Why land CONF 3 when in DCT LAW
 

Greetings all,

A question, why on the A320 must one land in CONF3 when in DIRECT LAW?

Thanks in advance.

Because Airbus says so. ;)


If in alternate law we are also only to land in Config three because when the gear handle is put down, the airplane reverts to direct law.

I believe roll response is more "forgiving" in Config three vice Config full. Config full activates roll spoilers and ailerons simultaneously. Config three activates ailerons first and then roll spoilers if side stick input commands it.

Insufficient pitch authority in CONF FULL, is what I was told.

I was told the same as TP above.
mcdhu

I think it has more to do with the trim change on approach.
Don't forget in direct law you have total control of the flying surface. Full back stick gives full elevator movement to the stop. That should be enough authority for anyone.

Although no Airbus document gives the reason it is true that they have submitted for certification that all abnormal landings will be in CONF3. It even goes to ridiculous extent of selecting Conf3 for approach when flaps are jammed where you only move the flap lever to 3 when nothing else moves. Flap full is mostly drag and for any situation like wind shear, gust etc. CONF3 is preferred. It gives better handling and control so why not in abnormals? Go around in many abnormals require configuration to be maintained which will be problematic in flaps full.

A320 roll response rate (in roll direct) is scheduled by flap handle position, not actual slat or flap configuration as I stated above.

I believe I am a pretty good 329/320 pilot but a lousy aerodynamic or computer engineer, or FCOM writer (as opposed to vilas). However I read a lot, especially incidents and accidents about the airplane I fly.

Those reading this thread, who fly a 319 or 320 and want to expand their knowledge about this topic, I suggest you read the following report:


http://ebook.lib.hku.hk/HKG/B35840213.pdf

CaptainMongo
Interesting incident. But I am afraid your conclusion that roll response rate is being governed by the flap lever position is incorrect. I reproduce section 3.1.19 of your report which is very clear:
The lateral control laws applied by the flight control computers corresponds to actual positions of slats and flaps(para 1.6.4). During the event the gains and kinematics corresponding to config full were not selected due to the fact that the slats were retracted.
During GoAround the crew retracted the lever to position 3 which moved the slat from full to three that reprogrammed the roll rate and not due to the flap lever position.

Not quite true, the QRH/ECAM gives the actual flap lever position for landing.
For instance single HYD failure can be in either, single engine can also be in either. Lots of abnormals that can be done in Config Full, and lets be honest, any brake system failure that affects stopping distance should probably be done Config Full.

Vilas you point is well taken, however the Airbus OEB - appendix 12 states:

"The selection of the flap lever to the position 3 (and consequently the retraction of the slats to CONF 3 position) leads to the selection of autopilot and flight controls gains which are not fully optimized with the real aircraft aerodynamic configuration. The consequence is an increased lateral sensitivity in manual control which can be avoided if CONF FULL is maintained for landing."

Therefore with the flap handle in Config 3, with slats at Config 3 and Flaps at Config full, gains are scheduled as if both flaps and slats are in Config 3, thus my contention that flap handle position is controlling.

If gains are controlled by actual position of slats and flaps (as you rightly pointed out from the report) , what gains are scheduled with slats in Config 3 and Flaps in Config Full? Slats Config 3 gains or flaps Config Full gains?

Suffice to say, the point is when in direct law, we fly Config 3 approaches to optimize lateral control.

Jonty
I missed out a word all complex failures, which off course lead to alternate law and gear down direct law landings are in CONF3. Even innocuous dual RA is in CONF3.

Mongo
The incident quoted by you is not appropriate for the topic because it is related to a mismatch of gain control when slats and flaps are in the wrong combination in normal law. This is not a problem when flaps are three or less and in correct combination. However the sentence given below in ECAM procedure F/CTL FLAPS FAULT/LOCKED is the reason:


FOR LDG (IF FLAPS ≤ 3)............................ USE FLAP 3


Do not select CONF FULL, so as not to degrade handling qualities.

If it degrades handling qualities in normal law then it is more so in direct law.

Vilas,

I disagree.

The OP asked a question of the FCOM. I explained my reasoning using an official source. I didn't use hear say from an instructor, I didn't use the FCOM to explain the FCOM (narrow body Airbus pilots already have an FCOM and can read it) I didn't even denigrate the FCOM "it even goes to the rediculous extent of selecting Config3 when flaps are jammed where you only move the Flap lever to three when nothing else moves."

I cited an official source which explained lateral gain control in Conf3 and Conf full.

Cite an official source backing whatever claim you are making, then I will listen. Until then, landing Conf3 is because of lateral stability - no other reason.

Regards,

Mongo

The flap handle does a lot more stuff than just move the slats/flaps up and down. It reconfigures the flight control systems as the HK incident crew found out when the flap handle was moved.

Mongo, Dale and Jonty



I had asked Airbus why land in Conf3 in specific case of Hyd G+Y Fail when flaps are jammed slats have no 3 position? I quote from the reply from Airbus"

I agree with you that selection CONF 2 during approach, and maintainingCONF 2 for Go-Around could have been less misleading for the pilot.
But the rationale of selecting CONF 3 for A320 family aircraft ishistorical, as this configuration has been proved, in terms of certification,as being the optimum one in terms of handling qualities in alternate law.”

Airbus removed this in A330, they leave the lever in Flap2 and land in Flap2. Flap lever doesn't do much more upto Conf3 only it reduce the roll rate in full. The incidence quoted is due to non standard configuration of flaps and slat and that to not because of the flap lever but actual movement of slat from full to three. Had the slats also jammed in full nothing would have happened