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Mastropiero
Topic Author
Posts: 113
Joined: Sat Dec 17, 2005 9:24 pm

Normal Law? Direct Law? Can Someone Explain?

Hi everyone!

I have come on many ocasions across this, without ever understanding what do you talk about when you refer to either "normal law" or "direct law". I have done some research but, alas, no useful results so far.

Would you mind explaining what do you mean when you refer to it?

I hope it is not something as simple as to make me blush with shame for not being able to figure it out myself....

Thanks!

deaphen
Posts: 1090
Joined: Wed Jul 13, 2005 3:57 am

RE: Normal Law? Direct Law? Can Someone Explain?

In what context do people use it??

Regards
Nitin
I want every single airport and airplane in India to be on A.net!

wilco737
Posts: 7275
Joined: Sun Jun 20, 2004 12:21 am

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting Mastropiero (Thread starter):I have come on many ocasions across this, without ever understanding what do you talk about when you refer to either "normal law" or "direct law". I have done some research but, alas, no useful results so far.

The "normal law" is the normal way or controlling Airbus Aircrafts. like 320s or 340s. Then the airplane trims on its own and maintains the bank and pitch etc etc. With the "direct law" the Airbus flies like a boeing.

I don't know too much about it, as I am flying MDs. But I am sure the Airbus pilots will explain it to you more detailed.

WILCO737 (MD11F)

Mastropiero
Topic Author
Posts: 113
Joined: Sat Dec 17, 2005 9:24 pm

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting Deaphen (Reply 1):In what context do people use it?? Regards Nitin

Cheers.

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: Normal Law? Direct Law? Can Someone Explain?

Normal and direct law are levels of control provided by the FBW system

Normal Law provides envelope protection (which prevents stalls, overspeed, excessive g, etc). It also provides autotrim so the aircraft stays in the attitude it was put when the control was released (within limits).

Direct Law provides no envelope protection and the sidestick moves the control surfaces in proportion to the control input. The stabilizer trim has to be moved manually by the pilot, there is no electric trim.

In between these two there is Alternate Law, which provides much reduced protection and speed stability. Pitch trim is automatic, as with Normal Law.

Degredation from Normal to Alternate or Direct Law is automatic, depending on system failures.

A full description can be found at:

http://www.airbusdriver.net/

Look under Airbus Info, Flight Control Laws.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

Tristarsteve
Posts: 3663
Joined: Tue Nov 22, 2005 11:04 pm

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting Jetlagged (Reply 4):Degredation from Normal to Alternate or Direct Law is automatic, depending on system failures.

In 20 years on the A320, I have only once encountered Direct Law in flight. An A320 had a double failure of an elevator posn transmitter, and went into direct law. Saw the words direct law on the PFR.
One and only time.

Starlionblue
Posts: 19937
Joined: Fri Feb 27, 2004 9:54 pm

RE: Normal Law? Direct Law? Can Someone Explain?

Here is the page at Airbusdriver that details the laws: http://www.airbusdriver.net/airbus_fltlaws.htm
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

tdscanuck
Posts: 8573
Joined: Wed Jan 11, 2006 7:25 am

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting WILCO737 (Reply 2):With the "direct law" the Airbus flies like a boeing.

That's not true for FBW Boeing's (i.e. the 777). A 777 has normal, secondary (equivalent to Airbus' "alternate"), and direct (same as Airbus' "direct").

Tom.

PGNCS
Posts: 2260
Joined: Sun Apr 15, 2007 5:07 am

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting WILCO737 (Reply 2):With the "direct law" the Airbus flies like a boeing.

Oh, it's not THAT bad, WILCO!

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting TristarSteve (Reply 5):In 20 years on the A320, I have only once encountered Direct Law in flight. An A320 had a double failure of an elevator posn transmitter, and went into direct law. Saw the words direct law on the PFR. One and only time.

I suspect most pilots only ever see it in the simulator. My experience of trying Direct Law out in the sim is that it is pretty ugly, certainly worse than hand flying a non FBW aircraft.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

Mastropiero
Topic Author
Posts: 113
Joined: Sat Dec 17, 2005 9:24 pm

RE: Normal Law? Direct Law? Can Someone Explain?

All right, thank you all for your replies.

Coal
Posts: 2588
Joined: Sun Aug 13, 2006 11:14 am

RE: Normal Law? Direct Law? Can Someone Explain?

Sorry to bring this back from way back, but is it just me or does the whole "law" term sound really creepy? Why do they call it normal law and alternate law? Why not call it normal mode or normal configuration and alternate mode or alternate configuration?

The fact that they call it law really creeps me out. As if the machines have taken over.

Cheers
Coal
Nxt Flts: SQ SIN-KIX | HD UKB-CTS | NH CTS-NRT | SQ NRT-SIN | AK SIN-DPS-SIN

rcair1
Posts: 1147
Joined: Wed Oct 28, 2009 8:39 pm

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting Coal (Reply 11):As if the machines have taken over.

Why we have, I mean they have....
I know my computer(s) seem to control my life, my car, my TV, my phone. Sometimes I wonder about my wife.

 Quoting Coal (Reply 11):Sorry to bring this back from way back, but is it just me or does the whole "law" term sound really creepy? Why do they call it normal law and alternate law? Why not call it normal mode or normal configuration and alternate mode or alternate configuration?

They reason they call it "law" is because you are talking about the control system 'laws' that dictate the behavior - the relationship between a/c performance, the controls and the control surfaces. In control theory, these relationships/behaviors (typically expressed as equations) are often called 'laws'.
The word 'laws' is used because it is often much more complex than a simple equation (linear or non-linear). It does represent active actions by the control system that goes beyond just relating motion of a stick to motion of a surface (that is what direct law does).
Airbus decided to use this term in their description.

When I was a senior in engineering, oh so many years ago, my senior project was to build an adaptive control system for a plant that represented a something like an antenna that moved according to a model that fit a pattern, but was unknown. I knew the basic 'equation' of the motion, but the values of the coefficients were unknown. My control system was programmed with a desired plant behavior (a different equation with known coefficients). The system had to read the behavior of the plant and based on what it saw the plant doing, adjust its inputs to cause the plant to actually operate like the desired plant.

Just to date myself - I used an analog computer http://en.wikipedia.org/wiki/Analog_computer to act as the plant. The control system was programmed in the firmware of an 8086 processor. The interface between them was some a/d's and d/a's.

Remember the three laws or robotics.....
Same thing.
I wonder if airbus follows them, and if so, when will the revolt be (See "I Robot")
rcair1

PhilBy
Posts: 834
Joined: Tue Aug 06, 2013 11:44 am

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting Coal (Reply 11):Why do they call it normal law and alternate law?

In this case 'Law' is a specific technical term used in control systems engineering.

BoeingGuy
Posts: 6313
Joined: Fri Dec 10, 2010 6:01 pm

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting Tdscanuck (Reply 7):Quoting WILCO737 (Reply 2): With the "direct law" the Airbus flies like a boeing. That's not true for FBW Boeing's (i.e. the 777). A 777 has normal, secondary (equivalent to Airbus' "alternate"), and direct (same as Airbus' "direct"). Tom.

Yes, not correct at all. In the Airbus FBW or Boeing FBW airplanes, when they are in Direct Mode they fly more like any non-FBW airplane.

The Direct mode on the 777 and 787 is basically an analog mode. Whatever input the pilot puts into the Flight Controls, the control surfaces move with no computer processing or envelope protection or anything. That's like a cable driven airplane.

Viscount724
Posts: 19316
Joined: Thu Oct 12, 2006 7:32 pm

RE: Normal Law? Direct Law? Can Someone Explain?

 Quoting Coal (Reply 11):Sorry to bring this back from way back, but is it just me or does the whole "law" term sound really creepy?

How did you find a 6-year-old thread to resurrect it?

wingscrubber
Posts: 823
Joined: Fri Sep 07, 2001 1:38 am

RE: Normal Law? Direct Law? Can Someone Explain?

A320-
http://www.smartcockpit.com/aircraft...ources/NORMAL_LAW_PROTECTIONS.html

A330-
http://www.smartcockpit.com/aircraft-ressources/A_0-Flight_Laws.html

After AF447 all Airbus pilots should be acutely aware of the meaning of 'alternate law' and the effect that has on stall protection in particular.
Sorry to lower the tone, and please don't flame me... but the flight control laws should be bread and butter of Airbus driving.
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