ZID Controller describes what he believes may be a glitch in A300 and A320 FMC's causing track deviations during arrivals to DTW.

Narrative

Possible FMS issue with Airbus aircraft. When the aircraft are vectored off of an arrival and then given a fix down the way with a crossing off of that fix; the FMS is reverting to the original route and turning the aircraft to rejoin the original route. This has been seen in Airbus A300 series aircraft and the Company involved has been notified through our QA department. QA at Indy Center has spoken with the Company about the issue and we thought it was a Company specific issue. This is the first time we have seen it happen with another carrier. The safety issue in this scenario is that aircraft have been seen to turn anywhere from 15 degrees to 40 degrees to rejoin the original route and sometimes the pilots are correcting and sometimes they are not. You can imagine the safety issue of having aircraft turning those amounts when pilots and controllers are not expecting this to happen. I do not know if the pilots all are aware that the aircraft are off course or not. If they are; why are they allowing the A/C to turn off course and if they don't know then all pilots need to be aware of this issue so they can correct. This is beyond me but it appears that it could be a software or programming issue with these FMS in the Airbus. Are these FMS in any other A/C as well? I believe the A300 operator has already run these scenarios in the simulator. They forwarded the reason why this is happening to our QA department. This issue needs to be brought up industry wide and find out if the FMS needs to be reprogrammed or if this can be solved through information dissemination through the pilot programs.