Originally Posted by paul21

Augmented 3 crew is probably 16 hour duty limit and 12 hour flight limit. The extra crew member is discounted because of on board sleep quality.

The duty period probably starts 1 hour before scheduled departure, so a 16 h limit on a 10.5h flight would give 5.5-1.0=4.5 hours of buffer. I believe IROPS gives a 2 hour extension for 6.5 hours of buffer if the pilot agrees. If there are 4 crew it goes up another 2.5 hours for 9 hours of buffer potentially. I say probably because I don't know BAs contract and I'm not sure the laws for international flights. Also there are other total rest and total hour constraints.