Q
I'll quote dirctly from the 747 OM:
"The main deck cargo handling bus is powered on the ground when external power 2 or APU generator 2 is available. An interlock prevents either source from powering main electrical busses and the main deck cargo handling bus simultaneously. Pushing the available Power source switch ON deenergizes the main deck cargo handling bus. When both external power 2 and APU generator 2 are available, external power is used. Pushing external power 2 switch ON transfers main deck cargo handling loads to AOU generator 2.
NOTE: When cargo handling is required, selecting only the number one auxiliary power source ON ensures the main deck cargo handling bus remains powered by the number two auxiliary power source."
End quote.
Operating Manual 6.20.6.
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MP