Actually,the B737 IRS can align up to (and down to) 78*15' N and S (by the special procedure for the high Lattitude alignment begin from 70*12' to 78*15' (~17 mins)) because it affected by the earth rotation force alone,but for some reasons that we require to convert the result from the IRU such as the true heading and so many things into the magnatic reference when we fly. So we require the MAG VAR data to convert those things which is become the limitation of the B737 CL because they have the internal MAG VAR data for B737 IRU only from 73*N down to 60*S.
Regards,
KANIT A.
