In the days when I was using the GNS-XLS (BAe146 '01-'05) the only arc it would fly was a database approach arc.
To fly the 15nm arc at Cairns (YBCS) from RAINY (about the 270 radial) to SUNNY (360 radial) we'd create defined points at 15 nm every 30 degrees (I think), and fly a series of straight lines, which kept one + 2 nm of the arc which in Australia, is within tracking tolerance.