Complex sentence: Trainees that sustain injuries or illnesses that disrupt their training are sent to the Medical Rehabilitation Flight, and once found medically fit, can join a flight that is at an equivalent place in the training cycle that the trainee left.
Split into simpler sentences: Trainees that sustain injuries or illnesses that disrupt their training are sent to the Medical Rehabilitation Flight. Once they are again medically fit, re-join a flight that was at an equivalent place in the training cycle that the trainee left.