LNAV 10 degrees off the FMS heading

[rlawrence]

 This is what I'm experiencing when engaging LNAV. It's always about 10-20 degrees off. I can use HDG SEL to fix it. The FMS  has 131 course, but the plane never flies the right heading unless I use HDG SEL. I never really thought much of this variation until I was flying into MSP and the APP controller said I was on the wrong heading.  My question is this magenta line versus what the plane is flying normal? 

Richard

 

 

 

[mmerelles]

This is called crosswind correction  

The aircraft is pointing the nose 10 degrees to the right of desired FMC trajectory/course due to the existing 88 knots of right crosswind pushing the entire aircraft to the left. If the aircraft a/p does not point the nose to the right to counteract those strong cross winds the aircraft trajectory will be deviating to the left of the desired course.

 

Try changing to HDG SEL mode instead of LNAV and set HDG 131 to coincide FMC course. The aircraft nose will be pointing to 131 as per the FMC, but after a few minutes your aircraft will be flying far to the left of the desired trajectory due to the winds.

 

I prepared a picture for you, hopefully this makes simpler to understand what is going on here.

 

 

 

Edited February 22, 2017 by mmerelles