On the additional Boeing 747-8, there is no need to switch off the air conditioning units (so-called “packs”) before starting the engines.
On the Boeing 747-8, the environmental control system features an automated pack shedding function. When you pull or turn the engine start switches to initiate the engine start sequence, the aircraft’s computer system automatically shuts off the packs (or isolates them) to ensure that all available pneumatic bleed air pressure from the APU or ground cart is directed entirely to the starter turbines.
Once the engines are successfully started and stabilized at idle, the system automatically restores air delivery to the air conditioning packs, meaning they will turn back on by themselves without pilot intervention.
While manual pack shutdown was a standard practice on older aircraft variants (like certain configurations of the older Boeing 747-400 to maximize duct pressure), the newer 747-8 completely automates this workflow.
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