I think you guys are speculating too much about how SIA picks their interview candidates. Ex-RSAF trainees have an advantage over others? I dont think so. They won't get chopped if they could fly. I, for one was a choppee too.
The first interview is held on every friday of the month. And the second interview is on every wednesday. The passing rate of the interviews is low and you will be surprised how many applicants you see on your interview day. Despite the low %, the company will try to fill about 8 courses a year to send to OBS/SFC.
If you are not being called up yet, perhaps there are too many applicants to filter through. As long as you meet the bottomline requirements, you will be shortlisted.
Anyway for those who got shortlisted, all the best and study real hard. Be honest, be confident and most importantly, be humble during the interviews.