Crosswind Landing and the Airbus

[chperplt]

I've had plenty of opportunity to watch airplane after airplane land in crosswinds. It seems that either the Airbus is a pain the butt to deal with in a crosswind, or I've just seen some real
sh!tty landings.

As I understand it, movement on the control stick requests a certain rate of roll from the computer. Once that rate is established, the computer keeps that rate until opposite control movement is applied. What happens in the crosswind situation? I would think that opposite rudder application would cause some problems with that.. Do you crab it in until the very last second?

What's the real deal??


Thanks

 

[Eagleflip]

Crosswinds in the 320 are not all that bad, actually. The side stick movement commands a roll rate proportional to the amount of stick input, e.g. more stick movement, more aileron/spoiler input. But it doesn't continue to roll once you take the input out.

By way of explanation, when you make a control input in the Airbus, you move the stick in the direction you want to move then release the input once the desired movement has ocurred. Were you to hold the stick over in, let's say, a left bank, the airplane would continue to roll. Instead, you move the stick left, wait for the bank attitude you desire, then release the pressure on the stick. The stick then centers by way of springs and small, nesting squirrels.

Not that I am a rocket scientist, but I figure that this control scheme is exactly the same in all aircraft, Airbus or not.

That being said, the 320 does not continue a roll once the stick is released from the bank input.

To do a textbook crosswind landing in a 'Bus, you maintain the crab until the guy on the left starts to fidget uncontrollably, then kick the nose out to align yourself with the centerline. Add a touch of upwind aileron to counter the extra lift generated by the advancing wing, flare gently, and...voila! A reasonable touchdown...

Ok, that's the textbook answer. All those crappy crosswind landings you saw?

That was probably me...

 

[F9 Driver]

As you get closer to the ground (200 ft I think) the airplane changes into a different "law" and the stick deflection creates alieron / elevator deflection porportional to the amount of deflection and autotrim stops.

Away from the ground stick deflection gives rate of roll and pitch and the plane remains in the attitude you leave it in (autotrim.)

What works for me is bumping or tapping the stick to keep the wing down and using the rudder conventionaly to allign the plane with the runway. Airbus does want the crab held until round out.

It sounds tougher than it is, it becomes second nature, and you pick it up quickly

As far as bad landings go... I seem to crash it on on CAVU days in no wind and greese it on in gusty nasty days in 200 and a half Go figure!

 

[side stick-n]

F9 driver,

If I'm not mistaken, pitch reverts to direct law, however roll remains in normal law.

I'm curious if anyone else senses that when you kick the nose around to straighten it up, the upwind wing actually drops a little. It appears that the computers automatically do this to maintain the same relative forward direction. Much in the same way it does during an engine failure. I may just be imagining things or the imputs are such second nature, that I don't even realize it.

 

[DoinTime]

Aren't those flight control computers designed so you don't have to think about it?

 

[Jeff G]

Whoa, guys, hang on. There is no flight control law change at low altitude on landing. That's a very common misconception, but it's not true. I know that because I almost had my head handed to me by a check airman on a fed observation for bringing that up. It seemed like a great explanation for a not-so-great crosswind landing.

There is a short blending transition on takeoff at 50 ft RA from ground mode (direct law) to flight mode (normal law), but not on landing. The only change that occurs on landing is a transition to flare mode at 50 ft RA, when there is a 2 degree pitch down bias introduced below 30 ft over 8 seconds to provide flare feel. But that's all. Roll rate and pitch g settings work in normal law all the way to the ground, and until 5 seconds after touchdown (ground mode again).

Somehow, knowing that the control laws don't change all the way to the ground makes the landing a lot easier. You just fly the thing without worrying about what the plane is doing behind your back.

Side stick-n, I've never noticed the upwind wing going down like that, unless somehow the computer translates a sudden yaw as also a small roll, which it corrects. I'm probably reaching. It's turned out to be a mistake in this airplane, at least for me, to kick out the crab too early, so I usually don't do that until 20 ft or so. So I don't have time to notice. If it needs it, I just bump the wing down a little and hope to not float.

I'm with F9 Driver on this one. My worst landings are usually the ones with little wind. The really windy ones, I'm wound up and alert enough to wrestle the darn thing all the way to a stop. It's the "easy" ones that make you hide in the cockpit afterwards.

 

[side stick-n]

Oooops,

You're right Jeff, I didn't read far enough. That dang FCOM again. Now, who's the boob?

 

[F9 Driver]

I just checked our FCOM out for the explanation 1.27.thirtysomething.

At 50 ft the aircraft does trim 2 degrees nose down over 8 seconds to get the pilot to pitch up into a flair, roll autotrim does go away, and the elevators do have a direct relationship to stick input (i don't know if it is considdered direct law, but that is in essance what the elevator is in.)

That is the 319. I don't know about the 320, but there are differences (the speed brakes extend fully when commanded with the autopilot on in the 319 where you get only 1/2 with the 320.)

I could fly this thing for the next 30 years and still have lots of beer bets to loose!

 

[Huck]

"I'm curious if anyone else senses that when you kick the nose around to straighten it up, the upwind wing actually drops a little."

All aircraft do that - it is a roll moment induced by the rudder.

Take a toy airplane, hold it straight and level, then imagine a big rudder deflection. Note that the force from the rudder is resolved somewhere around the center of the vertical stabilizer. That force is a good ways above the aircraft CG, and tends to roll the aircraft away from the desired turn.

It lasts until the greater lift from the "clean air" and wider effective span of the outside wing takes hold - that's how we roll with rudder up at altitude.

There's other factors at work, but it's been 15 years since Aero I down at the Middle Alabama Trade School....

 

[Eagleflip]

"I'm curious if anyone else senses that when you kick the nose around to straighten it up, the upwind wing actually drops a little."


Waaaaalll, down there in Nawth Caroliny, we got ourselves a dang fine enginering school too (and no, the nickname ain't got anything to do with a "Tarheel"...) and they taught us a few things about swept wing jets...

Don't forget about the aerodynamic effect of advancing a swept wing jet's wing into the relative air. That swept wing becomes, in effect, less swept as it moves into the relative air and actually creates more lift, not less.

In my experiences with the Airbus, and the Eagle, and every other swept wing aircraft, when you step in the rudder to straighten out the aircraft from a crab, the advancing wing (that makes it the upwind wing) normally climbs, not drops.

Dunno why Side stick n' has that experience--could be the same reason I automatically enter a slight left bank every time I rotate the jet, e.g. a slight "brain-hand disconnect."

Interestingly, those who have flown the U-2 tell me that they crab normally into the wind during a crosswind landing, but when they roll out (near the flare phase) they wind up banking in the opposite direction than normal. That means they bank to the left in a right crosswind. Has to do with the amount of weathervaning encounted by that huge tail and long fuselage. Or so I've been told, anyway.

The good ol' boys at State didn't cover that one, ya' know.

 

[Snakum]

"Waaaaalll, down there in Nawth Caroliny, we got ourselves a dang fine enginering school too (and no, the nickname ain't got anything to do with a "Tarheel"...) "

Wow Flip, I didn't know you went to Wake Tech. Small world ain't it?



Minh Thong
(Raleigh's finest ... Wake Technical Institute Class of '92)

 

[Eagleflip]

You're kiling me....

 

[Bubba Dog]

Flew the 319/320/321 for 18 mos and it takes a little getting used to but don't make it to complicated,it's just an airplane and not the space shuttle. Hold the power in a little longer than normal,kick it out of the crab about 30 ft. above the runway,cross control and push the nose down so the elevator forces the nosewheel to stick real good on the runway so weather vaning doesn't occur so easily. Might need to bump up the speed 5 or 10 kts on final approach to give you a little extra energy to finish out the process.Also at 50' crack the throttles out of the flex position so the autothrust doesn't try to push up the power to maintain you in the box approach speed you programmed,this will also eliminate the pendalum effect. By cracking it out of autothrust at 50 'you now have old fashion throttles and can reduce when you are ready. Don't try for a smooth touchdown,just get it on the runway with crosscontrols and get it stopped soon. A lesser flap setting helps also if the performance can stand it. Hope this helps and good luck!

 

[Jeff G]

Might need to bump up the speed 5 or 10 kts on final approach to give you a little extra energy to finish out the process.

This works pretty well when it gets gusty, but GS mini usually takes care of it for you.

By cracking it out of autothrust at 50 'you now have old fashion throttles and can reduce when you are ready.

The problem with this is that cracking the throttles doesn't turn off the autothrust, it only limits its range a bit. Since approach power is well below this setting, cracking the throttles doesn't actually do anything except invite an ECAM.

Don't try for a smooth touchdown,just get it on the runway with crosscontrols and get it stopped soon.

You can't cross control, because by doing that, you're asking for a roll rate instead of a bank angle. Instead, you have to bump the wing down to the angle you want and center the stick.

A lesser flap setting helps also if the performance can stand it.

Flaps 3 works very well in a stiff breeze. I've never seen where you take a performance hit for flaps 3 vs flaps full. The landing distance is almost always within about 20 feet. Just don't forget to pull power off a little later than usual with flaps 3 or you'll drop like a rock that last 15 feet or so. Ask me how I know that. I'm pretty sure there's still a dent in BUF from that.

 

[Eagleflip]

Just don't forget to pull power off a little later than usual with flaps 3 or you'll drop like a rock that last 15 feet or so. Ask me how I know that. I'm pretty sure there's still a dent in BUF from that.

AT LAST! The true reason I routinely hit hard when landing at BUF.... I'm landing on a dent!

I used to think it was me, but I can now blame it on someone else.

Come to think of it, JeffG, I land in a lot of places where you've been. Maybe you left dents in other runways as well. That might explain all of my landings...

 

[Bubba Dog]

JeffG with all due respect you pull power a little sooner with less drag out there on the wing,i.e. flaps 3. This comes from 8 years of teaching folks how to fly the KC-135,thrust and drag relationships are the same for all aircraft. Also by cracking your throttles at 50 ft you are limiting how much thrust can be applied,I never said it turned of the autothrust. It all happens in about 5 to 6 seconds,very quick,works for me but maybe not you. I always put the aileron into the wind after touchdown and keep the nose straight with the rudder and never had the first problem while maintaining forward stick pressure in up to 29 kts of crosswinds.. And your comment about ground mini is correct but sometimes ground mini can add a huge amount of airspeed and you need to be aware of this coming over the fence,i.e. sooner power pull or you will float forever. Also in some wind situations it hangs you on the edge of the approoach speed and you need to bump it up a bit. If I was landing at Reagan I would darn sure use flaps full versus 3 on a wet runway,are you sure there is only 20' difference,common sense tells you are at a lower approach speed,thus less diffuculty to stop the airplane.
I would rather try to explain why I went off the end with full flaps versus 3 flap anyday.

 

[Bubba Dog]

Also Jeffg I never got an ECAM warning when I cracked the autothrust,not enough time I guess.

 

[Jeff G]

The landing distance for flaps 3 vs. flaps full comes right out of the OPS system for every scenario I bothered to look up. It surprised me too. For a gross weight landing in calm winds, dry runway, sea level and standard conditions, landing distances are:

Flaps full: lo: 7174, med: 4883, man: 3970. vref: 134
Flaps 3: lo: 7167, med: 4837, man: 3942, vref: 138
Flaps 3 (+15 kt): lo: 8000, med: 5324, man: 4421, vref: 148

Go figure.

JeffG with all due respect you pull power a little sooner with less drag out there on the wing,i.e. flaps 3. This comes from 8 years of teaching folks how to fly the KC-135,thrust and drag relationships are the same for all aircraft.

This last bit is a true statement. But my guess is that the extra drag with flaps full doesn't have as much effect on float as the extra slats that also go out with flaps full. Which is why you (or at least, I) tend to drop in early with flaps 3 only: not as much lift. JMHO, and from empirical evidence only.

 

[Bubba Dog]

JEFFG my guess the reason you might be dropping in early is you are trying to see the same picture in the windscreen that you are for flaps full,i.e. in the middle. On a flaps 3 you aimpoint is more in the lower portion of the windscreen and sometimes we try to put it in the middle like a full flap resulting in an increased sink rate and dropping in as you say. Amazing about the landing distance.

Good luck and I wish I was still flying the Bus,it's a great machine for everyone concerned.

 

[Jeff G]

Could very well be the aimpoint. Good point.