+2 or Yaw Damper 1+2 fault, as soon as the the fault appears, the flight controls degrade to Alternate Law, and with these failures the AP disconnects with no chance to reengage. Now when selecting gear down, this exact degradation does not happen. Flight control remains in Alternate Law. That is my problem.  

Any further thoughts?
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Pihero
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RE: A320 Winglet: Flight Control Law Question

Fri Apr 12, 2013 9:43 am

Give us your source or a reference to it.
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flyinTLow
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RE: A320 Winglet: Flight Control Law Question

Sun Apr 14, 2013 9:55 am

It's in the Lufthansa Operating Manual for the A320 aircraft. Sorry, can't link that in any way...
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