CASA don't give a rats (no pun intended) about whether you can or can't get the cabin service done. They only care about SAFETY. If they believe (and in this case they do) that a 717 can be safely evacuated with 3 cabin crew and it fits within the 1:36 rule, then what is the problem?
If the company wants to put ridiculous service levels on board then that's an issue for the company, not CASA! If you are unable to complete your service duties, or if service duties begin to impede on your safety responsibilities then you need to address that with both the FAAA and the airline concerned.
Safe Flying.
SG
