Large aircraft auto rudder question

[Kliperoo]

Hi All,

Quick question for you. Saw a 777 a while ago in Christchurch NZ land in a good crosswind that required a crab appraoch. Myself and others saw it move from this style approach to aligning with the runway direction. Pretty simple so far.
I have since been told that this may not have been possible due to the auto coupling of the ailerons and rudder within large transport aircraft and that it might be impossible for pilots of these aircraft to control the rudder at all above normal taxi speed.

Could anyone that flies anything in this catagory (320, 767, etc) please let me know if it is possible to a) use the rudder independent of the ailerons; b) if not, is it possible to disconnect the auto-coupling system or c) if again not possible, does the computer automatically do this....

thanks

[18-Wheeler]

The 747's don't, though there's a gadget called the Turn Co-ordinator that applies a little bit of rudder for you with the flaps out, to help keep the skid ball in the centre.
But you can apply rudder and ailerons in a 747 in any direction at any time and they will follow the controls.

[Intruder]

I have since been told that this may not have been possible due to the auto coupling of the ailerons and rudder within large transport aircraft and that it might be impossible for pilots of these aircraft to control the rudder at all above normal taxi speed.

That may be the case during a coupled autoland approach, where the rudder is controlled by the autopilots. In fact the 747-400 (and likely others) will put in a certain amount of cross-control when the crosswind is in excess of about 5 knots.

Otherwise, the ailerons and rudder are independent, including when the autopilots are disconnected during the above-described autoland approach.

The turn coordination mentioned by 18-wheeler is a function of the yaw dampers, and is also independent of the primary rudder control.

[petitfromage]

Kliperoo,

Yes there is a auto couple between the aileron/rudder but....if the pilot inputs i) more rudder or ii) applys crossed rudder & aileron he/she can over ride it.

It is still possible (although ugly) to side-slip a heavy jet.

Crosswind approaches in Heavy jets are complex not because of the autocouple but because of sweepback.

Do a quick search in 'tech log' and you'll find a million reasons why we crab all the way until the flare.....and even then, we'd rather land slightly sideways than drop a wing to much and have a pod strike.

[Spearing Britney]

For the aircraft in question, 777, there is a wheel to rudder crosstie for low speed cases (primarily allowing a little bit of wheel input to help control an engine fail) and there is turn co-ordination - controlled by the same system that does the yaw damping.

However, just like a PA-28 you can fly it with left wheel and right rudder if you want to! So yes, taking off a crab before landing is usually done with rudder, and often in the flare. Manual landing are allowed at up to 40 knots crosswing, if doing an automatic landing (allowed with up to 25 knots, or sometimes 15 knots, of crosswind) then the autopilots will straighten the crab a bit earlier (500-200 feet) and land wing low.

Hope that helps!