B777 Drift on gnd?
 

Have any of you ever notice there is a slight drift (1-2deg) when on the ground in about 50% of the time? Couldn't for the life of me figure this one out! Could variation have any thing to do with it?

SR

IRS position wandering?

i apologise for not being specific enough. what i mean by drift is there is 1-2deg difference between the TRK and HDG? as far as i understand it, this should happen when there is a drift i.e crosswind. but this is when the acft is on the ground?!

ideas?

SR

just an ideea:
I saw the 737's from behind when taxiing.They are not alligned with the taxi line,they seem to deviate a little to the right,I think(due to gear design). If this is the case,the HDG will differ from the TRK with 1-2 degrees.Maybe it's the same with 777
alex:ok:

Sounds like the main gear shimmy dampers need bleeding !!

solved!

the reason is due to different Magnetic Variation Tables being loaded into the FMC and ADIRU. The FMC has 1995 MagVar Tables while the ADIRU has 2005 MagVar Tables. TRK comes from FMC while HDG comes from ADIRU.

SR

Gee, how do you know all that? Just kidding. But you said this apeeared only 50% of the time? How did you solve that ?:confused:

it was explained in one of boeing's bulletin! :O