Actually, it would be easier for GA aircraft with old avionics to accurately navigate to true north than it is for modern aircraft - read out the magnetic heading from the compass, add/substract the magnetic variation and set your DG accordingly, and voila - true north navigation.
On airliners, you rely on the database that could easily be 15 or 20 years old, and there is no way you can compensate for that from the flight deck.
Originally Posted by
India Four Two
A question for the experts. What is it about how the MagVar database is stored in the ADIRU, that requires a replacement unit? Is the database "knitted" like the ROM in the Apollo Guidance Computer?
AFAIK there is no way to update any part of the ADIRU software via onboard data loader, so the only way to update it, is to take it out and send it to manufacturer/avionics shop.