Originally Posted by fin 645

Are crew rest/layover requirements the impediment?

As an illustration, the recent rescue flight CMN-YUL appears to have operated as a same day YHZ-CMN-YUL turn with a crew day close to the limit. The operating aircraft had been ferried YYZ-YHZ the night before.

Originally Posted by songsc

Good point. It is possible that extra pilots are deadheading on the same plane to operate the return flight.

I recall certain countries have quarantine exemptions for airline crew members.