whether you are trained or not has absolutely nothing to do with whether you are covered is something goes wrong or you get hurt. That is a lame excuse and is not correct at all. Helping out once in a while is definately part of any job.
As a former union rep that dealt with this crap all the time, I can assure you that you are most certainly not covered when you are doing things outside of your job description. If you get injured while throwing bags, then workman's comp doesn't cover your injury. If you injure a wheelchair passenger while pushing them up the jetbridge, then that passenger can hold you personally liable, because you were not trained in the operation of the wheelchair and you were not authorized to handle that job in your job description.