Fortunately delays with QF21 can be contained by swapping the aircraft to QF8, provided that days QF7 is reasonably on time, as was the case here. The return QF22 was operated on-time by B789 VH-ZNE which had brought QF7 into Dallas. ZNH subsequently operated QF8 on time.Also on Wednesday 12, QF21 (B789 VH-ZNH), the 1245 hours SYD-DFW was not airborne until 1416 so gate arrival same day at 1442, 57 behind is the forecast.
It seems there is enough 'stretch' in the B789 Northern Summer Schedule to allow one aircraft to be missing - namely VH-ZND which has been in LAX maintenance since arriving with QF11 on Monday. This follows a 48 hour layover in Sydney Maintenance 1st to 3rd April (having positioned from Melbourne), and a 4½ day layover in Melbourne Maintenance 22nd to 26th March (having positioned from Sydney) !!