rudder yaw autoflight question

[Guest 767driver]

Folks,scenario - 17kt tailwind at 002/29kts, TAS=480, GS=497, so 17kt tailwind, from behind us, our heading should be 250, but our track is a left crab at 248 deg.So, my magenta line is 250, but my green diamond is at 248, so about 2 degrees due to the crosswind.Can you please tell me what the aircraft autopilot is doing to 'fight' the crosswind - is it employing rudder yaw? The reason I am asking, aside from my curiousity, is a gentleman posted earlier in the forum that contrary to popular belief, rudder yaw is NOT used to correct for a crosswind during the flight.Also, with that 2 degree difference, doesn't that add up over a long distance? If so, will the aircraft continue to fly that offset track until the crosswind is no longer present, and then correct back to the magenta line ?cheers,Josh

[marcom]

Hi Josh,what that person said, is correct. The AP won't add any rudder input to compensate for a xwind during cruise.The AP won't even care what your heading is!It knows what your track is (IRS info) and it knows what track it should fly (to stay on the magenta line). It then aligns the data (turns until track = track to fly) and voila you are flying on course.If the wind is stronger, you might see a difference of more than 5-6 degrees between heading and track. Hope that helps!Regards,Mark

[Guest 767driver]

MarkThanks for the explanation>> If the wind is stronger, you might see a difference of more than 5->> 6 degrees between heading and track. does this mean over distance there will be a large difference between heading and track ? (i.e. track error), or once the crosswindcomponent is no longer there, it will just return to correct track?cheers!Josh

[marcom]

"does this mean over distance there will be a large difference between heading and track ?"distance doesn't play a role in this, Josh.I think you are mixing up a few things:Heading is simply the direction in which the nose of the aircraft points. Track is the actual direction the aircraft is moving.Only in a 0 x-wind condition will track = heading.Track error is the lateral distance you are from your indented route (the magenta line)THe AP will always maintain the correct track. Its just your heading that will be affected by wind and not your track. (As long as you are in LNAV).Remember, the EHSI (the lower display) shows Track and not Heading as its reference! So when on course, the magenta line should be straight (until the next waypoint)Just a small example:YOu are in a 0 wind condition and are flying a heading of 170 which is also your track. Your next waypoint requires you to fly a track of 170.Now you have a x-wind which blows you 5

[Guest Mark McG]

Josh,Look at this photo. The aircraft is right on track and flying the magenta line even with a near 190Kt X-Wind. The white triangle on the EHSI shows the current aircraft heading. The dashed magenta line represents the HDG line, as selected on the MCP. The MCP HDG has been set to lie on top of the white HDG triangle on the EHSI - this is in case of LNAV mode dropping out. This photo is a good example of what the other Mark is meaning.http://www.jetphotos.net/viewphoto.php?id=212294

[Guest 767driver]

Mark & Mark :)thanks for the explanations, i think i am understanding this better, however, this is what i am refering to (photo below):You can see the the yellow heading is 250, and the track is 247, a difference of 3 deg, due to the 17 kt tailwind from behind the aircraft.So, I am wondering if this difference doesn't become culmulative over time as the difference between track and heading widens?http://forums.avsim.net/user_files/93834.jpgcheers!Josh

[marcom]

Josh,the Airbus ND is a bit different in this regard as it is heading oriented and not track oriented.As you can see, the track diamond (the green diamond at the top of the rose) is right on the green route line. Thus the aircraft is right on track (247

[Guest 767driver]

Mark, Ah.. i understand now. Boeing & Airbus confusion! Cheers, Josh

[Qavion]

"Boeing & Airbus confusion!"Not quite, Josh.Not sure about Airbus, but Boeing have optional "TRK UP" (PIC style) and "HDG UP" (your Airbus type) displays. It's simply a customer preference.Also, as far as I know, VOR and ILS displays are always shown as HDG up. So you can have both display types on the same aircraft.Cheers.Ian R>