A number of years ago I was in the position of being one of those who had a QRA committment. As an Engineer, we had to be available to deal with the jets out of hours - not a large pool of people so Duty Eng average was once every 4-6 weekends. This, along with QRA aircrew, made us exempt from SDO. So, over Crimbo we volunteered to cover SDO. Made no difference to us - we'd be at home, unable to drink and available for call-out anyway.
Not difficult, and meant that the minimum no of people were on duty.
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