...depends on the company's philosophy on "outside" flying and how hard they work you to the Flight Time Limitations (FTL).
In my previous existence on the 757/767 I wanted to do some freelance Twin Squirrel Public Transport work and the company said I could do it as long as I:
1) asked for the DFO's permission in advance (not every time, just once...
)
2) kept the rostering department informed of flying hours to make sure the total limits were not exceeded.
3) did not infringe any statutory days off requirement. e.g. after a Florida "bullet" long haul we needed 3 days off - couldn't legally do any freelance work then.
Depends on whether we are talking Public Transport flying or Aerial Work, I have no idea what category instruction comes under....
