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Why does Alternate becomes Direct law whenever landing gear down on Airbus?

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Why does Alternate becomes Direct law whenever landing gear down on Airbus?

Old 9th Sep 2010, 17:26
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Why does Alternate becomes Direct law whenever landing gear down on Airbus?

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?
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Old 9th Sep 2010, 17:51
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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.
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Old 9th Sep 2010, 18:09
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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
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Old 9th Sep 2010, 20:09
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Any why does the autopilot randomly fail for double, but insignificant failures in flight?
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Old 9th Sep 2010, 20:15
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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.
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Old 9th Sep 2010, 20:24
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........but Slasher, how are you going to contrive an Alt Law landing? Let me know and I'll have a go nxt week. Thanks
mcdhu
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Old 9th Sep 2010, 20:30
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It depends on the sim (early or late A320). Ask your checkie.
If hes worth his salt he'll know.
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Old 9th Sep 2010, 20:35
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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.

Dani
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Old 9th Sep 2010, 23:26
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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.
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Old 10th Sep 2010, 04:14
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The bus always lands in direct law
Yeah right
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Old 10th Sep 2010, 06:18
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AL Cz Green has finally stated the right reason
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Old 10th Sep 2010, 14:41
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Thankx Alt CrZ Green. That makes sense..
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Old 10th Sep 2010, 16:40
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Slasher.......I am the "checkie' - and obviously not worth my salt!!

So how to engineer an Alt law landing please?

Cheers
mcdhu
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Old 11th Sep 2010, 02:36
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Early or late A320 sim?
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Old 11th Sep 2010, 09:07
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Slasher - late - CAE Burgess Hill.
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Old 11th Sep 2010, 13:04
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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.
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Old 11th Sep 2010, 18:03
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the reason is there is no flare in alternate law so airbus give the aircraft in direct law like any coventional aircraft to fly
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Old 12th Sep 2010, 08:35
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Hi,
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.

Last edited by rudderrudderrat; 12th Sep 2010 at 12:31.
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Old 13th Sep 2010, 02:41
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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
l'imposible.

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
sidestick!)
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Old 13th Sep 2010, 06:20
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Direct Law is the simplest
Maybe simplest to understand but not simplest to fly

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

And if you want to push the envelope :

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