View Full Version : Why does Alternate becomes Direct law whenever landing gear down on Airbus?

9th Sep 2010, 18:26
In many failures cases on Airbus when u put the landing gear down the flight control law becomes direct law. I didnīt find anything written about it and i donīt understand why whould they do it, because no one wants to fly a plane in a worst condition.
There must be a logical explanation but i canīt find it. Any insights?

9th Sep 2010, 18:51
The bus always lands in direct law and it uses the radio alt to alert the prims that you are about to land. In the failure cases you mentioned, the radio alt is not available and the only way the plane knows you are going to land is by gear extention.

9th Sep 2010, 19:09
I understand the logics between the radio altimeter and the proximity sensors but not all the failures afect the RA, like the hydraulics, ADR, etc

9th Sep 2010, 21:09
Any why does the autopilot randomly fail for double, but insignificant failures in flight?

9th Sep 2010, 21:15
Next time your in the box on your next renewal ask your
checkie to let you try landing the damn thing in Alternate
and youll see why.

9th Sep 2010, 21:24
........but Slasher, how are you going to contrive an Alt Law landing? Let me know and I'll have a go nxt week. Thanks

9th Sep 2010, 21:30
It depends on the sim (early or late A320). Ask your checkie.
If hes worth his salt he'll know.

9th Sep 2010, 21:35
It has to be stated, and it has been before, that there is no alternate law, there is only normal law in pitch and direct law in roll, which is called then alternate law.

If you have certain failures your roll falls into direct, but your pitch not yet. It only does when you lower the gear. That's when your Airbussy is in direct law. It had to because the flight control computers are not able to do a normal landing anymore.

I'm sure they could have sorted it out differently, but that's how they designed it - and it's not bad.


Alt Crz Green
10th Sep 2010, 00:26
The bus always lands in direct law

Not true. The only direct law landings are if you are already downgraded to alternate law before gear down.
A normal landing is in the flare mode of normal law. There is no flare mode in alternate law, hence the reversion to direct law at gear down.
Normal law and alternate law both have the same characteristics in pitch, i.e. adjusting the sidestick adjusts the flight path of the aircraft. Without flare mode, an attempt to land in normal/alternate law would result in the aircrafts flight path shallowing as you pulled back the stick to flare and a resultant float until you ran out of speed.

John Citizen
10th Sep 2010, 05:14
The bus always lands in direct law

Yeah right :D

10th Sep 2010, 07:18
AL Cz Green has finally stated the right reason :D

10th Sep 2010, 15:41
Thankx Alt CrZ Green. That makes sense..

10th Sep 2010, 17:40
Slasher.......I am the "checkie' - and obviously not worth my salt!!

So how to engineer an Alt law landing please?


11th Sep 2010, 03:36
Early or late A320 sim?

11th Sep 2010, 10:07
Slasher - late - CAE Burgess Hill.

11th Sep 2010, 14:04
The way I understood it:

As the airplane approaches the runway in normal law, at a specific RA height it (50 feet IIRC) starts rolling in fwd trim, which the pilot overrides, by pulling back on the stick, much like a normal airplane. I suppose one could actually call it artificial feel.

With certain failures, the fwd trim input will not happen, since the airplane doesn't know it's height, so as a back up, it goes into direct law with the gear down. In direct law, the pilot has to trim the airplane, or I suppose one could accept much higher stick forces, however, this manual trimming gives the pilot a, dare I say it, a direct feel for the airplane.

Basically the point is, while it would be possible to land with the a/c in alternate law, due to the way pitch forces on the stick would would feel to the pilot, there is a much bigger chance of over controlling, since there is no real feel.

11th Sep 2010, 19:03
the reason is there is no flare in alternate law so airbus give the aircraft in direct law like any coventional aircraft to fly

12th Sep 2010, 09:35
the flight control law becomes direct law......no one wants to fly a plane in a worst condition
Direct Law is the simplest and most instinctive - ask any Boeing Pilot or even Wilbur Wright.

13th Sep 2010, 03:41
MCDHU simple - leave the gear up and ignore warns to
simulate what the flare is like and just do a wheels
up if you want to go that far. In the early sims the panel
could insert a failure of changeover to Direct with gear
down (mainley for EMERG CONFIG change with G/D at
1,000ft) but not any more since Airboos now regard it as

Direct Law is the simplest and most instinctive -
ask any Boeing Pilot or even Wilbur Wright.

Correct. I fly the frigging thing much better in Direct. In
certain hyd sim situations I'll drop the gear earlyer after I
got me flap 3. NPAs are then spot on (dispite the damn

John Citizen
13th Sep 2010, 07:20
Direct Law is the simplest

Maybe simplest to understand but not simplest to fly :confused:

No auto pitch trim :eek:
Elevator pitch deflection is proportional to stick deflection :eek:

And if you want to push the envelope :

No load factor protection :eek:
No angle of attack protection :eek:
No angle of bank protection :eek:
No pitch attitude protectionc:eek:
No high speed protection :eek:

13th Sep 2010, 08:48
Hi John Citizen,

Thanks for the info.
Sorry - I should have emphasised this bit then:
Direct Law is the simplest and most instinctive - ask any Boeing Pilot

John Citizen
14th Sep 2010, 01:32
Direct Law is the simplest and most instinctive

Lack of auto trim (as in direct law, or as in a Boeing) does not sound simple and instinctive to me. :confused:

For example, in Airbus normal law, you apply a control force, the aircraft will ALWAYS pitch at the same rate irrespective of speed, when you release the control force, it will hold the attitude. How simple is that :ok:

In a Boeing, pitch rate with control force depends on airspeed, and then you must TRIM to hold any new attitude that you have set. This does not seem to be simple and instinctive to me :confused:

14th Sep 2010, 02:29
Maybe simplest to understand but not simplest to fly

No auto pitch trim
Horrors! One might have to reach down and move the stab trim wheel! :bored:

Elevator pitch deflection is proportional to stick deflection
Bad news indeed - your back in a normal airplane!(almost)

And if you want to push the envelope :

No load factor protection )
No angle of attack protection )
No angle of bank protection ) actual hands-on piloting required
No pitch attitude protectionc )
No high speed protection )

I dont think youd enjoy flying my little PA18 unless I shoehorned an ELAC into it eh JR! ;)

John Citizen
14th Sep 2010, 03:48
Simple = easy to use, as per Dictionary.com | Find the Meanings and Definitions of Words at Dictionary.com (http://www.dictionary.com)

I did not ever mention horrors, bad news, dangerous or pilot inability/skill level. :ugh:

The discussion was purely about what is more simple.

What is more simpler (easier) :
1- adjust pitch attitude with only ONE control movement
2 - adjust pitch attitude with a control movement (primary control - elevator though the control column) THEN adjust another secondary control (pitch/stabilator trim) AS WELL

What is easier ?
1 or 2 control movemente/adjustments ? Its not that hard to work out :ugh:

14th Sep 2010, 11:25
Hi John C.

May I suggest you hire a simple (= uncomplicated) aircraft from your local flying club, and remind yourself what flying feels like again.

14th Sep 2010, 13:00

Didnt realise you know the AIRBUS systems that well. Whats your thoughts on the Phenom 300? I thought it was crap to be honest. Just the quality of the build. Soon Learjet will be bringing down a 45XR more speed, more range and bigger cabin.. Only slightly more on price! Cheaper to run also and they hold their value better. Didnt realise your boss was American.

16th Sep 2010, 10:46
when you release the control force, it will hold the attitude. How simple is that http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/thumbs.gifTo be picky, with sidestick neutral the FBW trys to maintain 1.0g and zero roll rate - it will not return to the previous attitude after atmospheric disturbance, although the effect is very close.
Pedant's hat back in cupboard.

17th Sep 2010, 14:13
Had a go at Alt Law landing in the box by way of Abnormal Attitude Law. It's different!

From the FCTM:-

'Takeoff and landing maneuvers are naturally achieved. For example, a flare requires the PF to apply permanent aft pressure on the sidestick, in order to achieve a progressive flare.'

- hence the need for the nose down pitch introduced by flare law at 50'. In Alt Law this does not happen so a permanent 'aft pressure on the sidestick' will send you flying again - and alpha floor is inhibited below 100'RA. So you need to arrest the ROD with small backwards movements of the stick - releasing between them. You'd think they might mention that somewhere!


18th Sep 2010, 02:26
To land the sim in ALTN law:
1. Fail Both RAs.
2. Extend flaps to posn 3 (Landing flaps) then fail both SFCCs.
3. Fail Both LGCIUs then lower gears by gravity.
4. Fail both FACs.

The landing can be done now in ALTN Law and it does not differ much than Normal law with flare mode.


21st Sep 2010, 09:47
The biggest problem with a direct law landing is the lack of feel through the side stick, and a tendency of most people to over correct (control) with the trim wheel. This can lead to the approach becoming destabilised in the latter stages, and the pilot has a natural tendency to not want to go-arround in direct law.