henkybaby
The 900 hours is a legal maximum that crew are allowed to fly (Civil Aviation Authority) - this takes into account the effects of working at altitude which as you know the cabin is pressurised to circa 7000 feet above sea level - and as a consequence any activity undertaken (pulling heavy carts and trolleys and even walking around ) takes more effort than if one was at sea level plus continuously crossing multiple time zones and the 'jet lag' this brings.