Why reinvent the wheel? Singapore Airlines A340-500 did it for years and now as you know the A350’s do it.
Ok it’s not 20/21 hours but is blighty close.
The Singapore aviation authority is different to Aussliland with duty, however it is done with how many crews? Used to know a few guys in the 340 era, landing currency was an issue.
2 flights a month... sounds like a pretty good roster to me!
Or will unions hold business to ransom? Get Red Q going and do KL - SYD - LHR
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